Engineering Rock Mechanics

392

60.0 -

• Pillar compressive strength


• 
  
  
  
  • 
    
    
    
    • Extraction ratio
    
    
    
    
  
  
  
  

.- ----.-... - .....---.__

• h
  0.0 "'1"'1"'1"'1"'1"111.11.1.111.1..11,,,r

We can see from this that with F = 2 the minimal pillar width for an
opening of 5 m is about 14 m, and hence the maximal extraction ratio is
2
(200 + wp) - wp' (5 + 14)2 -^142 165
(wo + WP)' (5 + 14)* 36 1

• 
  
  
  
  • =- = 0.46.
    (20.11)
    (c) If a room can be increased to 8 m wide, then the graph above shows
    that the pillar width will need to increase to about 22 m. The extraction
    ratio in this case is
  
  
  
  

1 .oo

-- 0.90

~~ 0.80

-- 0.70 .g

~- 0.60 E

-- 0.50 'g

-- 0.40

~~ 0.30

~- 0.20

-- 0.10

0.00

0

l3

420.8 In an attempt to improve the profitability of the mine in

420.7, the possibility of reducing both the opening width and the

pillar size is to be investigated. Plot the curve of extraction ratio

against opening width, for openings in the range 0.5 m to 4.0 m, and

hence determine the optimal opening width and the corresponding

extraction ratio.

If the extraction ratio thus identified is to be kept, what value of

the factor of safety is required if the opening width is to be changed

to 3.5 m?

A20.8 As the diagram below shows, the extraction ratio will be better

than about 0.85 if the opening width can be kept below about 2.5 m.

This is a result of the significant reduction in pillar strength with size.

An opening width of 2.5 m will require a pillar width of about 2 m but,

as the plot immediately above shows, at these sizes the pillar strength is

variable and so they are best avoided.