ENCE 361 Soil Mechanics
Triaxial Compression Tests
Principles of Triaxial Tests
The triaxial compression test is used to measure the shear strength of a soil under controlled drainage conditions.
Mechanism of Soil Shear in the Triaxial Test
A soil mass may be considered as a compressible skeleton of solid particles. Shear stresses are carried only by the skeleton of solidparticles, whereas the normal stress on any plane is carried by both the solid particles and the pore water. In a triaxial test, the shear strength is determined in terms of the total stress (intergranular stress plus pore water pressure), unless:
Complete drainage is provided during the test so that the pore pressure is equal to zero at failure, or Measurements of pore pressure aremade during the test.
Types of Triaxial Tests
The three types of basic triaxial compression tests are:
Unconsolidated-undrained Q (or U-U) Consolidated-undrained R (or C-U) Consolidated-drained S (or C-D)
They are derived from the drainage conditions allowed to prevail during the test. The type of test is selected to closely simulate, or to bracket, the conditionsanticipated in the field.
Various devices may be used to apply axial load to the specimen. Loading devices can be further grouped under controlled-strain or controlledstress types.
In controlled-strain tests, the specimen is strained axially at a predetermined rate In controlled-stress tests, predetermined increments of load are applied to the specimen at fixedintervals of time.
Triaxial Compression Chamber
Triaxial Compression Chamber
Side Friction for Top Bearings
May be useful to measure this friction before starting test
Specimen End Caps
Rubber membranes used to encase the specimen should provide reliable protection against leakage, yet offer minimum restraint to the specimen. Commercially available rubbermembranes having thicknesses ranging of 0.0025” (for soft clays) to 0.010” (for sands or for clays containing sharp particles) are generally satisfactory for 1.4” diameter specimens.
Trimming Frame (Cohesive Specimens)
Cohesionless soils use forming jackets as they cannot be practically trimmed
Pressure Reservoir MeasuringEquipment Deaired Water Vacuum and Air pressure supply Bourdon gages Timing Device Balances Water content apparatus
Preparation of Specimens
Three soil conditions for preparing specimens
Cohesive soils containing negligible amounts of gravel Cohesionless soils containing negligible amounts of gravel Soils Containing Gravel
Cohesive Specimens, Little Gravel
CohesiveSpecimens Little Gravel
Squaring ends using mitre box
Cohesionless Soils Little Gravel
Virtually impossible to trim into a specimen Freezing can be used; frozen specimens can be cut and trimmed, put into chamber and thawed before testing Specimens can also be compacted in a mould and then tested
Soils Containing Gravel
If the material to be tested is in an undisturbedstate, the specimens shall be prepared as with shown earlier. In testing compacted soils, the largest particle size is usually known, and the entire sample should be tested, whenever possible, without removing any of the coarser particles. However, it may be necessary to remove the particles larger than a certain size to comply with the requirements for specimen size, though such practice will resultin lower measured values of the shear strength and should be avoided if possible.
Q (U-U) Test
In the Q test the water content of the test specimen is not permitted to change during the application of the confining pressure or during the loading of the specimen to failure by increasing the deviator stress. The Q test is usually applicable only to soils that are not free draining,...
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