IN-SITU MEASUREMENT OF GROUND STIFFNESS ...reaaa.vms.my/images/e/e7/(5-21)IN-SITU_MEASUREMENT...IN-SITU MEASUREMENT OF GROUND STIFFNESS USING SUPER FALLING WEIGHT DEFLECTOMETER TEST

IN-SITU MEASUREMENT OF GROUND STIFFNESS USING SUPER FALLING WEIGHT DEFLECTOMETER TEST METHOD

Masaaki NAGASAWA, Hirotaka KAWASAKI

Geotechnical Group, Civil Engineering Technology Division, Shimizu Corporation

1-2-3 Shibaura Minato-ku, Tokyo, Japan [email protected]

ABSTRACT

In the design and construction of the road and structure foundations, in order to effectively prevent the residual settlement and the insufficiency of bearing capacity, reliable in-situ test methods which enable the measurement at high accuracy and repeatability are necessary. Plate loading test is the most commonly used method for in-situ measurement of ground stiffness. However, this method is considered to be time-consuming and costly for multi-point measurement.

Therefore, based on the simple principle of Falling Weight Deflectometer (FWD) method for pavement investigation, this Super Falling Weight Deflectometer (SFWD) method for measuring ground stiffness at high accuracy thanks to escalating rapid loading and accumulative displacement measurement was developed and applied at actual construction site. The ground stiffness measured by this method was found to be equivalent to that measured with plate loading test. In addition, with the advantage of simple and quick measurement using this method, it is now possible to early evaluate the ground stiffness to provide reliable information for consideration of construction method and process.

This paper describes the outline of this test method and the result of the actual application at some construction sites. 1. INTRODUCTION

For evaluating the stiffness of ground, there are two approaches as can be seen in Table 1: One is the direct calculation of ground stiffness based on load and displacement (Plate Loading Test, In-Situ CBR Test, FWD, Light Weight Deflectometer, SFWD), the other is the indirect evaluation using acceleration index (Automatic Compression Testing Machine, Intelligent Compaction).

The ground stiffness evaluation using loading plate is applicable for grounds of which maximum size of soil particles is less than one third, and measured depth is less than twice of plate diameter

Though plate loading test is the most commonly used method, it requires a large reaction base. Furthermore, it is quite labor and time-consuming, taking several hours for one measurement. On the other hand, though the indirect method using acceleration index value is quite simple, its range of applicable ground as well as the measuring accuracy remains great challenges.

FWD and Light Weight Deflectometer are common methods in which a weight is dropped repeatedly 6 times from the same height and the ground stiffness is evaluated based on the measured results. The first drop is preparatory loading for eliminating the bedding error. It also creates a loading history in the tested ground. Therefore, the ground stiffness obtained after first drop is the one under repeat loading, and is, therefore, different from the ground stiffness in term of K value obtained under monotonous loading of conventional plate loading test.

SFWD system (Figure 1) utilizes the principle of FWD but the way of applying load is quite different. In this system, the load is applied in multi stages with escalating rapid loading while the accumulative displacement is measured. The ground stiffness is defined as the envelope gradient of load-displacement curves.

As a result, it is quite simple to obtain equivalent K-value as in case of using plate loading test, and in-situ evaluation of ground stiffness is relatively easy with this method.

Table 1 In-Situ Measurement Methods for Ground Stiffness Correlation to

K-value of plate loading test

Measured item

displacement plate size Evaluation of K-value pre-loadingaccuracy （mm）

Direct

Plate Loading displacement φ300～ & load high φ750 no ── ── Test

In-Situ CBR Test

displacement & load high φ50 no conversion

low in gravelly soil

FWD displacement & load high

φ300～ φ450 applied cyclic loading

2. OUTLINES OF SFWD SYSTEM AND DATA PROCESSING METHOD

In SFWD measurement, a weight of 200kg is repeatedly dropped onto ground from gradually increased heights. It means that load with increasing intensity is applied continuously on ground and the ground stiffness is evaluated from the measured accumulative displacement. The maximum load corresponding to the maximum dropping height of 300mm is as large as 90kN, which means that a great impact on the ground is possible.

