The Constructor

INSITU STRESSES IN ROCKS AND MEASUREMENT METHODS

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What is in-situ stress in rocks?

In-situ stresses are the stresses which developed due to weight of the overlying materials and also due to the confinement and the past stress history at a point below the rock surface of the undisturbed rock mass. These stresses may vary considerably from one point to other. At some points these are zero whereas at other points they may be very high, even approaching the failure stress. At a point where in-situ stress is zero, joints are formed in the rocks and it will ease the underground excavations, constructions etc. but if In-situ stress is very high, any small disturbance to the stress field by tunnelling or even excavation may trigger violent release of stored energy in rock and cause failure. Generally, for the construction of tunnels or for any underground works we need a basic knowledge of in-situ stresses. Here are some important points regarding carefulness about insist stresses.

Orientation for a cavern

Care should be taken while selecting orientation for a cavern to avoid the alignment along the longer dimension which is perpendicular to the major principal stress (?1)

Selection of shape

Select a shape in which stress concentration is going to minimize when very high initial stresses are there.

Layout of complex underground works

Cracks in rocks tend to extend in the plane perpendicular to the minor principal stress (?3). So, knowledge about direction stress is required to prepare the layout of complex.

Saving in lining of tunnels

If the internal water pressure is greater than in-situ stress then there is no need to construct lining.

Large surface excavation

Substantial economy can be effected when making large surface excavation with pre splitting technique by orienting the excavation perpendicular to the minor principal stress.

Measurement of in-situ stresses in Rocks:

Here are three commonly used methods for the determination of in-situ stresses in rocks.
  1. Hydraulic fracturing method
  2. Flat jack method
  3. Over coring method

Hydraulic fracturing method

Flat jack method

Over coring method

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