Author: Tikeshwar Mahto,

Dy.Director of Mines Safety, Bilaspur egion(India)

+917898033693 tikeshwarmahto@yahoo.co.in

ECONOMICAL AND SAFE DESIGN OF ROOF BAR (GIRDER) FOR STRATA CONTROL IN UNDER GROUND MINES TO EXTRACT THICK SEAMS – A Case study in BG – Method.

Abstract

Blasting Gallery is a method of working to extract thick seams ( 8m - 15m ) in single lift. Strata control mechanism is a very critical aspect of BG-method, as the height of working is more than 10m. Natural support has very important role in overcoming dynamic load created by the hanging goaf, particularly in case of massive sand stone roof. Artificial supports are only for resisting separation of immediate roof. Hence, design of natural support as well as temporary supports are very- very important for the strata control point of view in Blasting Gallery Method. For controlling dynamic load, strength of pillars is a crucial factor and for strength of pillars, it’s height is very important. More the height of pillars, less will be it’s strength.. The author has explained elaborately about the relationship between height of galleries and strength of pillars in an article ‘’ Design of Pillras, a Critical Aspect for Strata Control in Underground Pillar Mining’’ published in CMTM (IICM ). In this paper, the author is concentrating only about the temporary supports used in Blasting Gallery Method. The author has critically diagnosed about the drawbacks and failure of existing supporting system and also suggested a modification in supporting system and design of roof bar (girder) for effective utilization of supports. If, the design of roof bar as suggested by the author is implemented effectively, a huge amount of rupees will be saved in purchase of roof bar every year and also an effective support resistance can be developed for the safe working of BG-method.

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Seam sections of NO.3 Seam of GDK.NO.10 Incline

EARLIER PRACTICES OF STRATA CONTROL MECHANISMS IN BG -METHOD

Supporting of main and split galleries while depillaring:

Main and split galleries are supported by 200mm x 200mm, M.S. roof bars strengthened at both ends and held in position by means of two Nos. of 40T Open Circuit hydraulic props. Such roof bars are placed at 1.0m interval and braced together by means of steel bracers, specially made for this purpose. The gap between girders and roof are lagged by wooden sleepers. The whole support assembly is shown below in Fig-3.

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Free body load diagram of support assembly of Fig.3 can be drawn in the following ways for clear representation of diffirent forces acting on support assembly

Where, AB is M.S. roof barL1, L2 are concentrated reactive forces (loads) on the roof bar, andR1, R2 are support resistances offered by the O.C Props (8-10Tons, while setting)

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We can see from the free body load diagram and fig-3 in which the support resistances (R1 & R2) offered by the Open Circuit props are directly acting on the steel roof bar and not on the roof of the gallery, because there is no contact between roof and roof bar. In this case total support resistance is utilized for bending the roof bar and not for resisting rock load. When R1 & R2 increases, L1 & L2 also increases which tries to bend the roof bar.

From, Fig-4, R1 + R2 = L1 + L2

It means total support resistances offered by O.C props are inversely transferred on the roof bar, which tend to bend the bar. After yielding of roof bar, support resistance decreases and adverse situations like, bed separation, side spalling, overriding, props dislodgment etc., are created.

The roof bar assembly in yielded condition is shown in Fig.5

CRITICAL STUDY OF FAILURE (OR PREMATURE YIELDING) OF ROOF BAR USED IN BLASTING GALLERY The roof bar of I- section used in BG working is loaded with different stresses like direct stress(compressive stress ), shear stress, bending stresses etc. and failure of which is caused by either any one of these or due to combined effect of these

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stresses. The roof bar of I – section is made of two different load bearing components, web and flanges. Flanges are for bearing bending moment and bending stresses and web is for resisting shear stress and direct stress (compressive stress). Case study of GDK. NO. 10 Incline made by the author reveals that the failure of roof bar is due to bending of flanges in centre and failure of web at the edges of the roof bar. The above-mentioned failures of roof bar are due faulty design & selection of roof bar. The author has critically diagnosed about it, and has made some modification in supporting system and also in design of roof bar, which are mentioned below.

