Soil Mechanics Topic – Triaxial shear test(CD, CU, UU tests) Created by Patel Jay K. 130010106033 Guided By Prof. Kumar Trivedi

Strength of different materials Steel Tensile strength Concrete Compressive strength Soil Shear strength Presence of pore water Complex behavior

When soil is loaded, shearing stresses induced in it When soil is loaded, shearing stresses induced in it. When shearing stresses reach limiting value, shear deformation take place, leading to failure of soil mass. Failure may be in the form of sinking of a footing or movement of a wedge of soil behind retaining wall or slide in an earth embankment.

Shear failure of soils Embankment Strip footing Failure surface Mobilized shear resistance

Shear failure of soils Retaining wall

Shear failure of soils Mobilized shear resistance Retaining wall Failure surface Mobilized shear resistance Retaining wall

Shear failure mechanism   At failure, shear stress along the failure surface () reaches the shear strength (f).

Mohr Circles & Failure Envelope  Soil elements at different locations Failure surface Y ~ stable X ~ failure ’

Triaxial compression test The Triaxial shear test introduced by casagrande and terzaghi in 1936, is far the most popular and extensively used shear strength test, both for field application as well as for the purpose of research. As the name suggest, the specimen is subjected to three compressive stresses in mutually perpendicular direction.

Stages and types of tests Consolidation stage in which normal stress applied the specimen and is allowed to consolidate Shear stage in which stresses is applied to specimen to shear it. Types of tests Unconsolidated un-drained test (UU test) Consolidated Un-drained test (CU test) Consolidated drained test (CD test)

Specimen preparation (undisturbed sample) Sampling tubes Sample extruder

Specimen preparation (undisturbed sample) Edges of the sample are carefully trimmed Setting up the sample in the triaxial cell

Specimen preparation (undisturbed sample) Sample is covered with a rubber membrane and sealed Cell is completely filled with water

Specimen preparation (undisturbed sample) Proving ring to measure the deviator load Dial gauge to measure vertical displacement

Consolidated- drained test (CD Test) Total, s = Pore water pressure, u Effective, s’ + Step 1: At the end of consolidation s3 s’3 = s3 Drainage Step 2: During axial stress increase s3 + sd s3 s1 = s3 + sd = s’1 s3 = s’3 Drainage Step 3: At failure s3 + sd s3 s1f = s3 + sd = s’1f s3 = s’3 Drainage

How to determine strength parameters c and f Mohr – Coulomb failure envelope Shear stress, t s or s’ s3c s1c s3b s1b (s2)b s3a s1a (s2)a

Failure envelopes For OC Clay, c ≠ 0 t s or s’ f s3 s1 (s2) c sc OC NC

Some practical applications of CD analysis for clays 1. Embankment constructed very slowly, in layers over a soft clay deposit Soft clay t t = in situ drained shear strength

Some practical applications of CD analysis for clays t = drained shear strength of clay core t Core 2. Earth dam with steady state seepage

Some practical applications of CD analysis for clays 3. Excavation or natural slope in clay t t = In situ drained shear strength

Important points about Triaxial CD test Since u = 0 in CD tests, s = s’ Therefore, c = c’ and f = f’ CD test simulates the long term condition in the field. Thus, c and f should be used to evaluate the long term behavior of soils

= + Consolidated- Undrained test (CU Test) s3 s3 + sd s3 s3 + sd s3 Total, s = Pore water pressure, u Effective, s’ + Step 1: At the end of consolidation s3 s’3 = s3 Drainage Step 2: During axial stress increase s3 + sd s3 s’1 = sVC + sd - ud s’3 = s3 - ud No drainage ud Step 3: At failure s3 + sd s3 s’1 = sVC + sd - udf s’3 = s3 - udf No drainage udf

Failure envelope f’ fcu s or s’ Shear stress, t s3b s1b (Dsd)fa s’3b Mohr – Coulomb failure envelope in terms of effective stresses f’ C’ Shear stress, t s or s’ Mohr – Coulomb failure envelope in terms of total stresses fcu s3b s1b (Dsd)fa ufb ufa ccu s’3b s’1b s3a s1a s’3a s’1a (Dsd)fa

Unconsolidated- Undrained test (UU Test) Step 1: Immediately after sampling Step 2: After application of hydrostatic cell pressure s’3 = s3 - uc sC = s3 No drainage uc = + Step 3: During application of axial load s’1 = s3 + Dsd - uc ud s’3 = s3 - uc ud s3 + sd s3 No drainage = uc ± ud +

Important points about Triaxial UU test Mohr circle in terms of effective stresses do not depend on the cell pressure. Therefore, we get only one Mohr circle in terms of effective stress for different cell pressures UU test simulates the short term condition in the field. Thus, UU test can be used to analyze the short term behavior of soils

Failure Envelope t s or s’ c s’3 s’1 s3a s1a s3b s1b s2 Failure envelope, f = 0 t s or s’ c s’3 s’1 s3a s1a s2 s3b s1b

Some practical applications of UU analysis for clays 1. Embankment constructed rapidly over a soft clay deposit Soft clay t t = in situ undrained shear strength

Some practical applications of UU analysis for clays 2. Large earth dam constructed rapidly with no change in water content of soft clay t = Undrained shear strength of clay core t Core

Some practical applications of UU analysis for clays 3. Footing placed rapidly on clay deposit t = In situ undrained shear strength

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