Triaxial Test Apparatus

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Figure10.2: Triaxial Test Apparatus

Stresses The stresses acting on a Triaxial sample during the test are as shown in Figure 10.3 From vertical equilibrium we have =   The term F/A is known as the deviator stress, and is usually given the symbol .
Hence we can write    Figure 10.3: Stresses on a Triaxial Test Sample
The axial and radial stresses are principal stresses. The object of the triaxial test is to determine the shear strength of the soil which is obtained using the Mohr-Coulomb theory. τ = c + σtanϕ 
10.2.1 Apparatus and Knowledge of Equipment
1. A constant rate
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Triaxial cell 50 mm 1700 kPa for samples up to 50 mm. The cell has 4 drilled holes at the base and is fitted with no-volume change valve (with loading piston and specimen pedestal).
11. Specimen trimmer (electrically driven lathe type) for cohesive soils if sample is undisturbed sample.
12. Base pedestal of non-corrosive material with drilled holes for both bottom drainage and pore water pressure measurement
13. Base disc for placing over the top of the base pedestal when no drainage is required
14. Top cap of non-corrosive material for both drained and undrained testing
15. Moisture content test apparatus.
16. A balance of 250 gm capacity accurate to 0.01 gm
17. Compressed air source
18. 3.8 cm (1.5 inch) internal diameter 12.5 cm (5 inches) long sample tubes.
19. Rubber rings
20. O-rings for sealing membrane to the top cap and base pedestal
21. Porous stones (for drained test)
22. Specimen trimmer for cohesive soils
23. Cartesian graph paper of 10 divisions per centimeter
24. Specimen mold for cohesionless soils
25. Specimen mold for cohesive soils
26. Membrane stretcher for cohesive soils: allows placement of membrane around sample and placement of
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Sand and clay soil specimens are prepared differently.
A. Sand Specimen
Cohesionless soils require special procedure to prepare the specimen for testing because it will not retain the required shape of specimen
a. install an O-ring at the bottom of the specimen pedestal
b. Place a rubber membrane in the specimen mold. Ensure surface is smooth and without wrinkle by applying a vacuum. While still applying the vacuum, insert the mold over the pedestal of triaxial cell and porous stone. Place some of the soil sample in the mold and compact it to desired density being careful not to puncture the rubber membrane.
c. Continue filling and compacting until the required specimen height is attained in the mold. Make top of the specimen flat and level. Place a porous stone on top of specimen level surface. Stop applying vacuum.
d. Take a representative sample of the soil for moisture content determination, w
e. Now place the loading cap on top of the porous stone. Roll up the top of the membrane to partly cover the sides of the loading cap and then install an O-ring over the membrane so that the membrane is tightly sealed between the O-ring and the loading

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