This document discusses techniques for blind backfilling of abandoned underground coal mines. It provides details on: 1) Common blind backfilling techniques including point support, pneumatic backfilling, hydro-pneumatic backfilling, and pumped slurry backfilling. 2) A gravity backfilling method developed by IIT Kharagpur that involves pumping a sand-water slurry by gravity through boreholes. 3) A field study applying the gravity method at an abandoned mine in India, monitoring the filled area using an underwater ROV camera. 4) Empirical relationships determined between filling parameters like area, flow rate, and sand concentration from both model and field studies.

1. Blind Backfilling of Goaf and Coal
Extraction below Built-up Areas /
Surface Structures.

2. • India’s Coal Production is dominated with OPENCAST MINING which
accounts for about 80% of total production
Mining is a process to extract valuable
minerals from the earth’s crust.
• Future Coal mining in India needs to emphasize coal Production from
Underground Mining
• Mining with Filling can increase Percentage of Extraction from
underground mines
• Underground Mining under Built-up Areas or Important Surface
Structures is possible with Mine Filling.

3. Blind Backfilling
• Old abandoned underground coal mines have left the
workings unapproachable.
• Surface instability and ground subsidence above such
workings has become cause of concern for the people
residing in this area.
• Conditions are acute in some parts in Ranigunj and
Jharia Coalfields.
• There is a need
(i) To develop a suitable cost efficient
backfilling method.

4. Trough Subsidence
Usually occurs over goaf of Longwall Workings

5. Stages of
Failure of
Pillars leading
to Subsidence
Pot Hole Subsidence
Usually occurs over old
workings worked by
Bord and Pillar Method

6. Common Blind Backfilling Techniques
Broadly classified into two types
(A) Point Support Technique:
 Support a relatively small area
 Use of cement grouted columns
Point Support

7. (B) Area-wide Blind Backfilling techniques
This is of three types:
(i)Pneumatic Backfilling
• Filled material carried underground
pneumatically
• Thrown all around the inlet hole
• Good for dry mines
• Packing area from one borehole is small

8. (ii) Hydro - Pneumatic Backfilling
• Mixture of solids and water is gravity fed
through borehole
• Air is simultaneously fed through pipe
placed concentrically to the feeder pipe
• Solids used:
sand, fly ash, small sized gravel/crushed
rock or washery rejects.
• Pipe diameter: 150 to 250 mm

9. Hydro-Pneumatic Method

10. Pumped Slurry Backfilling
 Mixture of solids and water is
pumped through borehole at high
pressures
 Large capacity pumps with high flow
are used
 Solid concentration is 10 to 20% by
weight
 Pipe diameters: 150 to 355 mm.

11. Compressed-
air PipeBromoform
Manometer
Mixing
Tank
Water
Pipe
Water
Flowmeter
Overflow Water
Tank
Overflow Water
Tank
Sand
Chamber
Slurry Injection
Pipe
Overflow
Pipes
Slurry
Pump
Water Pipe for slurry Mixing
Emergency
Water Line for
Jam Clearing
Compressor
Experiments on Slurry Pumping

12. The Gravity Backfilling Method
Developed and Proposed by Prof. S. K. Pal of IIT Kharagpur
after detailed Laboratory and Field Experiments
JAMMING PROBLEM
• Commonly jamming of feeder borehole, is sudden
and unpredictable
• Thus, it is difficult to plan for the location of the
next borehole in advance
COMMON DIFFICULTIES encountered in all Blind
Backfilling Process :

13. The schematic diagram of the experimental setup
for gravity blind backfilling

14. Features and Drawbacks
1. Slurry pumping method
 Superior method
 Quick and wider area filling at high slurry flow rate
Pumps - imported & costly
• High maintenance cost
2. The hydro-pneumatic method
 Less capital intensive
 Cost efficient
 Spread towards rise direction is more
• Filling rate comparatively slower
• Air injection may sometimes cause ground cracks

15. So evaluation of a pre-jamming indication
parameter is necessary
Pre-jamming indication parameter would certainly
facilitate a properly planned filling work
Benefit of blind backfilling
Maximization of the filling extent from a
single borehole by methodical reduction in sand
concentration as filling progress, thereby,
reduction of the overall filling duration