Failure of roof bar due to faulty supporting system (Exclusive study) Modes of failure of roof bar due to faulty supporting system are bending (sagging) of roof bar and shearing of web of the bar. It has been shown clearly about the supporting system and failure of roof bar in Fig.4 & Fig.5

Mathematical approach:

Taking free body load diagram of simply supported beam (roof bar) from Fig.4 in which, R1 &R2 = support resistances offered by O.C. props, L1& L2 = reactive concentrated loads on the roof bar at points A1 & A2 respectively, W = span or length of roof bar (generally 3.6m), C = mean distance of cogs erected on roof bar from the ends of bar.

From free body load diagram, ∑Fy = 0 Or, R1 – L1 –L2 +R2 =0 Or, R1+ R2 = L1+L2

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Taking R1 = R2 =R and L1 = L2 =L Hence, L1 = L2 = L =R.

(a)To construct shear force diagram (S.F.D.): V-X

Shear force at A (VA) ; VA- R1 = 0Or, VA = R1.

Shear force between A & A1 (VA1); VA1 – L1 = 0, Or, VA1 = L1 = R1

Shear force between A1 & B1 (VAB); VAB +R1 –L1 = 0 Or, VAB = L1- R1 =0

Shear force at B1; VB1- L2 = 0, Or, VB1 = L2 = R2 =R Shear force between B1 and B ;

VB – R2 = 0,

BA

A1B1

C W-2C C

W

R1 R2

L1 L2

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Or, VB = R2 =(b)To construct bending moment diagram (B.M.D.) ; Mb-X

Moment about clock-wise direction will be taken as positive.Taking section X- X1 between A and A1 for moment (Mb) calculation from left hand side

Mb = R1*X i.e. Mb =0, when X =0 Or , Mb = 0 at A

Moment (Mb) at A1= R1*C, where X = C

Taking section between A1 & B1 ;

Moment (Mb) = R1*X – L1*(X-C)

So, moment at B1 = R1(W-C) –L1(W-C-C), where X = W-C Or, Mb = R1(W-C)- L1(W-2C) ,since R1= L1

Or, Mb = R1*C

And moment at B =0 , where X = W,

Thus, S.F.D. and B.M.D. can be drawn as shown in figure below

BAA1 B1

C W-2C C

W

R1 R2

L1 L2

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(i) Loaded roof bar

(ii) S.F.Diagram

(iii) B.M. Diagram

Flexural strength calculationThis is for calculation of bending stresses in flanges of the roof bar due to bending moment as calculated above.

The section of the roof bar and stresses in flanges of the bar can be drawn in the following ways;

X

V

+ve

-ve

A BA1

B1

R1

R2

+ve

-ve

x

Mb

ABA1 B1

R1C R1C

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From the theory of simple bending;

M/I = α/y =E/R

Where, b= flange width, d= distance between the two flanges, t1= thickness of flange, t2= thickness of web,

αt = bending stress(tensile), αc = bending stress (compressive) ,

N1- N2 = neutral line M = moment of resistance or bending moment, I = moment of inertia, y = distance from neutral axis, E = Young’s modulus, and R = radius of curvature of internal surface of the deformed beam(roof bar).

Here, M/I = α/y Or, α = M*y/ISo `α ` will be maximum or minimum when ` y` is maximum or minimum.Thus , for y= 0 , α = 0 i.e. bending stress at neutral line is zero and bending stresses at flanges are maximum.Also ymax = d/2 αmax = (M/I) *ymax , or, αmax = Md/2I

Thus, maximum bending stress is at flanges of the roof bar, as shown in the figure above.

b

d

t1

t2

αc

αt

Neutral line N1-N2

ymax = d/2

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Moment of inertia(I) of I – section beam:

First, we will calculate moment of inertia of rectangular section beam of same dimension.

