SGI-R47 for pdf.p65

~~j STATENS GEOTEKNISKA INSTITUT V _.._ SWEDISH GEOTECHNICAL INSTITUTE

Geotechnical Properties of Clay at Elevated Temperatures LOVISA MORITZ

LINKOPING 1995

STATENS GEOTEKNISKA INSTITUT SWEDISH GEOTECHNICAL INSTITUTE

Rapport Report No47

Geotechnical Properties of Clay at Elevated Temperatures LOVISA MORITZ

This project is partly financed by the Swedish Council for Building Research (BFR), project numbers 900401-2 and 930537-7.

LINKOPING 1995

Report Swedish Geotechnical Institute (SGI) S-581 93 Linkoping, Sweden

Order Library SGI Tel. 013-20 18 04 Fax. 013-20 19 14

ISSN ISRN

0348-0755 SGI-R--95/47--SE

Edition 500

Printer Roland Offset, Linkoping, November 1995

SGI Report No 47 2

Preface

This report deals with the results from a laboratory study of the properties of clay at high temperatures. Parallel to this study full scale field tests with storage of heat at high temperatures were performed at the same site as the samples for the laboratory were taken. The project has been financed jointly by the Swedish Council for Building Research and the Swedish Geotechnical Institute.

The study has been carried out in the laboratory at the Swedish Geotechnical Institute under the authors guidance. The tests were performed by Dr Mensur Mulabdic', at present engaged at the Geotechnical Department of the University of Zagreb, Croatia, and Inga-Maj Kaller. Valuable views and suggestions were given by Ulf Bergdahl during the course of the project and Rolf Larsson has critically reviewed the report.

Linkoping, September 1995

Lovisa Moritz

Clay Properties at Elevated Temperatures 3

SGI Report No 47 4

Contents

Preface

Summary ............................................................................... ............ 7

1. Introduction ........... ......................................................... ................ 10

2. Previous Studies .......... ............................................. .................... 12 2.1 Summary ofthe literature study ................ .......... ........... ............. 18

3. Soil Material Studied ......................... .... .... .................................... 20 3.1 Test Site ............................................. ... ...... .. ............................. 20 3.2 Extracted Specimens ........... ........................................................ 20

4. Triaxial Compression Tests ..... .................................................... 23 4.1 Regular Triaxial Apparatus .................................. .. ...... .... ......... . 23 4.2 Triaxial Apparatus for High-Temperature Tests ....................... ... 24 4.3 Test Programme for Triaxial Compression Tests .... ..................... 25 4.4 Mounting, Consolidation ............................................................. 26 4.5 Heating and Undrained Tests ..................................................... . 27

5. Results of Triaxial Tests ............................... .. .. ........................... 28

5. l Results of Consolidation and Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 8 5 .2 Results of Rapid Undrained Tests ................................... ............ 31 5.3 Results of Slow Undrained Tests ................... .... .... .. .... .... ... ......... 34

6. CRS Tests ................. ...................................................................... 36 6.1 Standard CRS Equipment .................................. ... ..... ............. .... 36 6.2 CRS Equipment for High-Temperature Tests .............................. 39 6.3 Test Programme for CRS Tests ................. ...... ............................ 40


7. Results of CRS Tests ................................................ .. ..... ......... .... 41 7.1 Preconsolidation Pressure ................ .................. .......... ...... ......... 41 7.2 Compressibility Modulus ......... ......................... .... ... .. ..... ............ 44 7.3 Permeability ............ ................ ..... .......... .. .. .... ........ ... .. .... ......... .. 47

8. The Experimental Field .. .......................... .................................... 48 8.1 Description .............................. .. .... ............................................. 48 8.2 Results ......................... .................................... ......... ............ .... . 48

9. Comparisons and Discussion ..................................................... 54 9.1 Comparisons ofResults from the Laboratory and

the Experimental Field ......................... ......................... ...... ....... 54 9.2 Discussion on Creep ........... ............................... ................ .... ..... 55 9.3 Discussion on Temperature Cycling ................................... ..... .... 57 9. 4 Estimating the Magnitude of Settlement for a Heat Store . . . . . . . . . . . . . 5 8

10. Conclusions .... ........................... ...... ............ ................................... 63

References ............. ... ............ .......................................................... 65

Appendix 1 ........ .................. ....... ...... ................... ... ...... ............ 68

Appendix 2 ............................ .... .......................... ... .. ................ 69

SGI Report No 47 6

Summary

In order to investigate the properties of clay at high temperatures a laboratory study was conducted on samples from a site nearby a heat store. Simultaneously, full scale field tests were performed at this store. A literature survey of laboratory tests on clay at elevated temperatures has also been carried out. Finally, a calculation model for settlements in a heat store is presented.

Some ofthe most recently published articles on the subject are summarised in the report. In addition, an earlier article by Campanella and Mitchell in 1968 is included for its fundamental content about temperature effects on clay.

The laboratory investigation consisted of triaxial and CRS tests that have been carried out on 24 specimens taken at depths of 6 and 9 metres.

The triaxial apparatus has been modified to allow heating of the specimen to 70 °C. The tests were designed to simulate different situations when heating a heat store and determining undrained shear strength at different temperatures. Firstly, the specimens to consolidate for the in situ stress, then they were heated under drained and undrained conditions. Finally, undrained active compression tests were carried out at two different deformation rates and three different tem

peratures: 8, 40 and 70 °C.

To determine the compression characteristics of a clay in Sweden, CRS tests are usually carried out in the laboratory. In the CRS test, an undisturbed specimen is consolidated in a vertical direction at a constant rate of deformation and constant cross-section area. The tests in this study were conducted at room temperature about 20 °C, and also at 40 °C and 70 °C.

At the marina in Linkoping, the Swedish Geotechnical Institute, with the support ofthe Swedish Council for Building Research, established an experimental field for heat storage which was started in February, 1992. The experimental field was set up for the purpose of studying developments in settlement, pore pressure,


temperature, shear strength and other factors connected with high-temperature storage in clay. The experimental field results presented in the report are from the first two and a half years of operation. During this period oftime, store 1 has passed through five complete temperature cycles between 35 and 70 °C and store 2, which was started three months later, has attained a constant temperature of 70-75 °C.

According to the observations and results that have emerged from laboratory tests and field measurements, the amount of settlement in a heat store under design can be estimated by means ofa preliminary calculation model. In the model, it is assumed that the temperature is the only load effect that occurs. This calculation model is based on the assumption that the preconsolidation pressure decreases as the temperature increases. Alternatively, it is possible that creep starts at a lower effective stress level at elevated temperatures than under normal temperature conditions.

In order to use the model to calculate the settlements in a heat store, the temperature during operation in the middle of the store has to be known. The time the excess pore pressure exists in each temperature cycle has to be estimated. Furthermore, some deformation parameters for clay have to be known.

The deformation parameters are evaluated from standard CRS tests conducted at normal temperatures. Subsequently, a new preconsolidation pressure for the maximal temperature must be calculated as well as a new compressibility modulus. The soil's in situ vertical stress is calculated and compared with the original preconsolidation pressure and the calculated preconsolidation pressure at maximal temperature.

The model does not include heaving due to heat expansion of pore water and soil particles, nor does it account for the fluctuations of the temperature, which in reality takes place as a result of the fluctuation of the settlements .

Some ofthe observations from the field tests, the triaxial compression tests and CRS tests in the laboratory can be summarised in the following points:

• When the clay is heated the pore water and the clay particles expand, which gives rise to an increase in pore pressure and swelling ofthe clay ifthe possibility for drainage is limited.

SGI Report No 47 8

• When calculating pore pressure for a heat store, the fact that horizontal stress increases with rising temperature must be taken into account.

• The increase in temperature can start a creep process in the clay when excess pore pressure has been equalised. This creep process can be calculated by assumption of a lowered "preconsolidation pressure" and belonging changes in the creep parameters.

• In normal Swedish clay, shear strength decreases with rising temperature. Similarly, the modulus, M decreases before the preconsolidation pressure. In

0,

respect ofthe tested clay, the decrease is in the order of 0.5 % / 0 C.

• Preconsolidation pressure apparently decreases with rising temperature, according to normal interpretation ofthe oedometer tests in the laboratory. No field evidence for a lowering ofthe preconsolidation pressure exists. In fact, the measured pore pressure and deformations at cycling in store 1 contradicts the assumption of a lowering of the preconsolidation pressure at increasing temperature.


Chapter 1.

Introduction

Research has been conducted in Sweden since the end of the 1970s on the seasonal storage of energy in clay in the form of heat. Heat is introduced into the clay by means of vertical hoses pressed down into it and through which a heated liquid circulates. Low-temperature stores, i.e. stores that are heated up to a maximum temperature of 40°C, were studied initially. More recently, an interest has been shown in heat stores with temperatures up to about 70 °C, which are termed high-temperature stores. Since a heat pump is not needed for a hightemperature store, this makes the system solution somewhat simpler and cheaper. One of the problems of storing heat in clay is that settlement and lowering of the soil's shear strength can occur and the higher the temperature of the clay layer, the more likely it is that these problems will arise. Settlement can have an adverse effect on the serviceability of the hoses pressed down into the clay and reduce the usefulness of the upper surface, as well as causing damage to nearby buildings. The reduction of shear strength can give rise to problems from a sta

bility aspect.

Geotechnical studies of heated clay on a laboratory scale have been conducted since the early 1950s. These studies are comparatively few in number and more often than not carried out at low temperatures, probably due to their limited sphere of interest and the difficulty of handling hot clay in a laboratory environ

ment.

Follow-up field studies of pore pressure and settlement in low-temperature stores in clay have been carried out on several occasions, e.g. at Kungalv ( 1987) and Soderkoping (1992). The results of these studies show that pore pressure varied with temperature and that settlement was relatively small.

The change occurring in the properties of clay when heated is a complicated process and no field tests have previously been carried out at high temperatures. The Swedish Geotechnical Institute (SOI) therefore set up an experimental field

SGI Report No 47 10

at the marina in Linkoping where clay is heated to 70 °C for the purpose of studying the effects of energy storage at high temperatures. The experimental field has a large number of instruments for recording and measuring events in the clay in regard to settlement, temperature and pore pressure. Continuous sounding tests made are also made to check for changes in shear strength, etc. in the heated clay.

A desirable future objective is the ability to predict the magnitude of settlement and shear strength changes when planning heat stores, including high-temperature stores. To make this possible, it is necessary to know what geotechnical parameters undergo a change as a result ofthe application ofheat. Since research in this field is limited, the Swedish Geotechnical Institute decided, in parallel with the experimental field activities, to study the clay in the laboratory using methods suitably adapted for this purpose. The laboratory methods are calibrated against actual results obtained in the experimental field so that the results will have greater relevance.

The triaxial compression test is the most effective and widely used geotechnical laboratory method of studying the behaviour of soil in different stress situations. For this reason, triaxial compression tests were chosen for the initial studies. Later on, a change was made to the somewhat simpler CRS test, which is more widely used in Sweden, for determining the geotechnical compression and consolidation characteristics.

This report deals with triaxial compression tests and CRS tests at temperatures up to 70 °C and the results obtained with them. Some comparisons are also made with the results obtained from the experimental field. A method of estimating settlements in a proposed heat store in clay is also described.


Chapter 2.

Previous Studies

A survey ofthe literature in the Swedish Geotechnical Institute's database, SGILINE, reveals that geotechnical laboratory tests on clay at elevated temperatures have been conducted on a number of occasions. However, little in the way ofnew results has been published in recent years. The purpose of studying how the properties of clay vary with temperature has partly concerned extremely large temperature changes, 100-500 °C, with a view to the storage of spent nuclear fuel in clay, and partly concerned moderate temperature changes, 1050 °C, which can occur in connection with sampling, storage and tests in a laboratory environment. The latter can also be related to low-temperature stores. In Sweden, studies directly related to heat stomge in clay have also been conduct

ed.

