Ce 638 triaxial stress strain controlled


Published on

For M.Tech Geotechnical Engineering

Published in: Technology

Ce 638 triaxial stress strain controlled

  3. 3. SHEAR STRENGTH OF SOIL Determination of soil strength parameters ( c and f ) in the lab 1- Direct Shear Test 2- Unconfined Compression Test 3- Triaxial Compression Test To determine the shear strength of soil Mohr-Coulomb Yield Criterion is used t = c + sn tan f Soil Cohesion Angle of Friction Where : c & Ø are shear strength parameters 3
  4. 4. STATE OF STRESS IN TRIAXIAL TEST Pore Water Pressure u σc σc q q σc σc 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 4
  5. 5. BRIEF REVIEW Year Authors Title of Paper/ Research 1924 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 5
  6. 6. BRIEF REVIEW Year Authors Title of Paper/ Research 1975 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. 6
  7. 7. Example applications of triaxial test: Stability of shallow foundation or excavation in soft clay Cyclic loading of gravity base structure Effect of shallow gas on cyclic behaviour of dense sand 7
  8. 8. CONVENTIONAL TRIAXIAL TEST EQUIPMENT Conventional Triaxial Equipment (After K H Head, 1986) 8
  9. 9. Ref: Department of Construction Engineering Advanced Geotechnical Laboratory Chaoyang University of Technology MERCURY CONTROL DEVICE 9
  10. 10. 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 Confining Pressure s1 = Ds + s2 s2 Ds Ds s2 s2 e Ds s2 Ds Failure Deviator Stress 1. Consolidation Valve Closed 2. Shearing Valve Closed 10
  11. 11. Step 1 Step 2 Triaxial Compression Test 2- Consolidated Undrained Test (CU) fu sn t cu e Ds s2 Ds Failure s2 s1s2 s1 s2 s1 s1 - s2 s1 - s2 s2 s2s2 s2 s2 s2 s2 s2 s2 s2 Confining Pressure Ds Ds Ds s1 = Ds + s2 Deviator Stress s2 s2 1. Consolidation Valve Open 2. Shearing Valve Closed 11
  12. 12. Step 1 Step 2 Triaxial Compression Test 3- Consolidated Drained Test (CD) fd sn t cd s2 s1s2 s1 s2 s1 s1 - s2 s1 - s2 s2 s2 s2 s2 Confining Pressure s1 = Ds + s2 s2 Ds Ds s2 s2 e Ds s2 Ds Failure Deviator Stress 2. Shearing Valve Open 1. Consolidation Valve Open 12
  13. 13. 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 13
  14. 14. 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. 14
  15. 15. 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. 15
  16. 16. GRAPHICAL REPRESENTATION OF STRESS- CONTROLLED VS. STRAIN CONTROLLED TRIAXIAL TEST Strain ε Stressσ Strain Controlled Test Stress 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 16
  17. 17. 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. 17
  18. 18. 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. 18
  19. 19. 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. 19
  20. 20. 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. 20
  21. 21. SOIL PARAMETERS DERIVED FOR RESEARCH OF FIELD CONDITIONS LIKE…  Rainfall induced landslide.  Swelling soil.  Collapsible soil, and  Dynamic effect on geotechnical structures. 21
  22. 22. 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. 22
  23. 23. 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. 23
  24. 24. Shear Strength ??? 24