As can be seen in Figure 2, this system is an integration of load and displacement measuring functions. Devices for loading, displacement measurement, GPS signal, etc are controlled by a computer which is also used for analyzing measured results and evaluating ground stiffness. Table 2 shows the parameters of this system.

It is a totally automatic system which enables a fast, highly accurate and simple method for evaluation of ground stiffness. Therefore, the measurement of ground stiffness is largely optimized, the construction is more effective, and the quality assurance is very much simpler than before. Example of ground stiffness evaluation based on measured results is shown in Figure 3.

Figure 4 shows the load and displacement results in which the weight was dropped from different heights of 60, 175 and 300mm.As can be seen in this figure, the time duration from the start to the pick of loading is as small as 0.01 second. It means a great impacting load has been applied.

Figure 5 is a computer display showing the “Result Analysis” dialog of SFWD system. In the same display, one can see such information as load, displacement, load-displacement relation, and load–cyclic deformation modulus relation on the real time basis. Besides, the ground stiffness is also evaluated and shown at the same time.

medium

Light WeightDeflectomter

acceleration & load medium

φ100～ φ300 applied cyclic loading slightly low

SFWD displacement & load high

φ300～ φ450 no

equivalent to plate loading test high

Indirect

ACTM*1) acceleration ── φ50 applied conversion extremely

low

Intelligent Compaction acceleration ── roller width no conversion low

*1) Automatic Compression Testing Machine

Figure 1 Measurement at a Construction Site using SFWD

Figure 2 Super Falling Weight Deflectometer(SFWD)

Displacement sensor

GPS antenna GPS receiver

PC

Weight(200kg)

Damper Load Cell

Table 2 SFWD Specification Weight 200kg

Maximum Falling Height 300mm

Maximum Rapid Loading 90kN

Diameter of Plate φ 450、φ300

Loading Sensor load cell

Displacement Sensor GY & LVDT

Range of Displacement 0~30mm

Loading Plate(φ450mm, φ300mm)

Control

PositionMeasuring Data

Figure 3 Sample of a Distribution Map of Ground Stiffness

Deformation Modulus

2

Measured by SFWD

0.0

0.2

0.4

0.6

0.8

1.0

30 50 70 90 110

時　間　ｔ　（ms）

変　

位　

δ　

（m

m）

0

200

400

600

800

1000

荷　

重　

Ｐ　

（kN

/m

2）

変位h=60mm変位h=175mm変位h=300mm荷重h=60mm荷重h=175mm荷重h=300mm

Displacement (h= 60mm) Displacement (h=175mm) Displacement (h=300mm) Load (h= 60mm) Load (h=175mm) Load (h=300mm)

Dis

plac

emen

t, δ

(mm

)

Load

, P (k

N/m

2 )

Time (ms) Figure 4 Time History of Displacement and Load

s Elsf of SFWD system is calculated using the following formula in which the co

Elsf=0.25π D (1-ν ) Klsf (1) r sand or gravel, 0.4 for clay)

荷重・変位波形 Time History of Displacement and Load

Figure 6 shows the concept of [multi-stage rapid loading - accumulative displacement] method

used in SFWD system. As seen in this figure, the coefficient of bearing capacity, Klsf, is defined as the envelope gradient of all load and displacement peak values of each loading stage, and the coefficient of cyclic subgrade reaction, Krsf, is defined as the gradient of each load and displacement start and peak values of each loading stage.

The deformation moduluefficient of bearing capacity, Klsf, under plate loading condition is a variable.

2

Here, ν: Poison’s ratio (0.3 foD: Diameter of loading plate

荷重・変位関係 荷重・繰返し弾性係数関係 Relation between Relation between Load and Cyclic Deformation Modulus Load and Displacement

Figure 5 Sample of Output Obtained by SFWD Measurement

0

100

200

300

400

500

600

0.0 2.0 4.0 6.0 8.0変 位 δ （mm）

荷

重

P （

kN

/m

）2

2Lo

ad, P

(kN

/m)

Displacement, δ (mm)