Moment of inertia of rectangular section = b*d3/12

Where, b= width of section of the rectangular beam, d = height of section of beam. t1 = thickness of flange of I- section beam, t2 = thickness of web of beam. N1-N2 = neutral line

Now cutting the dotted portion of the rectangular section, as shown in the above figure for calculating moment of inertia (I) of I- section beam.

Hence, section of cut portion of the rectangular beam will be;

So, M.I. of two cut portions about N1- N2 = 2* (b- t2/2)(d-2t1)3/12 = (b-t2)(d- 2t1)3/12 Thus, M. I. of I – section beam (girder) will be;

d

b

t2

t1

N2

N1

d - 2t1

b-t2/2

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N1

N2

I = M.I. of rectangular beam – M.I. of cut portions.

Or, I = b*d3/12 – (b- t2)(d-2t1)3/12

Or, I = [ b*d3 – ( b- t2)(d-2t1)3]/12

Moment of resistance( bending moment ) can be taken from bending moment diagram(B.M.D.), as drawn in previous page.

So, maximum bending moment is at centre of the beam(or roof bar );

Or, M = R*C

Where, M = bending moment R = support resistance by O.C. props, and C = mid- distance of cogs from the edge of roof bar.

Case study:The author has done case study of support failure of GDK.NO.10- Incline, BG – panel, which is due to faulty support system and faulty design of roof bar.

Following are the data taken for the average loading of support assembly for clear picturisation of findings :

Average load on (R1 &R2 ) of O.C. props : 15tonne Length of girder (roof bar ) (W) : 3.6m Distance of cog from edge of girder ( C ) : 1.2m Section of roof bar (b*d ) : 200mm* 200mm Thickness of flange (t1) : 10mm Thickness of web (t2) : 7mm Cross- section of the web[(d-2t1)*t2)] : 180mm*7mm

Putting these data in the equations mentioned earlier for calculation of design parameters.

Maximum shear force in web of the roof bar: R1=R2= 15t = 15*1000*9.81N = 147.15KNMaximum shear stress in the web of the bar = shear force/cross- sectional area = 147.15KN/ 18cm*0.7cm =11.679KN/cm2

Maximum bending moment (M ) in centre = R*C = 147.15KN* 120cm = 17658KN-cm

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In this system maximum bending moment is at more than one point

Maximum bending stresses in the flanges (α) = M*d/2I

Where, d = 200mm = 20cm

I = moment of inertia

= [b*d3 - (b-t2)(d-2t1)]/12

= [20*203 – (20-0.7)(20-2*1)]/12 cm4

= [160000- 19.3*18]/12

= 47442/12 cm4

= 3953 cm4

Hence, max. Bending stress (α) = 17658*20/2*3953 KN/cm2

= 44.97 KN/cm2

Thus, the values of shear force (V), bending moment (M), and bending stresses (α ) are more and these are only due to faulty supporting system.

Disadvantages of this system are as follows:

Bending of roof bar, which eliminates re-utilization of it; Under utilization of O.C. props, because capacity of props are never

utilized fully due to yielding of roof bars; Bed separation due to under setting of supports ; Side spalling. Total roof loads are transferred on sides of the pillar,

because supports assembly are not in position to bear rock load, which enhances side spalling.

Dislodgment of props by hitting side spalled boulders. It happens because of O.C. props becoming slack after bending of roof bar. Side spalling is very severe in this mine, because of particular seam characteristic, which is shown in Fig.1 & Fig.2. clay and shale bands are exactly below the roof of the gallery which separate roof coal and pillar coal in two parts, i.e., there is no coherence between roof & pillar coal. Therefore when load comes from the roof strata in the abutment zone (sides), it is transferred into lateral thrust which causes side spalling ;

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Overriding of pillars, which creates unsafe working conditions for men and machinery. Supports used in the roadways for movement of LHDs are under loaded, because of yielding of roof bar, which may collapse while overriding of stooks. such type of incidence has occurred in one of the mines of SCCL.

