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Civil Engineering by Sandeep Jyani
FLUID MECHANICS
SSC JE 2018 Paper 2
Que. Determine the axial stress at failure for dry dense sand in case of a triaxial loading of ππ=300kN/m2, If previous test load is given as ππ=150kN/m2 and ππ=735kN/m2 at failure.
ππ = ππππππ ππ +
β
π+ ππ πππ ππ +
β
π
Que. A sample of saturated cohesionless soil tested in a drain triaxial compression test at an angle of internal friction 30Β°, the deviator stress ππ at failure for the sample of a confined pressure 200kN/m2 will be β¦.?
ππ = ππππππ ππ +
β
π+ ππ πππ ππ +
β
π
Que. In a drained triaxial compression test conducted on dry sand, failure occur when deviator stress was 218kN/m2 at a confined pressure of 61kN/m2. Find the angle of shearing resistance and the inclination of failure plane to major principle plane.
ππ = ππππππ ππ +
β
π+ ππ πππ ππ +
β
π
Shear Strength Tests
1. Direct Shear Testβ’ The test is carried out on a soil sample
confined in a metal box of square cross-section which is split horizontally at mid-height
β’ The soil is sheared along a predetermined plane by moving the top half of the box relative to the bottom half. The box is usually square in plan of size 60 mm x 60 mm.
β’ Tests on sands and gravels can be performed quickly
6Civil Engineering by Sandeep Jyani
Shear Strength Tests1. Direct Shear Test
7Civil Engineering by Sandeep Jyani
Shear Strength Tests
2. Triaxial TestThe triaxial compression test consists of two stages:First stage: In this, a soil sample is set in the triaxialcell and confining pressure is then applied.
Second stage: In this, additional axial stress (also called deviator stress) is applied which induces shear stresses in the sample. The axial stress is continuously increased until the sample fails.
8Civil Engineering by Sandeep Jyani
Test TypesThere are several test variations, mainly used in practice are:
1. UU (Unconsolidated Undrained) Test
β’ In this, cell pressure is applied without allowing drainage
β’ Then keeping cell pressure constant,deviator stress is increased till failure without drainage
β’ It takes overall 15 minutes
Test TypesThere are several test variations, mainly used in practice are:
2. CU (Consolidated Undrained) Testβ’ In this, drainage is allowed during
cell pressure application
β’ Then without allowing further drainage, deviator stress is increased keeping cell pressure constant
β’ 1st stage consolidation takes 24 hours
β’ 2nd stage consolidation takes 2 hours
Test TypesThere are several test variations, mainly used in practice are:
3. CD (Consolidated Drained) Testβ’ This is similar to CU test except that
deviator stress is increased, drainage is permitted
β’ Rate of loading should be slowenough to ensure that no excesspore water pressure develops
β’ It takes longest time
The triaxial test has many advantages over the direct shear test:β’ The soil samples are subjected to uniform
stresses and strains.
β’ Different combinations of confining and axial
stresses can be applied.
β’ Drained and undrained tests can be carried out.
β’ Pore water pressures can be measured in
undrained tests.
β’ The complete stress-strain behavior can be
determined
Shear Strength Tests
3. Vane shear Testβ’ In plastic cohesive soils, cohesion
is obtained with Vane Shear Testβ’ Vane is pushed into the soil and
twisted upto soil failureβ’ Failure is decided on the basis of
Torque Requiredβ’ Maximum torque applied
corresponds to Total Shear Resistance
β’ From Vane Shear Test, liquid limit can also be determined
πͺ =π»
π π«π ππ+π π
Shear Strength Tests
1. If vane is pushed till complete depth βHβ
πͺ =π»
π π«π π―π +
π«π
2. If Vane is pushed till depth βhβ only
πͺ =π»
π π«π ππ+
π«ππ
π«
π―
π«
π
Que: A 20cm long, 10cm wide in dia was pressed into a sub marine clay at the bottom of a bore hole. The torque was applied. At failure, torque was 1000 kg-cm. The cohesion of soil will be β¦ ?
πͺ =π»
π π«π π―π+π«π
β πͺ =ππππ
π (ππ)ππππ+πππ
β πͺ =π
ππ
Critical Void Ratio
β’ If initial void ratio of sand is greater than critical void ratio, then sand has volume decrease tendency
β’ If initial void ratio of sand is lessthan critical void ratio, then sand initially has volume decrease tendency and further volume increase tendency
β’ πππ depends upon effective stressβ’ At πππ no volume change occurs
π
πΊπππππππ ππππππ
πππ
πππππ ππππ
π ππππ ππππ
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