Geotechnical Properties of a Typical Collapsible Soil in South-Western Nigeria

Geotechnical Properties of a Typical Collapsible Soil in South-Western Nigeria

Owolabi, Titilayo. A1* and Ola, Samuel. A2

1. Postgraduate student at the department of Civil Engineering, Federal University of Technology Akure, Nigeria.

2. Professor of Geotechnical Engineering, Federal University Federal University of Technology Akure, Nigeria.

* e-mail of the corresponding author: [email protected]

ABSTRACT This research work has analysed the geotechnical properties of a typical collapsible soil. A portion of the highway along Osogbo-Iwo road at chainage 4+500 (Right hand side of the road) in southwestern Nigeria has had a history of perennial failure which has necessitated a special attention and hence this study. Direct shear tests were carried out on the undisturbed samples at various normal stresses and at 0.0375mm/min , 0.375mm/min and 3.75mm/min rate of strain. The shear strength increased with increasing rate of strain with C values of 5, 8, 10 𝑘𝑁 𝑚2⁄ and ∅ values of 120, 140 and 170 respectively.Consolidated Undrained triaxial tests with pore water measurement were carried out on the undisturbed samples at 1.5mm/min rate of strain and at the following cell pressures; 140𝑘𝑁/𝑚2, 280𝑘𝑁 𝑚2⁄ , 420𝑘𝑁 𝑚2⁄ . The angle of shearing resistance of ∅′ = 200 and cohesion 𝐶′ = 105𝑘𝑁 𝑚2⁄ for the effective stresses were measured. While the values were ∅𝑇 = 170 𝐶𝑇 = 100 𝑘𝑁 𝑚2⁄ respectively for the total stress. Unconfined compression test were carried out on the undisturbed samples at 0.06mm/min rate of strain. The shear strength of 11.85𝑘𝑁 𝑚2⁄ as measured in unconfined compression test. Oedometer consolidation test were carried out on dry and wet samples for the load of 20kg, 40kg, 80kg and 160kg. The oedometer test has shown that the soil sample is a collapsible soil with structural collapse at constant stress varying from 3.7% for stress at 100𝑘𝑁 𝑚2⁄ to 11.5% for a stress at 800kN/m2 when the collapse is triggered by adding water at constant load. The soil sample has low relative density of 28.6% with high void ratio and high porosity. The AASHTO classification is an A-5 material (silty soil) which is fair to poor rating in terms of general rating as subgrade material. The clay fraction was 10%, silt 35% and sand 55%. The natural moisture content is 17.5%, plastic limit 33%, plasticity index 10%, linear shrinkage 5%, maximum dry density of the soil sample is 1565kg/m3 with the corresponding optimum moisture content of 18.4%, group index 2 and specific gravity 2.67. The soil sample is a silty sand with little traces of clay having 45% of the soil passing through a No. 200 B.S sieve which is not suitable for subgrade material. The soil sample has low soaked CBR value of 0.86% with high liquid limit of 43% hence cannot be used as a subgrade material or construction material. Keywords: Collapsible soil, Oedometer test, particle size analysis test, Direct shear test, Atterberg limit test, consolidation undrained test, compaction test, specific gravity, California bearing ratio, soil classification, geotechnical properties

INTRODUCTION Many soils can prove problematic in geotechnical engineering because they expand collapse,

disperse, and undergo excessive settlement with a distinct lack of strength. Some characteristics may be attributable to their composition, the nature of their pore fluids, their mineralogy or their fabric (Bell and Culshaw, 2001). Collapsible soil can be defined as soil that is susceptible to a