Some of the most recently published articles on the subject are summarised below. In addition, an earlier article by Campanella and Mitchell in 1968 is included for its fundamental content.

Campanella and Mitchell ( 1968) conducted a series of triaxial compression tests on clay at different temperatures up to about 60 °C. They showed that in

undrained tests the pore pressure increases with temperature and if the temperature drops so also does the pore pressure. The graph describes a "hysteresis loop". They have also described a theoretical model for estimating pore pres

sure changes due to temperature changes. The pore pressure change, ..1u, can then be expressed as:

(2.1)

SGI Report No 47 12

where w = water content, Ps = specific density [t/m3],

11s = thermal coefficient ofcubical expansion ofmineral solids [0 C-1],

11w = thermal coefficient of expansion pore water [0 C-1],

7}31

= physico-chemical coefficient of structural volume change caused by a change in temperature [0 C-1],

mv = compressibility of soil structure [kPa-1]

L1T= temperature change [0 C].

The authors describe the pore pressure change as being directly proportional to the expansion ofthe pore water and clay particles due to the effect of the temperature change. The modulus used corresponds to a load-relief modulus since pore excess pore pressure results in a reduction ofthe effective stress, which can be regarded as unloading. Furthermore, the equation is complemented at the end with a coefficient, 7}

31, for taking into account any possible creep effects.

Burghignoli et al. (1992) have carried out experiments by varying the temperature cyclically for both natural and laboratory-manufactured clay under drained conditions. The temperature was varied between 15 and 60 °C. Three different kinds ofclay having a liquid limit which varied between 52 % and 63 % were tested. The experiments were performed in a triaxial cell. Temperature-controlled liquid circulated in a metal hose fitted round the specimen. A pore pressure sensor and a thermometer were inserted into the soil specimens. The effective stress was maintained at a constant level in each specimen throughout the entire test by varying the temperature extremely slowly. Each specimen underwent a complete temperature cycle.

The results ofthe tests show that temperature variations in clay produce irreversible changes in volume, the magnitude of which increases with the amplitude of the temperature cycle, see Fig. 2.1. The magnitude of the deformation is also dependent on earlier temperature cycles and the duration ofthe temperature increase. The total irreversible void ratio change in a temperature cycle of .1eTc can be expressed as:

(2.2)


-----0

C"l

E CJ 2 ------- It.Ver rt:.Vtc.._, I-. Q.) 4 h C

....., lU ~

-0 6 Q.) IB'C 48' C 1a· c c:: lU I-. 8

-0 Consolidation at ._ 0 10 p'=196.2 kPa Q.)

E I

-0 ~ 12 I•______!

> 14

101 103 104 105

Time (s)

Figure 2.1. Volume of drained water during a temperature cycle. Burghignoli et al (1992)

and the void ratio change that would have taken place due to creep at constant temperature during the same period oftime, Llecr can be expressed as:

(2.3)

where .1V TC is the total irreversible change in volume during a temperature cycle,

.1Vcr is the change in volume due to creep during the same period of tin1e and Vs is the volume of the solid substance.

The authors noted that the ratio between ecr and eTc was approximately constant, see Fig. 2.2, where ,1eTC refers to the difference in void ratio before and after a temperature cycle. The change in void ratio due to creep was about halfof L1erc·

Seneviratne et al. (1992) have carried out numerical calculations of stresses and deformation in respect ofheated clay and compared them with laboratory tests described in the literature. The FEM model that was used in the calculations is based on a modified Cam-Clay model and a thermoplastic portion. The material

parameters are taken from different experiments described in the literature.

SGI Report No 47 14

30 --e- D. T=40 ·c ---e,.-. 6T=30 ·c

----"' 0 20 --· D.T=20 ·c -* i< 10

Cl)

<l

-10 / /

~--I/ .:' /I-2QL--L--:.l_j[_.....L.__j__...L-_L-.J.__L__L..__L_---1.--'

-20 -10 0 10 20 30 40

D.ecR ( *1o-J)

Figure 2.2 . .1.De1c versus Ae0

, for temperature cycles of varying amplitudes. Burghignoli et al (1992)

In an undrained case, the authors find that wide differences exist between the calculated and observed results, which is partly because the calculation model does not take into account the irreversible change in volume that occurs after a temperature cycle. In drained tests, the calculated results coincide fairly well with the observed results, except in connection with low temperatures and cooling. In the case ofundrained shear tests, the results do not show good correlation. According to the experiments, the specimen is compressed while the calculations indicate that slight heaving ought to take place. The authors attribute this to creep. Shear strength diminishes with increasing temperature as a result of the increase in pore pressure. In the case ofdrained shear strength, the calculated results coincide well with the observed results. The heating phase in the calculation model appears to coincide fairly well with what happens in the experiments.

Tidfors ( 1987) has studied the way in which the deformation characteristics of Swedish clays vary with temperature. She has carried out some 80 CRS tests and incremental oedometer tests within a temperature range of7-50 °C. In the CRS tests, the specimen was heated under drained conditions and subsequently loaded. The rate of deformation was 0.84 %/h. In the incremental tests, the temperature was raised at the end of each load step in steps of 10 °C and afterwards lowered to 7 °C before the next load step was applied and the procedure repeated.


• •

Five different types ofclay were tested. The results show that the preconsolidation pressure decreases with increasing temperature. Tidfors expresses the preconsolidation pressure as a linear function with temperature, where the inclination coefficients vary between -0.22 and -0.73 kPa / °C for the different types of clay that were studied.

Tidfors suggests that the sedimentation environment, water content and clay content are three important factors influencing the way in which the deformation characteristics of clay change with increasing temperature. It is further recommended that the decrease in preconsolidation pressure be expressed as a function of the soil's liquid limit, see Fig. 2. 3. From the incremental oedometer tests, the

author concludes that the deformation at a certain temperature increase is due to the level of stress. The higher the stress, the greater will be the effect ofan increase in temperature.

14

be12

10 ::R0

u 8 0

0 0

--0

b

6b •<I

-4

• CTH2 X Lulea

0 0 20 40 60 80 100 120 140

Liquid limit wL, %

Figure 2.3. Relative apparent decrease in preconsolidation pressure at an increase in temperature by 10 °C as function of liquid limit. AfterTidfors (1987)

SGI Report No 47 16

Temperature, °C 5 30 45 60100

~ 80 "'-....

~-II) ~ -=-7; 60 {-

II....._ ...... ...... ~

t-·..,t) 40 '· I-.

20

0

Figure 2.4. Apparent change in preconsolidation pressure in percent versus test temperature as interpreted by Eriksson (1992).

Eriksson (1992) studied the compressibility properties of sulphide soils at different temperatures. Incremental oedometer tests and CRS tests were carried out on specimens ofclayey sulphide soil taken from three different localities. The tests were carried out within a temperature range of 5-60 °C and all ofthe specimens were stabilised at the temperature in question before being loaded.

From the results of these tests, Eriksson shows that preconsolidation pressure

decreases linearly by about I %/°C with increasing temperature up to about 50 °C, see Fig. 2.4. In addition, the compressibility modulus ML decreased by 0.3 %/ °C.

Eriksson also carried out a series of creep tests with at least 16 days per load step. These tests showed that creep, expressed as a function of the time logaritlun, increases after a certain time and that a definite breakpoint can be observed in the creep curve. Eriksson considers that creep settlement can be described by two tenns:

(2.4)


Temperature, °C

0 10 20 30 40 500 ,-----.....,,,-----"""T-----,------,------, 0.7 days ... .. ...... ...

~ 0

4

....

.. ...-,

... ... ... ·-·

... ..._..... .... ... _

----::-·-· ......... ..... .......__ ,,_

·-· -·

0.7 days ....

7 days

70 days 0 0

c 0

~ 8 2 years

tti c.. Q)

E 0 Q) -

0

... ... ', 7 days

·--·~·-·-· ·-· ....... .... ....

~ 0

20 years

.... 12 1----_..,.....__,.,__......_...,..____.,...._....-f-_,......_....__,--i..,.,------.....t 200 years

70 days ..... .... .........

16 .____________________.......____....._____~

Figure 2.5. Deformation and creep in sulphide clay versus temperature. Eriksson (1992)

where as is the creep parameter that applies from the time ofprimary consolidation, tP, up to the time ofthe breakpoint, tb. at is the other creep parameter that is applicable from the breakpoint. The tests further showed that only the creep parameter at is dependent on temperature, that it has a maximum somewhat above the preconsolidation pressure and that this maximum shifts towards lower stresses at higher temperatures. Eriksson presents a diagram for the creep deformation of sulphidic soils at different temperatures, see Fig 2.5.

2.1 Summary of the Literature Study A study of the literature shows that in undrained conditions a rise in temperature results in swelling and an increase in pore pressure. If the temperature is reduced, the pore excess pore pressure drops again and if the temperature is allowed to cycle between two values the pore pressure increase is repeatable. Consolidation is obtained in drained conditions instead ofan increase in pore pressure. Irreversible deformation will be obtained in a complete temperature cycle. In several cases this deformation has been described as a creep effect.

Modelling the behaviour of the soil at elevated temperatures in an FEM program, for instance, is perfectly feasible provided that all the effects that occur are taken

SGI Report No 47 18

into consideration. What appears to be most difficult of all is describing creep correctly.

Studies carried out using CRS tests show a definite tendency for the preconsolidation pressure to decrease with increasing temperature. Furthermore, this process is reversible so that the preconsolidation pressure returns when the temperature is lowered. However, it should be pointed out that all evaluation ofpreconsolidation pressure is empirical and based on ordinary oedometer tests (CRS and incremental) performed in accordance with standard procedure and compared with the behaviour of the soil under normal conditions. The evaluated preconsolidation pressures at elevated temperatures should therefore be regarded solely as pseudo-preconsolidation pressures and this should be recognised when applying the results to field conditions. Alternatively, the results can be interpreted as indicating that the creep processes in heated specimens start at lower stresses. These remarks should be borne in mind later on in the report and when mention is made ofpreconsolidation pressure it should be regarded only as a parameter and not as a characteristic.


Chapter 3.

Soil Material Studied

3.1 Test Site The specimens studied come from an area just outside the Swedish Geotechnical Institute's experimental field for heat storage in clay near the marina in Linkoping. The area is covered by grass and situated adjacent to the Stangan river close to where it flows into Lake Roxen. The Geotechnical Institute has conducted geotechnical studies using CPT tests, field vane tests, dilatometer tests and the extraction of undisturbed soil specimens from 12 different levels. The results of the field tests and laboratory examination of the extracted specimens are described by Bergenstahl et al. (1990) and (I 993).

The surface of the soil consists of 1.5-2.0 metres of dry crust clay. Under this is clay with plant remnants and then pure clay down to a depth of 8 metres. Below this level, sulphidic stains occur in the clay dovm to a depth of 11-12 metres and at greater depth there are occurrences of silt in the clay down to a firmer bottom layer situated about 18 metres below the surface, see Fig. 3. 1.

The water content in the clay under the dry crust varies between 70 and 85 %. The undrained shear strength is 17 kPa at a depth of 4 metres which then in

creases slightly to 20 kPa at a depth of I I metres. CRS tests previously carried out at six different levels show that the clay in this area is somewhat overconsolidated. This overconsolidation is about 30 kPa at a depth of 3 metres, after which it drops to 15 kPa at a depth of between 5 and I 0 metres, see Fig. 3. 2. The CPT soundings show that the clay is homogeneous and that no connected draining layers are present in the top 18 metres.

