Special Lab Department of Mining Engineering NIT, Rourkela, 769008

Title of the experiment: Determination of the periodic roof fall distance in the longwall mining.

SUBMITTED BY: GROUP NO- 5Members: Ravikant Kumar

Sanjay kumar Nooka Shashank

Field problem: As we all know, the longwall mining is one of the most prevalent methods being used in coal mines. SoFor the safety and success of such mines, one of the most important parameters is determination of the periodic roof weighting interval. Periodic weighting not only influences the safety but also the productivity in longwall mining.

Objective:To study the periodic loading and finding the periodic roof weighting in longwall mining in reference to the GDK 10A.

Background:The state of periodic loading and the interval of periodic roof weighting have an important role in geo-mechanical stability and, hence, in the continuity of longwall mining operations.

In this paper, the mechanism of roof caving in longwallmining—together with the effect of engineering and geo-mechanical properties of surrounding rock masses on themagnitude and timing of periodic loading—is studied.

Immediate roofThe immediate roof is the lowest part of the roof that is caved while advancing the power supports. When modeling the roof weighting interval, the thickness of the immediate roof should be calculated.

Roof caving mechanism and the rate of advance representadditional influences on the state of periodic loading.

The roof weighting interval increases due to a delay incaving (orun-caving) of the roof and a great deal of subsequent pressure on the power support system, which affects the mining operation. Therefore, it is necessary to study the process of periodic loading in longwall mining, especially in a mechanization mining operation.

What is main fall?

Main fall is the process of roof fall which occurs at some fixed distance from face as the longwall advances due to the phenomena called longwall weighting.

Difference between main fall and periodic fall

In main fall there is cantilever effect, means the roofis supported at only one end while other end remains unsupported. This cantilever effect applies extra load on the powered support.

But there is no cantilever effect in the periodic fall.Here the roof is supported at both the end, at one endthere is powered support and at other end the caved roof itself.

MAIN FALL

PERIODIC FALL

Note: in main fall 66m roof is falling while it is 34 nearly half of previous one.

Periodic roof weighting

The process of periodic loading in longwall mining is complex and significantly influences safety, stability,performance and the productivity of mining operations. Periodic loading and the periodic roof weighting interval are related to geo-mechanical conditions of the surrounding rock mass, particularly to the immediate roof. Periodic loading and the periodic roof weighting interval are related to geo-mechanical conditions of the surrounding rock mass, particularly to the immediate roof.

Mechanism of main fall1.As the shearer moves forward, the immediate roof

mudstone/siltstone cave over goaf.

2. Due to the fall of the immediate roof, it will still leave the higher sandstone above the support and face which act as the beam.

3. As the longwall advances, the beam moves downward due to its own weight.

4. It will cause the beam to act as a cantilever on the back of the support, which will cause the increased loading on the support.

5. Due to this increased loading, the sandstone gets fractured in the front of face just above. This allows the sandstone to move further downward to act as the cantilever.

6. Now the rock-mass tries to cantilever on support and it will cause freeting at face.

7. Due to freeting at the face longwall will advance faster and the sandstone above the support will now cave in, this will

cause the end of cantilever effect and thus the stress on the support gets vanished.

Methodology

Calculation of the main fall and periodic fall in reference to GDK 10A seam no 2

1. At main fall

TABLE.1PHYSICO-MECHANICAL PROPERTIES OF ROOF ROCK

BH NO.637

Depthfrom BoreHole Thickness Density

TensileStr. Youngs Mod

Surface(m) 637 (m) (gr/cc)

(kg/sq.cm)

x105(Kg/sq.cm)

163.4       0.1 1.4  

           

           

164.57       1.17 2.12 15.92 0.23

           

           

           

        10.43 2.12 17.07 0.35

           

175          

           

        8.2 2.21 25.31 0.41

           

183.2          

           

        4.13 1.56 36.72  

187.53       IMM    

        2 1.4 23.93  

189.33              

TABLE.2BORE HOLE LITHOLOGY

BHNO.637Depthfrom BoreHole

Thickn Lithology Bulking Bulk-up Cumu. Cumu.

Surface(m) 637 (m)  

Factor(m) Height

Bulk-upHt. Thickn

163.4       0.1 Coal      

               

               

164.57       1.17Carb sandy

Shale 1.4 1.638    

               

               

       Grey

Sandstone      

        10.43 (Medium - 1.15 11.9945 27.04 24.76

        Grained)      

175              

               

        8.2Grey

Sandstone 1.13 9.266 15.048 14.33

       (Fine

&Medium -      

183.2       Grained)      

               

        4.13 Shaly Coal 1.4 5.782 5.782 6.13

187.53       IMM        

      2 COAL     2

189.33         WorkingSec.        

REMARKS :Caving Height = 14.33mWorking Sec. = 2.0mImmediate Roof = 4.13m

( Which caves in as the supports are advanced)

SSt whichis expected to cave = 8.2m

CALCULATION FOR FIRST FALL DISTANCE FOR A FACE LENGTHOF 150 m

Vertical Pressure (Pv) = e1 x t1 + e’ x H

Where,

e1 – Density of bed which is expected to cave.

t1 - thickness of bed which is expected tocave

e’ – Density of the bed which is lying abovethe expected cavable bed

H – Depth upto the bed.

Pv = 2.21 x 8.2*+ 2.12 x 175

= 389.122 t/sq.m (* fromTable.2)

= 3.89 Mpa

Virgin Horizontal Stress:

= 175/d + 0.3

= 175/183.2+0.3

= 1.255

Horizontal Stress on (h) = m x Pv

= 1.255 x 3.89

= 4.881 Mpa

Weight per Unit Thickness of Beds w = e1 x t1

= 2.21 x 8.2

w = 18.122 t/sq.m

. Instiu Tensile Strength t = 0.9 x 2.531

= 2.278 Mpa

BY TRAIL ERROR PROCESS…

a) Let a= 66m b=150m

t = Thickness of Sandstone expected to cave in m

= 8.2 m (from Table.2)

a/b = 0.44

f(a/b) = 0.01991

Bending Stress (’) = f(a/b) x w x 1502 x 6 / t2

= 0.0199 x 18.122 x 1502x 6 /(8.2) 2

= 726.2t/sq.m.

= 7.262 Mpa

t = ’ – h

= 7.262 – 4.881

= 2.385 Mpa

The tensile Strength of the Rock is 2.278Mpa. Hence itcan be concluded the first fall will occur after aretreat of 66m..2. PERIODIC FALL :

Thickness of Bed1 (t1) = 8.2mTensile strength t = 2.278 MpaDensity e1 = 2.21 t/sq.m.Acceleration due to gravity(g)= 9.81 m/sec2

L12 = t1 x t / 3 x e1 x g= 8.2 x 2.278 x 103 / 3 x 2.21 x 9.81

L1 = 17 m

SUMMARY OF ALL THE ABOVE CALCULATIONS

The following points can be drawn from abovecalculations.

1.The main fall will occur after a retreat of about66m for a face length of 150m

2.The periodic fall will occur at an interval of17m