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oleh: A. Adhe Noor PSH, ST., MT Staf Pengajar Program Studi Teknik Sipil

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## oleh: A. Adhe Noor PSH, ST., MT Staf Pengajar Program Studi Teknik Sipil

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**Kuat Geser Tanah (Shear Strength)- Triaxial Test - (Courtesy**of COSC 323: Soils in Construction) oleh: A. Adhe Noor PSH, ST., MT Staf Pengajar Program Studi Teknik Sipil Jurusan Teknik Fakultas Sains dan Teknik Universitas Jenderal Soedirman**Piston (to apply deviatoric stress)**Failure plane O-ring impervious membrane Soil sample Soil sample at failure Porous stone Perspex cell Water Cell pressure Pore pressure or volume change Back pressure pedestal Triaxial Shear Test**Sampling tubes**Sample extruder Triaxial Shear Test Specimen preparation (undisturbed sample)**Setting up the sample in the triaxial cell**Edges of the sample are carefully trimmed Triaxial Shear Test Specimen preparation (undisturbed sample)**Sample is covered with a rubber membrane and sealed**Cell is completely filled with water Triaxial Shear Test Specimen preparation (undisturbed sample)**Proving ring to measure the deviator load**Dial gauge to measure vertical displacement Triaxial Shear Test Specimen preparation (undisturbed sample)**deviatoric stress ( = q)**c Step 2 Step 1 c c c c c+ q c Under all-around cell pressure c Shearing (loading) yes no yes no Consolidated sample Undrained loading Drained loading Unconsolidated sample Types of Triaxial Tests Is the drainage valve open? Is the drainage valve open?**Step 2**Step 1 Under all-around cell pressure c Shearing (loading) Is the drainage valve open? Is the drainage valve open? CD test UU test yes no yes no Consolidated sample Undrained loading Drained loading Unconsolidated sample CU test Types of Triaxial Tests**=**+ Total, s Effective, s’ Neutral, u sVC s’VC =sVC shC s’hC =shC 0 Drainage Drainage Drainage sVC + Ds sVC + Dsf s’V =sVC +Ds = s’1 shC shC s’h =shC = s’3 0 0 s’Vf =sVC +Dsf= s’1f s’hf =shC = s’3f Consolidated- drained test (CD Test) Step 1: At the end of consolidation Step 2: During axial stress increase Step 3: At failure**s1 = sVC + Ds**s3 = shC Consolidated- drained test (CD Test) Deviator stress (q or Dsd) = s1 – s3**Time**Expansion Volume change of the sample Compression Consolidated- drained test (CD Test) Volume change of sample during consolidation**Deviator stress, Dsd**Dense sand or OC clay (Dsd)f Loose sand or NC Clay (Dsd)f Axial strain Expansion Dense sand or OC clay Volume change of the sample Axial strain Compression Loose sand or NC clay Consolidated- drained test (CD Test) Stress-strain relationship during shearing**Deviator stress, Dsd**s1 = s3 + (Dsd)f (Dsd)fc Confining stress = s3c s3 Confining stress = s3b Confining stress = s3a (Dsd)fb Axial strain (Dsd)fa Shear stress, t f Mohr – Coulomb failure envelope s or s’ s3c s3a s3b s1a s1b s1c (Dsd)fa (Dsd)fb CD tests How to determine strength parameters c and f**Strength parameters c and f obtained from CD tests**CD tests Therefore, c = c’ and f = f’ Since u = 0 in CD tests, s = s’ cd and fd are used to denote them**fd**Mohr – Coulomb failure envelope Shear stress, t s or s’ s3a s1a (Dsd)fa CD tests Failure envelopes For sand and NC Clay, cd = 0 Therefore, one CD test would be sufficient to determine fd of sand or NC clay**NC**OC t f s or s’ c s3 s1 sc (Dsd)f CD tests