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TRIAXIAL TEST (STRESS CONTROLLED – STRAIN CONTROLLED)

Course: GEOTECHNICAL MEASUREMENT AND EXPLORATION

CIVIL ENGINEERING DEPARTMENTIIT KANPUR (INDIA)

TERM PAPER PRESENTATION

BY SUVRA SAMAL

Prof. SAMIRSINH P PARMAR

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OUTLINE OF PRESENTATION

INTRODUCTION: TRIAXIAL TEST REVIEW DETAIL TRIAXIAL TEST RESULT ANALYSIS OF TRIAXIAL TEST STRESS- STRAIN CONTROL TEST COMMENT ON STRESS- STRAIN CONTROL

TEST ADVANCEMENT IN TRIAXIAL TEST REFERANCES

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SHEAR STRENGTH OF SOIL

Determination of soil strength parameters ( c and f ) in the lab

1- Direct Shear Test2- Unconfined Compression Test3- Triaxial Compression Test

To determine the shear strength of soil Mohr-Coulomb Yield Criterion is used

t = c + sn tan f

Soil CohesionAngle of Friction

Where : c & Ø are shear strength parameters

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STATE OF STRESS IN TRIAXIAL TEST

Pore Water Pressure u

σc

σc

q

q

σcσc

σa = q +σc = σ1

Specimen Length L

Specimen Area A

ε = Axial Strain = ΔL/L

σ1 = Major Principle Stress = Axial Stress σa

σ3 = Minor Principle Stress = Radial Stress σr

q= Deviator stress = F/A = Axial Load/ Area

σr = σc =σ3 σ‘1 = Major Principle Effective Stress = σ1-u

σ‘3= Minor Principle Effective stress = σ3-u

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BRIEF REVIEW

Year Authors Title of Paper/ Research1924 Buisman and

Hveem Initial Triaxial device was invented

1930 Cassagrande, Karl Terzaghi

The first device that was similar to modern equipment was developed

1957 Bishop and Henkel

designed new test technique for Triaxial test

1962 Bishop and Henkel

published first book on Triaxial testing named, “The measurement of soil properties in the Triaxial test”.

1971 Seed and Silver

performed strain controlled test at very small strains

1975 Kovari V. and Tisa A.

described multiple failure state and strain controlled triaxial test

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BRIEF REVIEWYear Authors Title of Paper/ Research1975 Bishop and

Welsley carry out plane strain tests , true triaxial load.

1981 Deveaux D. , Vuez A., Amoros D.

developed technology for stress and strain controlled automatic parameter acquisition system for Triaxial test.

1988 Carrol, W.F developed fast Triaxial test device. performed nonlinear wave analysis to study the stress and strain rate effects on the results of soil parameters.

1988 Akitoshi M. Masato M. and Shinichi T.

developed independent principle stress control apparatus

1997 Claudio D. and Silvia I.

performed experimental analysis and theoretical interpretation of Triaxial load controlled loose sand specimen collapse.

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Example applications of triaxial test:

                                                                                                                                                                                                                                                                                         

Stability of shallow foundation or excavation in

soft clayCyclic loading of gravity base structure

Effect of shallow gas on cyclic behaviour of dense sand

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CONVENTIONAL TRIAXIAL TEST EQUIPMENT

Conventional Triaxial Equipment (After K H Head, 1986)

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Ref: Department of Construction Engineering Advanced Geotechnical Laboratory Chaoyang University of Technology

MERCURY CONTROL DEVICE

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Triaxial Compression Test

1- Unconsolidated Undrained Test (UU)

Step 1 Step 2

sn

t

cu

t= c

s2 s1s2 s1s2 s1

s1 - s2

s1 - s2

s2 s2

s2

s2

ConfiningPressure

s1 = Ds + s2

s2

Ds

Ds

s2

s2

e

Dss2

Ds Failure

Deviator Stress

1. Consolidation Valve Closed

2. ShearingValve Closed

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Step 1 Step 2

Triaxial Compression Test2- Consolidated Undrained Test (CU)

fu

sn

t

cu

t= c + sn tan

f

e

Dss2

Ds Failure

s2s1s2 s1s2 s1

s1 - s2

s1 - s2

s2s2s2 s2

s2 s2s2 s2

s2

s2

ConfiningPressure

Ds

Ds

Ds

s1 = Ds + s2

Deviator Stress

s2

s2

1. Consolidation Valve Open

2. ShearingValve Closed

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Step 1 Step 2

Triaxial Compression Test3- Consolidated Drained Test (CD)

fd

sn

t

cd

t= c + sn tan

f

s2s1s2 s1s2 s1

s1 - s2

s1 - s2

s2 s2

s2

s2

ConfiningPressure

s1 = Ds + s2

s2

Ds

Ds

s2

s2

e

Dss2

Ds Failure

Deviator Stress

2. ShearingValve Open

1. Consolidation Valve Open

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RATE OF STRAIN AND DRAINAGE CONDITION IN TYPES OF TRIAXIAL TESTS

Test Rate of axial strain Drainage condition

UU Fastest. failure occur within 20 to 25 minutes

Drainage valves are closed

CU Slow enough to adequate equalisation of excess pore

water pressure

Drainage valves are closed and excess pore water pressure

measured

CD Slow enough to result in negligible pore pressure

variation 

Drainage valves are opened and ΔV measured

 

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STRESS CONTROLLED TRIAXIAL TEST

In this type of test axial stress is increased by pre-decided increments and the vertical deformation of the sample measured.

