TRIAXIAL SHEAR STRENGTH TEST ON SOIL
This is the most widely used and is suitable for all types of soils. A cylindrical specimen, generally having a length to diameter ratio of 2, is used in the test and is stressed under conditions of axial symmetry in the manner shown in figure.
The specimen is subjected to an all round fluid pressure in the cell, consolidation is allowed to take place if appropriate, and then the axial stress is gradually increased by the application of compressive load through the ram until failure of the specimen takes place, usually on a diagonal plane.
Fig: Triaxial Test Apparatus
Fig: Mercury Control System
Fig: Pore Water Pressure Measurement Device
Fig: Volume Change Measurement
Fig: Preparation of Sample of Cohesionless Soil
The length of the soil sample is kept about 2 to 2.5 times is diameter. As the cell pressure, all round the specimen, net major stress on the top of the specimen is
During first observation, a particular fluid pressure is applied and corresponding value of is obtained at failure. With the help of each set of the values () different Mohr’s circles corresponding to failure conditions are drawn. A tangent to these circles gives failure envelop for the soil under the given drainage condition of the test.
(For drained Triaxial tests)
(for undrained test)
= initial area
= axial strain =
=initial volume of the specimen
= initial length of the specimen
= change in volume of the specimen
= change in length of the specimen
Stress conditions in soil specimen during Triaxial Testing:
The effective stresses acting on the soil specimen during testing are shown in figure. In this case effective minor principle stresses is equal to the cell pressure (fluid pressure) minus the pore pressure. The major principle stress is equal to deviator stress plus the cell pressure. Hence, the effective major principle stress is equal to the major principle stress minus the pore pressure.
Let the stress components on the failure plane MN be and and the failure plane is inclined at an angle to the major principle plane.
Let the envelop DF cut the abscissa at angle , C be the centre of the Mohr’s circle.
From and we get,
Principle stresses relationships at failure
Again from ,
On solving this equation, we get
Pore water pressure measurement:
Pore water pressure must be measured under conditions of no flow either out of or into the specimen, otherwise the correct pressure gets modified. It is possible to measure pore water pressure at one end of the specimen while drainage is taking place at the other end. The no flow condition is maintained by the use of the null indicator, essentially a U-tube partly filled with mercury.
Advantages of Triaxial Test
i. The stress distribution on the failure plane is uniform.
ii. The specimen is free to fail on the weakest plane
iii. There is complete control over the drainage.
iv. Pore pressure changes and the volumetric changes can be measured directly.
v. The state of stress at all intermediate stages upto failure is known. The Mohr circle can be drawn at any stage of shear.
vi. This test is suitable for accurate research work and the apparatus adaptable to special requirements such as extension test and tests for different stress paths.
Disadvantages of Triaxial Test
i. The apparatus is elaborate, bulky and costly.
ii. The drained test takes place a longer period in comparison with a direct shear test.
iii. It is not possible to determine the cross sectional area of the specimen at larger strains, as the assumption that the specimen remains cylindrical does not hold good.
iv. The strain conditions in the specimen are not uniform due to frictional restraint produced by the loading cap and the pedestal disc. This leads to the formation of the dead zones at each end of the specimen.
v. The consolidation of the specimen in the test is isotropic, whereas in the field, the consolidation is generally anisotropic.