unsaturated shear strength tropical volcanic residual soil r sari

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1 1 INTRODUCTION This research is a part of a study on the characteristic tropical volcanic residual soil. To determine the shear strength of the soil a series of triaxial tests were conducted. The triaxial test were Triaxial Consolidated Undrained Test (TXCU) for determined the saturated shear strength parameters and Triaxial Consolidated Drained Unsaturated (TXCD-UNSAT) for determined the unsaturated shear strength parameters. In attempt to measure unsaturated soil parameters, an extensive test is usually conducted using Modified Consolidation Drained Triaxial Test with sophisticated equipment. A simplified modified Consolidated Drained Triaxial Test were developed at the Geotechnical Laboratory of Parahyangan Catholic University Bandung. A high air entry value porous disc were installed to the modified Triaxial Cell equipment. An addition procedure of matrix suction equalisation were conducted after consolidation stage, resumed with sample compression under constant matrix suction. The results were analyzed based on the two methods of constitutive behaviour of unsaturated shear strengh : The Two Independent Stress Variable Method and Effective Stress Method was conducted. 2 UNSATURATED SHEAR STRENGTH TEST (TXCD-UNSAT) The Triaxial Consolidated Drained Test (TXCD) with constant matrix suction is a triaxial test in one series with four stages. The stages are saturation, consolidation, matrix suction equalisation and compression. The triaxial equipment consist of : 1. A triaxial cell with a modified pedestal to conduct unsaturated shear strength test with constant suction. 2. Cell pressure application system 3. Back pressure application system 4. Air pressure application system 5. Pore water pressure measurement system. 6. Volume change measurement system. The diagram of the system is shown in Figure 1-1 below. The pedestal of the modified triaxial test is shown in Figure 1-2. This test procedure was conducted in a lengthy period of time. One sequel of test need three to four weeks. Table 1-1 below listed the stages and Unsaturated Shear Strength Parameters of Tropical Volcanic Residual Soils R. Karlinasari Catholic University of Parahyangan Bandung, Indonesia P. Rahardjo Catholic University of Parahyangan Bandung, Indonesia ABSTRACT: The uppermost zone of tropical residual soil is a zone of unsaturated soil with a unique characteristic. The uppermost zone intensively change from unsaturated-saturated phase, because the intensity of sun heat is high and the intensity of rainfall is much higher than it’s soil permeability. A series of research on the unsaturated shear strength of tropical volcanic residual soil were conducted with Triaxial Consolidated Drained Unsaturated (TXCD-UNSAT) Method. The research is resulted on unsaturated shear strength parameters. These parameters were evaluated with another research’s results on unsaturated shear strength. An analysis based on the two methods of constitutive behaviour of unsaturated shear strengh ; The Two Independent Stress Variable Method and Effective Stress Method was conducted. From the nature of the uppermost zone which is intensively change from unsaturated-saturated phase, the Effective Stress Method concluded has a better preferences. Keywords : Unsaturated, Shear Strength Parameters, TXCD-UNSAT, Independent Stress Variable Method, Effective Stress Method

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Page 1: Unsaturated Shear Strength Tropical Volcanic Residual Soil R sari

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1 INTRODUCTION

This research is a part of a study on thecharacteristic tropical volcanic residual soil. Todetermine the shear strength of the soil a series oftriaxial tests were conducted. The triaxial test wereTriaxial Consolidated Undrained Test (TXCU) fordetermined the saturated shear strength parametersand Triaxial Consolidated Drained Unsaturated(TXCD-UNSAT) for determined the unsaturatedshear strength parameters.

In attempt to measure unsaturated soilparameters, an extensive test is usually conductedusing Modified Consolidation Drained TriaxialTest with sophisticated equipment. A simplifiedmodified Consolidated Drained Triaxial Test weredeveloped at the Geotechnical Laboratory ofParahyangan Catholic University Bandung. A highair entry value porous disc were installed to themodified Triaxial Cell equipment. An additionprocedure of matrix suction equalisation wereconducted after consolidation stage, resumed withsample compression under constant matrix suction.

The results were analyzed based on the twomethods of constitutive behaviour of unsaturated

shear strengh : The Two Independent StressVariable Method and Effective Stress Method wasconducted.

2 UNSATURATED SHEAR STRENGTH TEST(TXCD-UNSAT)

The Triaxial Consolidated Drained Test(TXCD) with constant matrix suction is a triaxialtest in one series with four stages. The stages aresaturation, consolidation, matrix suctionequalisation and compression.

