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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 cavernCare 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 shapeSelect a shape in which stress concentration is going to minimize when very high initial stresses are there.
Layout of complex underground worksCracks 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 tunnelsIf the internal water pressure is greater than in-situ stress then there is no need to construct lining.
Large surface excavationSubstantial 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.
- Hydraulic fracturing method
- Flat jack method
- Over coring method
Hydraulic fracturing method
- In this method, water pressure is used to create a crack in bore hole. Water is pumped into a section of the bore hole isolated between two packers.
- As the water pressure is increased, initial compressive stress in rocks starts reducing and it becomes tensile at some stage
- When the tensile stress & tensile strength of rock are equal a crack is formed in the bore hole wall.
- By continuing pumping, the hole extends further and eventually the pressure down the hole falls to a steady value, shut-in pressure.
- This method is used to determine in-situ stresses in rock only when the point is at considerable depth below the surface.
Flat jack method
- In flat jack method, a flat jack is inserted in the rock slot which was formed by cutting the rock with cutting tool. When the cutting is done deformation takes place surrounding the slot. Here, Insitu stress is related to the pressure needed to balance this deformation.
- Firstly mark the location of slot and one or more measuring points are installed such that the location of the slot is midway between them.
- The spacing (d) of the measuring points is kept equal to the gauge length of extensometer.(which is generally 6 inches)
- Then slot is to be cut deeply, as a result of this cutting, the pins installed at measuring points move towards each other and spacing reduces between them due to initial compression of rock.
- Now place the flat jack and cemented it. The pressure in jack is gradually increased. When the pins are returned to initial spacing, then we can say that pressure in jack is equal to the Insitu stresses.
- This method is used only when there is an access to a rock face.
Over coring method
- In this method, a small diameter bore hole is drilled into the rock and deformation gauge is inserted into it which measures the change in diameter.
- Now, drill the large diameter bore hole concentrically over the small bore hole. Thus a thick cylindrical rock is formed, which is detached from the rock mass and is free of stress.
- If the rock was under initial compression, there would be enlargement of diameter, which is measured with the help of deformation gauge. From this deformation values we can calculate the Insitu stresses in rocks.
- This method can be used only to measure the Insitu stresses at some distance (up to 5m) away from the rock face.