E=0.25π･D(1-ν2)･K

Coefficient of Bearing Capacity, Klsf

Coefficient of Cyclic Subgrade Reaction, Krsf

Figure 6 Multi-Stage Rapid Loading-Accumulative Displacement Method used in SFWD

3. LABORAT N OF MEASURING ACCURACY

CBR test nd SFWD measurement were carried (Figure 7).

acting silica into a large

ylinder with a com and it has diameter of 650mm

comparison with in

tion obtained in SFWD measurement and plate ading test. This figure also shows a good agreement of deformation modulus obtained by SFWD

measu

st and the SFWD measurement. Cyclic deformation modul

ORY TEST FOR VERIFICATIO

In order to verify the accuracy of SFWD system, laboratory plate loading test, in-situ a

3.1 TEST CONDITIONS

Tested ground with dry density, ρd, of 1.537g/cm3 was created by comppacting energy of 4.5E (E : compactive effort ≈ 550kJ/m3)

mpacting with energy of 0.15E . On the other hand, for

Figure 7. Laboratory SFWD Test

c c c and thickness of 200mm. For comparison with plate loading test, tested soil was modified by replacing silica with

crusher-run (Dmax = 40mm) and co c-situ CBR test, the compaction energy was varied from 0.5Ec to 4.5Ec and part of silica in tested

soil was replaced by crusher-run (Dmax = 40mm).

3.2 COMPARISON WITH PLATE LOADING TEST

ＳＦＷＤ試験

Figure 9 shows the load-displacement rela

lorement, Elsf, and plate loading test, Elpl. Figure 10 shows the relationship between initial deformation modulus, El, and cyclic deformation

modulus, Er, obtained in cyclic plate loading teus for comparison is the one at the cyclic load of 280kN/m2, which is around the middle value of

load range. In the same figure, the initial and cyclic deformation moduli obtained by SFWD measurement and by cyclic plate loading test have correlation coefficient from 0.93 to 0.95, which

載荷板φ450

試験領域

貫入体φ50

ＳＦＷＤ試験 現場 験ＣＢＲ試

Tested area

Silica (4.5Ec) ρd=1.537g/cm3

φ50 φ450

SFWD CBR

Figure 8 Laboratory SFWD Test Configuration

a) Cross Section b) Top View

載 50荷板φ4

６号硅砂・4.5Ecρd=1.537 g/cm3

試験領域Tested area

SFWD

φ450

Silica (4.5Ec) ρd=1.537g/cm3

現場ＣＢＲ試験

貫 50入体φ

６号硅砂・4.5Ecρd=1.537 g/cm3

試験領域

φ50

Tested area

CBR

Silica (4.5Ec) ρd=1.537g/cm3

means relatively high correlation. Therefore, it can be concluded that it is highly reliable using SFWD in replace of plate loading

test for quality control in compacting work of man-made ground like fill.

and in-situ CBR test in case tested area contains rusher-run (1.5Ec). On this load-displacement relation line of in-situ CBR test, CBR deformation

modul

easurement and C

3.3 COMPARISON WITH IN-SITU CBR TEST

Figure 11 shows the results of SFWDc

us, Ecbr, is defined as gradient of linear part at initial stage under monotonous load. In this case, in-situ CBR test and SFWD measurement yields almost the same deformation modulus.

Figure 12 shows the correlation between deformation modulus obtained by SFWD measurement and CBR. As can be seen in this figure, deformation modulus by SFWD m

BR are in very good agreement.

SFWD

PLT

Load

, P (k

N/m

2 )

Elpl=13.78MN/m2

Elsf=14.20MN/m2

Displacement, δ (mm)

Figure 9 Comparison of Load Displacement obtained in SFWD Measurement and Plate Loading Test

0

20

40

60

80

0 20 40 60 80

初期弾性係数Eℓ

繰返し弾性係数Er

Initial deformation modulus

Ersf=1.099Erpl r=0.95

Cyclic deformation modulus

Elsf=0.960Elpl r=0.93

Initial deformation modulus, El Cyclic deformation modulus, Er

SFW

Df（

MN

/m

）の

弾性

係数

　Ｅ

s2

Def

orm

atio

n M

odul

us o

f SFW

D,

E sf (

MN

/m2 )