3.2 Extracted Specimens Triaxial and CRS tests have been carried out on specimens taken just outside the experimental field at depths of 6 and 9 metres using a standard piston sampler, St I. The specimens were taken on three separate occasions from six different holes at a distance of 2-5 metres from each other. Specimens from middle and lower sample tubes, which are normally least disturbed, were used for

SGI Report No 47 20

SHEAR STRENGTH [kPa] WATER CONTENT [ %]

20 40 60 80 100 12000 10 20 30 I CLAY WITH SILT

LAYERS AND ROOTS l~

~

~' I / I T CLAY WITH ROOTSI

I

I ' '

~) I

5 'I

I I I I I I

\1 1·~ I I CLAY

le.~- I

I' I .!. 1

I l I I I I '\"I'-.,.l ~ '

I ,

' , ; /1/v:10 I

CLAY COLOURED ' II \lj BY SULPHID

.'. ~ \I,;

I

I I

1l ""'I I

,1! \

I

.,..,, VARVED CLAY WITH I

'1 I I

SILTYLAYERS15 ,, ~1· ,,' ' ~, II ,,, , I ,.II/ j.! I~

I

I I

' \

0 20 40 60 0,5 1,0 1,5 2.0 2,5 3.0

SENSITIVITY BULK CONTENT [ t/m3 J

SHEAR STRENGTH ----b-- WATER CONTENT

( FALL CONE TEST]

LIQUID LIMIT (ACC TO FALL CONE TEST}

SHEAR STRENGTH ( F IELD VAN E TEST)

---0---- BULK DENSITY-- ---- SENSITIVITY (ACC.TO FALL CONE TEST}

-- -* -- SENSITIVITY ( ACC. TO FIELD VANE TEST}

Figure 3.1. Soil profile in the test field at the Linkoping marina.


a' lkPo)

50 100

,, /

/c

/I

5 b \ \ \

E

C I \ ~ \ 0 w \

(c) \

10 \ \

\ C

\ \

C

15 c =preconsol idot ionpressure

from oedometertest

Figure 3.2. Preconsolidation pressure versus depth in the test field.

the tests. The specimens from a depth of 6 metres display a variation in water content of between 81 and 87 % and specimens from the 9-metre level have water contents of between 77 and 85 %. All the specimens from the 9-metre level displayed visible sulphide stains, while ocular inspection of the specimens taken from a depth of 6 metres showed them to be perfectly homogeneous.

Dilatometer tests showed that the relationship between horizontal and vertical effi:d:::i'Je SJ:e:E :h re S)J1, re K value, is about O. 7 at a depth of 6 metres and

0

0.6 at a depth of 9 metres. These figures have been used as a basis for calculating the horizontal stress used in the triaxial compression tests.

SGI Report No 47 22

Chapter 4.

Triaxial Compression Tests

4.1 Regular Triaxial Apparatus The triaxial apparatus is used mainly for determining the shear strength in soil under different conditions of stress. Stress conditions are simulated by applying vertical pressure, av, and all-round horizontal pressure, aH. The former is normally created by a vertical load and the latter is applied by pressuring the liquid in the cell. Both pressures can be controlled independently of each other. The soil specimen is surrounded by an impermeable rubber membrane. The specimen is placed on a porous filter which permits drainage of pore water from the specimen and regulation of the pore water pressure. In the case oflow permeable specimens, spiral strips of filter paper are placed round the specimen inside the rubber membrane to facilitate drainage, see Fig. 4.1. The diameter of the specimen is 50 mm and its height is 100 mm.

Figure 4.1 . Test speciemen with filter strips.


4.2 Triaxial Apparatus for High-Temperature Tests The triaxial apparatus used for the high-temperature tests has a cell modified to allow heating of the specimen to 70 °C, see Fig. 4.2. The liquid in the cell consists of water with a 20 mm layer of oil on top. A somewhat taller cell than usual has been used in order to provide space for cables for the heating device and thermometers without these touching the specimen. Draining of the specimen takes place at the bottom and a volume gauge measures the amount of drained pore water to an accuracy of± 0.05 ml.

LOAD CELL y VERTICAL DISPLACMENT TRANSDUCER

ROTATING BUSHING INSIDE

TO HEAT SERVO CONTROLER

THERMOMETER 2 ( REGISTRATION)

FIXING ROD 3 PARTS HEATING ELEMENT ,.._..........._.._ THERMOMETER 1 ::.=::..c.:..:::.:...c..c.____,_1-JA,14!11-,1.+-..,,c.....'--,L~ii-o11 ( FOR HEAT SERVO

HO LOI NG CAGE CONTROLER)

POROUS STONE HOLDING CAGE

CELL PRESSURE TRANSDUCER

POR E PRESSURE TRANSDUCER

Figure 4.2. Layout of the rebuilt triaxial cell.

A heating device and holder have been installed inside the cell. Two thermometers are also built into the cell. One of them is connected to the heating device only and the other is for recording cell temperature. The bottom plate is supplemented with three guide rods so that the cell can be mounted without touching the specimen.

SGI Report No 47 24

Heating of the specimen is effected by means of three sheets of heat foil measuring 5 0 x 100 mm. The sheets ofheat foil are mounted on a holder to prevent

them from corning into direct contact with the specimen and the walls of the cell. The sheets of foil are also connected to a locating device to which a thermometer is also connected, see Fig. 4.3. The thermometer is placed between the

foil and the specimen. The power supply for the sheets of heat foil is controlled so that the water in the cell will reach a predetermined temperature of± 0. I 0 C. Furthermore, an additional thermometer can be installed in the specimen itself

to record its temperature gradient. This was tested before the final tests were begun in order to obtain an idea of how quickly the temperature of the entire clay specimen increased. It turned out that a suitable heating rate to avoid tem

perature gradients in specimens was about 10 °/30 minutes.

control equipment

~ ~

0 0 0 computer 0 l )

00000011

aooooou

Figure 4.3. Temperature regulation.

4.3 Test Programme for Tri axial Compression Tests The tests were designed to simulate different situations when heating a heat

store and determining undrained shear strength at different temperatures.

The tests were carried out on 12 specimens, six each from depths of 6 and 9 metres. The specimens first had to undergo consolidation for in situ stress, eight at room temperature and four at 8 °C. The specimens that had undergone consolidation at room temperature were then heated under drained and undrained

conditions. Finally, undrained active compression tests were carried out at two different defom1ation rates and three different temperatures: 8, 40 and 70 °C.


4.4 Mounting, Consolidation The soft clay called for considerable care when mounting the specimen. The specimen was pressed out of its sampling tube and placed in a cradle of the Geonor type, where it was cut to an exact length of 100 mm. The specimen was then weighed and placed in the cell on a filter. The specimen was fitted with four spiral-shaped strips of filter paper. The three locating pins on the bottom plate were utilised when the membrane was mounted so that there would be no danger of disturbing the specimen.

Consolidation was carried out in three stages in order to recover the stress situation in situ, see Fig. 4. 4. Each stage lasted for 24 hours, which was judged suitable with regard to the properties of the clay. The first stage consisted of an isotropic effective pressure of 10 kPa for specimens from a depth of 6 metres below the surface and 11 kPa for specimens from a depth of 9 metres. The second stage was anisotropic with o-'v = 29 kPa and cJ'H = 22 kPa for specimens from a depth of 6 metres and a'v = 40 kPa and a'H = 26 kPa for specimens from a depth of 9 metres. The third and last stage was also anisotropic to the final stresses of a'v = 48 kPa and a'H = 33.6 kPa for specimens from a depth of 6 metres and a'v = 70 kPa and a'H = 42 kPa for specimens from a depth of 9 metres.

Pore pressure was maintained the whole time at 200 kPa through an applied "back pressure" to ensure complete water saturation in the specimen. Skempton' s pore pressure parameter, B, was measured before the specin1ens were to be heated. Parameter Bis a measure ofthe saturation of the soil and should be close to 1.0 for water-saturated soil.

in situ

stage 1: isotropic stage 2: anisotropic I stage 3: anisotropic II

3

Figure 4.4. Consolidation in steps.

SGI Report No 47 26

4.5 Heating and Undrained Tests When the specimens had undergone consolidation for in situ stress, eight of them were adapted to temperatures of 40 and 70 °C. Heating took place under drained and undrained conditions. Temperature, vertical deformation and volume change were measured under drained conditions and temperature, vertical deformation and pore pressure under undrained conditions. Heating was carried out in steps of 10 °C / 30 minutes in order to achieve as uniform heating of the specimen as possible. After the heating steps the specimen was maintained at a constant temperature for 24 hours.

The specimens that had been heated under undrained conditions were loaded in undrained state to failure at a deformation rate of 1 % / minute, which is known as a quick test. Testing was carried out at temperatures of 8, 40 and 70 °C.

The specimens that had been heated under drained conditions were loaded to failure under undrained conditions at a deformation rate of 0.006 % / minute,

which corresponds approximately to the rate normally used for consolidated undrained triaxial compression tests. These tests were also carried out at temperatures of 8, 40 and 70 °C.


Chapter 5.

Results of Triaxial Tests

5.1 Results of Consolidation and Heating Consolidation to in situ stress conditions resulted in normal deformation for all specimens except two in which the change in volume during consolidation amounted to roughly double that of the other specimens, see Appendix 1. These two specimens had been extracted from a depth of 9 metres in the same borehole and in all probability were somewhat disturbed in connection with the sampling operation. Consolidation of these specimens was carried out at 8 °C, following which one was compressed rapidly and the other slowly.

In connection with heating the specimens, an increase in pore pressure occurred during the undrained tests and a decrease in volume during the drained tests. In the undrained tests at a temperature of 40 °C, the increase in pore pressure was 13 kPa for specimens from a depth of 6 metres and 15 kPa for specimens from a depth of 9 metres. At 70 °C the corresponding increase in pore pressure was 27 kPa and 35 kPa, see Fig. 5.1. At the higher temperature, pore pressure dropped with time although the temperature was maintained at a constant level and the test was undrained. If pore pressure is calculated in accordance with the equation (2 .1) for the different levels and temperatures, the following will be

obtained:

Test temperature Calculated excess pore Actual maximum excess and sampling level pressure with equation (2.1) pore pressure from triaxial

compression tests

40° and 6 m 15,1 kPa 13 kPa 40° and 9 m 19,5 kPa 15 kPa 70° and 6 m 28,8 kPa 27 kPa

70° and 9 m 37,3 kPa 35 kPa

This shows that the equation gives a fairly accurate estimate of the change in pore pressure resulting from the heating of clay in a triaxial cell.

SGI Report No 47 28

Time, min Time, min 0 OJ '< 0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400 -u -0.30 -0.015-+--'~~~~~~~~~~~~~~~~-'-'---t_l

0 "'O -0.25(l)

-0.010 70°a.

(l) ~_-0.20 ------···-···----·. .. . -- . . ..... ·-- ~0.005 CJ) --··--···· ·- ....------ -·--··----· ---·-· .. ---··- -··-··--·- ·· -. - ·- - ... .. ~ -~ -0.15 -~ 0.000 -·---· -----------·--·-······ -- ·- ····--- -_-.·-:-::_-.-:·:.:.:--:-:..--·.:: 40° m 1-0.10--1 _. ro I 0.005 < i: ... ll) .;:-0.05.., ~ 0.010~ 0. -0 40° -0

~ 0.00 ~ 0.015 ·-· -·- -.·- ..-l 70° (l) CJ CJ

3 ·;: 0.05 a) ·E 0.020 c)"'O ~ .;_ 0.025 i.____________________~(l) > 0.10§.

C Time , min Time, min ro CJ)

0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400 40 40

35 35 -----------70° 0 030 30 7 .~a.. r a..

,:., : ...........__ ,:., . 70° . .., 25 .., 25 t,I) t,I)!'C: C:20 20_g.a ____________...,...----40° CJ r CJ15 40° ., 15t ...!"' .,,= =.,, .,, 10 .,, 10 .,r f.~ ... ~ 5 b) c. 5 d).., OJ ... .. 0 0 0 0

0.. 0..

