PRESSURE ARCH THEORY (LONGWALL)

Prepared by

P. VINAY CHARY

THE MINING ENVIRONMENT

IN-SITU STRESSES

No excavation No Problem

All rock in the ground subjected to compressive

stresses and if excavation is made the rock left

standing has to take more load because the

original support provided by the rock within the

excavation has been removed.

Natural stresses which exist prior to excavation. The natural stress field

can be composed of gravitational stress, tectonic stresses and residual

stresses.

Induced stresses are man-made stress component due to removal or

addition of material.

Vertical and horizontal stresses

An excavation is made in the strata, the pre mining system

of forces is disturbed, and the latent energy which existed in

the strata is released and the pressure redistributes itself.

The bending of roof beds will cause the beds to sag away

from each other and thus become distressed.

The deflected vertical compressive fore P will skirt the

distressed region along the line of contact of beds A, B, C,

D.

The line of increased pressure is known as ‘Pressure arch’ or

‘Pressure dome’ or ‘Pressure ellipse’, the shape of which will

largely be determined by the thickness and strengths of the beds.

Similar distribution of the compressive forces P-1 and P-2, and

the shearing force S, will develop in the floor and these will

tend to force the floor bed into the roadway and cause floor lift.

As the face advance the roof beds converge in the working

place.

Front and back Abutment pressure

When an opening is created in a coal seam, the

stress that was present before the opening was

created is re-distributed to the adjacent coal pillars

that are left.

The areas within the remaining coal where the

vertical stress is greater than the average are called

abutments and hence the stresses in those areas are

called abutment pressures.

The beds which become detached from the beds above are

relieved of the weight of the higher strata and the load which

was originally acting vertically over the excavated area then

deflects and bridges over the working area and transmits its

weight forward to some region ahead of the coal face known

as front abutment pressure.

Depending on the local condition, the front abutment

pressure can first be detected at a distance of one half to one

times the overburden depth out by the face.

When the face approaches to within 30m, it begins to increase

rapidly. It reaches the maximum value, ranging from 0.5 to 6.0

times the overburden pressure, when the face is 1-6m in bye.

After that the pressure drops drastically and vanishes at the face

line.

Front abutment loads, 1.8-8.0 times the overburden pressure have

been observed over a 10-30 m wide strip, parallel to the extraction

front.

This overpressure may manifest itself up to even 60 m from the

extraction front.

The growth of side abutment pressure resembles that of the front

abutment pressure.

The maximum side abutment pressure, ranging from 0.4 to 3.5 times

the overburden weight, occurs in the first pillar near the goaf edge.

It decrease exponentially away from the goaf edge. The width of the

side abutment pressure zone can be approximated by

Ws = 9.3 √h

Where h is seam depth.

Convergence is generally less than 50-75 mm before the face moves in.

The beds which become detached from the beds above are

relieved of the weight of the higher strata and the load which was

originally acting vertically over the excavated area then deflects

and bridges over the working area and transmits its weight

forward to some region backward behind the coal face at a

region where the again make contact by subsidence of the higher

beds known as back abutment pressure.

It is located in the goaf area bridging between the main gate tail

gate road. In this abutment pressure the maximum pressure

which is felt is the overburden pressure.

For the analysis of stability of underground openings, the

knowledge of stresses, strength and failure mechanism are

important.

The idea of the stress concentrations and their effects on the

surroundings of the openings helps the design engineers to plan

a suitable method of support system.

However the knowledge of rock mass properties are still to be

acquired and rock mass classification systems is an attempt

towards the purpose. And also knowing of creep phenomena

or time dependent behavior of the rocks.

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