Stress controlled tests were carried out by imposing finite load increments.

The load increments are followed by time periods at constant stress level.

The Stress-controlled test at the stress level characterized by a mobilized friction angle suffers a sudden collapse.

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STRAIN CONTROLLED TRIAXIAL TEST

Soil specimen sheared at a uniform strain rate.

Shear force acting on soil specimen measured using proving ring.

The stress vs. strain curve observed after the peak point.

Continuous stress vs. strain curve available so at every corresponding value of strain, stress is available and vice a versa.

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GRAPHICAL REPRESENTATION OF STRESS- CONTROLLED VS. STRAIN CONTROLLED TRIAXIAL TEST

Strain ε

Stre

ss σ

Strain Controlled TestStress Controlled Test

Stress increased from one stage to another

Continuous behavior of stress vs. strain observed

Failure Point

ε1 ε2

σ1

σ2

Stress increased from σ1 to σ2 and in between the failure occurs then the corresponding value of strain at failure (εf) not available in stress controlled test

εf

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COMMENT OF STRESS CONTROLLED TRIAXIAL TEST

Peak shear resistance can be only approximated because failure occurs at a stress level somewhere between the pre failure load increment and the failure load increment

(i.e. the failure load may be in between the two stages of load increment and exact failure load may be approximated)

Only the peak shear resistance can be observed and plotted.

Simulated field conditions like… Foundations, stage wise construction of structure

on soil etc.

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COMMENT OF STRESS CONTROLLED TRIAXIAL TEST Stress controlled test are preferred for

conducting shear test at very slow rate. Why? Because applied load can easily be kept

constant for any given period of time.

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COMMENT OF STRAIN CONTROLLED TRIAXIAL TEST Most widely used. Focus on interdependence of peak strength

and loading history. Advantage: Peak shear resistance (at failure) as well as

lesser shear resistance (residual stress – ultimate strength) can be observed and plotted.

Simulates field Conditions like… Embankment, retaining wall, slope stability

etc.

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ADVANCEMENT IN TRIAXIAL TEST

Triaxial test options now a days Isotropic, anisotropic and Ko-consolidation Monotonic and cyclic loading Drained and undrained shearing Stress-strain-strength properties Creep tests Special testing programs Temperature controlled test Frozen triaxial test High pressure- high temperature triaxial test Large scale triaxial test.

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SOIL PARAMETERS DERIVED FOR RESEARCH OF FIELD CONDITIONS LIKE…

Rainfall induced landslide. Swelling soil. Collapsible soil, and Dynamic effect on geotechnical structures.

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CONCLUSION:

More realistic Reasonably accurate results Better control over drainage Various modifications can be possible to

simulate field conditions. Advance instruments are available. Ease of Combination with computer

software's. Use of data loggers, gauges and sensors

makes it fool proof for research, where accuracy is important.

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REFERENCES Bishop, A. W., and D. J. Henkel, “The Measurement of Soil Properties in the

Triaxial Test,” 2d ed., Edward Arnold, London, 1969. Kovari,K : “Multiple Failure State and strain controlled Triaxial tests” Rock

Mechanics 7,pp. 17-33(1975) , Springer-Verlag 1975. Vaid,Y. P. , Negussey, D., and Zergoun, M.,”A Stress-and strain-controlled

monotonic and Cyclic Loading System,” Advanced Triaxial testing of Soil and Rock, ASTM STP 977, Robert T. Donaghe, Ronald C.Chaney, and Marshall L. Silver, Eds. , American Society for Testing and Materials, Philadelphia, 1987 , pp.119-131,(1987)

Mochizuki, A. , Mikasa, M. , and Takahashi, S. , ”A New Independent Principal Stress Control Apparatus,” Advance Triaxial Testing of Soil and Rock, ASTM STP 977, Robert T. Donag7he, Ronald C. Chaney, and Marshall L. Silver, Eds., American Society for testing and Materials,Philadelphia,1988, pp. 844-858.(1988).

Prisco, Di Claudio., and Imposimato, Silvia. “Experimental Analysis and Theoretical Interpretation of Triaxial Load controlled Loose Sand Specimen Collapses” Mechanics of Cohesive Frictional materials, VOL. 2, 93-120(1997)

Sitharam, T.G. , Ravishankar B.V. , Patil S.M. “Liquefaction and Pore Water Pressure Generation in Sand: Cyclic Strain Controlled Triaxial Tests”,3(1), pp.57-85,January-June 2012.

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Q…. ?Shear Strength ???