The triaxial equipment consist of :1. A triaxial cell with a modified pedestal to

conduct unsaturated shear strength test withconstant suction.

2. Cell pressure application system3. Back pressure application system4. Air pressure application system5. Pore water pressure measurement system.6. Volume change measurement system.

The diagram of the system is shown in Figure 1-1below. The pedestal of the modified triaxial test isshown in Figure 1-2.

This test procedure was conducted in a lengthyperiod of time. One sequel of test need three tofour weeks. Table 1-1 below listed the stages and

Unsaturated Shear Strength Parameters of Tropical Volcanic ResidualSoils

R. KarlinasariCatholic University of Parahyangan Bandung, Indonesia

P. RahardjoCatholic University of Parahyangan Bandung, Indonesia

ABSTRACT: The uppermost zone of tropical residual soil is a zone of unsaturated soil with a uniquecharacteristic. The uppermost zone intensively change from unsaturated-saturated phase, because the intensityof sun heat is high and the intensity of rainfall is much higher than it’s soil permeability.A series of research on the unsaturated shear strength of tropical volcanic residual soil were conducted withTriaxial Consolidated Drained Unsaturated (TXCD-UNSAT) Method. The research is resulted on unsaturatedshear strength parameters. These parameters were evaluated with another research’s results on unsaturatedshear strength.An analysis based on the two methods of constitutive behaviour of unsaturated shear strengh ; The TwoIndependent Stress Variable Method and Effective Stress Method was conducted. From the nature of theuppermost zone which is intensively change from unsaturated-saturated phase, the Effective Stress Methodconcluded has a better preferences.Keywords : Unsaturated, Shear Strength Parameters, TXCD-UNSAT, Independent Stress Variable Method,Effective Stress Method

Page 2: Unsaturated Shear Strength Tropical Volcanic Residual Soil R sari

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Figure 1-1. Modified triaxial cell to test unsaturated soils

The application of back pressure on samplewas conducted by water pressure application fromtop and from the bottom of the sample. Onsaturation and consolidation stages the backpressure were applied via the top cap, and onmatrix suction equalisation stage via the watercompartement at the bottom of the sample. Theapplication of the air pressure on sample wasstarted to conducted at matrix suction equalisationstage via the top cap, and will continue to thecompression stage.

Figure 1-2. The water compartment at the bottom ofceramic disc triaxial cell pedestal.

Table 1-1. The example of pressure values applied at everystages of TXCD unsaturated test.

STAGE 1 ua uw -ua ua-uw

Consolidation 1.8 0.8 1

Matrix suction equalisation

1.8 0.8 0.4 1 0.4

Shearing 1.8 0.8 0.4 1 0.4

STAGE 2

Consolidation 2.6 1.2 1.4

Matrix suction equalisation

2.6 1.2 0.4 1.4 0.8

Shearing 2.6 1.2 0.4 1.4 0.8

STAGE 3

Consolidation 3 1.4 1.6

Matrix suction equalisation

3 1.4 0.4 1.6 1

Shearing 3 1.4 0.4 1.6 1

At the beginning, the sample is saturated, thenit is consolidated with cell pressure of 1.8 kg/cm2

and back pressure of 0.8 kg/cm2 . After dissipationof pore water pressure achieved, matrix suctionwas applied. The matrix suction was modelling asthe soil condition on nature, where the top of thesoil is dry and the bottom of the soil is wet. Themoisture tension then happen between the top andthe bottom of the sample. The compartment at thebottom of the sample is functioned as the watertable in nature.

The sample is separated from the water byhigh air entry value disc, this lay out makes the aircannot escaped from the bottom of the sample,meanwhile the water will attracted to the top of thesample by capillarity. The back pressure were keptat 0.4 kg/cm2, air pressure from air compressorwere applied at the top of the sample at 0.8 kg/cm2,so a matrix suction of 0.4 kg/cm2 was attained.After the water volume change was stabilized, thenthe compressions were begin(axial compressionapplication).

Below is the water volume change curves andthe total volume change curves at matrix suctionequaliasation stage. It is interesting that on thesecond stages curve, a drop of volume change wereoccured, this phenomenon may be related to themacropore of the soils as mention by Feurharmel,Gehling & Bica, 2006.

Page 3: Unsaturated Shear Strength Tropical Volcanic Residual Soil R sari

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Figure 1-3. Water volume change and total volume change atmatrix suction equalisation stage.