16. The Project Site
Abandoned Krishnanagar
Colliery of ECL, has been
selected to verify the
findings of laboratory
scale model study
BLIND BACKFILLING by Gravity Backfilling Technique

17. Drilling of Boreholes
Fig. 5.3 The underground map of Krishnanagar Colliery marking the
positions of different types of boreholes

18. The Movable Blind
Backfilling Laboratory
Fig. 5.12 Photograph of the caravan, inlet water pipelines and the overhead tank
5000 litre
overhead tank
for hole-flushing
in case of power
failure

19. Two Submercible Pumps of 600 gpm capacity were installed
in the two boreholes drilled outside the area to be filled up
Wireless
antenna
Fig. 5.6(b) Main switches and
starters
Fig. 5.6(a) Pump room with pump
outlets

20. Sand Feeding Arrangement
SAND STORAGE – SAND BUNKER – BUCKET ELEVATOR –
MIXING VAT
Sand Bunker
Bucket
Elevator
Fig. 5.10 Photograph of sand bunker feeding sand to bucket elevator

21. Movable Backfilling Laboratory

22. Continuous Data Recording System
Fig. 5.16 A close-up view of data logger during data recording
Continuous graph of piezo head with time

23. Air Movement during filling Operation
The entrained air from the inlet hole moves along the roof of
the underground roadway and produce an effusive release
from the nearest air-release hole.
Fig. 6.2(a) Air bubbles moving along
underground mine roof
Fig. 6.2(b) Effusive release of air
bubbles and water

24. Details of sand-filling through
different Mother( Inlet) Boreholes
Sl.
No.
Borehole No.
Sand
deposited
(m3)
Cumulative amount of
sand deposited (m3)
Remarks
1. 17 6689 6689 Unhindered filling
2. 23 1427 8116
Hindered filling due to
stoppings
3. 28 6236 14352 Unhindered filling
4. 29 1544 15896
Hindered filling due to
stoppings
5. 27 4223 20119 Unhindered filling
6. 3 1357 21476
Incomplete filling (no
jamming)

25. FILLING through
Boreholes
Fig. 5.3 The underground map of Krishnanagar Colliery marking the
positions of different types of boreholes
Sl.
No
.
Borehol
e No.
Sand
deposit
ed (m3)
Cumulative
amount of
sand
deposited
(m3)
Remarks
1. 17 6689 6689 Unhindered filling
2. 23 1427 8116
Hindered filling
due to stoppings
3. 27 6236 14352 Unhindered filling
4. 29 1544 15896
Hindered filling
due to stoppings
5. 28 4223 20119 Unhindered filling
6. 3 1357 21476
Incomplete filling
(no jamming)
1
2
5
4
3
6

26. Monitoring of filled-up area
Position of filled-up sand bed at the end of
Filling through the first mother borehole
Shape of filled-up sand bed at the end of
Filling in the MODEL STUDY
6,689 m3

27. Monitoring of filled-up area
Position of filled-up sand bed at the end of
filling through the second mother borehole
Position of filled-up sand bed at the end of
filling through the third mother borehole
2
1
4 3
9,660 m3
15,896 m3

28. Monitoring of filled-up area
Position of filled-up sand bed at the end of
filling through the fifth mother borehole
Position of filled-up sand bed at the end of
project when filling through the fifth
mother borehole was continuing.
5 6
20,119 m3 21,476 m3

29. Monitoring of Filled-up Area in the
Field Study
Fig. 6.7 ROV camera system with float block removed for clear
viewing of propellers
Control panel for ROV camera system contro
panel

30. Monitoring of filled-up area
Position of filled-up sand bed after the first
stage of filling through the first mother borehole
Position of filled-up sand bed after the second
stage of filling through the first mother borehole

31. Monitoring of filled-up area
Position of filled-up sand bed after the fourth
stage of filling through the first mother borehole
Position of filled-up sand bed after the seventh
stage of filling through the first mother borehole

