The document discusses subsidence control in coal mines, highlighting two main types of subsidence: pothole and trough subsidence, and their impacts on surface structures. Various control measures are detailed, including plane fitting, trenching, tension cables, hydraulic sand stowing, and partial extraction methods, each aimed at minimizing subsidence and protecting surface infrastructures. The effectiveness of these methods, particularly hydraulic sand stowing, is emphasized, with specific strategies for extraction that balance coal recovery and surface stability.

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