Mining Subsidence Engineering

The field of subsidence engineering stretches from the firm rock at the mining horizon to the surface layer of loose ground on which buildings stand and farming is conducted. The cavity artificially created underground by the extraction of mineral removes the natural support from the overlying strata. As a result, successive layers of rock over the mine workings bend under the influence of gravity, until finally the movement reaches the upper earth surface. At the same time, the underground cavity is closed up to a greater or lesser degree. The extent of movement in the upper layers thus depends on the closing up, over a period of time, of this cavity. The size of the latter is consequently the basic dimension in the calculation of strata and ground movement. By contrast, convergence between roof and floor in mine roadways plays only a minor part in movements occurring in the upper layers of overlying rock, because of the insignificant width of roadways.

Settlement of the immediate roof over the workings, as described in the previous section, continues in higher strata if the mine excavation is so wide that it can no longer be bridged by the overlying rock and the pressure arch in the roof beds collapses (Figs. 12 and 49). In settling, the lower roof beds, if they become detached in sections from the rock bond along horizontal slip planes and loosened bedding planes, withdraw their support from higher beds. Being no longer fully supported, these immediately give under their own dead weight and the extraneous load bearing on them and lay themselves on the beds that have already gone down. The downward movement spreads in this way very rapidly until it reaches the upper earth surface (see Fig. 118).

Mining causes damage not only at the surface but also to mine structures in the interior of the rock mass. In mine roadways and working areas, it is true, this damage is caused principally by rock pressures which belong in the domain of rock mechanics. At the shaft, however, it is caused solely by the strata movements discussed in the last chapter. The mining subsidence engineer must therefore concern himself also with the kinematics of shaft damage, on which there is already a considerable literature. Protection of the shaft, the prime haulage and ventilation route linking the working levels with the surface installations, is after all of the highest importance to the maintenance of the mine undertaking.

Discussion so far has assumed a virgin rock mass that is being worked through for the first time on a large scale in the course of extracting a seam. In most cases, however, the subsidence engineer is concerned with strata that have been mined repeatedly — as, for example, when there have been previous workings in the same seam, or when a deposit consisting of several seams has been worked through from top to bottom, seam by seam, for decades. In a rock mass that has been fragmented to a greater or lesser degree in this way, the strata overlying a working will undoubtedly move differently — more intensely, and also more rapidly — than over a first working, for which these pre-calculation methods were devised (Fig. 102). The degree of previous working thus affects both the development of movement and the field of stress in the rock mass as a whole. This “degree of previous working” is determined by the number and area of seams worked, the amount of convergence in the panels, the perpendicular distance between workings, and the spatial distribution of unmined parts of the deposit (peripheral zones, panel ribs, residual coal pillars). With increasing mining, the structure of the rock mass is altered and the overlying strata weakened and broken up. Furthermore, this structure is affected by the mining practices adopted: a rapid sequence of workings concentrated on one block of strata, a rapid face advance, and caving are all conducive to fragmentation of the rock mass, and all accelerate the development of movement.

Since the middle of last century, when the coal industry went over from drift mining to deep-level mining, attention has been given in Europe to the ground movements which occur on the extraction of coal. As early as 1860, the railway companies in the Ruhr were taking ground-level measurements, in order to bring evidence of the lowering of their tracks in the event of disputes about mining damage. On the basis of these and similar levellings, there appeared from 1870 onwards, in Germany and in other European countries, a number of scientific publications on ground movements in mining and on mathematical formulae for the prediction of ground subsidence.

At the beginning of this century, the mine surveyor was faced with the task of estimating, in increasingly thickly populated mining districts, the mining damage likely to be done to buildings, transportation installations, and agricultural land lying over new mines. He had little more to go on than the mine plans, and had to institute procedures for observing ground movement processes and to develop prediction methods suited to particular mines. In doing so he could confine himself, as was shown in the previous section, to the vertical component of movement — subsidence v_z — and the horizontal component of changes in position — displacement v_x and v_y. As is known, from these components as worked out for two or three points near the surface object under consideration, it is possible to derive all the other factors of movement, such as tilt and curvature on the one hand and extension and compression on the other. The difficulty in covering the movement process in the whole rock body, from mine to surface, in a closed calculation persists even today. Consequently it was necessary to do without a mechanical model and find instead abstract or analytical calculation procedures which were simple to apply to deformation at the surface. The only mechanical characteristics of the rock body available for the calculation were details of the mine geometry such as area, thickness of seam, and depth, which served as quantitative parameters for strata movement, and the results of observations such as limit angle and angle of break. At first only the static end-state of movement could be considered. Later, attempts were made to derive intermediate trough outlines from final shapes by means of time factors.