平板試験の弾性係数　Ｅpl（MN/m2）Deformation Modulus of Plate Loading Test, Epl (MN/m2)

Figure 10 Relation between Deformation Modulus of Plate Loading Test and SFWD

Therefore, it can be concluded that it is possible to use SFWD in replace of in-situ CBR test for quality control in compacting work of man-made ground like road foundation.

dy soil (decomposed granite oil) are selected as typical examples of in-situ measurement and stiffness evaluation using SFWD.

easurement is the first ecessary step in which SFWD measurement (φ450mm or φ300mm), Plate Loading Test (φ750mm or

φ300m

4. EXAMPLE OF ACTUAL APPLICATION ON MAN-MADE GROUND

Actual applications on mudstone soil, cement-stabilized soil and sans

4.1 METHOD FOR EVALUATION OF GROUND STIFFNESS USING SFWD

For evaluating the ground stiffness using SFWD, the calibration mn

m), in-situ CBR test (φ50mm), density and water content measurement are carried out. Based

Figure 12 Relation between Initial Deformation Modulus of SFWD and CBR

y = 5.680x0.594

r = 0.957

0

20

40

60

80

100

SF

0 20 40 60 80 100

CBR　（％）

2）

WD

初期

弾性

係数

　Eℓ（

MN

/m

砕石

硅砂

Initi

al D

efor

mat

ion

Mod

ulus

o

f SFW

D, E

lsf (

MN

/m2 )

Crusher-run Silica

Figure 11 Relation between Normalized Load and Displacement of CBR and SFWD

0

0.0 1.0 2.0 3.0 4.0 5.0

m）

25

50

75

100

125

150

175

正規

化荷

重

P＊

（kN

/

Nor

mal

ized

load

, P* (

kN/m

2 ) P＊

＝0.

25π

D(1

-ν2 ) P

Initial deformation modulus of CBR test

Ecbr=46.0MN/m2

Initial deformation modulus of SFWD

Elsf=48.0MN/m2

CBR

SFWD

変 位 δ （mm）Displacement, δ (mm)

on the results obtained in these tests and measurements, correlation between entities like deformation modulus, bearing capacity coefficient, CBR, dry density, water content, etc can be figured out.

As shown in Figure 13, the measurement points are arranged in order to avoid the effect of load history caused by plate loading test, and the results of measurement are the average values of SFWD measu

4.2 STIFFNESS EVALUATION OF MUDSTONE GROUND, CEMENT STABILIZED GROUND

an-made ground at an airport, including lower subgrade d and roller-compacted upper subgrade of fill (t = 300mm),

upper

ues of RI measurement after compacting work are dry density (ρd) of 1.89

nd compacting by 20-ton

.2.2 MEASURED RESULTS AND DISCUSSION

ts at the same position using plate loading test and s of plate loading test and SFWD have similar cyclic

history

ding test, respectively.

in-situ CBR test is quite sensitive to the fluctuation of ground condit

evalua

OMPOSED GRANITE GROUND)

nd including a fill made of decomposed granite soil and a

rement at 4 positions, in-situ CBR test as well as measurement of density and water content at 2 positions.

Figure 13 Outlines of Calibration Tests

200

200

Plate Loading Test（φ750 or φ300）

SFWD(φ450 or φ300)

CBR(φ 50)

Density and Water Content

4.2.1 OUTLINES OF MAN-MADE GROUND

The measured ground was 4 types of mf fill, lower subgrade of cut, cement-stabilizeo

subgrade of cut (t = 150mm). Lower subgrade of fill is formed by mudstone soil (Dmax = 150mm) and compacted with 20-ton

vibrating roller (4 times). Average val7g/cm3, water content(w) of 11.6%, and degree of compaction (Dc) of 98.7％ Lower subgrade of cut is made of new mudstone and leveled with bulldozer. Upper subgrade of

fill and cut are cement-stabilized layers created by mixing with cement (50kg/m3) a vibrating roller (6 times)

4

Figure 14 shows the results of measuremenFWD measurement. This figure shows that resultS

loops. Figure 15 and 16 show the correlation between SFWD measurement and in-situ CBR test as

well as plate loaAs shown in figure 15, a correlation of CBR values is relatively low. The result is believed due to

the fact that the loading φ50mm rod usedion. As shown in figure 16, a correlation between results of SFWD measurement and those of

plate loading test is extremely high. By using this correlation, it is possible to use SFWD instead of plate loading test for investigating

the ground stiffness. It makes the work on quality control at site, especially for ground stiffness tion at large construction site, simpler and speedier.

