Laboratory Tests to Determine Shear Strength of Soils

Transcription

1 Triaxial Testing

2 Laboratory Tests to Determine Shear Strength of Soils Geotechnical Engineering II (ENGI 6723) Presented by Rodney P. McAffee, Ph.D., P.Eng.

3 Laboratory Tests to Determine Shear Lecture Topics Strength of Soils o Brief overview of direct shear test o Determine soil shear strength parameters from triaxial tesnng: Unconsolidated Undrained Consolidated Undrained o Triaxial test setup and behaviour o Use of results in engineering pracnce o Examples of triaxial test results

4 Shear Strength Shear strength of soils is required to solve problems of stability o Bearing capacity, earth pressures, slope stability, etc. Shear strength is a funcnon of effecnve normal stress

5 Laboratory Tests to Determine Shear Direct Shear Test Strength of Soils Triaxial Tests o Pore water pressure measurement o TesNng under back pressure Types of Triaxial Tests o Unconsolidated Undrained o Consolidated Undrained o Consolidated Drained

6 Direct Shear Test

7 Direct Shear Test Disadvantages o Failure plane is forced to be horizontal o Cannot control drainage o Stress concentranons at the sample boundaries o Uncontrolled rotanon of principal planes and stresses Advantages o Test in inexpensive o Fast and simple o Easy to prepare for cohesionless samples

8 Triaxial TesNng Casagrande developed triaxial tesnng in the 1930s More complicated than direct shear tesnng o But more versanle Drainage can be controlled No rotanon of principal stresses o Some small shear stresses do act on the boundaries Stress concentranons are limited Failure plane can occur anywhere Stress paths to failure can be controlled

9 Triaxial Test Apparatus and assumed Stress CondiNons

10 Load frame with stepper motor drive Triaxial cell Data acquisiton system Electronic measurement transducers o Load, displacement, pressure, and volume change Computer SoZware for triaxial tesnng Control panel and de aired water control system

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12 Control Panel to regulate pressure and flows during tesnng

13 Drainage Paths in Triaxial TesNng The 3 permissible drainage paths are: Q-Test (for quick test) S-test (for slow test)

14 Consolidated Undrained (CU) Test Behaviour Sample is first consolidated under desired stresses AZer consolidanon complete, drainage valves closed Typically, pore water pressures are measured o Calculate total and effecnve stresses Excess pore water pressure (Δu) can either: o Increase (+ive): specimen contracts or consolidates o Decrease ( ive): specimen expands or swells Axial stress increased incrementally or at constant rate of strain

15 Back Pressure during TesNng To ensure 100% saturanon (necessary to give accurate pore water pressures), a back pressure is applied to the pore water Cell pressure also increased by same amount to maintain the same effecnve consolidanon stresses

16 Typical Stress-Strain Curves for CU Tests Principal (effective) stress ratio is a simple way to normalize the stress behaviour with respect to σ 3 during the test

17 With pore water pressures measured, we can calculate both total and effecnve stresses at failure Typically, a number of tests over a range of stresses is carried out NC clay specimens develop posinve pore water pressures (σ = σ Δu)

18 OC clays tend to expand during shear causing decreased pore water pressures (σ = σ (-Δu))

19 TesNng on saturated sand with measurement of pore water pressure

20 Mohr envelopes in terms of both total and effecnve stresses for consolidated undrained tests on normally consolidated clay samples

21 Mohr envelopes in terms of both total and effecnve stresses for consolidated undrained tests on over consolidated clay samples

22 Use of CU Strength in Engineering PracNce Soils are fully consolidated and at equilibrium with the exisnng stress system Then, addinonal stresses are applied quickly

23 Example Test for NC Clay: o Consolidated under a stress of 150 kpa o Then sheared undrained in axial compression o Principal stress difference at failure = 100 kpa o Induced pore water pressure at failure = 88 kpa c and c T are assumed to be zero

24 Unconsolidated Undrained (UU) Test Behaviour Sample is placed in the triaxial cell with the drainage valves closed No consolidanon occurs when confining pressure is applied Usually, pore water pressures are not measured Sample is loaded to failure in 10 to 20 minutes o Called Q test (for quick) Test is a total stress test and it yields the strength in terms of total stresses

