Laboratory Manual - Triaxial CD Test

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Aim of the experiment To determine shear strength parameters i.e. angle of shearing and cohesion of a given soil sample. Theory In triaxial test a cylindrical soil specimen is covered by a rubber membrane, put under pressure in a confining chamber, and then loaded on the main axis until the soil specimen fails. During testing, several parameters are measured including the confining pressure, the axial force, the axial deformation, the generated pore water pressure and the specimen volume change. The test is repeated on similar specimen at different confining pressures. The results are then used to draw the Mohr’s circles of each of the specimens at failure and then to determine the shear strength parameters: the shear intercept and the friction angle. There are three different types of triaxial tests: unconsolidated-undrained (UU) tests, consolidated- undrained (CU) tests, and consolidated-drained(CD) tests. The difference between all these tests is the drainage condition and they refer to saturated soil specimens. Different parameters are measured under different specimen condition: a) during undrained and unconsolidated conditions, the pore water pressure is measured; b) during drained and consolidated conditions, the volume change in the specimen is measured. Apparatus 1. Triaxial test cell with base, perspex cell and head. Compression machine. 2. Lateral pressure assembly. 3. Proving rings. 4. Dial gauge. 5. Rubber membranes. 6. Rubber ‘O’ rings. 7. Split mould of 3.8 cm dia and 7.6 cm height. 8. Deaired water supply. 9. Porous stone. 10. Filter paper. 11. Balance of 0.01 gm accuracy. 12. Drying oven. 13. Stop watch. 14. Volume change burette. 15. Scale and vernier calipers Procedure of CD Test for sandy soil specimen First the weight of the specimen i.e., sand is obtained. Then a split mold is used to form the specimen during preparation. The rubber membrane is already located to the lower palten using the ‘O’ rings. The split mold is placed around the membrane, and a porous stone is placed on the lower palten inside the membrane. Next, the mold is filled with sand, which is introduced using a funnel to achieve a very loose condition. A densely packed specimen can be achieved by tamping moist sand. The porous stone and top cap are put in place, and the membrane is sealed to the top cap. Finally, the split mould is dismantled. After that the cell is placed, bolted, and filled with water. Pressure is applied to the water and the vacuum inside the specimen is released.

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The test must be conducted following the stress path that closely simulates the stress history of the specimen in the field. The most common stress path consists of applying the confining pressure (by means of the control panel) followed by the deviator load. The dial indicators used for measuring the force and deformation are zeroed. Valves connecting the top and bottom of the specimen are kept open for drained tests or closed for undrained tests conditions. In the simplest case, only the dial indicator for the force and dial indicator for the deformations are read at predetermined increments. For more elaborate testing, volumetric changes must also be monitored as indicated on the control panel, using the pipettes as well as changes in pore pressure if they are allowed. The test ends when the sample get failed or when the strain exceeds 20%. The cell is unloaded and dismantled, and the specimen is removed. Calculations and results: A) Specimen Data : (Specimen No.________) 1) Type of test performed :  Consolidated drained (CD) 2) Type of specimen : 3) Diameter of specimen, D o (mm) : 2 4) Initial area of specimen, A o (mm ) : 5) Initial height of specimen, H o (mm): 3 6) Volume of specimen, V o (mm ) : 7) Mass of specimen (gm) : 8) Wet unit weight of specimen : 9) Water content of the specimen : 10) Dry unit weight of specimen : B) Triaxial Compression Data: 1) Consolidation pressure on test specimen, 2) Rate of axial strain 3) Initial height of specimen, H 0(mm) 4) Proving ring calibration

2

σ3 (N/mm

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: : : :

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Observations: Axial Deformation

(Div) (1)

∆H

Axial Strain, a (3)=∆H/H0

Volume change, ∆V (4)

Volumetric strain v (5)= ∆V/V0

(2) = (1)x LC (mm)

Corrected area Ac (6)=A0x[(1 -v)/(1-a)]

2

Minor principal stress (i.e. chamber pressure, σ3) (N/mm ) = Unit axial load at failure, ∆p (N) = 2 Major principal stress at failure (i.e., σ1= σ1+∆p) (N/mm ) =

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Proving Ring Dial (Divn) (7)

Applied Axial Load

Deviator Stress ∆σ

(8)= (7)x proving ring calibration (N)

(9) =

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(N/mm2)