Dedicated to my best teacher Dr.Safi Ur Rehman

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2. Design of Pillar
By
Abdullah
Abdullahktk@aol.com
Mob# +92341-4164951
DEPARTMENT OF MINING ENGINEERING
UNIVERSITY OF ENGINEERING AND TECHNOLOGY,
PESHAWAR
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3. Contents
 Pillar and design
 Laboratory test
 Scale effect
 In-situ test
 Different expressions for strength of pillar
 Pillar load
 Factor of safety
 Extraction ratio
 Design procedure
Design of pillar 3

4. Design of pillar
 Pillar is the portion of rock mass left in place to support opening .
 Design is the creation of a plane or convention for the construction of
an object, structure, system, machine etc.
 To design a pillar strength of rock mass or coal is determined.
 The strength of rock mass may be determined by,
• Laboratory Test
• In-situTest
Laboratory Test
 To determine the strength in laboratory, at least ten specimens are
taken.
 Specimen may be cylindrical or cubical.
 UCS of specimen is determined by universal testing machine(UTM).
 Laboratory value is not actual representative of rock mass.
Design of pillar 4

5. Scale effect
 Rock mass is large in size and volume comprising of weak zones or
geological discontinuities therefore laboratory value is not the actual
representative of rock mass, this is called scale effect.
 UCS decreases with increase of size.
 The laboratory value must be scale downed in order to make it
representative of rock mass.
 Most common approach for scaling the laboratory value to field value
is the following.
휎1= k ∕ ℎ
k = 휎c 퐷
Where, 휎1 = Strength of rock mass
h = Height of pillar
휎c = UCS of specimen tested in laboratory
D = Diameter or size of specimen
Design of pillar 5

6. In-situ Test
 There is a system and mechanism for in-situ test in order to
determine strength of rock mass or coal, but it is very expensive and
time consuming.
 Various investigators from different countries of the world
performed in-situ tests and then they proposed different expressions
for strength of pillars in order to design pillars.
 The most important and commonly used expressions are of Obert-
Duvall/Wang formula, Holland-Gaddy formula, Holland formula,
Salamon-Munro formula and Bieniawski formula.
Design of pillar 6

7. 1. Obert-Duvall/Wang formula
휎p = 휎1 (0.778 + 0.222푤 ℎ)
where,
휎p = Pillar strength
휎1 = UCS of cubical specimen( w/h=1 )
w = Pillar width
h = Pillar height
Design of pillar 7

8. Holland-Gaddy formula
휎p = k 푤/ℎ
Where,
휎p = Pillar strength in psi
k = Gaddy factor = 휎c 퐷
w = Pillar width in inches
h = Pillar height in inches
Design of pillar 8

9. Holland formula
휎p = 휎1
푤
ℎ
Where,
휎p = Pillar strength
휎1 = Strength of cubical pillar( w=h=1) = k / 36
k = 휎c 퐷
w = Pillar width
h = Pillar height
D = Size of specimen
Design of pillar 9

10. Salamon-Munro formula
휎p = 1320 푤0.46/ ℎ0.66
Where,
휎p = Pillar strength in psi
w = Pillar width in ft
h = Pillar width in ft
Design of pillar 10

11. Bieniawski formula
휎p = 휎1 ( 0.64 + 0.36푤 ℎ)
Where,
휎p = Pillar strength
휎1 = strength of cubical specimen of critical size or greater
i.e about 1 meter
w = Pillar width
h = Pillar height
Design of pillar 11

12. Pillar Load
 Pillar load (the average stress on pillar) is determined on the bases of
tributary area approach.
푠푝 = 1.1H (
푤+퐵
푤
) (
푤+퐿
퐿
)
Where,
푠푝= Pillar load in psi
H = Depth below ground surface
w = Pillar width
L = Pillar length
B = Entry span
Design of pillar 12

13. Factor of Safety
F.O.S = 휎 p 푠푝
 Factor of safety should be between 1.3 to 2.
Extraction Ratio
 The ratio of mined area to unmined area is called extraction ratio.
 It is represented by e and given as,
e = 1 − (
푤
푤+퐵
)
2
Where,
e = Extraction ratio
w = Pillar width
B = Entry span
Design of pillar 13

14. Design Procedure
 The following step by step procedure was recommended by
Bieniawski (1983) for planning new room and pillar mining or other
engineering practices including long wall mining.
Step-1
 From geological data, borehole logs and rock and coal specimen
tabulate the following,
• UCS of roof rock and coal i.e 휎c
• Spacing of geologic discontinuities
• Condition of geologic discontinuities
• Orientation of geologic discontinuities
• Ground water condition
Design of pillar 14

15. Step-2
 Determine the rock mass quality for roof rock and select the roof
span B.
Step-3
 Based on UCS (휎c ) of coal determine the value of k for pillar locality.
k = 휎c 퐷
Step-4
 Select the pillar strength formula to estimate the pillar width w for a
known seam height h.
휎p = 휎1 ( 0.64 + 0.36푤 ℎ)
Step-5
 Determine pillar load(average stress on pillar) based on tributary area
approach.
푠푝 = 1.1H (
푤+퐵
푤
) (
푤+퐿
퐿
)
Design of pillar 15

16. Step-6
 Select a factor of safety F ( usually ranging from 1.3 to 2 ) and equate
휎p
퐹
= 푠푝 and solve this for w.
Step-7
 For economic consideration, check whether the percentage
extraction is acceptable for economic mining.
e = 1 − (
푤
푤+퐵
)
2
Step-8
 If the %age extraction is not acceptable and need to be increased by
decreasing the pillar width w, select from step-7 a pillar width which
would give the require coal extraction and determine whether this is
acceptable for mine stability.
F.O.S = 휎 p 푠푝 > 1.3
Design of pillar 16

17. Step-9
 Check the results by Obert-Duvall formula, Holland-Gaddy formula,
Holland formula and Salamon-Munro formula.
Step-10
 Exercise engineering judgment, by considering a range of mining and
geological parameters, to asses the various options for mine
planning.
Design of pillar 17

18. Reference
 Strata control in mineral engineering book by Z.T. Bieniawski
 Coal mine ground control book by SYD S Peng
 Google
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19. THANKS
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20. ANY QUESTION
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