Figure 5.1 . Pore pressure changes and deformations at undrained heating of speciemens N from 6 (a and b) and 9 m (c and d) depth. Observe that the scales vary. co

Fig. 5.1 also shows the vertical deformation resulting from undrained heating, which displays significant vertical swelling of about 0.2 % for the 6 metre level at 70 °C. For the 9 metre level, practically no swelling at all occurred and minor vertical compression was obtained instead. This is assumed to be because the effective horizontal pressure has decreased too much in relation to the vertical effective pressure on account of the large increase in pore pressure. At a depth of 9 metres, the effective vertical pressure, cr'v, was 70 kPa and the effective horizontal pressure, cr'H was 42 kPa. When pore pressure increased due to the effect of heating, the effective pressure dropped and cr'v = 70-35 = 35 kPa and cr'H= 42-35 = 7 kPa were obtained with the result that the stress situation ended up close to the line for active shear failure, see Fig. 5.2. If this had been avoided by continuously increasing crH , swelling would no doubt have been found here also. No noticeable vertical swelling was measured at a temperature of 40 °C in connection with heating, neither for specimens from 9 metres nor for those from 6 metres.

100

I I

C1 = C1Ymax c

plastic

A deformation

A= in situ 8 = after heating

el ast ic deformation

100 r:1~ ( kPa)

Figure 5.2. The stress path at heating the speciemen from 9 m depth to 70 °C in undrained conditions.

SGI Report No 47 30

From the diagrams in Fig. 5.3, which show the deformation occurring in connection with drained heating (note that the stress situation is constant and the same as originally in situ), it can be seen that consolidation for 24 hours is inadequate since the deformation curves never level out. This indicates that creep occurs in these specimens at high temperatures. According to studies of creep in clay (Larsson, 1986), this begins in the case of clays of normal temperature at about 0.8 cr'c, where cr 'c is the preconsolidation pressure. If this was valid at high temperatures also, noticeable creep would hardly occur in these tests since they are overconsolidated by about 15 kPa which corresponds to an overconsolidation ratio of 1.3-1.2 at the test levels. The result indicates that creep in clay would start at a lower effective stress when the temperature is raised.

If the changes in volume and vertical deformation are compared, it is possible assuming that the measured change in volume is the same as the specimen's change in volume to calculate the specimen's horizontal defom1ation. The decrease in the diameter of the specimens at 40 cc will then be 0. 18 % for both specimens from depths of 6 and 9 metres and at a temperature of 70 °C the decrease will be 0. 94 % and 0.92 % for specimens from depths of 6 and 9 metres. In these calculations, the expansion of the pore water and mineral substances due to the rise in temperature has been taken into account. If these assumptions are correct, it would mean that horizontal deformation in connection with the heating of clay can be related largely to consolidation and that no effects of shear deformation or creep in a horizontal direction can be seen.

5.2 Results of Rapid Undrained Tests The results of the rapid triaxial compression tests show that the tests were carried out too quickly for shear strength to be evaluated in a satisfactory manner. Furthermore, the 9 metre and 70 cc test was unusable because the relationship between the vertical and horizontal effective pressures was already excessive in connection with heating. As mentioned earlier, the test carried out at 8 °C was disturbed in connection with sampling. The results are shown in Fig. 5.4. No definite trend in regard to the change in shear strength with rising temperature can be seen from these results. Since these tests were preceded by undrained heating, the stress situation was completely changed at the start of the tests due to the increase in pore pressure and in consequence the results do not lend themselves readily to interpretation. Clearly defined fractures were formed in the rapid tests, see Fig. 5. 5. The angles of these fractures to the horizontal plane varied between 45 and 55 °, the smaller angles being obtained primarily in re

spect of the warmest specimens, see Appendix 1.


Time, min Time , minw N

0 200 400 600 800 1000 1200 1400 1600 1800 2000 0 200 400 600 800 1000 1200 1400 1600 1800 20000.0 0.0 ..

0.1 , ·¥.0.1 \ ·-"""""";:-~-:~-=--=~ 40° 0.2 '-- ' ~ 0.2 \ ~ 0.J '~.

I - 0.4 ·, -~.. C 0.5 '•, ~

-~ 0.J \ ._g 0.6 '~- -~400' \~ 0.4 E 00.87 ....Y, i: '\. ... . ·,

,.:2., 0.5 ·-...... .:2 0.9 -..,_ ~ 1.0 .......____

:'.: 0.6 - 1.1 ...........__0: ' ---..... o: 12 .._

-~ 0.7 -~ . """"~ t: 1.J -..y c) t ----· ~~ 70° a) ., 1.4 --...._ 70° > 1.5 ...,___ _ ________ ____ _______,> 0.8

Time, min Time , min

0 200 400 600 800 1000 1200 1400 1600 1800 2000 0 200 400 600 800 1000 1200 1400 1600 1800 2000J.5 ~

70°J .0

2.5 ~ ~ ------70°

~

.., - 2.0 ~

., - W ~ C ~ 0: I 0: ~ t5 j ' -5 1.0 ; V

., I ~

., 1.0 ------- 40°§ 0.5 I: 4~ ! M 0

~

(/) > 0.0 b) >0 ~o ---G) d)

-0.5 - M ;;o (D

-0 0 ;:i.

z Figure 5.3. Vertical deformation and volume change in % at heating under drained 0

.I>, conditions for speciemen from 6 (a and b) and 9 (c and d) meters depth. -..J

0

t 70 70

lJ 0

0 Cl.. -"' 60

0 Cl.. -"'_ 60

" (1) ;:iro· en --:' 50

b --:' 50 b

I I ~ rn b 40 ~ 40 ro < ~

"' e30 "' "' ~ 30

(1) a. -l (1)

3 (1) " iil c rn

"' u -~ 20

gI: j 0

Assumed fa il ure prior to the initiation of the test

2 3 4 5 6 7

. ' 8 9 10

a)

11 12

"' u ·;:.8 "' ·;.,

Q

20

10

0 0 2 3 4 5 6 7 8 9 10

c)

11 12

40 Vertical deformation € 1 • % 40 Vertical deformation €1 '%

0 ~

~

30

20 go

~ -"'

~

30

20

go -

., ~ 10 "' -5 0 ~ ~ 7 2 3 4 5 6 7 8 9

40°

70° 10 11

b)

' 1~ 13 14

5'n= ;: u .,...

10

0 4 5 6 7

70°

8 9 10

40°

11 12

d)

13 14 ~ t

-10 ~ -10"'.,... =

t .,... 0 0 ~ Q.,

Figure 5.4. Results from the undrained triaxial quick tests on speciemen from (.,.) (.,.)

6 (a and b) and 9 (c and d) meters depth.

Figure 5.5. Test specimen with and without rubber membrane after a quick test at 40 °C. Specimen from 9 m depth.

5.3 Results of Slow Undrained Tests The results of the slow triaxial compression tests show a decreasing trend in the peak shear strength values with rising temperature for specimens from a depth of 6 metres, see Fig. 5. 6. On the other hand, the trend of the results of tests with specimens from a depth of 9 metres are more doubtful. The specimen from a depth of 9 metres that was tested at 8 °C had probably been disturbed in connection with sampling. Unfortunately, since the number of specimens extracted was limited, the disturbed specimen was never replaced by a new specimen. For an undisturbed specimen the curve would probably be similar to that for the specimen taken from a depth of 6 metres. Residual shear strength appears to be independent of temperature.

SGI Report No 47 34

() 60 60 c\: 55-j go .0,:.

- 50

-45.. ? 40 i:; 35

.,, 30 "' :: 25

~ 20 -~ 15

·;;::,:

10 ., Q 5

0 0

40

c\: 30 .0,:.

~ 20 ., gf' 10 "' ., '-' 0 ... ~ -10 t :" 0

c..

uJ u,

Figure 5.6. Results from slow undrained triaxial tests on specimen from 6 (a and b) and 9 (c and d) meters depth.

~ -0 0 "'O (l)

::.io" (/)

!!l. !!! (l) < Q)

(l) -c..

-l (l)

3 "'O ~ !!l. C

~

a)

2 3 4 5 6 7 8 V crtical deformation

9 €,

10 '%

11 12

go

-40° 70°

b)

2 3 4 5 6 7 8 9 10 11 12 13 14

~ 55 J 700 - 50

-45.. b 40!.. b 35-"' 30"' i: 25

~ 20 -~ 15

"'·;;:., Q

10

5 c)

0 0 1 2 3 4 5 6 7 8 9 10 TI ~

Verticaldeformation€1 ,%40 go70°

c\: 30 .0,:. 40°

~ 20 ., ~ 10 co: d).c ~ 0+-....-~,--,--.-.,.....,-~,-.-,-....-,~-,--,:~~-::-r;;:~~.. ::, 2 3 4 5 6 7 8 9 10 n ~ D M ., ~ -10 ... Q. ., ... 0

0-.

Chapter 6.

CRS Tests

To determine the compression characteristics of a clay in Sweden, CRS tests are usually carried out in the laboratory. In the CRS test, an undisturbed specimen is consolidated in a vertical direction at a constant rate of deformation and constant cross-section area. The vertical load needed to deform the specimen is measured continuously. In addition, pore pressure is measured at the specimen's undrained lower surface, i.e. the test is performed under only partially drained conditions.

6.1 Standard CRS Equipment The equipment consists of an oedometer ring having a diameter of 50 mm and a height of 20 mm in which the specimen is mounted. This ring prevents the specimen from expanding horizontally. The ring with specimen is placed on a filter stone. A top cap with filter stone which fits in the ring is placed on the specimen, see Fig. 6.1. During the test, the specimen is drained on one side through the top cap while pore pressure is measured at the specimen's lower surface.

A graph plotting effective vertical pressure against vertical deformation is prepared from the results of the test. From this graph, it is possible to evaluate the preconsolidation pressure, cr'

0 which corresponds empirically to the effective

pressure to which the soil may be subjected under nonnal conditions without major consolidation settlement occurring. Following this, the compressibility modulus is evaluated after the preconsolidation pressure, ML, the limit pressure where the modulus begins to increase again, cr'L and the relationship between the modulus increase and the increase in effective stress, modulus number M', see Fig. 6. 2.

Additionally, a logarithm for the evaluated permeability is plotted against deformation, where permeability is calculated from the measured pore pressure and deformation rate.

SGI Report No 47 36

LOAD CELL

DEFORMATION TRANSDUCER

STAMP BALL COCK

...+--++--GUIDE RING+ CLAMP RING

-+-----4- -++- TEFLON RING

L---4==-J..J.--r:,:,a~---\J,oe:~=::!....I-- 0 - RING SE AL

--.....----+-- FILTER STONE

OEDOMETER CASING

Figure 6.1. CRS oedometer.


EFFECTIVE PRESSURE kPa

oc 20 40 60 80 100 120 140 160

5

~ 0

z 10 0 v.i C/'.l

~ ~ 15 0 u PERMEABILITY. k m/s

10-1 1 10-10 10-9K,

10 -s 10-1 10-s 20 0 t---'--------'--,/---<------'---'-

5

z Q 10

2000t-~-...&..+__.,_...._____._...L.........Js-_, C/'.l C/'.l

~ 15 Cl) C ;E ~ 1600 0 t:. log k C/'.l u 20 ::J

5 Cl 1200 250

z ~

0 800 C/'.l C/'.l

~ ~ 400 0 U ML---f----...---'----_,.,

a o[.

