The triaxial test

The triaxial test

This is the most widely used shear strength test and is suitable for all types of soil. The test has the advantages that drainage conditions can be controlled, enabling saturated soils of low permeability to be consolidated, if required, as part of the test procedure, and pore water pressure measurements can be made. A cylindrical specimen, generally having a length/diameter ratio of 2, is used in the test and is stressed under conditions of axial symmetry in the manner shown in Figure. Typical specimen diameters are 38 and 100 mm. The main features of the apparatus are shown in Figure. The circular base has a central pedestal on which the specimen is placed, there being access through the pedestal for drainage and for the measurement of pore water pressure. A Perspex cylinder, sealed between a ring and the circular cell top, forms the body of the cell. The cell top has a central bush through which the loading ram passes. The cylinder and cell top clamp onto the base, a seal being made by means of an O-ring.

Triaxial test

The specimen is placed on either a porous or a solid disc on the pedestal of the apparatus. A loading cap is placed on top of the specimen and the specimen is then sealed in a rubber membrane, O-rings under tension being used to seal the membrane to the pedestal and the loading cap. In the case of sands, the specimen must be prepared in a rubber membrane inside a rigid former which fits around the pedestal. A small negative pressure is applied to the pore water to maintain the stability of the specimen while the former is removed prior to the application of the all-round pressure. A connection may also be made through the loading cap to the top of the specimen, a flexible plastic tube leading from the loading cap to the base of the cell; this connection is normally used for the application of back pressure (as described later in this section). Both the top of the loading cap and the lower end of the loading ram have coned seating, the load being transmitted through a steel ball. 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. The load is measured by means of a load ring or by a load transducer fitted either inside or outside the cell. The system for applying the all-round pressure must be capable of compensating for pressure changes due to cell leakage or specimen volume change.

In the triaxial test, consolidation takes place under equal increments of total stress normal to the end and circumferential surfaces of the specimen. Lateral strain in the specimen is not equal to zero during consolidation under these conditions (unlike in the odometer test, as described in Section). Dissipation of excess pore water pressure takes place due to drainage through the porous disc at the bottom (or top) of the specimen. The drainage connection leads to an external burette, enabling the volume of water expelled from the specimen to be measured. The datum for excess pore water pressure is therefore atmospheric pressure, assuming that the water level in the burette is at the same height as the centre of the specimen. Filter paper drains, in contact with the end porous disc, are some-times placed around the circumference of the specimen; both vertical and radial drainage then take place and the rate of dissipation of excess pore water pressure is increased.

The all-round pressure is taken to be the minor principal stress and the sum of the all-round pressure and the applied axial stress as the major principal stress, on the basis that there are no shear stresses on the surfaces of the specimen. The applied axial stress is thus referred to as the principal stress difference (also known as the deviator stress). The intermediate principal stress is equal to the minor principal stress; there- fore, the stress conditions at failure can be represented by a Mohr circle. If a number of specimens are tested, each under a different value of all-round pressure, the failure envelope can be drawn and the shear strength parameters for the soil determined. In calculating the principal stress difference, the fact that the average cross-sectional area (A) of the specimen does not remain constant throughout the test must be taken into account. If the original cross-sectional area of the specimen is A' and the original volume is V' then, if the volume of the specimen decreases during the test.

Pore water pressure measurement

The pore water pressure in a triaxial specimen can be measured, enabling the results to be expressed in terms of effective stress; conditions of no flow either out of or into the specimen must be maintained, otherwise the correct pressure will be modified. Pore water pressure is normally measured by means of an electronic pressure transducer.A change in pressure produces a small deflection of the transducer diaphragm, the corres- ponding strain being calibrated against pressure. The connection between the specimen and the transducer must be filled with de-aired water (produced by boiling water in a near vacuum) and the system should undergo negligible volume change under pressure. If the specimen is partially saturated a fine porous ceramic disc must be sealed into the pedestal of the cell if the correct pore water pressure is to be measured. Depending on the pore size of the ceramic, only pore water can flow through the disc, provided the difference between the pore air and pore water pressures is below a certain value known as the air entry value of the disc. Under undrained conditions the ceramic disc will remain fully saturated with water, provided the air entry value is high enough, and enabling the correct pore water pressure to be measured. The use of a coarse porous disc, as normally used for a fully saturated soil, would result in the measurement of the pore air pressure in a partially saturated soil.