Group#323-33

Topic :Techniques for measurement of in situ stresses.

TECHNIQUES FOR MEASUREMENT OF INSITU STRESSESStresses can be measured in borehole, on an outcrop and

in the walls of underground galleries.Three of the best known and most used techniques are:HYDRAULIC FRACTIURING FLAT JACK METHODOVER CORING

Each of these techniques have their advantages and disadvantages.

These techniques exert pressure and disturb the rocks to create a response that can be measured and analyzed.

They are explain in the next slides one by one.

HYDRAULIC FRACTURING

This technique is performed on Dams, Onshore buildings etc.

It is designed for cable tool drilling.In hydro fracturing decision is taken from

shut in pressure. and shut in pressure is defined as:

“The pressure which is exerted by water in bore hole when becomes equal to resistive forces is called shut in pressure”.

MECHANISMThe rock is cracked by pumping water into a

bore hole.The known tensile strength of the rock is

inferred and concentration of stress at the well bore are processed to yield the initial stresses in the plane perpendicular to bore hole.

Increase in pressure burst the walls and Final pressure Ps gives safe passage to cracks for the pressure release and it become zero.

After all processes factors of safety are applied by the formula:

n/3+sq.ft

FLAT JACK METHODThis technique is applied on the rock face of

sub surface rocks.Rock faces are found in tunnels, mining or in

excavations.In this method points are marked on equal

distances depending upon tens meter.When marking is completed then bolting is

applied.Plates of gauges are attached after bolting

with the help of hammering.The micro fractures that are developed by

hammering are given by the attached gauges.

MECHANISMThe rock is partially unloaded by cutting a

slot and then reloaded;The insitu stress normal to the slot is related

to pressure required to null the displacement that occurs as result of slot cutting.

Over coringThis technique is used to identify tensile stress.

It is also applied on rock face.Bore hole is made on rock face.The gauges are installed in the hole.These gauges gives us the results that how

much is the hole changing its diameter or shrinkage of diameter.

Diameter is increased via trimming.

Trimming is done on one side of the hole to increase the diameter.

Then change in stresses is checked.Resulting in low pressure due to increase in

volume.

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