MODIFICATION IN SUPPORTING SYSTEM SUGGESTED BY THE AUTHOR:

The author has done nothing extra, but has made some changes after deep study in BG method of working. In the changed system of supporting, the wooden lagging are exactly above the O.C. Props to make direct contact of the O.C. props with the roof of the galleries. The support capacity or strength of the O.C. props are directly transferred to the roof of the galleries and not to the roof bar, which eliminates the chances of bending of roof bar and the O.C. props remain always tightened against the roof. Also, support resistance increases, which can improve strata condition.

The modified system of support assembly is shown in Fig.6, given below. The support resistance can further be increased by strengthening roof bars properly at both ends.

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Modified supporting system

Free body load diagram of the modified system of supporting is shown below in Fig.7

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Where,

AB is roof barR1 & R2 are support resistances offered by OC propsL1 & L2 are reactive support resistances transferred to the roof rock, andL3 , L4 & L5 are concentrated reactive support resistances offered by roof bar to the roof rock

In Fig.7, the length of the arrows shows the magnitude of loads.

L1 and L2 have greater length in comparison to L3 , L4 & L5. It means major portion of the support resistances (R1 & R2) offered by OC props are transferred to the roof to bear rock load and small amount (L3, L4 & L5) of support resistances (R1 & R2)to the roof bar for preventing separation of immediate layers.

i.e. R1 +R2 = Load transferred to the roof rock + Load transferred to the roof bar = ( L1 + L2 ) + ( L3 +L4 +L5 ) = L1 +L2 +L3 +L4 +L5

In this system strength of props are not utilized for bending the roof bar and props are always remain intact with roof in tight position. Also, rock load

L1L3 L4 L5

L2

BA

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Fig. 7R1 R2

comes directly on the OC props and not on the roof bar, which eliminates the bending of roof bar.

From the modified system of supporting, as shown in Fig.6 free body load diagram of support assembly, shear force diagram (S.F.D. ) & bending moment diagram (B.M.D.) can be drawn as shown below;

(i) Loaded beam

(ii) S.F.Diagram

(iii) B.M.Diagram

A B

A1 A2 A3

R1 R2

L2L1

L3 L4 L5

CC W-2C/2 W-2C/2

W

X

V

A

BA3

A2

A1

+ve

-ve

R1-L1

R2-L2

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X

Mb

ABA1 A2 A3

W (R1-L1)/2 - L3*C

(a)To construct shear force diagram (S.F.D.):

Summation of forces along Y- axis;∑FY = 0,Or, R1+R2-L1-L2-L3-L4-L5 =0,Or, R1+R2 = L1+L2+L3+L4+L5

Assuming R1 = R2 = R, L1 = L2 =L, and

L3 = L4 = L5 = 2(R-L)/3

Shear force at A VA = R1- L1

= R- L

Shear force between A and A1,

VA = R1- L1 = R- LShear force at A1, VA1 = R1- L1- L3

= (R- L) - 2(R-L)/3 = (R- L)/3

Shear force between A1 and A2, VA1 = (R- L)/3 Shear force at A2,

VA2 = VA1- L4

= (R- L)/3 – 2(R- L)/3 = -(R- L)/3, Shear force between A2 and A3

VA2 = -(R- L)/3

Shear force at A3,

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VA3 = VA2 – L5

= -(R-L)/3- 2(R- L)/3 = - (R-L) Shear force between A3 and B , VA3 = -(R- L)

Shear force at B, VB = VA3 + R2 –L2

= -(R- L) + R –L = 0

(b)To construct bending moment diagram (B.M.D. ):

Moment along clock-wise direction is assumed as +ve and anti clock-wise as –ve and from left to right i.e. from A to B. Bending moment between A and A1;

