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A number of methods are used for conducting the triaxial test on cohesive soils. The commonly used methods are described as below:

A deaired, coarse porous disc or stone is placed on the top of the pedestal in the triaxial test apparatus. Then a filter paper disc is placed over the porous stone. The prepared cohesive soil specimen is then placed over the filter paper disc of the stone. The usual size of the specimen is about 37.5 mm diameter and 75.o mm height. A porous stone is also placed on the top of the specimen. After that a deaired vertical filter strip papers are placed at regular spacing around the entire periphery of the porous stone such that these assembly touch both the stones. The cohesive soil sample in then covered in a rubber membrane, in which it can slid over the specimen with the help of the attached membrane stretcher. The membrane stretcher is completely covered with the specimen using O-rings.

The triaxial cell used for test is then placed over the base and fixed to it firmly. The triaxial cell is then filled with water by means of a pressure supply. The space left in the top portion of the triaxial cell is filled by filling oil through an oil valve. Both oil valve and air-vent valve are closed till they overflow with oil supply. To pressurize the water inside the cell, it will be connected to a mercury-pot system. The soil specimen starts to consolidate as soon as the pressure starts building up. The soil specimen is connected to the burette through pressure valves for the measurement of volume changes during the test. The consolidation can be considered as complete when there is no more volume change occurs in the test.

After the complete consolidation of the specimen it can then be used for shearing. The drainage valve is closed. A pore-water pressure measurement system is attached with the specimen. Initially, the proving ring dial gauge will be set to zero. The loading frame has a manual control. Using that, the ram can be pushed back into the cell to make it not to touch the loading cap. The loading machine will then operate in the selected speed. The proving ring dial gauge records the force due to friction and the upward thrust produced on the ram. Then the machine is halted and with the help of manual control, the ram is pushed further back into the cell and bringing it in contact with the loading cap at the next round. After that the dial gauge for the measuring axial deformation of the soil specimen is set to zero.

The soil sample is then sheared by applying the deviator stress in the loading machine. The proving ring readings are generally taken corresponding triaxial strains of 1/3%,2/3%,1%,2%,3%,4%,5%…until failure or 20% axial strain.

After completion of the test, the loading is stopped. With the manual control of the machine, all other acting additional axial stress is removed. The triaxial cell pressure is then reduced to zero to completely empty the cell. The triaxial cell is then unscrewed and removed from the attached base.O-rings are taken out, and the stretching membrane is removed. The specimen is can be recovered after removing the final loading cap and top porous stone. The filter paper strips will be peeled off. The post-shear mass and length of the soil specimen are determined.

The procedure is almost similar to that for a consolidated-undrained test as explained above, with one specific difference that the specimen is not allowed to consolidate in the first step as in consolidated-undrained. As the name suggests, the drainage valve used in the test will be completely closed during the whole procedure. However, the specimen can be connected to the pore-water pressure measurement system if needed.

The whole procedure is very similar to that for a consolidated-undrained test, with one specific difference is that the specimen is sheared very slowly in the second step. In this test, the drainage valve kept open after the consolidation step in the first stage of the test. Drainage valve remains connected to the burette throughout the whole test. The volume changes occurring during the shearing stage of the specimen are measured with the help of the burette. Since the permeability of cohesive soils is much low, it takes 4-5 days for the consolidated drained test to complete.

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Kanwarjot Singh

Kanwarjot Singh is the founder of Civil Engineering Portal, a leading civil engineering website which has been awarded as the best online publication by CIDC. He did his BE civil from Thapar University, Patiala and has been working on this website with his team of Civil Engineers.

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