Figure 6.2. Results from a CRS-test and evaluation of parameters. Larsson, (1982)

SGI Report No 47 38

6.2 CRS Equipment for High-Temperature Tests Since it may be feared that the expansion ofmaterial in the apparatus can produce undesirable effects in connection with heating, as has been described in the literature, extra care was taken. The parts round the specimen consist primarily of stainless steel. These parts were heated to 70 °C, following which the expansion was measured and the parts then assembled to check that they did not bind at the high temperature. A different material was chosen for the most crucial component, the oedometer ring, so that the effect of material expansion would not influence the test results to an unnecessary extent. The material chosen was invar, which consists of 64 % steel and 36 % nickel and has a coefficient of thermal expansion of 0. 15·10-5 /

0 C. The other parts of stainless steel have a coefficient of thermal expansion of 1.15 · 10-5 /

0 C.

The heating device consists of the same type ofheat foil as used in the triaxial apparatus. The foil was connected to a thermostat from which it is possible to control the temperature. To make room for the heat foil, the oedometer cup with water was raised about 50 mm, see Fig. 6. 3.

Figure 6.3. CRS oedometer adapted for tests at high temperatures.


6.3 Test Programme for CRS Tests Twelve tests were performed on the specimens taken from depths of 6 and 9 metres. The tests were conducted at room temperature of about 20 °C and also at 40 °C and 70 °C. Two tests from the same level were performed at each temperature.

The specimens tested at 40 °C were heated in a single stage and the specimens tested at 70°C were heated in two stages, first to 40 °C and then to 70 °C. Excess pore pressure arose during the heating process. This excess pore pressure had to be equalised after each stage and before the compression test was begun.

The tests were otherwise conducted in accordance with the procedure described in the Swedish Standard for CRS tests, SS027126. The rate of deformation varied between 0.6 % /hand 0.8 % / h. Testing continued until 28-40 % deformation was attained. Evaluation of the deformation parameters has been carried out in accordance with the procedure described in the Swedish Standard. However, this evaluation is based on empirical relations which presuppose that the tests are carried out at room temperature.

SGI Report No 47 40

Chapter?.

Results of CRS Tests

Two of the specimens from a depth of 9 metres which were loaded at 20 °C and 40 °C show doubtful results throughout and are therefore marked in parentheses in each diagram.

Pore pressure was recorded during the heating phase in some of the series of tests. The maximum pore pressure then attained was 1.1 kPa at 70 °C. Pore pressure was only measured at 20-minute intervals, however, and the maximum pressure was probably higher. Since the excess pore pressure had to be equalised before the compression test could be started, a certain degree of consolidation had already taken place by the start of the test. This is roughly the same as off loading a certain stress and then on loading it again. The significance of this is limited as long as the pore pressure does not give rise to negative effective stress in the clay specimen. It is therefore extremely important for the specimen to be heated slowly so that no unnecessary excess pore pressure arises.

The results in terms of stress deformation curves show that the curves are displaced towards lower effective stress at higher temperatures, see Fig. 7.1 . This should under normal circumstances be interpreted as a decrease in preconsolidation pressure and in the following discussions it is assumed that the displacement of the curves towards lower effective stresses can be interpreted as a decrease in preconsolidation pressure at higher temperatures.

7.1 Preconsolidation Pressure From the results it can be seen that the evaluated preconsolidation pressures decrease with rising temperature, see Fig. 7. 2. On the other hand, this decrease does not appear to be linear with the temperature. Shown in the figure is a calculated curve for the preconsolidation pressure, cr'

0 T, which varies with temper

ature T in accordance with:


~ N

0 20 40

0 ~~~---- ~'-:::::~-~ --~

5

'o'210

C 0

·.;::: ctl

E ....0 1 5 Cl)

Cl

20

(/) 25

G)

::u CD 'O

Effective stress [kPa]

60 80 100 120 140

0 Figure 7.1. Stress-deformation curves from CRS tests on specimens from 6 m depth.;:i.

z 0

.I::,. --'1

T )o,1sI I 0

cr cT = cr cT0 (T

(7.1)

where cr'cTo is the measured preconsolidation pressure at room temperature [kPa] and T

O is the corresponding room temperature [0 C].

60(0 a. I ..:,(. (T r.15(I).... a-' 0 :::, '-- '-- cTo T (/) (/) 50 ~

..

... --0. -- - - C ! --,__0 -- ·.;:: co -0 40 -0 (/)

C 0 (.) (I)

~ 30 -·

0 20 40 60 80 100

Temperature [°CJ

80co Ia. 15..:,(.

(I) 70 -~--- ·0 ,-- , o --- .... :::, '-- (j ( -T r(/) --- :!o T (/)

(I) "ir- - .... 60 ---·-- 0. r--~- --- - .--. C j - 0 •·.;:: co 50 ---

0 -

-0

0 (/)

C 40 ---~· 0 (.) (I).... a. 30

0 20 40 60 80 100

Temperature [°CJ

Figure 7.2. Preconsolidation pressure versus temperature for tests from 6 m (top) and 9 m (bottom).


This equation has also been compared with the results obtained by Tidfors (1987) where agreement is good except for tests down to 7 °C. It has also been compared with the results ofthe CRS tests on sulphide soils carried out by Eriksson (1992). There it turns out that preconsolidation pressure at higher temperatures is overestimated by the curve, but ifthe exponent is changed from 0 .15 to 0.22 then agreement will be fairly good. In all probability, the decrease in the evaluated preconsolidation pressures is not only due to the temperature but also to the type of soil or some other soil-dependent parameter.

7.2 Compressibility Modulus The modulus before preconsolidation pressure, M

0, has been evaluated as the

maximal modulus value before the preconsolidation pressure and is shown in Fig. 7. 3 at different temperatures. A decrease in M

0 with increasing temperature

can be seen. M is generally described as an elastic modulus. According to em0

pirical relations, M0

can be evaluated on the basis of preconsolidation pressure in accordance with M

0 "" 5 0 · cr '

0 or on the basis ofundrained shear strength, 'Cfu, in

accordance with M "" 250 · 'Cfu (applicable to highly plastic clays). The evaluated 0

preconsolidation pressures obtained in the tests performed at room temperature and the shear strength results obtained in the triaxial compression tests are applied in the empirical equations and presented in Fig. 7. 3. Since the measured shear strength results stem from active tests and do not exactly correspond to the 'Cfu used empirically, this line has been shifted downwards in parallel to show the trend in the graph. The triaxial compression tests performed with specimens from a depth of 9 metres produced somewhat dispersed results and at 8 °C the specimen was probably disturbed, so no shear strength has been evaluated for that temperature. The decrease in shear strength at a depth of 6 metres shows fairly

good correlation with the measured decrease in the modulus M , which is about 0

0.5 % I 0 C. The compressibility modulus before preconsolidation pressure cr' cT at temperature T can thus be expressed as:

(7.2)M 0 T = M 0 (1-0.005· ~T)

where M is the compressibility modulus before the preconsolidation pressure 0

[kPa] and ~T is the difference in temperature between T and the temperature prevailing when M

0 was measured [0 C].

The measured values of the compressibility modulus ML are presented in Fig. 7. 4. ML displays a slight tendency to increase but can be regarded as practically unchanged with temperature. The modulus number M' also appears to be independent of the temperature.

SGI Report No 47 44

- -

4000 X - ....________co 250·-cru

a. 3500 ,___

= ~1___ :!: 0 3000 --- r---x (/) 1t :_ - - ::i - - - -- - 50·cr'c~ - -- - - -::i

2500 - - - - -- - - - - ' - ~

E -0 0 2000 - -

I------- -

I - - - ... 1500 11)..., M

0 (1-0.005-!iT)

11) 1000 -E 0

~-0 50011)

0 ,______ --10

0 20 40 60 80 100

Temperature [°C)

co 4000 --------2so·-cru 1------x

~ 3500 x1--------'----e-- ~~..,.______ ,_ 50-cr'c~ 3000 • - -·-~ -- ---

- - - ! ~ 2500 -- -,-

t _:.:..-T _ ] 2000 I -

: 1500 ---- -- -·---"-- --- M 0

( 1 - OD05 · !1T)

i11)

1000 --_ ___.__ ~~~- I -_:_-_1-~.g 500 11)

0 0 ·-- --------1- =t 0 20 40 60 80 100

Tempe rature [°C]

Figure 7.3. M versus temperature for tests from a) 6 m and b) 9 m. 0


400 co c. 350 .:.:: (/) 300 :J :i "O 0 E C

f.---.,250

200 i

,. I

I

0 (/) 150

, C/l

~ c. 100 E 0 50

(.)

0

0 20 40 60 80 100

Temperature [°CJ

600 co Q.

500 ,. ...=:J (/)

~-----

i

,--- •:J 400 --- ,."O 0 E 300 C 0

'cii (/) 200 - -~ c. E 100 ----- 0

(.)

0

0 20 40 60 80 100

Temperature [°C]

Figure 7.4. ML versus temperature for tests from a) 6 m and b) 9 m.

SGI Report No 47 46

•• •

7.3 Permeability The variation ofpermeability with temperature is shown in Fig. 7. 5 in the form ofmeasured initial permeability ki. In theory, permeability ought to increase with temperature since the viscosity ofwater decreases with rising temperature, but the measured values indicate that permeability decreases with rising temperature. The explanation for this observation could be due to microscopic bubbles, which are normally dissolved in the pore water, being released and filling out individual larger pores during the heating process. These effects could be eliminated by applying back pressure to ensure water saturation of the specimen.

2, 1 0E-09 ~------ -------~-~

1 , 90E-09 '- -----+-----+----f----+-- -------j

1,70E-09 '~ E 1,50E-09 >- 1 , 30E-09 - ------l--------1---4------l----1

~:o 1, 1 0E-09

~ 9,00E-10

~ 7,00E-10 1

0. 5, 00E-1 0 - _____,______ --------+- - -+-----

3, 00E-1 0 ~ ---'-------_j--'-----+------.......;1---__,_____

1,00E-10 - ____,____

0 20 40 60 80 100

Temperature [°CJ

2, 1 0E-09

1,90E-09

1, 70E-09 --------~---!----+---~ vi --E 1, 50E-09

>- 1,30E-09 - - - - ___ _;______--J--_. _ __,______j

:.0 1,l0E-09 - --·--· ·----- - _ _ ____,____,____ _ __, co Q)

E... Q)

0.

9,00E- 10

7,00E-10

5,00E-10

3,00E-10

1,00E-10 -- -

I

- - ----..---+----<

' --- ----f..----,-~.··-

•-~ ---1-----4---....._ •___,_______,_____,_______,

0 20 40 60 80 100

Temperature [°CJ

Figure 7.5. Pemability versus temperature for tests from a) 6 m and b) 9 m.


Chapter 8.

The Experimental Field

8.1 Description At the marina in Linkoping the Swedish Geotechnical Institute, with the support of the Swedish Council for Building Research, established an experimental field for heat storage which was started in February, 1992 (Bergenstahl et al, 1990). The experimental field was set up for the purpose ofstudying developments in settlement, pore pressure, temperature, shear strength and other factors connected with high-temperature storage in clay. The temperature of the clay amounts to a maximum of 75 °C and heating is carried out by means ofhoses introduced into the clay through which a liquid circulates. The experimental field consists oftwo stores measuring 10 x 10 x 10 metres. In store 1 the temperature is varied cyclically between 35 and 70 °C and in store 2 it is maintained at a constant level of 70-75 °C. Through this arrangement, the effect oftemperature cycling alone can be distinguished. Temperature cycling in store 1 is performed in periods of three months.

Instrumentation in the experimental field consists of different types of deformation meters, piezometers and temperature sensors . The instruments are situated in the centre of the stores, at the edge of the stores and outside the stores. Pore pressure and temperature are measured at five different levels in the centre of the stores, 1.5, 3.5, 6, 9 and 12 metres below the surface. In addition, pore pressure and temperature are measured at a depth of six metres at the edge of store 1 and outside both stores.

Two different methods of determining shear strength have been used in situ in the stores using dilatometer and field vane equipment.

