it shows the, what are the required preventive measures to minimize damage due to subsidence while working

1. SUBSIDENCE CONTROL IN
COAL MINES
By B.SUNIL FROM JNTUHCEM

2. INTRODUCTION
 Any deformation occurred above the surface
due to underground excavations is called
SUBSIDENCE.
 SUBSIDENCE due to underground coal mines
mainly categorized into two forms
 Pothole subsidence
 Trough subsidence

4. INTRODUCTION
 POTHOLE SUBSIDENCE is caused due to
the sudden collapse of overburden into the
underground voids.
 It is very hazardous to life as it occurs without
any prior indications.
 TROUGH SUBSIDENCE is occurred over a
larger area which is mainly depend on the
depth of the extraction.

5. INTRODUCTION
 Due to subsidence surface structures are
greatly disturbed and damaged.
 So it is necessary to take control measures
while working in underground to minimize the
subsidence.
 The main objective of this presentation to
show what are the control measures to be
taken while working.

7. SUBSIDENCE CONTROL
MEASURES
 PLANE FITTING METHOD
 TRENCHING
 TENSION CABLE METHOD
 HYDRAULIC SAND STOWING
 PARTIAL EXTRACTION METHODS

8. PLANE FITTING METHOD
 Surface structures undergo curvature and
twisting due to subsidence. so to minimize
strain due to twisting force, plane is
considered on which structure is held .
 Under this plane height adjustment devices
are placed which maintain inclination with time
dependant along the line of subsidence.
 No of device are necessary given by formula

13. PLANE FITTING METHOD
 N=1.5xW/J
 N= no of devices
 W= weight of the structure
 J= capacity of jacks
 jacks placed at interval not exceeding 3 m in
for heavy structure jacks are placed densely.

14. TRENCHING
 In this method a trench is made around the
house such that the compressive forces on the
wall and floor are reduced.
 The dimensions of the trench to be made are
0.3m wide, 0.9 to 2 m from the wall, and 0.15
to 0.2 extended below the foundation.
 Wider the trench has been made greater the
compressive force can be reduced.

16. TRENCHING

17. TENSION CABLE
 Tension cable is used to reduce damage due
to the twisting force.
 In this cable is used having high compressive
strength and low tensile strength.
 For proper design tension in the cable must be
checked before.

20. HYDRAULIC SAND STOWING
 Hydraulic stowing is the best method for
controlling the subsidence.
 In India HSS play vital role in subsidence
control.
 Subsidence of the stowed panel is below 5%
in Indian coal mines.
 Subsidence may be increased by delay of
stowing.

21. HYDRAULIC SAND STOWING
 As the stowing takes much time the immediate
rock should have minimum strength to hold the
load for a span of time.
 In India mostly the roof is sand stone so it is
efficient to go with sand stowing.
 But in foreign countries the roof is easily caved
so the subsidence in stowed panel ranges
from 5% to 35%.

22. PARTIAL EXTRACTION
 Partial extraction is done to minimize both the
subsidence and strain such that surface
structures are fully protected from the damage.
 These are again classified into
 Non effective width extraction
 Chess board method
 Wide and stall method
 Splitting of pillar with stowing
 Harmonic mining

23. NON EFFECTIVE WIDTH
EXTRACTION
 Non effective width of extraction is
underground width of extraction which does
not cause any subsidence above ground.
 As opening made in the seam stress are
redistributed and pressure arch is formed,
the height of the dome is depend up on the
width of opening.
 As the width of extraction increases the
dome touches the surface and the movement
of surface starts.

25. NON EFFECTIVE WIDTH
EXTRACTION
 The maximum width of extraction in a seam
without any movement above the surface is
called NON EFFECTIVE WIDTH.
 This expressed in depth of cover, in Indian
mines NEW is 0.3 to 1.17 times of cover.
 The pillars are to be extracted in such a way
that at any time width does not exceed NEW.
 Pillars in rows are left alternatively maintaining
factor of safety 2.

26. CHESS BOARD METHOD
 When NEW method is not possible chess
board method is done.
 In this alternative pillars are extracted in a
chess board pattern.
 By this method extraction up to 60 to 65% is
possible.
 The safety factor of the pillars left should not
be less than 2.

28. WIDE AND STALL METHOD
 When the depth of cover is moderate, the
recovery of coal is very low because more coal
has to be left to minimize the subsidence.
 So to get more recovery compared to the
conventional method wide and stall method is
used.
 In this method gallery size is maintained to
match the strength of rock and width and
height ratio of pillar increased to stabilize the
condition
 The galleries are widened up to which the
pillar can bear the load to improve the

30. SPLITTING OF PILLAR WITH
STOWING
 The DGMS does not permit to split the pillar
below structures by caving.
 But the splitting can be done with stowing.
 The size splits depend on the strength of
pillars.
 The safety factor of the pillars is to be 1 to
maintain stability up to the time of completion
of stowing.

32. HARMONIC MINING
 Extraction of panel causes tensional and
compressive strain above the surface.
 To overcome the strain counter strain to be
acted so this can achieved by extraction of two
seams simultaneously in particular manner.
 Strain due first working is balanced by the
strain induced in the another working.
 This method of mining is known as
HARMONIC MINING.

34. PROUD TO BE
A
MINING
ENGINEER