LAB MANUAL 11Triaxial

LAB MANUAL 11Triaxial - CIVE 310 Soil Mechanics Triaxial...

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CIVE 310 Soil Mechanics Triaxial Test Section Assigned Due Before Lab: Read this Handout 108
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SHEAR STRENGTH Why do we need to know the strengths of soils? 1) Bearing Capacity Calculations: q ult = cN c + γ DNq +0.5 γ BN γ 2) Slope Stability Calculations In geotechnical engineering we like to know what the Factor of Safety against failure is. How do we determine this Factor of Safety (FS) in geotechnical problems? FS= ______Strength of soil________ Pressure (stresses) applied to soil Where do soils get their strength? 1) From cohesion – usually associated with clays 2) From frictional forces – These are the normal stresses (or confining stresses) that act on a soil (remember the sliding block on a plane from physics?) We can define soil strength as: τ failure = c + σ n tan φ Where; c = the cohesion of the soil σ n = the normal (confining) stress on the soil What kinds of tests are used for evaluating the shear strength of soils? 1) Direct Shear 2) Direct Simple Shear 3) Triaxial Shear 4) Unconfined Shear What information do we get from these tests? If we do a graph of the deviator stress vs. axial strain, we can find the peak deviator stress ( σ d ). Knowing this and the confining pressure ( σ 3 ), we can draw the Mohr’s circles for the soil and determine the failure envelope for the soil. 109
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Triaxial Shear Test 1) Form a cylindrical sample or cut it from an “undisturbed” Shelby tube sample, and place a latex membrane over it. 2) Place the sample inside a cell and apply a confining pressure to it ( σ 3 ). 3) Apply an axial load and measure axial deformation and deviator load. Stresses on the sample: 4) Data Reduction Axial Strain = є a = Change in the sample height = H i –H (H i – H = H) Initial sample height H i Deviator Stress = σ 1 ’ - σ 3 ’ = Applied axial load (this in units of pressure) Sample area 110
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Since the sample undergoes deformation, the area of the sample changes. We assume that the
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This note was uploaded on 04/03/2008 for the course CIVE 310 taught by Professor Iforget during the Spring '08 term at Drexel.

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LAB MANUAL 11Triaxial - CIVE 310 Soil Mechanics Triaxial...

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