8.2 Results The experimental field results presented below are from the first two and a half years of operation (Gabrielsson et al, 1995). During this period ohime, store I has passed through five complete temperature cycles and store 2, which was started three months later, has attained a constant temperature of 70-75 °C.

SGI Report No 47 48

Pore pressure changes in stores I and 2 are shown in Fig 8.1, which also includes temperature curves. The figures for the pore pressure changes are evaluated from automatic BAT gauges and also from open pipes.

Pore pressure increases during the first heating phase and then drops when the consolidation process takes over. A distinct difference can be seen between stores I and 2 in the first heating phase. The increase in pore pressure in store 1 is much greater than in store 2 because heating of store I was carried out at a faster rate initially. When cooling of store 1 was begun, the pore pressures decreased so markedly that they even dropped below the original pressures. This indicates that the effective pressure in the clay increased to levels that are largely equivalent to the original preconsolidation pressure. The pore pressure curves in store I closely follow the temperature curve. It can be noted that negative excess pore pressure is not equalised as rapidly as positive excess pore pressure. In store 2 the excess pore pressures were equalised after only five months from the time when heating of the store was commenced.

Fig 8. 2 shows the results produced by automatic total settlement gauges on the surface of the ground in the centre of stores 1 and 2 and the temperature sensor in the middle of the stores. Distinct swelling can be seen from the outset in store 1 during the first phase of heating. Following this, the soil is consolidated at a steady rate until cooling starts. The rate of deformation accelerates during the cooling phase. The settlement curve follows the cycling of the temperature curve with a steadily downward trend. In store 2, on the other hand, swelling is much less prominent at the beginning, since heating of this store was carried out at a slower rate and the concurrent consolidation process thereby took over. Although the pore pressures were equalised, settlement of the clay took place at a uniform but somewhat higher rate than in store 2.

Undrained shear strength measured with dilatometer and field vane tests during the heating period is shown for stores 1 and 2 in Fig 8.3. No clear trend parallel to the temperature can be discerned. The results are difficult to interpret since different excess pore pressures occurred at the various times the measurements were recorded. In other respects, the tests were carried out and evaluated in the same manner at all temperatures.


0

Heat store No 1 (Jl

:r (\ A ~ A (\ r: 50~ / \ 50 uU V V \ 1 V 401 r \ j ~ 40 i

I /I ;1 ..., & 30 .\ v--\ /I / l f\ /\ 30 f c.. .\/ ·-'\ \ _; I / l / \ A I \ ~ ~ 20

/ j 1 _r-'...... 1I'_ 1 1 ,.... 1 L1 . /\ 1 20 (/) ' ,-, ·

1l 1 1 \ , 1 · \I

~ 10 ~~' ' L fr· · " j 11 -' \ 1· \' '.' \ ~ 10 0..'-,

II \ -~1

/,• ,.,./ "'\

, \ \ r 1 -

/, 1

\ L_,.J..

f ' - , / '" ' • J _,,,,,,., ....l:: 0 ' ,· ,. ~- ~ ,. 0 g__ \ //. \', ,.,.. ......,,,- r \·'-..J/ , . -

(/) j\ /--- - ,./ I I I- - /I \ _/ J10~ - Ii \, ,_;- J \\ , I I\ · / \\ / - _,.,.,.

Q ., I ~ I I \' I I \\ . / -20 \ l \ i I \\,· / \ :J /

.\ . ' I I -30 ~ l( I \ I \ 1 \ IU --- --- =Pwp 3.5m J ~ v

-<Ol --- aPwp 6-0 m ~ ---=Pwp 9.0 m --=Temperature - -50 , 1 1 1 , , , :1 1 1 1 1

~ FEB MAY AUG NOV FEB MAY AUG NOV FEB MAY AUG NOV 'O

g_ 1992 1993 1994 1995 z 0

~ Figure 8.1 a. Pore pressure changes and temperatures in store 1. -...J

Heat store No 2 0

~ 80 ---------------------------~80 7J

"O 0 701 / V '' w"'~ "-. ,,--\-70 (1)

3: i (1) en 60 60 ~ m 50 50 E ~ r,

(l)

~ f \ ..... (1) 40

I \ 40 _,::, 0.

ctl . I I\ .....-I (l)(1) r\ I ..--;;- 30 30 0...I -.; .J ~3 0... 8"O r r 1· (l)--'<::(1)

~ 20 I ._ri \] 20§. b

.....C ::, 'JJ \\

(/)ro (/)en (l) 10 10 ..... p0... 1-~~.,.J--~.. ,,~.;,-:--~___.:--~--:-, __ --...t-'"\:-\-, .,.--~-/- 0.....

0 0(l)

\ '---------- .../0... . ~~--· rJ (/)

- \ -../; - ' \ I --. \..I~ -10

'-' '-v>< ----- =Pwp 3.5 mw -20 ---=Pwp 6.0 m

-30 --- =Pwp 9.0 m

- - =Temperature - 40

-50 +-.---T~-,--.-.--. 1 r r I , 1 1

FEB MAY AUG NOV FEB MAY AUG NOV FEB MAY AUG NOV 1992 1993 1994 1995

0, Figure 8.1 b. Pore pressure changes and temperatures in store 2. ....l.

-10

U1 N 0 1= Settlement store No. 1

2 = Settlement store No. 210

Tl = Temperature store No. 1 20

T2 = Temperature store No. 2 s s 30 ....,

C Q,)

80~ 40 :;:::;...., ---,,,,,..--- 70Q,) Cl) / I

/ l / 1\50 Tlt i / /. I \ 60 :2:I \ J ' ~I' \II \ I .1 (1.)\ I \I \ I I ~

I \ I I I I/""\ \ 50 _,;::I60 / f \ II

( /\ ,..- \I \\ / ~ (1.)t / '-,i.. -,r - -.:I ' \ 1/ \ 40 (1j

\p 0.. I I 870 30I I (1.)

I I

I I

20 e

1 T2 /80 -- ___ j 10

G) 0(/)

90 -t--,--r----,------.--,----,------,-------,----,---,----.--,-----,-----,-----,-----,---,-----,---.----,---,--,---r-------r- . . . . ::0 (1) FEB MAJ AUG NOV I FEB MAJ AUG NOV FEB MAJ AUG

"O 0 ;:+ 1992 1993 1994 z 0 .I>, Figure 8.2. Settlements and temperatures in stores 1 and 2. ---.J

Shear strength (kPa) 0 5 10 15 20 25 30 3.5

O+--~..........~~......_.~.......~~.....__.~..........~~..........~----+ 0

XXX Vane, 8'e

+++ Vane, we 1st cycle

*** Vane, we 2nd cycle

-5 000 Vane, we 4th cycle

181181181 Dilatomer, 8'e

EBEBEB Dilalomer, 70'e

1st cycle

Bili!HEI -10

Dilalomer 70'e

I I

2nd cycle -l'-

Shear strength (kPa)

0 5 10 15 20 25 30 3.5 0+-'-~..........~~........_~..........~~....._~.........~~-'---~--'--1 0

XXX Vane, 8'e

+++ Vane, 23'e

*** Vane, 45'e 000 Vane, 66'e t:,t:,t:, Vane, 74'e

-5 -5

~181181 Dilatomer, B'e

EB EB EB Dilatomer, 23'e

illBi!Bil Dilatomer, 45'e

CCC Dilatomer, 66'C

_ store depth II!10 - 10

Figure 8.3. Estimated undrained shear strength in store 1 (top) and store 2 (bottom).


Chapter 9.

Comparisons and Discussion

9.1 Comparison of Results from the Laboratory and the Experimental Field

In a comparison ofpore pressure changes in the experimental field, see Fig. 8 .1, with those calculated theoretically from equation (2 .1) and those measured in the triaxial compression tests, it will be seen that the pore pressure change in the experimental field at a depth of 9 metres is much greater in both store 1 and store 2, see Table 9.1 . Furthermore, a certain amount of drainage occurs in the experimental field and in consequence the pore pressures measured here are somewhat underestimated in comparison with the other pressures measured and calculated.

The large difference in pore pressures is probably due to the fact that in actual field conditions there is a comparatively high passive soil pressure against the surrounding soil, which inhibits the possibility for the soil to expand horizontally on account of an increase in temperature and instead increases the excess

pore pressure still further. If the horizontal stress, crH , did not increase, soil failure would occur at a certain degree ofheating, as is illustrated in the case ofundrained heating to 70 °C in the triaxial apparatus with specimens from a depth of 9 metres, Fig. 5.2.

Table 9.1. Calculated and actual excess pore pressure at different temperatures.

Excess pore pressure, L1u=

Temperature Pressures measured Pressures measured Pressures measured and depth in triaxial tests in store 1 in store 2

40° and 6 m 13 kPa 23 kPa 20 kPa 40° and 9 m 15 kPa 49 kPa 34 kPa 70° and 6 m 27 kPa 29 kPa 34kPa 70° and 9 m 35 kPa 58 kPa 47 kPa

SGI Report No 47 54

In a comparison of the settlement curves from the experimental field, see Fig. 8.2, with the vertical deformation measured in the triaxial compression

tests, it can be established that heating in the experimental field was partially drained. The maximum vertical swelling in store 1 was about 0.06 % and in store 2 it was 0. 02 %. The maximum vertical swelling in connection with undrained heating to 70 °C in the triaxial apparatus was 0.23 %. Swelling was thus not at all as great in the field as in the laboratory.

Studying the total settlement that occurred in store 2 up to the point where excess pore pressure was equalised, it will be found that it amounted to 0.30 %. In the laboratory, the settlement in drained tests amounted to 0.75 %. If it is as

sumed that the concluding part of the curve, see Fig. 5.3, is due entirely to creep, a primary vertical deformation of 0.6 % will be obtained. It can still be assumed that the "primary" settlement obtained in the triaxial compression tests

includes creep effects, while it may be assumed that the figure from the field is free from creep since excess pore pressure then prevailed, thus unloading the clay. Another way of expressing this is to say that in the triaxial test creep can

be assumed to constitute one half and primary consolidation the other half of settlement after a certain point in time. This can be compared with the results reported by Burghignoli et al. (1992) in their article, where they state that creep

accounts for about half of the effect in a temperature cycle. The results of the triaxial compression tests are taken as a mean of the tests conducted with speci

mens from a depth of six metres.

Preconsolidation pressure apparently decreases with rising temperature, according to normal interpretation of the oedometer tests in the laboratory. The appar

ent decrease in preconsolidation pressure is limited and under normal circumstances the temperature changes in a heat store will not lower the apparent "preconsolidation pressure" below the in situ vertical stress. Consequently, no field evidence for a lowering of the preconsolidation pressure exists. In fact, the measured pore pressure and deformations at cycling in store I contradicts the assumption of a lowering of the preconsolidation pressure at increasing temper

ature.

9.2 Discussion on Creep In this study, no laboratory tests of incremental oedometer type have been carried out in an attempt to analyse creep. An indication that creep occurs in the drained triaxial compression tests, where the curves never seem to flatten out during the consolidation phase, can nonetheless be discerned. A study of the


literature also shows that creep must be taken into consideration at high temperatures.

Creep is called secondary consolidation when it is time-dependent and takes place so slowly than no hydraulic gradient arises. Larsson (1986) has shown that the creep parameter a8 is dependent on deformation as shown in Fig. 9.1. Early on, a8 has an extremely low value, which later rises rapidly to a maximum and then declines slowly with increasing deformation. The initial value coincides at normal temperatures with a vertical stress below about 0.8 · cr'c and the maximum value coincides with the preconsolidation pressure. In addition, the magnitude of <Xs has proved to vary with the water content and to a certain extent also the type of soil, see Appendix 2. Creep settlement is calculated from the relation:

t a = a ·log-1

s s t2 (9.1)

where <Xs is the creep parameter and the time t 1 > ½·

-:,!?.0

2.S-(!) 0_, -0

'w -0 2J) z ~ .... < ~ _, 0 1.S U'l z 0 u

> QC

<0

1,0

z 0 u w U'l

~ 0,5 0

u: ~ w 0 u

0 -0 5 10 15 20 25 30

COMPRESSI ON %

Figure 9.1. Coefficient of secondary consolidation as versus compression for Backebol clay, Larsson (1986).