25 Typical Stress-Strain Curves for UU Tests Note: max. Δσ occurs at relatively low strains a) Remolded and some compacted clays b) Medium sensitive undisturbed clay c) Highly sensitive undisturbed clay

26 Typical Mohr failure envelopes for UU Tests 100% saturated clay All samples have same water content and void ratio therefore the same undrained shear strength Increased cell pressure compresses any air voids Partially saturated clay

27 Change in both strength and deformanon azer remolding an undisturbed sample of NC clay φ u = 0 when results are ploged with respect to total stress

28 Use of UU Strength in Engineering PracNce Engineering loading is assumed to take place so rapidly that Δu cannot dissipate or for consolidanon to occur Change in total stress does not affect the in situ undrained shear strength

29 Scope Terminology Significance of use Apparatus Test Specimens Procedure CalculaNon Report

30 ASTM D 2850: Unconsolidated Undrained Terminology o Failure is defined as the maximum principal stress difference or that measured at 15% axial strain Test Specimens o Minimum diameter of 3.3 cm o Height to diameter rano between 2 and 2.5 o Procedures to prepare undisturbed and compacted samples Procedure o Axial strain: 1%/min for plasnc and 0.3%/min for brigle o Test should last approximately 15 to 20 minutes

31 ASTM D 2850: Unconsolidated Undrained CalculaNons o Axial strain, ε = ΔH/H 0 o Average cross secnonal area, A = A 0 /(1 ε) o Principal stress difference, σ 1 σ 3 = P/A o CorrecNon equanons for: If all around pressure changes specimen length CorrecNon for snffness of the rubber membrane Report o Index propernes of material being tested o IniNal H 0, Diam., γ d, void rano, w.c., saturanon, etc. o Rate of axial strain, strain and stresses at failure o Stress strain curve and failure sketch

32 ASTM D 4767: Consolidated Undrained Terminology o Failure is defined as the maximum principal stress difference or that measured at 15% axial strain, or o Maximum stress obliquity, σ 1 /σ 3 Test Specimens o Same as for UU test Procedure o SaturaNon procedure to ensure: Pore Pressure Parameter, B > 0.95 (B = Δu/Δσ 3 ) o ConsolidaNon procedures to ensure specimen reaches equilibrium in a drained state at the effecnve consolidanon stress required

33 ASTM D 4767: Consolidated Undrained Procedure o Axial loading to produce equalizanon of pore water pressures throughout the sample at failure o Assuming failure will occur at 4% axial strain Rate of strain = 4% / (10 x t 50 ) o Details for measuring pore water pressures CalculaNons o EquaNons for height and area azer consolidanon (H c & A c ) o Axial strain, ε = ΔH/H c o Average cross secnonal area, A = A c /(1 ε) o Principal stress difference, σ 1 σ 3 = P/A

34 ASTM D 4767: Consolidated Undrained CalculaNons o Calculate effecnve stresses based on Δu measured o CorrecNon equanons for: Report CorrecNon for filter paper strips CorrecNon for snffness of the rubber membrane o Index propernes measured for material being tested o EffecNve consolidanon stress, t 50 o H c, A c,diam., γ d, void rano, w.c., saturanon azer consolidanon o Rate of axial strain, strain and stresses at failure o Stress strain curve and failure sketch

35 Interpreting Scatter in Test Results US Army Corps of Engineers Procedure: Draw strength envelope in a posinon such that data from two thirds of the tests lie above the failure envelope

36 Primary References: o A.W. Bishop and D.J. Henkel, The Triaxial Test o R.D. Holtz and W.D. Kovacs, Introduc:on to Geotechnical Engineering o US Army Corps of Engineers: Engineering and Design Manual: Slope Stability, Appendix D Shear Strength Characteris:cs (EM , Oct 31, 2003) o ASTM D2850 and D4767 Standard Test Methods

37 Summary / QuesNons Lecture Topics Brief overview of direct shear test Determine shear strength parameters from triaxial tesnng: Unconsolidated Undrained Consolidated Undrained Triaxial test setup and behaviour Use of results in pracnce Examples of triaxial test results