for Mining Engineers

1. Natural
Supports
Design of pillars
1

2. Classification of pillars
1. Square and rectangular pillars
2. Long-wall pillars
3. Irregular pillars
4. Rib and crown pillars
5. Safety and barrier pillars
2

3. 1. Square and Rectangular Pillars
 These are integral part of room and pillar mining.
 Design is based on uniaxial stresses from supper
incumbent load.
 Common in flat dipping mineral deposit.
 Where the horizontal span exceeds the critical span,
 Vein/seam allows one pass mining and
 Overlying strata can not support itself without
strong pillars.
 Application include coal, potash, uranium ( Canada),
copper (USA).
3

4. 1. Square and Rectangular Pillars
 Mining is carried out in two phases.
i. Rooms are mined in advance sequence
ii. Pillars are mined in all or partially in retreat
sequence.
o Fill may be used or the back is caved.
o Average pillar stresses are estimated on the bases of
o Supper incumbent loading and
o Pillar strength on the avg. rock compressive
strengths.
o Safety factor between 4 to 6 is considered adequate.
4

5. 2. Long Pillars
 Integral part of slot mining—a variant of room
and pillar mining method.
 Used in gently dipping ore bodies.
 Also called stope and pillar mining in Canada.
 Room and pillar slot mining in Australia.
 If ore is too thick to be mined in one pass,
 Ore is extracted by benching.
 Roof span and pillar design is similar to that of room
and pillar design.
 Pillars either fail as a result of rock burst or
progressive slabbing at the pillar sides.
5

6. 3. Irregular Pillars
 Common to old methods such as breast
stoping.
 Pillars are randomly placed.
 Design follows the principal of room and
rectangular pillars.
 Locations are dependent on ore grade.
 Pillars are restricted to low grade and areas of
bad ground.
6

7. 4. Rib and crown pillars
 These structure provide support in manner
similar to room and pillar operations.
 Geometry is different, since
 Pillars are common to steeply dipping ore bodies
where open fill methods are used.
 For example cut and fill method.
 Rib and crown form the skeleton around the stope
 Secondary recovery is common as not more than
50% extraction is possible in primary mining.
7

8. Rib and crown pillars
 Stopes are filled before secondary mining.
 Rib and crown support can be approximated
as a series of columns supporting a panel.
 Pillar load depend on its position relative to
the center of the panel where maximum
deflection occurs.
8

9. 5. Protective pillars
 These pillars protect----
 Surface and underground structures due to pillar
failure.
 They can be classified as
a. Barrier pillar:
 Protect the active mining area from the adverse
effects of nearby mining.
 Normally highly stressed.
 Stresses extend to hanging and footwall rocks.
 Be careful while designing nearby openings such as
haulage drifts.
9

10. Rib and Crown pillars
b. Safety pillars:
 Solid ore blocks left intact beneath surface
structures to protect the structures from
subsidence.
 Size is determined on the basis of allowable
surface displacement for the of structure
overhead.
c. Shaft pillars:
 Protect shafts.
 Their design should take into account ground
movement induced by nearby mining.
10

11. 11

12. 12

13. 13