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Vol. 19 [2014], Bund. H 1722 large and sudden reduction in volume upon wetting. These soils are typically silt and sand size with a small amount of clay and low plasticity index (Pawlak, 1983). Collapsible soils are unsaturated soils which present a potential for large deformation and a complete change to the whole particle structure after wetting, with or without loading. These soils are characterized with loose structures composed of silt to fine-sand-size particles. Collapsible soils are generally deposited in arid and semi-arid regions. According to Clemence (1981), Collapsible soils are soils that compact and collapse after they get wet. The soil particles are originally loosely packed and barely touch each other before moisture soaks into the ground. As water is added to the soil in quantity and moves downward, the water wets the contacts between soil particles and allows them to slip past each other to become more tightly packed. Water also affects clay between other soil particles so that it first expands, and then collapses like a pack of cards. Another term for collapsible soils is "hydro-compactive soils" because they compact after water is added. The amount of collapse depends on how loosely the particles are packed originally and the thickness of the soil that becomes wetted. Collapsible soils consist of loose, dry, low-density materials that collapse and compact under the addition of water or excessive loading. Soil collapse occurs when the land surface is saturated at depths greater than those reached by typical rain events. This saturation eliminates the clay bonds holding the soil grains together (Mulvey, 1992). A collapsible soil is one in which the constituent parts have an open packing and which forms a metastable state that can collapse to form a closer packed, more stable structure of significantly reduced volume. The most important property that a collapsible soil possesses is that of low interparticle bond strength. It is when these bonds are destroyed (through either soil loading, soil saturation or a combination of both) that the collapse of the soil occurs (Rogers, 1994). Kakoli (2011), highlighted that collapsible soils experience significant volume decrease due to the increase of soil moisture content, with or without an increase in the in-situ stress level. Foundations on collapsible soils suffer from sudden settlement, which may contribute to serious damage or catastrophic failure due to inundation. According to Dudley (1970), Barden et al. (1973) and Mitchell (1976), four factors are needed to produce the collapse in soil structure:

• An open, partially unstable, unsaturated fabric.

• A bonding or cementing agent that stabilizes the soil in the unsaturated condition.

• The addition of water to the soil, which causes the bonding or cementing agent to be reduced and the inter-aggregate or intergranular contacts to fail in shear, resulting in a reduction in total volume of the soil mass. The collapsible behaviour of compacted and cohesive soils depends on the percentage of fines (especially clay fraction), the initial water content, the initial dry density and the energy as well as the process used in compaction. There are three main types of collapsible soil: collapsible loess, collapsible alluvial soils (usually high in silt content) and collapsible manmade fills.

Location Of The Study Area Undisturbed and disturbed soil samples were collected at one section of the road; chainage 4 +

500 at the right hand side along Osogbo-Iwo road in Southwestern Nigeria (see Figure 1). The location falls within longitude 4.465050 E , latitude 7.779800N and distance of 4.5km from Osogbo city-centre and 40.5km from Iwo city-centre. The offset from the centre of the road to the soil location is 9m.The undisturbed and disturbed soil samples were collected at a depth of 1.5m from the surface using sampler test pits and hand auger respectively, when the ground water was at a depth of 0.74m from the surface.

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Figure 1: Location of the collapsible soil in Southwestern Nigeria

[Source: Google Earth (2012)]

GEOLOGY OF THE STUDY AREA Osogbo-Iwo road is located within the crystalline precambrain basement complex of

Southwestern Nigeria. Rahaman (1973 ) revealed that the Precambrian basement rocks in Southwestern Nigeria consist of the magmatite-gneiss complex, charnokitic rocks, older granites, unmetamorphhosed delorite dykes, slightly magmatised to unmagmatised paraschists and metaigneous rocks. The major rock that underlies the Osogbo-Iwo ,Osun state is charnockites rock. The Charnockites in general outcrop as smooth widely distributed rounded boulders although in place as e.g. at Oke-Patara and Osuntedo form of isolated hills. The Charnockites are composed of quartz, alkali feldspar, plagioclase, orthopyroxene, clinopyroxene, hornblende, biotite and accessory amounts of opaque ore apatite, zircon and allanite. The climate of Osogbo-Iwo road is subtropical Rain Forest type, with a mean annual temperature of about 280C and a mean annual rainfall of over 1600 mm (Oluwagbenga et al, 2009).