Taking section, X – X1 between A and A1. Hence, bending moment Mb = (R1- L1)*X Thus B.M. at A = 0, as X =0, and

B.M. at A 1 = (R1- L1)(W-2C)/2 , where X = (W-2C)/2 = (R- L)(W-2C)/2

Bending moment between A1 and A2

Taking section, X-X1 between A1 and A2

Bending moment (Mb) = (R1-L1)*X – L3*[X- (W-2C)/2]

Thus, M b at A2 = (R1- L1)*W/2 – L3*[W/2- (W-2C)/2], where X = W/2 Or, Mb = (R- L)*W/2 - 2/3(R-L)* C Or, Mb = (R- L) (W/2- 2/3C)

Bending moment between A2 and A3,

Taking section, X- X1 between A2 and A3,

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Bending moment (Mb) between A2 and A3 = (R1- L1)*X – L3 [ X - (W-2C)/2)] – L4(X-W/2)

B.M. at A3 = (R1- L1) (W/2+C) – L3 [W/2+C- (W-2C)/2] – L4(W/2+C- W/2),

Where, X = W/2+C,

Thus, B.M. at A3 = (R1- L1) (W/2+C) – L3(2C) – L4*C = (R- L) (W/2+C) – 2/3(R- L)*2C – 2/3(R- L)*C = (R- L) (W/2+C) – 2(R- L)*C

= (R- L) (W/2- C)

Bending moment between A3 and B; Taking section, X- X1 between A3 and B; Bending moment (Mb) betweenA3 and B = (R1- L1)*X–L3[X-(W-2C)/2]– L4(X-W/2)- L5[X-(W/2+C)]

Hence, B.M. at B = (R-L)*W – 2/3(R-L)[W- (W-2C)/2 +(W-W/2)+ (W-W/2-C)] = (R-L)*W – 2/3(R-L)[3W –3W/2] = (R- L)*W- 2/3(R- L)*3W/2 = (R- L)*W – (R- L)*W = 0Where, X = W, L3 = L4 = L5 = 2/3(R- L), R1 = R2 = R, and L1 = L2=L

Case study:

cross - Section of roof bar (200mm *200mm)

d = 200mm

b = 200mm

t2 = 7mm

t1 =10mm

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Let R1 = R2 = 15 tonne (setting load), L1 = L2 = 10.5 tonne (assumed),

Hence, L3 = L4 = L5 = 2/3(R1- L1) =3 tonne

W = 3.6mtr. and C = 0.8mtr.

Thickness of flange (t1) of the roof bar = 10mm,Thickness of web (t2) of the roof bar = 7mm, Width of flange (b) = 200mm, and Distance between two flanges (d) = 200mm

Max. Shear force at A and B = R1- L1

= 15- 10.5 = 4.5 tonne = 4.5*1000*9.81 N = 44.145KNMax, shear stress at A and B = 44.145KN/(18cm*0.7cm) = 3.504 KN/cm2 MAX. Bending moment at A2 = (R- L)(W/2- 2C/3) = (15 – 10.5) (3.6/2- 2*0.8/3) t-m = 6.5t-m = 3.8*1000*9.81*100 N-cm. = 3727.8KN-cm.Maximum bending moment in modified system is at only one point in centre, and lesser than the old system.

Maximum bending stress (α) in the flanges = Md/2I Where, M = maximum bending moment = 3727.8KN-cm. d = distance between two flanges = 20cm, and I = moment of inertia = 3953 cm4.

Thus, α = 3727.8*20/2*3953 KN/cm2 = 9.43 KN/cm2. Thus, if we compare the bending stresses, it is very less in modified system.

Compressive(direct)stress in the roof bar : Compressive force at both the ends of roof bar =10.5tonne = 10.5*9.81KN

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= 103.005 KN The compressive force is beared by web of the roof bar, so this compressive force is responsible for the failure of the web of the bar.

