Question

11. Briefly describe three methods used to determine the in-situ stress state for rocks masses that are commonly used in rock mechanics.

Answer 1

Answer:- Three methods for determining in situ stress for rock masses:-

(1) Flat Jack method

(2) Over coring method

(3) Hydraulic fracturing method

(1) Flat jack method:- It is very important method for determining in situ stress. In it flat jack consist of two plates of steel that are welded together. Flat jack method consists of measuring the displacement between strain gauges at the surfaces of an excavation created by cutting a nearby slot. The flat jack is inserted in slot, grouted in place and pressurized until the pin or strain gauge readings have returned to their original position, which is called cancellation pressure. Cancellation pressure is used as an estimate of the stress normal to the jack. To know the complete state of stress in the area of interest, six flat jack tests need to be conducted in six different directions.

(2) Over coring method:- It is also a method of stress calculation. Over coring methods are also in practice to determine the in situ stresses of rock mass, which involves drilling a large diameter hole (60-150 mm) in the volume of rock, sufficiently at a distance so that the effect of the excavation or ground surface will be negligible. It will be followed by the small pilot hole at the end of larger hole. The pilot and large diameter holes must be as concentric as possible. Pilot hole length vary between 300 and 500 mm. The large diameter hole is resumed, partially or totally relieving stresses and strains within the cylinder of rock that is formed. The changes in strains or displacements are then recorded.

(3) Hydraulic fracturing method:- In this method stress is measured by drill hole. The pressure applied to the borehole wall is increased to a value, where existing fractures l open or new fractures are formed. The fluid pressures required to open, generate, propagate, sustain and reopen fractures in rock at the test horizon are measured and related to the existing stress field.