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SURFACE SUBSIDENCE HAZARD EXERCISE
You are working as an engineering geologist for a geotechnical engineering consultant
specialized in problems of surface subsidence due to mining.

SURFACE SUBSIDENCE HAZARD EXERCISE
You are working as an engineering geologist for a geotechnical engineering consultant
specialized in problems of surface subsidence due to mining.
In the area of the map abandoned mine workings are present in two coal seams. One of
the seams is about 1m, the other is 2m thick. Mining was done with partial extraction in
room and pillar workings.
It is supposed that areas with less than 10m of rock above a coal seam have risk of
surface subsidence.
13 drill holes have been made.
1. Which type of drilling technique would you have recommended for drilling the 13
boreholes?
2. What is the stratigraphic sequence of layers found with the data of the 13
boreholes?
3. Make a cross-sectional sketch to illustrate the definition of safe and unsafe
conditions
4. Draw the bedrock topography contour lines on the map*
5. Determine the dip angle and dip direction of the coal seams of 1 and 2 meter
thickness and indicate which assumption you made on the geometry of the
layers*
6. Divide the map into “safe” and “unsafe” zones, using the risk evaluation
prescription mentioned above*.
7. Give a critical judgment of the accuracy of your result
8. In the area within boreholes 1, 9 and 10 somewhere a 400m deep, 3m diameter
abandoned mine shaft is present. Describe the hazards due to the presence of
this shaft with respect to the building of houses on the surface.
9. Suggest a program of site investigation to locate the position of the shaft. Among
other things, indicate the geophysical techniques which may be used. Explain the
properties you want to discover and which may lead to the deduction of the
location of the shaft.
10. Describe any hazard mitigating measures.
* Draw your constructions clearly on the map or using separate sheets if necessary.
Borehole records
Boundaries of layers found in the boreholes are given as meters above sea level.
Bh. 1
76- 67
67-40
Clay
Sandstone
Bh. 2
22-15
15-14
14-12
12-5
Bh. 7
63-50
50-49
Mudstone
Void
Clay
Shale
Coal
Sandstone
Bh. 8
102-85
85-76
76- 74
Clay
Shale
Coal
Bh. 3
29-24
24-4
4-2
Clay
Shale
Coal
Bh. 9
113-84
84-50
Clay
Sandstone
Bh. 4
53-45
45-30
30-28
28-10
Clay
Shale
Coal
Sandstone
Bh. 10
76-58
58-30
Clay
Sandstone
Bh. 5
43-40
40-39
39-10
10-8
Mudstone
Coal
Shale
Void
Bh. 11
43-31
31-10
Clay
Sandstone
Bh. 6
24-23
23-0
0- -1
Bh. 12
19-9
9-3
3-1
Clay
Shale
Coal
Clay
Mudstone
Coal
Bh. 13
54-46
46-30
Clay
Sandstone
Data 2: 1:5000 Topographic map
85-
contours 10m intervals, grid NS/EW at 100m intervals and borehole locations
Next page
1100
1000
0
10
0
900
20
10
800
30
20
12
6
3
700 30
40
600
50
50
11
5
13
60
70
7
400
80
1
70
300
90
10
100
80
110
200
90
9
8
100
0
60
4
500
100
40
2
120
110
120
0
100
200
300
400
500
600
700
Correction
1. Percussion drilling and analysis of cuttings, cheap and quick but less accurate or
rotary core drilling
2. The stratigraphical sequence is clay/mudstone/coal/shale/coal/sandstone
3. Sketch: section with dipping seam of constant thickness, rock overburden less
and above 10m, and 10m high caving zones
4. Recognise the zone where the rock outcrops and play around with the bedrock
topography in order to have a convincing distribution. The bedrock head map has
to be drawn because:
a. the rock does not outcrop except in the zone of BH 5, (6) and 7 and
b. a zone is assumed to be safe if the seam cover is more than 10m.
In the area of boreholes BH5, 6 and 7, the bedrock topography is the same as
the surface topography.
The bedrock contours can be drawn at 10 m intervals.
5. The thin seam is pierced by BH 5, 6 and 7 (1m thick void). This is enough info to
obtain the strike and dip of this seam using the 3 points construction. The thicker
seam is pierced with boreholes 2, 3 ,4 and 8 so that the structural contours of this
seam can also be drawn. 2 constructions are needed because the seam lie
underneath/above each other.
When you use the 3-points construction, you assume that the layers are not
folded.
1m coal seam: 15/315 – 2m coal seam: 12/329 (dip/dip direction)- check
6. The hazard areas are determined by intersecting strike lines and bedrock
topography contours.
A hazard zone is a zone where the vertical distance between the surface
topography and the top of a seam is between 0 to 10m => draw:
a. delineation related to bottom of seam xm structural contact with x m
bedrock contour (0m)
b. delineation related to bottom of seam, xm structural contour with x+10m
bedrock contour
Note that the seams have slightly different strikes.
Also include the triangle between BH 1, 9 and 10 (shaft) in the hazard zones.
Show legend and north direction
7. Accuracy depends very much on bedrock contour lines. For ex, BH 11 is not in
the danger zone for the 2m thick seam; this depends on the accuracy of the
bedrock map.
8. If you knew where the shaft was, you would not built on it except if expensive
security measures would have been taken against risk of collapse, methane and
creeping/flowing of clay towards shaft.
9. Site investigation for locating shaft:
a. Morphology study (subsidence, troughs and depressions 4-5m depth are
not visible on 10m contour map, inclined trees indicating creeping
towards shaft)
b. Structural model has to continue under sea
c. Aerial photos
Geophysics:
Gravimetry: mass contrast between void and ground mass
Magnetometry: magnatic contrast between shaft lining and ground
Electric: resistance contrast between void and ground
GPR: electromag contrast between void/lining/ground
Seismics: velocity contrast (due to elasticity param or density)

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