Failure envelopes For OC Clay, cd ≠ 0**Soft clay**t Some practical applications of CD analysis for clays 1. Embankment constructed very slowly, in layers over a soft clay deposit t = in situ drained shear strength**t**Core t = drained shear strength of clay core Some practical applications of CD analysis for clays 2. Earth dam with steady state seepage**t**Some practical applications of CD analysis for clays 3. Excavation or natural slope in clay t = In situ drained shear strength Note: CD test simulates the long term condition in the field. Thus, cd and fd should be used to evaluate the long term behavior of soils**=**+ Total, s Effective, s’ Neutral, u sVC s’VC =sVC shC s’hC =shC 0 Drainage s’V =sVC +Ds ± Du = s’1 sVC + Ds sVC + Dsf No drainage shC shC s’h =shC ± Du= s’3 ±Du s’Vf =sVC +Dsf± Duf = s’1f No drainage s’hf =shC ± Duf = s’3f ±Duf Consolidated- Undrained test (CU Test) Step 1: At the end of consolidation Step 2: During axial stress increase Step 3: At failure**Time**Expansion Volume change of the sample Compression Consolidated- Undrained test (CU Test) Volume change of sample during consolidation**Deviator stress, Dsd**Dense sand or OC clay (Dsd)f Loose sand or NC Clay (Dsd)f Axial strain + Loose sand /NC Clay Du Axial strain - Dense sand or OC clay Consolidated- Undrained test (CU Test) Stress-strain relationship during shearing**Deviator stress, Dsd**(Dsd)fb s1 = s3 + (Dsd)f Confining stress = s3b Confining stress = s3a s3 (Dsd)fa Axial strain Total stresses at failure Shear stress, t fcu Mohr – Coulomb failure envelope in terms of total stresses s or s’ ccu s3b s3a s1a s1b (Dsd)fa CU tests How to determine strength parameters c and f**s’1 = s3 + (Dsd)f -uf**s’3= s3 -uf Mohr – Coulomb failure envelope in terms of effective stresses Shear stress, t f’ ufb s or s’ ufa C’ s’3b s’1b s3b s3a s’3a s1a s1b s’1a (Dsd)fa (Dsd)fa CU tests How to determine strength parameters c and f uf Effective stresses at failure Mohr – Coulomb failure envelope in terms of total stresses fcu ccu**Strength parameters c and f obtained from CD tests**CU tests Shear strength parameters in terms of effective stresses are c’ and f’ Shear strength parameters in terms of total stresses are ccu and fcu c’ = cd and f’ = fd**Mohr – Coulomb failure envelope in terms of effective**stresses f’ fcu Mohr – Coulomb failure envelope in terms of total stresses Shear stress, t s3a s1a s or s’ s3a s1a (Dsd)fa CU tests Failure envelopes For sand and NC Clay, ccu and c’ = 0 Therefore, one CU test would be sufficient to determine fcu and f’(= fd) of sand or NC clay**Soft clay**t Some practical applications of CU analysis for clays 1. Embankment constructed rapidly over a soft clay deposit t = in situ undrained shear strength**t**Some practical applications of CU analysis for clays 2. Rapid drawdown behind an earth dam Core t = Undrained shear strength of clay core**t**t = In situ undrained shear strength Some practical applications of CU analysis for clays 3. Rapid construction of an embankment on a natural slope Note: Total stress parameters from CU test (ccu and fcu) can be used for stability problems where, Soil have become fully consolidated and are at equilibrium with the existing stress state; Then for some reason additional stresses are applied quickly with no drainage occurring**Specimen condition during shearing**Initial specimen condition sC = s3 sC = s3 No drainage s3 + Dsd s3 No drainage Unconsolidated- Undrained