MATERIALS AND METHODS

Preparation of Specimens Samples and specimens were prepared in accordance with BS1377 of 1990.

Test procedures The following laboratory tests were conducted on the disturbed soil samples: Atterberg limits

test, specific gravity test, sieve analysis test, moisture content test, compaction test and California bearing ratio test (CBR). Prior to preparing the test specimens, the materials were air-dried and

Vol. 19 [2014], Bund. H 1724 broken into smaller fragments, care being taken not to reduce the sizes of the individual particles. All the tests were carried out at Geotechnical Engineering Laboratory in Federal University Technology Akure, South western Nigeria.

Direct shear tests were carried out on the undisturbed samples at 0.0375mm/min, 0.375mm/min and 3.75mm/min rate of strain and at the following vertical pressures; 50kN/m2, 100kN/m2, and 150kN/m2. Consolidated Undrained test with pore water measurement were carried out at 1.5mm/min rate of strain and at the following cell pressures; 140kN/m2, 280kN/m2, and 420kN/m2 .Unconfined compression test were carried out on the undisturbed samples at 0.06mm/min rate of strain. Oedometer consolidation test were carried out on dry and wet samples for the load of 20kg, 40kg, 80kg and 160kg. All the tests were carried out in accordance with British standard code of practice (BS1377:1990).

X ray diffraction analyses were carried out on the fraction of the soil passing the No. 200 B.S. Sieve using two approaches namely:

• Air dried preferred orientation (ADPO): Water was added to the soil powder to obtain soil/water suspension which was sucked on a porous disk for air drying.

• The air dried preferred orientation (ADPO) soil sample was heated to 6000C for 1hr.

The above samples were loaded inside the sample holder of the Radicon MD 10 Mini-diffractometer machine. The machine was allowed to run for 20minutes, the diffractogram produced were analyzed using international centre for diffraction data (ICDD, 1999). The tests were carried out at the centre for energy research development at Obafemi Awolowo University, Ile-Ife Osun state in Nigeria.

RESULT AND DISCUSSION The result of the grain size analysis for the unstable soil in Osogbo-Iwo road is shown in

Figure 2. The result shows that 45% of the soil passed through a No. 200 B.S sieve .The clay fraction was 10%, silt 35% and sand 55%. This particle size distribution can be taken as a classical particle size distribution for an unstable soil with a large component of the soil being silty fine to medium fine sand and with 10% clay acting as binder to bind the particle together. The part played by the clay can be seen in the photograph of the unstable soil shown in plate 1 and 2 .The natural moisture content is 17.5% , the liquid limit is 43% , plastic limit 33% , plasticity index 10%, linear shrinkage 5%, group index 2, and specific gravity 2.67 .The soaked CBR is 0.9% while the unsoaked CBR is 3.6%. The maximum void ratio is 1.1 while the minimum void ratio is 0.4. The void ratio as found in situ is 0.9 which is close to the maximum void ratio, to give a relative density of 28.6%. These are shown in Table 1 where the summary of the physical properties are given.

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Plate 1: photograph of the unstable soil Plate 2: showing the part played by the clay

Figure 2: Particle size distribution of the unstable soil in Osogbo-Iwo road

Table 1: Physical properties of the study area in Osogbo-Iwo road Tests Results

Clay,% 10

Silt, % 35

Sand, % 55

Natural moisture content, % 17.5

Liquid limit, % 43

Plastic limit, % 33

Plasticity index, % 10

Specific gravity 2.67

Optimum moisture content, % 18.4

CLAY BOULDERS

Fine Medium

Coarse FIne Medium Coars

e Fine Medium Coars

e SILT SAND GRAVEL

Test method BS 1377: Part 2: 1990: 9.2/9.3/9.4/9.6/9.7*

Hydrometer wet sieve analysis

m

Perc

enta

ge P

assi

ng

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Maximum dry density kg/m3 1565