4.3 STIFFNESS EVALUATION OF SANDY GROUND (DEC 4.3.1 OUTLINES OF MAN-MADE GROUND

The tested ground was man-made groucut made of decomposed granite soil.

0

100

200

300

400

0

50

100

150

200

SFWDの

弾性

係数

E

sf (MN/m2)

0 50 100 150 200

平板試験の弾性係数 Epℓ (MN/m2)

盛土/下部路床/無処理切土/下部路床/無処理盛土/上部路床/C処理t=300

切土/上部路床/C処理t=150

y=3 .534x0.806

　r=0 .876

Figure 16 Relation between Initial Deformation Moduli of SFWD and that of Plate Loading Test

Initial Deformation Modulus of Plate Loading Test, Elpl (MN/m2)

Initi

al D

efor

mat

ion

Mod

ulus

of S

FWD

, E

lsf (

MN

/m2 )

Fill/Lower subgrade Cut/Lower subgrade Fill/Upper subgrade Cut/Upper subgrade

0

0 50 100 150 20

50

100

150

200

0

現場ＣＢＲ (％)

SFWDの

弾性

係数

E

sf (MN/m2)

盛土/下部路床/無処理

切土/下部路床/無処理盛土/上部路床/C処理t=300

切土/上部路床/C処理t=150

y=1 .198x　r=0 .530

Figure 15 Relation between Initial Deformation Modulus of SFWD and CBR

Initi

al D

efor

mat

ion

Mod

ulus

of S

FWD

, E

lsf (

MN

/m2 )

CBR (%)

Fill/Lower subgrade Cut/Lower subgrade Fill/Upper subgrade Cut/Upper subgrade

Figure 14 Relation between Load and Normalized Displacement

500

2）

600

0.0 2.0 4.0 6.0 8.0 10.0 12.0

正規化変位 δ*

[δ*=δ/{0.25π(1-ν2)D}]

荷重

強さ

　p　

（kN

/m

平板載荷試験φ750

SFWDφ450

E=0.25π･D(1-ν2)･K

SFWD E

sf=65.0MN/m

2

平板 E

pl=39.7MN/m2

Plate Loading Test(PLT) (φ750)

Load

, P (k

N/m

2 )

Normalized Displacement , δ* (mm)

The decomposed granite soil for m imum size of 19mm. Each 30cm layer is compacted by 10-ton roller in 8 times. After completing the required compaction, the sand replacement method is applied for confirming that the degree of compaction is at least 90%.

Cut is created by motor scraper. 4.3.2 MEASURED RESUL D DISCUSSION

Figure 17 shows the s of plate loading test and SFWD measurement on the same position. It can be seen that the plate loading test has similar record with SFWD measurement.

Figure 18 shows the correlation between SFWD result and that of plate loading test. It is clear that there is a good agreem t between these methods. Therefore, the evaluation of bearing capacity coefficient and deformation modulus by using SFWD is as accurate as by using plate loading test.

SFWD was applied on the whole 5.6-hectre area of man-made ground for evaluating the deformation modulus. Figure 19 shows the contour of SFWD results that have been converted into initial

aking fill has the max

TS AN

result

en

0

200

400

0.0

600

m）

800

1.0 2.0 3 5.0

変位 δ （

荷重強さ

p （kN

/2

平板載荷試験φ300

.0 4.0

mm）

SFWDφ300

E=0.25π･D(1-ν2)･K

平板 E

pl=30.9MN/m2

SFWD E

sf=35

/m

.4MN

2

Plate Loading Test (PLT) (φ300)SFWD (φ300)

Load

, P (k

N/m

2 )

Displacement, δ (mm)Figure 17 Relation between Loa nt d and Displaceme

0

20

40

60

80

0 20 40 60 80 100

平板試験の弾性係数 Epℓ (MN/m2)