Pre-calculation of the horizontal components of ground movement causes more difficulty, even today, than the calculation of subsidence. There are a number of reasons for this. For one thing, the measurement of linear change along lines of markers, unlike the more frequent operation of taking levels, can rarely be pursued over an extended period, as it requires a great deal of time and trouble, and many markers become lost in the course of time as a result of earth works. The values obtained in practice cannot yet therefore be regarded as statistically sound for all shapes and sizes of extraction area. Above all, the magnitude of critical displacement V_Max, which cannot be deduced as easily as critical subsidence from mining concepts such as seam thickness and type of stowage, is known only approximately. This is because a half or whole critical area, over the ribside of which this peak displacement value occurs, is frequently interrupted at greater depths by fault zones. Depending on the direction of the line being measured, the displacement component v_x or v_y measured is larger or smaller, and it agrees with the resultant displacement v only when observation lines run diametrically across the working (Fig. 121). This fact seriously complicates the comparison and analysis of measurement results. By contrast, the subsidence of points is independent of direction. A further difficulty is that displacement values are very small, not only at the trough margin but even at its centre, and are thus particularly sensitive to errors in measurement or to being overlain by secondary movements of the marker pegs, unconnected with mining.

The pre-calculation methods dealt with in the preceding sections are based on the final state of ground movement over a single extraction area. In coal-mining this movement process can be regarded as complete 6 months to 5 years after the halting of operations, but in room-and-pillar working in, say, potash or other ore mining it may not be complete until after 100 years. Consequently, in mine planning and the protection of structures, account must also be taken of the interim development of ground movement resulting from the constant enlargement of the extraction area and its long-term influence. As is known, the local values for subsidence, tilt, and curvature of the ground, as well as of displacement and tensile or compressive strain, alter during the period of working, and a structure which, for example, may end up under compression can in the intervening period be exposed to tensile stress. Figures 237 to 240 show how greatly the trough elements alter during mining activity and how the time-related movement and deformation at a point in the trough can be graphically represented.

The components of ground movement arrived at in earlier sections — namely, subsidence v_z and horizontal displacement v, together with the factors of tilt v _z ^’ , surface curvature v _z ^” , and linear change s derived from them — serve the subsidence engineer as a basis for determining surface damage. Here again, as in consideration of the interaction between shaft and surrounding strata, the question arises as to how far a given structure follows the movement and deformation of the ground. We know that the lie of the structure alters with the movement of the ground — it subsides, shifts laterally, and tilts with the ground on which it stands (Fig. 276); we also know that a stiff but elastic structural foundation undergoes less bending, stretching, and compressing than the ground; but we do not know the magnitude of the difference in deformation as between ground and structure in each case. There is a need therefore for closer investigation into the soil mechanics of the transmittal of deformation forces from ground to structure.

Movements and deformation of the ground produce various forms of damage at the populated surface, depending on whether residential houses, industrial installations, communications networks, public utility supply mains, or agricultural land are concerned. Damage to buildings results from tilting, curvature, and linear deformation of the ground built on. With the majority of communications installations and canalized waterways, it is subsidence and the compression or extension of the subgrade which alter the gradient of their alignment or deform the rails, road surfaces, and conduits concerned to the point of failure. With underground cables and pipelines, on the other hand, it is only linear deformation which causes damage. Subsidence on its own can cause mining damage to fields, meadows, drainage channels, canals, and watercourses. Damage resulting from ground displacement is found only in service piping branching off to individual buildings and in extensive conveyor-belt installations (Table 20).

Where there is underground mining over a wide area, ground movement and mining damage cannot be avoided in densely populated districts. For example, the cost of repairs, compensation for loss of value, and structural precautions in the Ruhr District alone amounted in 1981 to some 770 million DM (say US $ 320 million). Of this total, around 50% was for the control of drainage (Fig. 340). In addition, the Ruhrkohle AG group set aside three times the annual requirement in a reserve fund for mining damage. In the U. K., the cost of mining damage in the year 1964 amounted to some £4 mn ($ 8 mn), of which almost 50% went on minor damage to houses. In potash mining and ore mining, expenditure for mining damage is somewhat less.

The 14 chapters of this book bear witness to the great advances that have been made in mining subsidence engineering since the turn of the century and even since the Second World War. Leaving aside extremes of working and strata conditions such as are to be found in steep-lying strata, in the area of geological faults, and where there is a high concentration of working in one place, it is possible today, with the known calculation methods, to predict ground movements with an operationally adequate precision of ±10% in flat-lying, and ±20% in steep-lying strata. These fruits of research are of value not only to mining but also to those areas of activity which stand in close technical association with it. Mention may be made here of construction engineering, communications, town and country planning, land ownership, and the administration of justice. In the common endeavour to uphold the safety of communications and the possibilities for working and living in localities exposed to the risk of mining damage, neither the mining industry nor associated agencies can afford not to pursue a constant improvement of techniques developed in the past. The work of research will never reach a definitive conclusion, because new factors are constantly being introduced into the calculations and considerations by developments in mining and construction techniques, such as concentration today on the best working areas economically, and the use of pre-stressed concrete or pre-fabricated construction elements in civil engineering — to mention only one or two innovations — but also by changes in legal and economic views.