SGI Report No 47 56

In all probability, the creep curve follows any changes in preconsolidation pressure when the soil is heated. That is to say, the maximum value of the creep parameter at higher temperatures occurs at a lower effective stress if the preconsolidation pressure decreases. Alternatively, what may happen is that creep occurs at lower stresses if the temperature is raised without what is normally meant by preconsolidation pressure undergoing any change. This means in both cases that only small increases in the temperature of a completely normally consolidated clay could cause major creep deformation to occur. Since the majority of soft clays in Sweden are somewhat overconsolidated, however, the effects will not be so great at moderate temperature increases since no largescale primary consolidation takes place. According to previously presented relations, a clay with an overconsolidation ratio of 1.25 would have to be heated to at least 85 °C for the clay to behave as though it were normally consolidated and so display primary deformation of any appreciable magnitude.

9.3 Discussion on Temperature Cycling In this study, no laboratory tests with temperature cycling have been perfonned. From observations made at the Swedish Geotechnical Institute's experimental field in Linkoping and references from the literature, certain conclusions can be drawn in regard to the effect of temperature cycles on deformation.

Temperature cycles do not necessarily accelerate the deformation process, which depends largely on the drainage paths. When drainage takes place rapidly, the settlement process is accelerated through temperature cycling, but otherwise not.

Burghignoli et al. (1992) have observed irreversible volume changes in specimens exposed to extremely slow temperature cycles where the size increases with the amplitude of the temperature cycle. The magnitude of the deformation is also dependent on earlier temperature cycles and the duration of the temperature increase.

The phenomenon observed by Burghignoli et al. is due to creep in the clay. Since heating and cooling of the clay took place so slowly, no positive or negative excess pore pressure arose. If excess pore pressure is obtained during the heating phase, creep is brought to a halt since the effective stress diminishes. The amount of creep occurring in connection with cycling and rapid drainage, i.e. when no noticeable excess pore pressure arises, mostly depends on the mean temperature of the temperature cycle. The speed at which creep settlement occurs depends on the current creep parameter, i.e. the extent to which the creep curve is shifted towards lower vertical stresses.


9.4 Estimating the Magnitude of Settlement for a Heat Store

According to the observations and results that have emerged in this study, the amount of settlement in a heat stor~ under consideration can be estimated by means of the following preliminary calculation model. In the model, it is as

sumed that the temperature is the only load effect that occurs. Should the effect of some other load, e.g. in the form of fill on top of the heat store, come into play, a far more complex problem would arise. This calculation model is based

on the assumption of a preconsolidation pressure decreasing as the temperature increases. Alternatively, it may be that creep starts at a lower effective stress level at elevated temperatures. Strong reservations should therefore be made in

connection with the use ofthe specified equations when cr'cT < cr' , which as 0

mentioned earlier normally never occurs and for which no empirical evidence therefore exists.

Calculation Procedure: The deformation parameters are evaluated from standard CRS tests conducted at normal temperatures. Subsequently, a new "preconsolidation pressure" for the maximal temperature can be calculated according to equation (7 .1) as well

as a new compressibility modulus, M0 r, according to equation (7.2). The soil' s

in situ vertical stress is calculated and compared with the original preconsolidation pressure and the calculated "preconsolidation pressure" at maximal temperature.

During the first heating process, the stress-deformation process is shifted from point A to point Bin Fig. 9.2. This is on condition that the pore pressure has time to equalise. Otherwise, it will be situated somewhere above point B on the curve for T max. As long as the vertical stress is on the elastic part of the curve, the excess pore pressure will be equalised relatively fast since the compressibility modulus is high . .1.c.r is the settlement caused by the rise in temperature and subsequent consolidation and can be expressed as:

cr' cr'0 . 0A 0Llc.T =---- (9.2)MoT Mo

if cr ' 0T < cr'

0 , the expression would (with the aforementioned precautions) be

SGI Report No 47 58

()'~

,-;::-----+----- er' ( kPa)

£(%)

Figure 9.2. Stress-deformation curves for temperatures TO and Tmax·

A _ cr I cT cr I o ( cr I o -cr IcT )

LlcT------+ (9.3)MoT Mo M1

Added to this is the creep settlement. During the period of time that excess pore

pressure prevails, it is assumed that no creep occurs. Creep is assumed to start

at a vertical stress~ 0.8 · cr·cT and attain its maximum at cr·cT· The next steps is

to determine the value of cxS,max' which depends on the water content and type of soil and can be estimated from Appendix 2 together with the inclination ficxs on the remaining part of the curve. On the basis of cr ·

0T, the creep settlement

parameter cx5 for the prevailing effective vertical pressure can now be calculated. Following this, the creep deformation, cxs, is calculated with equation (9.1)

for the desired period of time. The time t2 is the point in time when full pore

pressure equalisation took place. This time can be calculated approximately using Therzaghi's consolidation theory, where the compressibility modulus corresponds to M , since the clay is unloaded and loaded again. Permeability

0

also ought to be corrected, bearing in mind that viscosity increases when the

temperature is raised. The total deformation in a heat store will then be

(9.4)


With equations (9.1), (9.2) and (9.3) the total deformation can be expressed as

(9.5)

and when cr 'cT < cr' 0

, the total deformation could (with aforementioned precautions) be expressed as

cr' cr' ( cr' -cr' ) t!>=____£I_ __o + cT o +as·log_!._ (9.6)MoT Mo ML t2

As an example, the figures obtained from the experimental field can be inserted in the above equations. At store 2, the temperature was increased to 70 °C and

then maintained at this level. The store extends down to a depth of 10 metres. The clay is overconsolidated by about 15 kPa from a depth of 5 metres and above this level the overconsolidation is 30-50 kPa. Let us study the settlement between 5 metres and 10 metres below the surface. At a depth of 7.5 metres, cr'

0

is 61 kPa and cr'c at this depth is 76 kPa. ML is 250 kPa and 'Cfu is 18.5 kPa. Calculate the "preconsolidation pressure" for 70 °C according to equation (7 .1)

20)0,15 cr 'cT = 76 ·( = 63 kPa

70

and the compressibility modulus before the preconsolidation pressure at 70 °C according to equation (7.2) where M can be expressed as 'Cfu·250. The original

0

temperature can be assumed to be 7 °C and LiT is thus 63 °C.

=18.5 · 250 =4625 kPa M 0

M 0 r =18.5-250-(1- 0.005-63) =3168 kPa

Now the deformation due to the increase in temperature can be calculated with equation (9.2) since cr·cT > cr ' .

0

61 61 0L1Er =----- = 0 6 1/o

3168 4625 '

SGI Report No 47 60

For the 5 metre thick clay layer, this gives a primary settlement of 0.03 m.

It now remains to calculate the deformation due to creep. The water content for this somewhat sulphide-stained clay is around 80 %, which gives an CX

8 max of

0.018. The maximum value for creep is around the "preconsolidation pressure" for the temperature in question, which here corresponds to 63 kPa, and the starting value begins at about 80 % ofthis, which corresponds to about 50 kPa. For a water content of 80 %, the creep parameter drops by 0.04 · L\E after the preconsolidation pressure has been exceeded. The actual stress in this example is 61 kPa, which is less than the "preconsolidation pressure", cr'cT = 63 kPa, and results in a reduction ofthe creep parameter of 0.003. Fig. 8.1 shows the pore pressure trend at the experimental field, which was measured over a period of about two years. Deformation due to creep can now be calculated with equation (9.1) for the 23 months or so that creep occurred. Creep started after about five

months when the excess pore pressure had been equalised.

23 es = (0.018-0.003)-log- = 0.0099

5

For a 5-metre thick layer of clay, this gives a settlement of 0.05 m. The total settlement for layer 2 after two years would thus be 0.08 m, which is comparable

with the figure obtained in Fig. 8.2. i.e. just over 0.08 m.

Ifthe calculation is to be performed for layer 1 where the temperature was cycled

between 35 and 70 °C, it will be necessary to take into account what happens during the cycling process. Deformation due to the increase in temperature, L\Er, is calculated in the same way and accordingly amounts to 0.03 m for layer 1

also. This is an irreversible deformation as distinct from the deformation that is caused by the volume of pore water and clay particles increasing and decreasing as a result of the temperature fluctuations. When a heat store is actively cooled, negative excess pore pressure can occur, as was the case here. This negative excess pore pressure arises if free water is not available which can be sucked up at the same rate as the cooling process. 1n ordinary Swedish low penneable clays, however, the availability of free water is usually very limited. These negative excess pore pressures give rise to a corresponding effective pressure increase and an immediate decrease in volume takes place at the same rate as the pore pressures change. When the clay is again heated, the pore pressures increase and a corresponding immediate increase in volume occurs. Provided that the stresses are located in the overconsolidated range the whole time, this is on the whole a


reversible process in regard to changes in both volume and pore pressure, apart from negligible permanent consolidation related to effects of repeated loading in each heating phase. What remains is deformation due to creep. Creep is assumed not to occur when positive excess pore pressure prevails. For layer I, excess pore pressure occurs during each reheating process. It is then to be expected that creep will occur only during the cooling periods, which in this case is a question of totally about ten months. The time, tz, for equalisation of the initial pore pressure is about five months for layer I. During the cooling-down periods, the mean temperature is roughly between 70 and 35 °C and a new "preconsolidation pressure" for 5 2 °C will then have to be calculated according to equation (7. I)

20)0,15 cr'cT = 76· = 66 kPa (

52

The starting point for creep is 80 % ofthe preconsolidation pressure, which in this case gives a stress of 53 kPa. The maximum value for creep occurs around the "preconsolidation pressure". With the prevailing stress level ofcr' =61 kPa,

0

this gives a decrease in as of 0.007. as max is chosen according to Appendix 2 for the type ofsoil and water content in question and a current value of as gives 0.011 . We can now calculate the deformation due to creep for layer I. This then gives

15Es= 0.011 ·log-= 0.0052

5

For a 5-metre thick layer ofclay, this gives a settlement of about 0.03 m. The total settlement for layer 2 after two years would thus be about 0.06 m, which shows good agreement with the measured value of 0.065 m in Fig. 8.2. Added to this is the cyclical swelling and compression of the clay due to the volume expansion and contraction of the pore water and soil particles during the heating and cooling processes, which appears as regular vertical fluctuations with an amplitude ofabout ±15 mm round the mean settlement.

SGI Report No 47 62

Chapter 10.

Conclusions

Observations from the field tests as well as the triaxial compression tests and CRS tests in the laboratory can be summarised in the following points:

• When clay is heated the pore water and clay particles expand, which gives rise to an increase in pore pressure and swelling of the clay if the possibility for drainage is limited.

• When calculating pore pressure for a heat store, the fact that horizontal stress increases with rising temperature must be taken into account.

• The increase in temperature can start a creep process in the clay when excess pore pressure has been equalised. This creep process can be calculated by assumption of a lowered "preconsolidation pressure" and belonging changes in the creep parameters.

• In normal Swedish clay, shear strength decreases with rising temperature. Similarly, the modulus, M

0 decreases before the preconsolidation pressure.

In respect of the tested clay, the decrease is in the order of 0.5 % / °C.

• Preconsolidation pressure apparently decreases with rising temperature, according to normal interpretation of the oedometer tests in the laboratory. No field evidence for a lowering of the preconsolidation pressure exists. In fact, the measured pore pressure and deformations at cycling in store 1 contradicts the assumption of a lowering of the preconsolidation pressure at increasing temperature.