SFWDの弾性係数

Esf (MN/m2)

100

盛土

切土

y=1.347x0.991

　r=0.836

Fill

Initi

al D

efor

mat

ion

Mod

ulus

of S

FWD

, E

lsf (

MN

/m2 )

Cut

2Initial Deformation Modulus of PLT, Elpl (MN/m )

Figure 18 Relation between Initial Deformation Moduli of SFWD and Plate Loading Test (PLT)

deform

ethod on r result of =40mm),

r and mely feed

ctual

compared with plate loading test or in-s several hours. Therefore, it can be considered a great e ffness evaluation is always time-consuming and costly.

The final purpose is to ensure that a man-made ground satisfies the required performance by on-site quality control using this SFWD system. In addition, by digitalization and visualization of information, it is expected to be very useful for works related to management, maintenance and renovation after starting the utilization of the target ground.

It is also ex ted that SFWD method will be applied more and more in the coming time and has a great contribu on the development of road constructing technologies. REFERENCES 1. Japan Society of Civil Engineers. Operation Manual of FWD and Portable FWD. Pavement

Engineering Lib . Japan. 2002 2. H.Kawasaki.et.al. The Outline of Automatic Ground Stiffness Evaluation System(SFWD) and the

Result of the Application at Construction Sites. The Foundation Engineering and Equipment. Vol.34. Japan. 2006

3. H.Kawasaki.et.al. The Method for Measuring Ground Stiffness by AutomatiC Ground Stiffness Evaluation System(SFWD). JSCE 2004 Annual Meeting & Annual Conference. Japan. 2004

4. A.Saragai.et.al. Accuracy validation of Automatic Ground Stiffness Evaluation System(SFWD) by Laboratory Test. JSCE 2 n. 2004

5. H.Kawasaki.et.al. In-Situ Measurement for Stiffness of Gravel Fill by using SFWD Test Method. Proceedings o eering. Japan.2006.

6. H.Kawasaki.et.al. sing SFWD Test Method. Proceedings of st on Geotechnical Engineering.

ation modulus as in plate loading test. From this result, it can be concluded that SFWD is effectively applicable for large construction

area and the evaluation of ground stiffness is undoubtedly very accurate.

5. CONCLUSION

This pape ports the results of laboratory tests and the application of SFWD mman-made ground. It was found that the measurement using SFWD method yields similaground stiffness in comparison with plate loading test. In addition, for small-size gravel (DmaxSFWD measurement has a good correlation with in-situ CBR test.

By applying SFWD method, the measurement of ground stiffness has become simplespeedier. Besides, it facilitates an effective confirmation of construction process, providing a tiback for evaluating the quality of ground making works on the real time basis.

The required time for each measurement using SFWD is approximately 10 minutes (atime depends on the number of loading ng distance.). It is relatively short

S

r re

stages as well as the moviitu CBR test which takes

nhancement which changes the old idea that ground sti

pection

rary 2

004 Annual Meeting & Annual Conference. Japa

f the 41st Japan National Conference on Geotechnical EnginIn-Situ Treated Fill by u Measurement for Stiffness of Cement

the 41 Japan National Conference

Figure 19 Contour of Initial Deformation Modulus obtained by SFWD

480m

260m

Japan.2006. 7. S.

. H.Kawasaki.et.al. Relation between In-Situ CBR and SFWD Test Method. JSCE 2007 Annual Meeting & Annual Conference. Japan.2007.

9. T.Sugimoto.et.al. In-Situ Measurement of Mudstone Ground Stiffness by using SFWD Test Method. Proceedings of the 43rd Japan National Conference on Geotechnical Engineering. Japan.2008.

10. K.Iwamura.et.al. In-Situ Measurement of Decomposed Granite Ground Stiffness by using SFWD Test Method. Proceedings of the 43rd Japan National Conference on Geotechnical Engineering. Japan.2008.

Horita.et.al. In-Situ Measurement for Stiffness of Airport Fill by using SFWD Test Method. JSCE 2007 Annual Meeting & Annual Conference. Japan.2007.

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