Group index 2

Activity, (P.I / % of clay) 1.0

Linear shrinkage, % 5

Soaked CBR, % 0.9

Unsoaked CBR, % 3.6

Void ratio eo 0.90

Minimum void ratio emin 0.4

Maximum void ratio emax 1.1

Relative density, % 28.6

Compaction Characteristics The value of the maximum dry density of the soil sample is 1565kg/m3 with the corresponding

optimum moisture content of 18.4%.This is shown in figure 3 below

Figure 3: Compaction Characteristics

Figure 4 shows the X-ray diffraction analysis on the fraction of the soil passing the No. 200

B.S. Sieve. This figure shows that most of the soils are composed predominantly of quartz and kaolinite. The presence of quartz was identified by the 3.34A0 peak in the ADPO heated to 6000C for 1 hr. and kaolinite was also observed by the presence of 1.36A0 in the ADPO.

1400.00

1420.00

1440.00

1460.00

1480.00

1500.00

1520.00

1540.00

1560.00

1580.00

1600.00

9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0 21.0

Dry

Den

sity

(kg/

m3 )

Moisture Content (% Dry Weight)

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Shear Strength Characteristics The unconfined Compressive strength, qu for the soil sample is 23.70kN/m2 and the shear

strength is 11.85kN/m2. The result of the test is given in figure 5. Failure occurs at a rate of strain of 4.5%. For the direct shear test, at a vertical pressure of 50kN/m2, 100kN/m2 and 150kN/m2 using0.0375mm/min, 0.375m m/min and 3.75mm/min rate of strain respectively on the undisturbed samples, the angle of shearing resistance varied significantly with the rate of strain. For example, at the rate of strain of 3.75mm/min the angle of shearing resistance was 170. This was reduced to 140 when the rate of strain decreased to 0.375mm/min. This was further reduced to 120 for a rate of strain of 0.0375mm/min.

Figure 4: X-rays diffraction traces of the unstable soil

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Figure 5: Unconfined compressive strength of undisturbed unstable “collapsible” soil sample

Results of direct shear tests on the undisturbed samples at different rates of strain

Figure 6: Direct shear stress-strain curve for normal stress of 50kN/m2 at three different rates of strain for the undisturbed sample

0.375 mm/min

0.0375 mm/min

Shear strain (%)

Shea

r stre

ss

kN/m

2

3.75mm/min

Axial strain (%)

Unc

onfin

ed C

ompr

essi

ve S

treng

th, q

u

2

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Figure 7: Direct shear stress-strain curve for normal stress of 100kN/m2 at three different rates of strain for the undisturbed sample

Figure 8: Direct shear Stress-strain curve for normal stress of 150kN/m2 at three different rates of strain for the undisturbed sample

3.75mm/min

0.375 mm/min

0.0375 mm/min

3.75mm/min

0.375mm/min

0.0375mm/min

Shea

r stre

ss

kN/m

2

Shear strain (%)

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Figure 9: shear stress against normal stress for the undisturbed samples at 3.75mm/min rate of strain for direct shear test

Rate of strain = 3.75mm/min

Figure 10: shear stress against normal stress for the undisturbed samples for direct shear test

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Figure 11: shear stress against normal stress for the undisturbed samples for direct shear test

Rate of strain = 0.0375mm/min

Figure 12: Influence of the rate of strain on shear strength for direct shear test

Vol. 19 [2014], Bund. H 1732 Table 2: Effect of rate of strain on shear strength parameters of the unstable soil for direct

shear test Rate of strain (mm/min) Cohesion C, kN/m2 Angle of internal friction,ϕ degree

0.0375 5 12

0.375 8 14

3.75 10 17

Rate of strain = 1.5mm/min

Figure 13: Analysis of effective stresses based on maximum deviator stress

Figure 14: Analysis of total stresses based on maximum deviator stress

Rate of strain = 1.5 mm/min

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OEDOMETER CONSOLIDATION RESULT

Figure 16: Graph of log pressure against void ratio (wet and dry)

Figure 15: Stress-strain curves for the unstable soil at different confining pressures

Dry

Wet

Failure strain, ϵf =5 %

Failure strain, ϵf =3.6 %

Failure strain, ϵf =7.5 %

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SUMMARY OF FINDINGS

Classification Characteristics The soil sample is a silty sand with low plasticity index and little amount of clay which serves

as the binder. The silt controls the behaviour of the soil, from the physical structure of the soil observed, there is apparent cohesion between the soil which collapses upon wetting. Classified as ML group (inorganic silts, fine sand with low plasticity) according to the Unified soil Classification System, USCS and as an A-5 material (silty soil) which is fair to poor rating in terms of general rating as subgrade material according to AASHTO classification.