3 UNSATURATED SHEAR STRENGTHPARAMETERS ANALYSIS

The analysis of the TXCD-Unsat test resultswere conducted by two methods : The TwoIndependent Stress Variable Method after Fredlund& Rahardjo,1993 and Unsaturated Effective StressMethod after Khalili & Khabbaz, 1998.

3.1 TXCD-UNSAT results in the TwoIndependent Stress Variable Method.

The Independent Stress Variable Method is amethod that separated the effect of the nett normalstress and the effect of matrix suction on soil’sshear strength in a diffeent axis of stress.

The results of Unsaturated Triaxial CD test ofBH02 Neglajaya 0.5-1.0 m on 3D Mohr-Coulombdiagram was shown in diagram of Figure 1-5below. On Figure 1-4, it is shown the increase ofsoil’s shear strength in the raise of matrix suction.

Figure 1-4. The failure envelope of versus (ua-uw) plane ofBH02 Neglajaya 0.5-1.0 m

It is shown on the diagram above that at BH02Neglajaya 0.5-1.0 m results, the b almost thesame magnitude as ’ angle, at 0.09 kg/cm2 to0.29 kg/cm2 matrix suctions (or below the air entryvalue (AEV) of the soil at 0.3 kg/cm2). After theAEV the b decrease almost a quater of the b at0.29 kg/cm2 to 0.69 kg/cm2 matrix suctions.

3.2 TXCD-UNSAT results in UnsaturatedEffective Method.

The Unsaturated Effective Stress Methodanalysis started by the determination of efectivestress parameters (. Khalili & Khabbaz (1998) inKhalili, Geiser & Blight (2004), were collectedsome formerly research results and determined therelationship between effective stress parameters(and suction ratio as shown in Figure 1-6 below.Khalili concludes that the effective stressparameter (is a parameter strongly related tosoil’s structures.

Figure 1-5. Mohr Coulomb failure envelop for unsaturated soil samples of BH02 Neglajaya 0.5-1.0 m

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Figure 1-6. Effective stress parameters () versus suctionratio (Khalili & Khabbaz (1998))

The suction ratio is the magnitude of matrixsuction over suction value at the time that air startto enter the pore in soil structure (sae, suction AirEntry Value (AEV). The relation on Figure 1-6has a top limit of y = 1.045 (s/sAEV)-0.46 andbottom limit of y = 1.008 (s/sAEV)-0.68. Khalili &Khabbaz take the relation and then define therelation on drying process as follow :

(1)

The results of BH02 Neglajaya 0.5-1.0 msamples, then expressed in failure envelope planeof q’-p’ and the diagram shown at Figure 1-7below. The diagram shown that the maximumpoint of unsaturated TXCD stress path from 0.090.09 kg/cm2 to 0.69 kg/cm2 matrix suction lay on

the top of state line for saturated shear strength ofM= 1.048.

By taking a’ and ’ of saturated soil samples,a prediction of the relationship between deviatoricstress q’ and matrix suction will be defined. Theprediction was defined by calculated deviatoricstress q relations as follow :

(2)

Figure 1-8 below shown the prediction and theactual deviatoric stress over suction for BH02Neglajaya 0.5-1.0 m soil. For the 0.09 kg/cm2 and0.29 kg/cm2 matrix suctions, the actual results arefit on the prediction line, meanwhile the actualresults of 0.69 kg/cm2 matrix suction is higher thanthe prediction line.

Figure 1-7. The relation of matrix suction and deviatoricstress on effective method analysis for BH 02 Neglajaya 0.5-

1.0 m sample

Figure 1-8. The failure envelope of q’-p’ for a saturated soil and the maximum stress path pointof stress from unsaturated soil of BH02 Neglajaya 0.5-1.0 m

Page 5: Unsaturated Shear Strength Tropical Volcanic Residual Soil R sari

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4 THE COMPARATION OF THE TXCD-UNSAT RESULTS

The shear strength of tropical volcanic residualsoil based on the TXCD-Unsat in this research havea range of value as follow :(a) Based on the Two Independent Stress-State

Variable Method the results was resumed inTable 4-2 below. The maximum value of b isapproaching the value of ’. The magnitude ofb drastically decrease on the further increase ofsuction value. The same results was obtained byRahardjo’s research on residual soil ofSingapore (Rahardjo, et all, 2004), the resumeof the results is shown on Table 4-1.

(b) Based on effective parameters of (s/sAEV) -0.55

all the results were resumed in Figure 4-1. Asgeneral the point of maximum stress path is layabove the saturated state line. The distance ofthe points from the saturated state line isindicating a function of the magnitude ofsuction. There is a specific shapes of the dot linecompare to the previous research collected byKhalili at Figure 4-2.