On the basis of these results, it can be stated that for a heat store built in Swedish clay, which is normally somewhat overconsolidated, settlement will be relatively small for moderate increases in temperature. Settlement can be estimated by means of equations 9.2 and 9.5. This is subject to the condition that no load is placed on the surface. In connection with loading of the surface, particular


attention must be paid to the fact that the undrained shear strength may be lowered so that stability is not jeopardised. Following this, it should be borne in mind that the preconsolidation pressure may have decreased and/or creep propensity increased, which would result in greater deformation than usual in the event of a load being placed on the surface.

The surface of a heat store can be used to advantage as a car park, football field, grassed area or park, etc. Heat stores ought not to be located on slopes where even a moderate decrease in shear strength could initiate a landslide. In addition, the distance between a heat store and a large building, high road embankment, noise barrier, etc. should be so great that the rise in temperature does not spread and affect them or their stability even in the long term. To give an example, settlement for the experimental field is estimated at 0.17 metres after 25 years of operation at temperatures varying between 35 and 70 °C. The temperature increase of 1 °C in the soil extends about 40 metres from the edge of the heat store during the same time. The nearest building ought not to be situated closer to the heat store than this unless a special study is conducted.

SGI Report No 47 64

References

Bergenstahl, L, Lehtmets, M & Sundberg, J (1990). Forsoksfalt for hogtemperaturlagring i lera, forprojektering. Swedish Geotechnical Institute, Varia 278. Linkoping, 35 p.

Burghignoli, A, et al. (1992). Deformability of clays under non isothermal conditions. Rivista Italiana di Geotecnica, No 4, pp 227-235.

Campanella, RG & Mitchell, JK (1968). Influence of temperature variations on soil behaviour. ASCE, Vol. 94, No SM3.l , pp 709-734.

Eriksson, LG (1992). Sulfidjordars kompressionsegenskaper, Inverkan av tid och temperatur. Licentiatuppsats, Tekniska Hogskolan i Lulea, avdelningen for geoteknik. Lulea, 151 p.

Gabrielsson, A, Bergdahl, U, Lehtmets, M & Moritz, L (1995). Forsoksfalt for varmelagring, Driftsrapport 2. Swedish Geotechnical Institute, Linkoping. Varia 375. Linkopning, 81 p.

La Rochelle, P, m.fl. (1988). Observational approach to membrane and area corrections in triaxial tests. ASTM STP 977, Phiadelphia, pp 715-731.

Larsson,R, Bengtsson, PE & Eriksson, L (1993). Sattningsprognoser for bankar pa los finkomig jord - berakning av sattningars storlek och tidsforlopp. Swedish Geotechnical Institute, Infonnation 13. Linkoping, 51 p.

Larsson, R (1986). Consolidation ofsoft soils. Swedish Geotechnical Institute, Report No 29. Linkoping, 174 p.

Larsson, R (1982). fords egenskaper. Swedish Geotechnical Institute, Information I. Linkoping, 48 p.

Seneviratne, HN, et al. (1992). A review ofmodels for predicting the thermomechanical behaviour of soft clays. The University of Sydney, Research Report No. R655, Australia, 25 p.

Tidfors, M (1987). Temperaturens paverkan pa leras defonnationsegenskaperen laboratoriestudie. Licentiatuppsats, Chalmers Tekniska Hogskola, Inst. for geoteknik med grundlaggning. Goteborg, 119p.


SGI Report No 47 66

Appendix


APPENDIX 1

Properties and results from triaxial tests at elevated temperatures

sample bore hole/ water ratio density vertical volume failure D=normal test tube deformation change angle C=quick test (%) (t/m3) (%) (%)

D610 2 / middle 83,0 1,53 1,0 1,6 52° D640 2 / under 87, I 1,51 1,5 2,0 52° D670 12 / under 86,1 1,53 5,6 4,7 55°

C610 I / middle 81 , 1 1,53 1,3 2,5 54° C640 I /under 81 ,6 1,53 1,7 2,4 52° C670 11 / middle 81 ,3 1,52 1,2 2,4 C671 11 / under 85,3 1,53 1,5 2,7 480

D910 13 I middle 82,9 1,54 7,3 9,0 D940 12 I under 77,5 1,56 2,8 3,8 26° D970 2 / middle 84,7 1,53 3,5 3,8

C910 13 / under 79,6 1,55 6,3 7,1 38° C940 I I under 81,9 1,53 1,8 2,4 55° C970 I /middle 85,7 1,54 4,0 5,0 45°

before consolidation after consolidation Iafter failure I

SGI Report No 47 68

APPENDIX2

Values on the parameters <X and Ps for secondary consolidation, 5

taken from SGI Information 13.

These values are recomended These values are recomended for clay for gyttja, gyttjy clay and

sulphidic clay

WN O:s max f3o:s WN O:s max f3o:s o/o o/o 25 0.000 0.000 25 0.000 0.000 30 0.002 0.027 50 0.007 0.030 40 0.006 0.031 75 0.016 0.033 50 0.010 0.035 100 0.021 0.035 60 0.014 0.039 125 0.026 0.038 70 0.018 0.043 150 0.030 0.040 80 0.021 0.046 200 0.036 0.046 90 0.025 0.049 250 0.040 0.051

100 0.029 0.053 300 0.044 0.055 110 0.033 0.0.57 3.50 0.047 0.0.58 120 0.037 0.061 400 0.0.50 0.061

Peat are using asmax= 0,025 and~.= 0,000


STATENS GEOTEKNISKA INSTITUT SWEDISH GEOTECHNICAL INSTITUTE

Serien "Rapport" ersatter vara tidigare serier: "Proceedings" (27 nr), "Sartryck och Preliminara rapporter" ( 60 nr) samt "Meddelanden" ( 1 0 nr).

The series "Report" supersides the previous series: "Proceedings" (27 Nos), "Reprints and Preliminary Reports" (60 Nos) and "Meddelanden" (10 Nos).

RAPPORT/REPORT No Ar/Year

1. Grundvattensankning till foljd av tunnelsprangning 1977 P. Ahlberg & T. Lundgren

2. Pahangskrafter pa langa betongpalar 1977 L. Bjerin

3. Methods for reducing undrained shear strength of soft clay 1977 K.V. Helenelund

4. Basic behaviour of Scandinavian soft clays 1977 R. Larsson

5. Snabba odometerforsok 1978 R. Karlsson & L. Viberg

6. Skredriskbedomningar med hjalp av elektromagnetisk 1978 faltstyrkematning - provning av ny metod. J. Inganas

7. Forebyggande av sattningar i ledningsgravar -en forstudie 1979 U. Bergdahl, R. Fogelstrom, K.-G. Larsson & P. Liljekvist

8. Grundlaggningskostnadernas fordelning 1979 B. Carlsson

9. Horisontalarmerade fyllningar pa los jord 1981 J. Belfrage

RAPPORT/REPORT No

10. Tuveskredet 1977-11-30. lnlagg om skredets orsaker

l la. Tuveskredet - geoteknik

llb. Tuveskredet - geologi

llc. Tuveskredet - hydrogeologi

12. Drained behaviour of Swedish clays R. Larsson

13. Long term consolidation beneath the test fillsat Vasby, Sweden Y.C.E Chang

14. Bentonittatning mot IakvatteR T. Lundgren, L. Karlqvist & U. Qvarfort

15. Kartering och klassificering av leromradens stabilitetsforutsattningar L. Viberg

16. Geotekniska faltundersokningar Metoder - Erfarenheter - FoU-behov. E. Ottosson (red.)

17. Symposium on Slopes on Soft Clays

18. The Landslide at Tuve November 30 1977 R. Larsson & M. Jansson

19. Slantstabilitetsberakningar i !era Skall man anvanda totalspanningsanalys, effektivspanningsanalys eller kombinerad analys? R. Larsson

Ar/Year

1981

1984

1981

1981

1981

1981

1982

1982

1982

1983

1982

1983

RAPPORT/REPORT No

20. Portrycksvariationer i leror i Goteborgsregionen J. Berntson

21. Tekniska egenskaper hos restprodukter fran kolforbranning - en laboratoriestudie B. Moller, G. Nilson

22. Bestamning av jordegenskaper med sondering en litteraturstudie U. Bergdahl & U. Eriksson

23. Geobildtolkning av grova moraner L. Viberg

24. Radon i jord - Exhalation - vattenkvot - Arstidsvariationer - Permeabilitet A. Lindmark & B. Rosen

25 . Geoteknisk terrangklassificering for fysisk planering L. Viberg

26. Large diameter bored piles in non-cohesive soils Detennination ofthe bearing capacity and settlement from results of static penetration tests (CPT) and standard penetration test (SPT). K. Gwizdala

27. Bestamning av organisk halt, karbonathalt och sulfidhalt i jord R. Larsson, G. Nilson & J. Rogbeck

27E. Determination of organic content, carbonate content and sulphur content in soil R. Larsson, G. Nilson & J. Rogbeck

Ar/Year

1983

1983

1983

1984

1984

1984

1984

1985

1987

RAPPORT/REPORT No Ar/Year

28. Deponering av avfall fran kol- och torveldning 1986

T. Lundgren & P. Blander

28E. Environmental control in disposal and utilization of 1987 combustion residues T. Lundgren & P. Blander

29. Consolidation of soft soils 1986 R. Larsson

30. Kalkpelare med gips som tillsatsmedel 1987 G. Holm, R. Trank & A. Ekstrom

Anvandning av kalk-flygaska vid djupstabilisering av jord G. Holm & H. Ahnberg

Orn inverkan av hardningstemperaturen pa skjuvhallfastheten hos kalk- och cementstabiliserad jord H. Ahnberg & G. Holm

31. Kalkpelarmetoden 1986 Resultat av I O ass forskning och praktisk anvandning samt framtida utveckling. H. Ahnberg & G. Holm

32. Two Stage-Constructed Embankments on Organic Soils 1988 D Field and laboratory investigations D Instrumentation D Prediction and observation of behaviour W. Wolski, R. Larsson et al.

33 . Dynamic and Static Behaviour of Driven Piles 1987 (Doctoral thesis) Nguyen Truong Tien

34. Kalksten som fyllningsmaterial 1988 J. Hartlen & B. Akesson

RAPPORT/REPORT No

35. Thermal Properties of Soils and Rocks (Doctoral thesis) J. Sundberg

36. Full-Scale Failure Test on a Stage-Constructed Test Fill on Organic Soil W. Wolski, R. Larsson et al.

37. Pore Pressure Measurement -Reliability of Different Systems M. Tremblay

38. Behaviour of Organic Clay and Gyttja R. Larsson

39. Gruvavfall i Dalalvens avrinningsomrade -Metallutslapp och atgardsmojligheter RAPPORT TILL DALALVSDELEGATIONEN

T. Lundgren & J. Hartlen

40. Shear Moduli in Scandinavian Clays - Measurement of initial shear modulus with seismic cones - Empirical correlations for the initial shear modulus in clay R. Larsson & M. Mulabdic'

41. Overvakningssystem - Slantbeteende - Skredinitiering Resultat fran ett fullskaleforsok i N orrkoping B. Moller & H. Ahnberg

42. Piezocone Tests in Clay R. Larsson & M. Mulabdic'

43. Footings with Settlement-Reducing Piles in Non-Cohesive Soil Phung Due Long

Ar/Year

1988

1989

1989

1990

1990

1991

1992

1991

1993

RAPPORT/REPORT No

44. Agnesbergsskredet R. Larsson, E. Ottosson & G. Sallfors

45. Agnesbergsskredet - Skredforebyggande atgarder H. Sandebring & E. Ottosson

46. R&D for Roads and Bridges International Seminar on Soil Mechanics and Foundation Engineering B. Rydell (Editor)

Ar/Year

1994

1994

1995

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