Triaxial Compression Test ( Part 01 ) - Determination of soil strength parameters

Triaxial compression test is the most reliable and widely used shear strength test that is suitable for almost all types of soils.

The test consists of a cylindrical soil specimen ( 1.5'' diameter and 3'' height ) and on special occasions large diameter samples are used.

The specimen is wrapped with an impermeable rubber membrane and O-rings are used at the top and bottom to provide a water tight seal, thus allowing drainage from only the top and bottom sides of the soil sample. the membrane prevents the cell water from penetrating the pores of the soil. The sample is placed on a metal pedestal and is encased inside a plastic cylindrical chamber that is filled with water.   

Procedure

First of all we need to saturate the sample 100% and then subjected to equal all around confining (cell) pressure (Sigma 3), by the compression of water in the chamber and this cell pressure is usually held constant during the test.

 
To cause shear failure an axial load P is steadily applied to the specimen through a vertical loading (axial load) ram until failure occurs. this load ''P'' is known ad " Deviator force"

The axial load P causes axial stress (sigma d) and it is called as thee " Principal stress difference" or "Deviator stress".





Where,
delta L = vertical displacement
L = initial displacement
delta V = volume change (we can measure this by water quantity coming out)
V = initial volume
A0 = initial cross section area of the soil specimen
Ac = corrected cross sectional area of the soil specimen
delta D = lateral displacement
D = initial diameter of the soil specimen

The test is performed a number of times on several specimens of the same soil using different initial cell pressures, and corresponding deviator stresses, determined. so that the axial stresses at failure are calculated.


The results are used to obtain the shear strength parameters using the Mohr's circle of stress (later)

Drainage can be permitted via porous stones placed on top and bottom of the soil specimen and through tubes connected to the top and base of the specimen. the pore water pressure can be measured via pressure gauges connected to the tubes.

There are basically three types of triaxial tests which are commonly assigned by a two letter symbol. The first letter refers to what happens before shearing ( stage 1 - consolidate ) - whether the specimen is consolidated or not, and the second letter to the drainage during shearing ( stage 2 - shearing ).



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