What are the Factors of Subsidence in a Mine?
Field observations have shown that both the amount and the nature of subsidence are not same in all cases. They vary with the change of geo-mining conditions. The factors that regulate the amount and the nature of subsidence are listed below.
- Angle of draw of the strata above the coal seam.
- Method of working.
- The quality of packing in the goaf.
- Effects of depths.
- Geological factors.
- Area of extraction.
- Nature of the overlying strata.
- Inclination of the strata.
- Effects of time.
- Rate of face advance.
Discussed below all the above mentioned points in some details.
Area on the surface affected by subsidence depends on the angle of draw
the higher the value of angle of draw, the more will be the surface area affected by subsidence. Angle of draw is always positive, and varies between 4 and 420. The highest angle of draw has been found in very strong ground, such as, sandstone/quartzite; and least has been found in very loose ground, like running sand. Experience has shown that angle of draw is more on the dip-side than on the rise-side in inclined strata. One may follow Statham’s rule in determining the angle of draw on the dip and rise side respectively. The rules are as follows:
Angle of draw on the dip side: a +[ (24 –a)/24 *d ];
Angle of draw on the rise side: a – (a/24 *d ); where in both cases, a is the angle of draw of the horizontal strata, and d is the thickness of the coal seam.
In case of flat, single seam extractions in normal coal bearing strata, the maximum angle of draw varies between 20 and 25 degrees. It may be worth mentioning that the ground movement outside the 180 and 200 is negligible from the point of view of structural damages. Angle of fracture is generally smaller than angle of draw by about 5 and 15 degrees. Experience has further shown that angle of draw is more on the starting side of a panel than on the finishing side.
The effects of faults on the angle of draw are not very clear. Earlier it was thought that a well-defined fault will arrest subsidence, but field observations have shown that subsidence can extend beyond a fault.
Method of Extraction
surface subsidenceis erratic, irregular and more damaging with bord and pillar method of mining—due to the presence of remnant coal pillars in the goaf. In contrast, Longwall workings yield a smooth and gradual subsidence profile and can be planned to super-critical dimension for strain free settlement of the subsiding strata.
In longwall method of mining, the amplitude of subsidence, as well as subsidence factor will be much higher than those in Bord and Pillar method of mining. The reason is simple: in longwall method of mining, subsidence can take place freely in absence of any resistance in the goaf. But, in Bord and Pillar method of mining, the presence of remnant coal resists the free subsidence of the ground. Even though amplitude of subsidence is more in longwall method of mining, damage to the surface may not be as high as with bord and pillar method of mining – this is due to the gentle slope of the subsided ground over longwall workings.
Even in bord and pillar method of mining, the amount and nature of subsidence will vary, depending on a host of factors, including whether depillaring is taking place with or without stowing. Depillaring with caving will have subsidence factor in the region of 70-80%. But depillaring with stowing will have that figure in and around 10 to 20%. With partial stowing, that figure will be somewhat higher. Another difference: in case of depillaring with stowing, the subsidence profile will be smooth and gradual and the angle of draw flatter. But in case of depillaring with caving, the subsidence profile will be steep and irregular. Smooth, uniform and gradual subsidence does not cause that much damage to the surface structures.
Type of packing of the goaf
if a material of equal resistance can quickly replace the coal removed by mining underground, there will be no subsidence on the surface. The amount of vertical subsidence depends on the amount and the quality of packing in the goaf. A complementary factor in this case is the thickness of extraction. Efficient packing in the goaf can reduce the amplitude of subsidence.
It has been seen in many instances that improved packing in the goaf has increased the angle of draw and the surface subsidence is more gradual and uniform, which is advantageous from the point of view of the structural stability.
It seems obvious that the greater the thickness of a coal seam, the greater will be the amplitude of subsidence, the amount and quality of packing are the modifying factors. Generally, the quality of packing is better in thin seams than in the thick seams. On the effects of seam thickness on the angle of draw, the opinions of the experts vary. But the majority are of the opinion that the more the thickness of a seam, the flatter will be the angle of draw. At critical area of extraction, the amplitude of subsidence is directly proportional to the thickness of extraction.
Effects of Depths
the greater the depth of a coal seam under extraction, the larger will be the critical area of extraction required for full subsidence at a point in the subsidence basin. Besides, for deeper seams, the area affected on the surface due to subsidence basin will be larger, though the slope of the subsidence basin will be more uniform and gentle.
But in shallow workings, the ground subsidence would be abrupt, discontinuous and steep, and hence more damaging. Besides, subsidence movement at the surface starts early in case of shallow workings.
Geological Factors
lateral strain is more intensified, where coal extraction takes place in the region of geologically uneven ground. Faults, particularly if they extend to the surface, induce uneven and rapid subsidence. Washout, if they are filled with loose deposits, will tend to increase the angle of draw – possibly to about 450. This widens the subsidence basin. Workings alongside old goafs result in un-symmetrical subsidence basin—subsidence basin shifts towards the goaf
Area of Extraction
the greater the area of extraction underground, the greater would be the area of the subsided ground on the surface. Amplitude of subsidence also depends on the area of extraction underground, till the critical area of extraction is reached. At critical area of extraction, subsidence will take place only at one point in a subsidence basin. When the area of extraction exceeds the critical area of extraction, full subsidence takes place at multiple points in the subsidence basin – giving the subsidence basin a flat-bottomed shape.
Nature of Overlying Strata
when extraction of coal and mineral takes place under a strong roof of massive sandstone, the angle of draw and the subsidence basin is not as extensive as it would be while working under a weak roof of shale/claystone or even bedded sandstone. Additionally, strong roof – due to its rigidity – doesn’t come down into the goaf that easily. The result is a large area of goaf with uncollapsed roof hanging over it. But, when the roof finally comes down – when the weight of the roof overcomes the shearing strength of the strata – it sets up excessive strain and induces sudden subsidence on the ground surface.
Inclination of Strata
angle of draw tends to be more on the dip-side than on the rise-side, so shifting the subsidence basin towards the dip side. The point of maximum subsidence, instead of being vertically above the centre of the goaf, lies at right angle over the centre of the goaf.
Effects of Time
generally, subsidence movement starts slowly, then gathers momentum, and remains in active state for quite some time. Then it slows down and finally stops when the subsiding ground attains some sort of stability or equilibrium. A given point on the surface will continue to subside till it reaches stability or equilibrium. How long the subsidence movement will persist depends on the depth of the workings and the nature of the strata. In this context, nature of strata implies that whether the strata have already been disturbed by extraction in other coal seams in this area.
Extraction of shallow seams causes the subsidence to start and stop earlier than those for extraction of deep seams.
In a virgin area, subsidence movement continues longer than in areas already disturbed by earlier mining activity. One can assume that about 95% of the subsidence movement takes place within 3 to 5 years of extraction, and then remaining may extend for another year or so, unless speeded up by workings in other seams in the vicinity.
Rate of Face Advance
the magnitude of travelling strain varies with the rate of face advance. The magnitude is higher and near the limiting value for a slow advancing face – which may cause serious damages to the surface structures. A regular and fast rate of face advance causes less damage to the surface structures. (Strain value E is proportional to S/H, where H is the thickness of extraction, and S is the maximum subsidence)
Where the rate of face advance is regular and steady, the strata above tend to yield uniformly behind the face. In consequence, subsidence tends to be uniform and regular. When the face movement stops, subsidence doesn’t stop, but continues behind the face. What is more, the ground displacement gets pronounced, angle of draw increased. So the surface structures at the edge of the working face – which might have survived the subsidence wave if the face moved rapidly and regularly – are more likely to be damaged.