California Bearing Ratio The CBR of the soil sample under saturated condition was tested. Hence the soaked and

unsoaked CBR was obtained, the soaked CBR value was 0.86% while the unsoaked CBR value was 3.61. The implication of the results obtained shows that the soil sample cannot be used as a subgrade material or construction material because of its low CBR values. There is decrease in the

Vol. 19 [2014], Bund. H 1736 CBR value from 3.61% to 0.86% which shows that the intergranular mineral to mineral contact is weakened by the water and thereby the particles slide over one another and consequently collapsed when wet.

Compaction Characteristics The soil sample has low maximum dry density of 1565kg/m3 and low optimum moisture

content of 18.4%. Hence cannot be used as a subgrade material or construction material.

Shear Strength Characteristics Comparing Results of Direct Shear and Triaxial Tests for a Collapsible soil

• Both results give angle of internal friction ϕT for a total stress analysis and rate of strain between 1.5min/mm and 3.75mm/min of ϕT = 170

• There is a significant difference in the cohesion values while the direct shear gives a values of 10kN/m2, the triaxial test gives 100kN/m2

• Increasing the rate of strain i.e shearing the soil faster gives an increase in both the cohesion and angle of friction 100% increase in cohesion from rate of strain of 0.0375 to 3.75mm/min while only 42% increase in the frictional angle was found

Oedometer Consolidation The Oedometer test shows the results of consolidating the sample when dry and when wet

(Figure16) It also shows that the soil sample is very loose when dry with high void ratio but when water is added to it (when wet), the soil collapses and compact. As the load applied increases, deformation increases while the void ratio decreases. The oedometer test also indicate that the soil sample is a collapsible soil with a structural collapse at constant stress varying from 3.7% for stress at 100kN/m2 to 11.5% for a stress at 800kN/m2 when the collapse is triggered by adding water at constant load ( figure 17 to figure 20).

CONCLUSION Consequent upon the tests carried out on the soil sample obtained at chainage 4 +500 in

Osogbo-Iwo road, the following conclusion can be drawn:

• The soil sample has low relative density of 28.6% with high void ratio and high porosity which indicate that the soil is weak and compressible, wetting reduces the strength, increase in ground water level and vibration from vehicle liquefies the soil hence, it collapses.

• The soil sample is typically silty sand with little amount of clay having 45% of the soil passing through a No. 200 B.S sieve which is not suitable for subgrade material. The soil also has low plasticity index of 10%, low moisture content and low dry density.

• The Oedometer test shows that the soil sample is very loose when dry with high void ratio but when water is added to it (when wet), the soil collapses by up to 11.5%.

• The soil sample has low CBR value of 0.86% with high liquid limit of 43% hence cannot be used as a subgrade material or construction material.

Intergranular mineral to mineral contact

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• The shear strength of the sample is low, Both the direct shear and consolidated undrained triaxial test give angle of internal friction ϕT for a total stress analysis and rate of strain between 1.5min/mm and 3.75mm/min of ϕT = 170. The shear strength increases with increase in the rate of strain. There is a 100% increase in cohesion for an increase in rate of strain from 0.0375 to 3.75mm/min while only 42% increase in the frictional angle was obtained for the same strain range in direct shear. The structure of the soil has metastable texture as the bonds between the grains breakdown when the soil is wet. Hence, the collapse process represents a rearrangement of soil particles into a denser state of packing; also the soil has an open, partially unstable and unsaturated fabric.

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