SUBSIDENCE CONTROL IN COAL MINES

By B.SUNIL FROM JNTUHCEM

INTRODUCTION Any deformation occurred above the

surface due to underground excavations is called SUBSIDENCE.

SUBSIDENCE due to underground coal mines mainly categorized into two forms

Pothole subsidence Trough subsidence

INTRODUCTION POTHOLE SUBSIDENCE is caused due to

the sudden collapse of overburden into the underground voids.

It is very hazardous to life as it occurs without any prior indications.

TROUGH SUBSIDENCE is occurred over a larger area which is mainly depend on the depth of the extraction.

INTRODUCTION Due to subsidence surface structures are

greatly disturbed and damaged. So it is necessary to take control

measures while working in underground to minimize the subsidence.

The main objective of this presentation to show what are the control measures to be taken while working.

SUBSIDENCE CONTROL MEASURES

PLANE FITTING METHOD TRENCHING TENSION CABLE METHOD HYDRAULIC SAND STOWING PARTIAL EXTRACTION METHODS

PLANE FITTING METHOD Surface structures undergo curvature and

twisting due to subsidence. so to minimize strain due to twisting force, plane is considered on which structure is held .

Under this plane height adjustment devices are placed which maintain inclination with time dependant along the line of subsidence.

No of device are necessary given by formula

PLANE FITTING METHOD N=1.5xW/J N= no of devices W= weight of the structure J= capacity of jacks jacks placed at interval not exceeding 3

m in for heavy structure jacks are placed densely.

TRENCHING In this method a trench is made around

the house such that the compressive forces on the wall and floor are reduced.

The dimensions of the trench to be made are 0.3m wide, 0.9 to 2 m from the wall, and 0.15 to 0.2 extended below the foundation.

Wider the trench has been made greater the compressive force can be reduced.

TRENCHING

TENSION CABLE Tension cable is used to reduce damage

due to the twisting force. In this cable is used having high

compressive strength and low tensile strength.

For proper design tension in the cable must be checked before.

HYDRAULIC SAND STOWING Hydraulic stowing is the best method for

controlling the subsidence. In India HSS play vital role in subsidence

control. Subsidence of the stowed panel is below

5% in Indian coal mines. Subsidence may be increased by delay

of stowing.

HYDRAULIC SAND STOWING As the stowing takes much time the

immediate rock should have minimum strength to hold the load for a span of time.

In India mostly the roof is sand stone so it is efficient to go with sand stowing.

But in foreign countries the roof is easily caved so the subsidence in stowed panel ranges from 5% to 35%.

PARTIAL EXTRACTION Partial extraction is done to minimize both the

subsidence and strain such that surface structures are fully protected from the damage.

These are again classified into Non effective width extraction Chess board method Wide and stall method Splitting of pillar with stowing Harmonic mining

NON EFFECTIVE WIDTH EXTRACTION

Non effective width of extraction is underground width of extraction which does not cause any subsidence above ground.

As opening made in the seam stress are redistributed and pressure arch is formed, the height of the dome is depend up on the width of opening.

As the width of extraction increases the dome touches the surface and the movement of surface starts.

NON EFFECTIVE WIDTH EXTRACTION

The maximum width of extraction in a seam without any movement above the surface is called NON EFFECTIVE WIDTH.

This expressed in depth of cover, in Indian mines NEW is 0.3 to 1.17 times of cover.

The pillars are to be extracted in such a way that at any time width does not exceed NEW.

Pillars in rows are left alternatively maintaining factor of safety 2.

CHESS BOARD METHOD When NEW method is not possible chess

board method is done. In this alternative pillars are extracted in

a chess board pattern. By this method extraction up to 60 to

65% is possible. The safety factor of the pillars left should

not be less than 2.

WIDE AND STALL METHOD When the depth of cover is moderate, the

recovery of coal is very low because more coal has to be left to minimize the subsidence.

So to get more recovery compared to the conventional method wide and stall method is used.

In this method gallery size is maintained to match the strength of rock and width and height ratio of pillar increased to stabilize the condition

The galleries are widened up to which the pillar can bear the load to improve the recovery.

SPLITTING OF PILLAR WITH STOWING

The DGMS does not permit to split the pillar below structures by caving.

But the splitting can be done with stowing.

The size splits depend on the strength of pillars.

The safety factor of the pillars is to be 1 to maintain stability up to the time of completion of stowing.

HARMONIC MINING Extraction of panel causes tensional and

compressive strain above the surface. To overcome the strain counter strain to

be acted so this can achieved by extraction of two seams simultaneously in particular manner.

Strain due first working is balanced by the strain induced in the another working.

This method of mining is known as HARMONIC MINING.

PROUD TO BE A

MINING ENGINEER