Hence, compressive stress = comp. force/area of web under compression = 103.005KN/ (width of wooden slippers * thickness of web)

=103.005KN/(10cm*0.7cm) = 14.715 KN/cm2

Comparison between loading parameters of old supporting system and modified supporting system;

Loading parameters

Old supporting system

Modified supporting system

Remarks

Max.shear stress in web of bar.

11.679 N/cm2 3.504 KN/cm2 Shear stress has reduced to 1/4th

Max. bending moment

17658 KN-cm 3727 KN/cm2 Max. B.M. in modified system is at only one point and about 1/5th, whereas in old system it is on many points

Max, bending stress(tensile or compressive) in flanges of the bar.

44.97 KN/cm2 9.43 KN/cm2 Max. bending stress in modified system is less than 1/4th of old system.

Compressive (direct) stress in web of bar

--------- 14.715 KN/cm2 There is no compressive stress in old supporting system.

Comparing both the systems, it can be inferred that the modified system is better in all respect, except one parameter i.e. compressive stress, which may cause the failure of web of the roof bar. This problem can be solved by changing design parameter of the roof bar and by strengthening the bar at

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its ends. We know that the shear stress and compressive stress are responsible for failure of web. Hence, design of the web should be changed for meeting required support resistance.

Failure of roof bar due to faulty design of roof bar:

The author has studied about the failure of roof bar in the BG- panel in GDK-NO. 10-Incline, which is only due to faulty design of roof bar. Roof bar used in early years was of 150mm * 150mm section, but the use of this roof bar is discontinued without any technical reason. Currently BG- panel is using 200mm*200mm girder of I – section. The thickness of web is about 7mm. It has become use and throw i.e. after using once, it is being thrown in scrap, because after failure of web there is no further use in supporting. The author has observed that ,using such type of roof bar is not only wastage of money , but also decreasing support resistance and increasing heap of scrap in the mine. In author’s view, old roof bar (150mm*150mm) is very purposeful and economical also. Only thing to be done is to rectify the faulty supporting system and proper strengthening of roof bar at its ends. The I- section 200mm*200mm roof bar will be useful only when strengthened properly at its ends.

Mode of failure of roof bar observed by the author:

GDK- 10 Incline is currently using 200mm*200mm I-section roof bar, which is failing in its web due to faulty design of roof bar and also due to improper strengthening at its ends. The web failure observed by the author is shown in figure given below.

Section of the failed roof bar (web failure )

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MODIFICATION IN DESIGN OF ROOF BAR SUGGESTED BY THE AUTHOR

In author’s view, the old type of roof bar ( 150mm*150mm ) is very economical and purposeful for the cost and salvaging point of view. Only weakness is in bending part, which can be rectified by using modified system of supporting. This weakness can also be rectified by changing the design of the roof bar, suggested by the author as mentioned below;

Design of web of roof bar:

Design of web is very important for resisting shear stress and compressive stress. When roof bar is tightened against the roof, the web is under compression. Therefore, the strength of web should be such that, it can bear a load upto designed strength of the O.C. props (about 30t). For the designing of web, two things are important. One is web thickness (t2) and another is its height (h).

So, if we increase the web thickness (t2), the strength of web will increase and, if we increase the height of web (h), the strength of web will decrease.

Section of web of the roof bar

The strength of web can be expressed mathematically in the following ways;

S α t2, and S α 1/h , so combining these two equations we get;

S α t2/h

Or, S =K*t2/h , where S = strength of web, K = proportionality constant, t2 = web thickness, and h = height of web.

t2

h

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P = Load on web (value of P Varies in between 10t and 30t)

Here, S should be greater than P, and for this the web shall be strengthened as shown in figure below.

The author hase observed that the value of web thickness (t2 ) should not be less than 10mm and distance between two flanges (d) not more than 150mm .

