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Design of PillarBy

Abdullah

Abdullahktk@aol.com

Mob# +92341-4164951

DEPARTMENT OF MINING ENGINEERING

UNIVERSITY OF ENGINEERING AND TECHNOLOGY,

PESHAWAR

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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

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 ofan 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.4Design of pillar

Scale effect

Rock mass is large in size and volume comprising of weak zones orgeological discontinuities therefore laboratory value is not the actualrepresentative 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.

𝜎1= k ∕ ℎ

k = 𝜎c 𝐷

Where, 𝜎1 = Strength of rock mass

h = Height of pillar

𝜎c = UCS of specimen tested in laboratory

D = Diameter or size of specimen5Design of pillar

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 andtime consuming.

Various investigators from different countries of the worldperformed in-situ tests and then they proposed different expressionsfor 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.

6Design of pillar

1. Obert-Duvall/Wang formula

𝜎p = 𝜎1 (0.778 + 0.222 𝑤 ℎ)

where,

𝜎p = Pillar strength

𝜎1 = UCS of cubical specimen( w/h=1 )

w = Pillar width

h = Pillar height

7Design of pillar

Holland-Gaddy formula

𝜎p = k 𝑤/ℎ

Where,

𝜎p = Pillar strength in psi

k = Gaddy factor = 𝜎c 𝐷

w = Pillar width in inches

h = Pillar height in inches

8Design of pillar

Holland formula

𝜎p = 𝜎1 𝑤

ℎ

Where,

𝜎p = Pillar strength

𝜎1 = Strength of cubical pillar( w=h=1) = k / 36

k = 𝜎c 𝐷

w = Pillar width

h = Pillar height

D = Size of specimen

9Design of pillar

Salamon-Munro formula

𝜎p = 1320 𝑤0.46/ ℎ0.66

Where,

𝜎p = Pillar strength in psi

w = Pillar width in ft

h = Pillar width in ft

10Design of pillar

Bieniawski formula

𝜎p = 𝜎1 ( 0.64 + 0.36 𝑤 ℎ)

Where,

𝜎p = Pillar strength

𝜎1 = strength of cubical specimen of critical size or greater

i.e about 1 meter

w = Pillar width

h = Pillar height

11Design of pillar

Pillar Load Pillar load (the average stress on pillar) is determined on the bases of

tributary area approach.

𝑠𝑝 = 1.1H (𝑤+𝐵

𝑤) (

𝑤+𝐿

𝐿)

Where,

𝑠𝑝= Pillar load in psi

H = Depth below ground surface

w = Pillar width

L = Pillar length

B = Entry span

12Design of pillar

Factor of SafetyF.O.S = 𝜎p 𝑠𝑝

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 − (𝑤

𝑤+𝐵)2

Where,

e = Extraction ratio

w = Pillar width

B = Entry span

13Design of pillar

Design Procedure The following step by step procedure was recommended by

Bieniawski (1983) for planning new room and pillar mining or otherengineering 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 𝜎c

• Spacing of geologic discontinuities

• Condition of geologic discontinuities

• Orientation of geologic discontinuities

• Ground water condition

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Step-2 Determine the rock mass quality for roof rock and select the roof

span B.

Step-3 Based on UCS (𝜎c ) of coal determine the value of k for pillar locality.

k = 𝜎c 𝐷

Step-4 Select the pillar strength formula to estimate the pillar width w for a

known seam height h.𝜎p = 𝜎1 ( 0.64 + 0.36 𝑤 ℎ)

Step-5 Determine pillar load(average stress on pillar) based on tributary area

approach.

𝑠𝑝 = 1.1H (𝑤+𝐵

𝑤) (

𝑤+𝐿

𝐿)

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Step-6 Select a factor of safety F ( usually ranging from 1.3 to 2 ) and equate

𝜎p𝐹

= 𝑠𝑝 and solve this for w.

Step-7 For economic consideration, check whether the percentage

extraction is acceptable for economic mining.

e = 1 − (𝑤

𝑤+𝐵)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 = 𝜎p 𝑠𝑝 > 1.3

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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 mineplanning.

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Reference Strata control in mineral engineering book by Z.T. Bieniawski

Coal mine ground control book by SYD S Peng

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THANKS

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ANY QUESTION

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