An investigation was made to study marlstone failure phenomena as it tested in the conditions of conventional triaxial compression at confining pressure up to 30 MPa. Also a special triaxial cell was designed and built to test the rock samples. The cell was constructed of high strength, stainless steel which is eight inches in diameter, for samples with 70mm diameter. A number of tests, were conducted, employing them lateral pressures. Core samples were marlstone from south east of Iran. As we know in theoretical analysis, determining a suitable strength criterion for the tested rock is ideal. The tests are interpreted to propose an empirical failure criterion and in this way twelve empirical, mathematical and physical criteria, expressed in the convention of principal stresses, were considered.


for design of slopes and foundations on rock masses it is well recognised that engineers need a failure criterion which is applicable over the stress range from brittle to ductile behaviour of rocks. A strength criterion is a relation between stress components which will permit the strengths developed under various stress combinations to be predicted. Given that stresses control the stress - strain behaviour of rocks, strength criteria are best written in effective stress form. However, around most mining excavations, the pore - water pressure will be low, if not zero and so s ij ˜ s ij. For this reason, it is common in mining rock mechanics to use total stresses in majority of cases and to use effective stress criteria only in special circumstances.


The testing equipments consist of three parts, triaxial cell, loading machine and hydraulic pressure system.

2.1 The triaxial cell

A special triaxial cell was designed and built to test the rock samples under lateral pressures from zero to 30 MPa the cell was constructed of high strength, stainless steel, which is eight inches in diameter, two valve located at the top and bottom of the cell, the bottom for the lateral confining pressure and the top allowed air to escape as the cell was being filled with hydraulic fluid through the entry valve in the bottom. A cylinder of high strength steel was threaded on to the base and the connection sealed by one O - ring, the upper seal of the cell was provided by a steel packing gland threaded into the top of cylinder and sealed in the same manner as the base the centre of the packing gland contained a movable piston of stainless steel sealed by three O - rings. The sample was placed between two small loading block of stainless steel 70 mm in diameter, flat on the side in contact with the sample and sepherical on the opposite sides which were matched to sepherical depressions in the base and in the piston, which allowed for slight misallignments incurred during sample prepration.

2.2 Hydraulic pressure system

This system include a pump used to apply and to control the hydmlic pressure up to 33 MPa.

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