Therefore, minimum thickness of web (t2) = 10mm, and Maximum height of web (h) = (150- 2*10) mm = 130mm

Design of flanges of roof bar:

As the author has compared the loading parameters of old roof bars and new roof bar, the bending stresses are less in new type of the bar, which is because of its larger width of flange.

Design of flange of roof bar includes the design of flange thickness (t1) and width of flange (a).

Hence, minimum thickness of flange (t1) = 10mm , and Minimum width of flange (a) = 200mm.

Thickness of flange (t1)

b

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P (Load on Web)

P

t2

Final design sample of roof bar:

The author has done only thing in modified design, that the web thickness (t2) has been increased from 7mm to 10mm and distance between two flanges has been decreased from 200mm to 150mm.

Proper Strengthening of Roof Bar:

Strengthening of roof bar is very important and essential for the strata control point of view in Blasting Gallery method. Strata load is transferred vertically on the O.C. props through the roof bar at both ends. Capacity of the O.C. prop is 40tons, therefore roof bar should be capable to bear the load coming on the O.C. props. For this the roof bar is to be strengthened properly, otherwise the roof bar will yield prematurely at the ends and the support assembly will be ineffective. The scheme of proper strengthening of roof bar is shown in the figure given below:

Section of Roof Bar Longitudinal view of the Roof Bar

150mm

200mm

10mm

10 mm

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Section of the strengthened roof bar longitudinal view of the Strengthened roof bar

Web of the girder Edge of the web strengthened with pieces of C- channel (2``×4`` or 3``×6``) Plan view of the longitudinal section of the strengthened roof bar

Comparison of design parameters of different types of roof bars:

Design parameters Old type of roof bars

150mm*150mm 150mm*200mm

Currently using roof bar200mm*200mm

Modified roof bar200mm*150mm

Web thickness(t2) 9-10mm 7mm 7mm 10mm

Thickness of flange(t1)

10.5mm 10.5mm 10mm 10mm

Width of flange(b) 150mm 150mm 200mm 200mm

Distance between two flanges (d)

150mm 200mm 200mm 150mm

Moment of inertia(I) of bar

1714.28 cm4 3050.2 cm4 3953.53 cm4

2146.42cm4

Cross-sectional area of bar

4525mm2 4403mm2 5260mm2 5300mm2

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Cross-sectional area of web

1240mm2 1218mm2 1260mm2 1300mm2

Advantages of the modified supporting system and modified design of roof bar:

It eliminate the bending of roof bar, which can be re –utilized ; Strengthened roof bar can bear a minimum of 30t (compressive) load; fully utilization of strength of OC props, because props are tightened

against the roof and not to the roof bar ; support resistance offered by OC props are improved tremendously after

modification in supporting system and design of roof bar. Hence less chances of bed separation ;

rock loads are shared by the OC props, hence abutment pressure at side will be less which will minimize side spalling ;

props will be tightly intact with roof, therefore no chances of props dislodgment by hitting side spalled boulders ;

overriding of pillars and stooks will be reduced ; It will provide safe working conditions for men, machinery and property. Recently the 150mm x 150mm M.S.roof bar has been replaced by

200mm x 200mm roof bar, because of incapability of previous roof bar to bear rock load after getting yielded, which was only due to wrong supporting system. Hence, if the supporting system suggested by the authors is followed, the 150mm x 150mm roof bar will be adequate to bear rock load in BG panel and extra expenditure on supportmen & roof bar can be saved.

Conclusion:The authors have given valuable suggestion regarding supporting system in BG working . After applying the author’s suggestion, support resistance has been improved in BG working. The improvement in support resistance has decreased the chances of layer separation and over riding of pillars. It is very economical and purposeful. It can save about Rs. 50 Lacs per annum on purchase of roof bar.

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Declaration:

The above observations and comments are of author and not necessity to the organization.

Signature of author

Tikeshwar Mahto ) Date- Dy. Director of Mines Safety,

Bilaspur Region(India)tikeshwarmahto@yahoo.co.in

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