test (UU Test) Data analysis Initial volume of the sample = A0 × H0 Volume of the sample during shearing = A ×H Since the test is conducted under undrained condition, A × H = A0 × H0 A ×(H0 – DH) = A0 × H0 A ×(1– DH/H0) = A0**0**0 s’3 =s3 - Duc sC = s3 s’3 =s3 - Duc Duc sC = s3 No drainage Increase of cell pressure Increase of pwp due to increase of cell pressure Skempton’s pore water pressure parameter, B Unconsolidated- Undrained test (UU Test) Step 1: Immediately after sampling Step 2: After application of hydrostatic cell pressure = + Duc =B Ds3 Note: If soil is fully saturated, then B = 1 (hence, Duc = Ds3)**s’1 =s3 +Dsd- DucDud**s’3 =s3 - DucDud Duc ±Dud s3 + Dsd Increase of pwp due to increase of deviator stress Increase of deviator stress s3 Skempton’s pore water pressure parameter, A No drainage Unconsolidated- Undrained test (UU Test) Step 3: During application of axial load = + Dud =ABDsd**Dud =ABDsd**Duc =B Ds3 Skempton’s pore water pressure equation Du=B [Ds3 + A(Ds1 – Ds3] Unconsolidated- Undrained test (UU Test) Combining steps 2 and 3, Total pore water pressure increment at any stage, Du Du=Duc + Dud Du=B [Ds3 + ADsd]**=**+ Total, s Effective, s’ Neutral, u s’V0 =ur 0 s’h0 =ur -ur 0 s’VC =sC +ur -sC=ur sC -ur + Duc = -ur + sc (Sr = 100%;B = 1) s’h =ur sC s’V =sC +Ds + ur - scDu No drainage sC + Ds sC s’h =sC +ur - scDu -ur + sc ± Du s’Vf =sC +Dsf+ ur - scDuf = s’1f sC + Dsf No drainage No drainage s’hf =sC +ur - scDuf = s’3f sC -ur + sc ± Duf Unconsolidated- Undrained test (UU Test) Step 1: Immediately after sampling Step 2: After application of hydrostatic cell pressure Step 3: During application of axial load Step 3: At failure**=**+ Total, s Effective, s’ Neutral, u Step 3: At failure t s’Vf =sC +Dsf+ ur - scDuf = s’1f sC + Dsf s’ No drainage s’1 s’3 s’hf =sC +ur - scDuf = s’3f Dsf sC -ur + sc ± Duf Unconsolidated- Undrained test (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**=**+ Total, s Effective, s’ Neutral, u Step 3: At failure Failure envelope, fu = 0 t cu s’Vf =sC +Dsf+ ur - scDuf = s’1f ua ub sC + Dsf No drainage s1b s3b s’3 s1a s3a s’1 s or s’ s’hf =sC +ur - scDuf = s’3f Dsf sC -ur + sc ± Duf Unconsolidated- Undrained test (UU Test) Mohr circles in terms of total stresses**t**s3b s1c s3c s1b s3a s1a s or s’ Unconsolidated- Undrained test (UU Test) Effect of degree of saturation on failure envelope S < 100% S > 100%**Soft clay**t Some practical applications of UU analysis for clays 1. Embankment constructed rapidly over a soft clay deposit t = in situ undrained shear strength**t**Core t = Undrained shear strength of clay core Some practical applications of UU analysis for clays 2. Large earth dam constructed rapidly with no change in water content of soft clay**t = In situ undrained shear strength**Some practical applications of UU analysis for clays 3. Footing placed rapidly on clay deposit Note: UU test simulates the short term condition in the field. Thus, cu can be used to analyze the short term behavior of soils**Example**• Given • Triaxial compression tests on three specimens of a soil sample were performed. Each test was carried out until the specimen experienced shear failure. The test data are tabulated as follows: • Required • The soil’s cohesion and angle of internal friction**Example**8 6 4 2 0 2 4 6 8 10 12 14**Example**8 6 4 2 0 2 4 6 8 10 12 14**Example**8 6 4 2 0 2 4 6 8 10 12 14**Example**8 2 6 4 4 2 0 2 4 6 8 10 12 14