32. Monitoring of filled-up area

33. Variation of maximum sand throughput
‘Vs’ with sand concentration ‘C’
in the model study
Vs = 227.88e-0.1092C
Vs = 408.35e-0.1232C
Vs = 404.39e-0.0944C
Vs = 551.57e-0.1133C
Vs = 604.41e-0.0822C
0
50
100
150
200
250
300
350
400
0 5 10 15 20 25
Sand Concentration C (%)
MaximumSandThroughputVs(litres)
Q=15lpm
Q=20lpm
Q=25lpm
Q=30lpm
Q=35lpm
in the Field study

34. Relative spreads of sand in model
study and field study
Model
inclination
Multiple
regression type
Regression
coefficient (R2)
Empirical relationships Remarks
7.50 Power 0.936
Model Study
3.50 Power 0.897
4.80 Power 0.987 Field Study
149.1378.00603.0
** SU LLeA 
752.1753.0537.0
** SU LLeA 

764.0509.0356.2
** SU LLeA 
LU
LS
An approximation of the shape of sand filled area

35. Variation of Area of the filled-up
portion ‘A’ with Slurry Flow Rate ‘Q’
and Sand Concentration ‘C’
Model
inclina
tion
Multiple
regression
type
Variables
involved
Regression
coefficient
(R2
)
Empirical
relationships
Remarks
7.50
Power A, Q, C 0.785
8.0294.1
2236.0 
 CQA
Model study
3.50
Power A, Q, C 0.837
23.1234.1
6832.0 
 CQA
4.80
Power A, Q, C 0.967
02.2313.1
041.1 
 CQA Field study
A = Area of the filled out Area
Q = Slurry Flow Rate
C = Sand Concentration

36. Pressure-signature Analysis
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
0 2000 4000 6000 8000 10000 12000
Time(s)
Pressure(Pa)
Jamming
General pattern of pressure signature during laboratory experiments
in the LABORATORY

37. CONCLUSIONS
 Filling by Simple Gravity Backfilling Method is
an efficient method of backfilling, where filling
may start with 15 % sand concentration and
then gradually be reduced to 9 % or less, as
and when the pre jamming indication arises.
 Under Favourable Geo-mining conditions about
6000 m3 of sand can be delivered through a
single mother borehole.
 Sand packing in the filled-up areas occurs
tightly up to the roof as monitored by the
Underwater ROV Camera.
 The shape of the filled-up area is very similar to
that obtained in the model study.

38. CONCLUSIONS
 Different empirical relationships obtained in Field study are
also similar to those obtained in the Model study, but the
values of the constants are different due to the reasons
mentioned earlier.
 Similar to model study the Standard deviation ratio in the
pressure signature curve may be used an indicator for pre-
jamming condition, but its magnitude should be increased
from 4 to 20.
This technique being very simple yet efficient, may be used in
future for filling existing voids in water-filled abandoned
mines with proper scientific monitoring arrangements in order
to reap the benefits of the present study.

39. Hydraulic Stowing or Hydraulic Filling
Problem at Mosboni Mine : Although High Head is available
• Slurry flow is pulsating and
• Frequent jamming and Pipe-Joint Breakage

40. Critical Velocity vc For Slurry Flow
Slurry Density and Concentration
Durand’s Equation
Prof. A. P. Ujfin’s Empirical Equation

41. Actual Flow Velocity with Recommended Flow
Velocity for 3”, 4”and 4.5” Pipe Diameters

42. Frictional Pressure Loss

47. Schematic details of stowing pipelines
Route C
Route D
Route B
i. e., Geometric Profile
Hydraulic Profile is drawn by using the
Pressure loss data to find equivalent
length of the horizontal pipeline that
would cause the same pressure loss as
that occurs in that part of pipeline.
Hydraulic Gradient Line is drawn by joining the feed point and the discharge point of each route

48. Route B

49. Route C

50. Route D

51. Some Concepts of Paste Backfilling
Paste Backfilling would be another suitable technique for
filling of active mines under built-up Area

52. Characteristics of Fill with respect to Concrete
Young’s
Modulus in
MPa
Compaction in %
Concret 3000-9000 0
Cemented fill 300-6000 3-5
Hydraulic fill 10-60 10-20
Uncemented rockfill 10-30 15-50
e

54. UCS with binder content
Effect of water on strength generation

55. Hardening Process
Water to cement ratio = 7

57. Filling Area

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