Rajiv Gandhi College Of Engineering, Research & Technology

By Sunil Wasade

Guided By Prof. P.K. Singh

H.O.D. Dr. M.D. Uttarwar

Mining Engineering Seventh semester

2015-2016

Introduction

Overburden Movement in Longwall Mining

Abutment Pressure in Longwall Mining

Bed separation

Concept of Immediate Roof And Main Roof

Theories of rock pressure and strata movement

Conclusion

References

In this presentation, the overburden movement in longwall mining of shallow coal seams with thin top soils and key strata group was studied. The

results show that the overburden movement changed with the variation of mining height, panel

width and depth under the specific geological conditions. Also explain Abutment Pressure in

Longwall Mining Including Immediate Roof And Main Roof & Pressure arch theory (longwall)

INTRODUCTION

Figure 1: Stratigraphic sequence used in the model

The longwall panel studied in this presentation is located in the Appalachia Coalfield, United States

where most coal seams are shallow. With a depth of 575ft (175m)

the overburden strata are mostly bedrock

the panel width can reach up to 1,430ft (435m)

When a longwall panel of sufficient width and length is excavated, the roof is fractured and “convergence” between roof and floor occurs at much greater distance in advance of the faces.

Maximum rate of convergence is noticed at about 5 to 20m form the face.

overburden can generally be divided into four zones

1) Caved zone

2) Fractured zone

3) Dilated zone or Continuous deformation zone

4) Surface extension zone

Figure - Overburden Movement above a Longwall Panel

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.

Front and back Abutment pressure

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.

Bed separation cavities: As the excavation made in coal seam, the roof bed above the excavation tends to curve into the opening, and because of different radii of curvatures, lose contact with, or separate from the beds above, creating cavities between the adjacent beds.

Roof beds bend downwards from over the solid coal across the working area.

A certain amount of separation is inevitable when the natural support is removed

It is essential that is must be controlled by close and rigid supports as uncontrolled movement my give rise to dynamic peak loading in the some parts.

cavities will also act as reservoirs of a gases, dangerous occurrence.

The total load coming on support in longwall working

a) Static Load

b) Dynamic Load

Static Load : It is the dead weight of immediate roof rock

The height of immediate roof (h) is given by

h=(m-d)/(k-1)

Where,

h = height of immediate roof

m= height of exacation

d = maximum allowable convergence

k = swelling factor

Beam or Plate Theories

plate movement Theories

Arch Theories

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.

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.

Winning and working coal in India vol.II –

R.T. Deshmukh and D.J. Deshmukh

Principle and practice of modern coal mining - R.D. Singh

Coal mining and management vol II. –

S.P. Mathur

Slideshare.com/arch theory

Thank you