AIM OF THE EXPERIMENT
Determination of Shear Strength of Rock by Direct Shear Method
SCOPE OF THE TEST
The objective of this test is to determine the shear strength of discontinuity planes or weak horizons in rock as a function of the stress perpendicular to the sheared plane. Peak and residual strength are
distinguished.
a) This test measures peak and residual direct shear strength as a function of stress normal to the sheared plane. Results are employed, for example, in the limiting equilibrium analysis of slope stability problems or for the stability analysis of dam foundations.
b) The inclination of the test specimen with respect to the rock mass, and its direction of mounting in the testing machine are usually selected so that the sheared plane coincides with a plane of weakness in the rock, for example a joint, plane of bedding, schistosity or cleavage, or with the interface between soil and rock or concrete and rock.
c) A shear strength determination should preferably comprise at least five tests on the same test horizon with each specimen tested at a different but constant normal stress.
d) In applying the results of the test, the pore-water pressure conditions and the possibility of progressive failure must be assessed for the design case as they may differ from the test conditions.
APPARATUS USED
A mould to set the test specimen at the required orientation and position in the filling material (plaster or cement), leaving the shear plane free. This mould must have exactly the same form as the shear box, to ensure perfect fitting when sample plus filler material are placed in the shear box.
Testing equipment (a shear box) incorporating:
a) A means of applying the normal load, typically a hydraulic, pneumatic or dead-weight mechanical system, designed to ensure that the load is uniformly distributed over the plane to be tested. The resultant force should act normal to the shear plane, passing through its centre
of area. The system should have a travel greater than the amount of dilation or consolidation to be expected, and should be capable of maintaining normal load to within 2% of a selected value throughout the test.
b) A means of applying the shear force, typically a hydraulic jack or a mechanical gear-drive system, designed so that the load is distributed uniformly along one half face of the specimen with the resultant applied shear force acting in the plane of shearing. The equipment should
be designed for a shear travel greater than 10% of the specimen length. It should include rollers, cables or a similar low friction device to ensure that resistance of the equipment to shear displacement is less than 1% of the maximum shear force applied in the test.
c) Equipment for independent measurement of the applied shear and normal forces, with an accuracy better than ±2% of the maximum forces reached in the test. Recent calibration data applicable to the range of testing should be appended to the test report.
d) Equipment for measuring shear, ·normal and lateral displacement, for example micrometer dial gauges or electric transducers. The shear displacement measuring system should have a travel greater than 10% of the specimen length and an accuracy better than 0.1 mm. The normal and lateral displacement measuring systems should have a travel greater than 20 mm and an accuracy better than 0.05 mm. Resetting of gauges during the test should if possible be avoided. If electric transducers or an automatic recording system is used a recent calibration should be included in the report.
Instructor: Dr. Radhakanta Koner, Department of Mining Engineering, IIT(ISM) Dhanbad
Students: MTech students of Mining Engineering, IIT(ISM) Dhanbad
Course Name: Geomechanics Practical (MNC505)
Session: 2020-21, Monsoon
Acknowledgment
Prabhat Kumar Mandal
Krishna Murari
Birju Gope
Vijay Karanga