Table 4-1. Shear strength parameter of Singaporeresidual soils (Rahardjo, et al, 2004)

Granitic Bukit Timah Formation

Depth(m)

c’(kg/cm2)

’(o)

b

(o)

5-9 0.26 27 29 to 8

10-15 0.13 35 35 to 6

15-21 0.12 38 35 to 4

Table 4-2. Shear strength parameters from TXCD-UNSATtests

BH02 Cij 0.5-1.0 m

MatriksSuction(ua-uw)

(kg/cm2)

c’(kg/cm2)

’(o)

b

(o)

0.29 0.23 22 16.9

0.68 0.35 22 14.4

0.89 0.40 22

BH01 Cil 4.5-5.0 m

0.40 0.66 32 4.88

0.80 0.68 32 4.88

1.00 0.71 32

BH02 Neg 0.5-1.0 m

0.09 0.38 26.5 26.69

0.29 0.48 26.5 9.51

0.69 0.55 26.5

BH02 Neg 13.0-13.5 m

0.40 0.93 33 33.26

0.60 1.06 33 4.545

0.90 1.09 33

BH03 Neg 2.5-3.0 m

0.07 0.16 33 20.63

0.26 0.24 33

0.35 0.30 33

BH05 Neg 0.5-1.0 m

0.00 0.46 23 9.255

0.20 0.49 23 1.556

0.60 0.50 22

Figure 4-1. Failure envelope on q’-p’ plane for saturated soil and the maximum point of stress path for unsaturated soils in thisresearch.

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Figure 4-2.The evolution of critical state line over suction onp’-q’ plane, kaolin clay soil (Wheller Sivakumar, 1995) on

Khalili et all, 2004

5 THE APPLICATION OF THEUNSATURATED SHEAR STRENGTH ONTROPICAL VOLCANIC RESIDUAL SOILSCONDITION

The uppermost zone of tropical residual soil is azone of unsaturated soil with a unique characteristic.The uppermost zone intensively change fromunsaturated-saturated phase, because the intensity ofsun heat is high and the intensity of rainfall is muchhigher than it’s soil permeability. From this nature ofthe uppermost zone which is intensively changefrom unsaturated-saturated phase, it is important tohave a method that could modelling this behaviour.The Effective Stress Method not as the TwoIndependent Stress State Variable Method couldunified the saturated and unsaturated condition onone method.

Figure 5-2 shown the failure envelope of q’-p’for a saturated soil and the maximum stress pathpoint of stress from unsaturated soil of BH02 Cij0.5-1.0 m, and Figure 5-1 shown the stress-straincurve for saturated and unsaturated soil of BH02 Cij0.5-1.0 m. The saturated soils is on positive porepressure of 90 kPa, it is clearly that the method cancompared the stress path of the saturated and theunsaturated condition of soils.

Figure 5-1 The failure envelope of q’-p’ for a saturated soil

and the maximum stress path point of stress from unsaturated

soil of BH02 Cijengkol 0.5-1.0 m

Figure 5-2. The stress-strain curve for saturated andunsaturated soil of BH02 Cij 0.5-1.0 m

6 CONCLUSION

The shear strength parameters obtained fromthe simplified TXCD–Unsat have been compared tothe previously result both from the Two IndependentStress State Method and the Unsaturated EffectiveMethod, and the results is conformer.

7 REFERENCE

Melani, I., 2004. Shear Strength Charactheristics of ACompacted Kaolin Under Infiltration Conditions,Disertation, School of Civil and EnvironmentalEngineering, Nanyang Technological University

Karlinasari, R, 2009, Karakteristik Tanah Residual TropikVulkanik Formasi Gunung Api Tua (Qob) Jawa Barat,Disertasi pada Program Doktor Teknik Sipil UniversitasKatolik Parahyangan, Bandung.

Rahardjo, H, Aung, K.K, Leong, E.C, Rezaur, R.B.,2004,Charachteristic of Residual Soils in Singapore as formed byweathering, Engineering Geology 73, 157-169

Khalili N, Geiser F, Blight GE., 2004, Effective stress in unsa-turated soils: A critical review with new evidence, Interna-tional Journal of Geomechanics. ASCE; 4(2), 115-126.

Khalili, N. and Khabbaz, M.H., 1998, A unique relationship forfor shear strength determination of unsaturated soils, Geo-technique, 48(5), 681-688.