Introduction Wherever minerals are won by underground mining extending over any significant area, the overlying rock mass subsides into the underground cavities opened up by mining, and the upper surface of the ground subsides correspondingly, forming hollows and trenches, open cracks in the earth, abrupt steps, and extensive subsidence troughs. Ground can sink vertically or be displaced horizontally - or both by as much as several metres. Since the middle of last century, and particularly over the coalfields of Europe, ground movements on this scale have led to severe damage to buildings, communications, and agriculture, for which the aggrieved land and property owners have demanded compensation from the mine operators and, when necessary, have pressed their claims in the courts. To be in a position to present an effective defence against unjustified claims, mine surveyors have since the beginning of this century made numerous measurements of underground excavations and observed ground movements with extreme care. From the experience thereby gathered, and the theoretical conclusions drawn on how ground movements develop, there has gradually been evolved a new branch of science and technology - mining subsidence engineering which has been taught in German mining academies since 1931. At first all that was required was a knowledge of the extent of surface damage and the duration of surface influence of a mine working, together with the ability to attribute an appropriate share of costs to neighbouring mines, which very often were jointly responsible for the damage; but with increasing mining activity underground and increasing residential development on the surface, it became necessary, both to the mine operator and to the surface developer, to do everything possible to minimize mining damage. Today therefore, it is the task of subsidence engineering to develop procedures for a) predicting strata and ground movements over mine workings; b) ascertaining the effects of such movements on building structures, mine shafts, etc.; c) minimizing subsidence damage by means of improvements in mining, protection of structures, regional planning etc. From this description of its role, it follows that mining subsidence engineering, although certainly to be counted among the earth-engineering sciences, takes in not only the study of ground movements and rock structure but also areas bordering on the sphere of mine surveying, like mining law, property law, mining engineering, constructional engineering, rock and soil mechanics, communications engineering, agriculture, hydrology, town planning, etc. (see Table 1). 2 Introduction Table 1. Division of the earth-engineering sciences as between construction and mining With the increasing depth of mining, its impact began to be felt even by the numerous mine structures, and especially by mine shafts; and so steps had to be taken to protect shafts and other important installations in mines from the damaging effects of a mine's own excavations. Since the 1930s therefore, the planning of mining procedures near the shaft and of safety measures at the shaft itself, both designed to protect the structure from mining damage, were added to the responsibilities of subsidence engineering. One result was that a planned and economic recovery from among the large reserves of unworked mineral locked up in shaft pillars became possible for the first time. Finally, subsidence engineering also has a safety role. Its measures in relation to both mining and construction help to protect communications networks, public utilities installations, important public buildings and historic monuments within mining areas from damage which could impair their functioning or even render them dangerous. Subsidence engineering can be differentiated from the closely related field of strata control in the following way. In strata control, what is chiefly studied is stress changes and rupture processes in the immediate vicinity of underground excavations, with the object of minimizing convergence in mine roadways, avoiding rock bursts, sacrificing as little, mineral as possible in support pillars, and utilizing rock pressure in the dislodging of mineral for extraction. It is thus the interaction between solid rock and roof supports or pillars in circumstances of disturbed load equilibrium which is at the centre of consideration in strata control (i.e., a load model). In subsidence engineering, on the other hand, what is principally being investigated is the interaction between loose ground and structural foundations or shaft linings under the influence of strata movements at a distance from underground workings (a movement model). The inclusion of strata control in the sphere of subsidence engineering as “subsidence engineering underground", which was still being advocated by O. Niemczyk in 1949 in his textbook on subsidence engineering, "Bergschadenkunde", cannot be sustained today in view of the very different legal position and practical objectives - leaving aside the common objectives of minimizing costs and preventing accidents – of the two disciplines. In the discussion which follows therefore the only thematic distinction drawn is between strata movements, triggered off by mining, in the interior of a predominantly solid rock body stretching from workings to caprock and shaft side (Part I), and on the other hand, movements in the loose ground of the upper surface layer in which surface structures have their foundations (Part II). Introduction 3 At the centre of the subsidence engineering stage stand the thickly populated coal-mining areas of Europe. Their numerous coal seams, often metres thick, are today mined across extensive areas 300-1000 metres underground, in fronts of 200-300 m length, without supporting pillars. The result is that the rock mass overlying them drops like a sagging plate behind the advancing coal-faces, immediately after extraction, and breaks up only on reaching the floor of the workings. The rock strata overlying massive orebodies and salt deposits subside in a similar way when mined, except that parts of these deposits, left standing like islands, often retard the progress of subsidence. Even in opencast mining, significant settlement and consequent damage can occur in ground adjoining a mine as a result of a fall in the water-table. Thus the ground movements, visible damage, and protective measures so familiar in relation to coal-mining are basic attributes also of other branches of mining – even of oil and natural gas production, which can lower the surface of the ground by a metre or more. It is only in the working of vein ore occurring in strong country rock that mining damage is negligible. In today's active competition between domestic mining and foreign sources of energy and minerals, subsidence engineering is of mounting importance. More than ever is it becoming necessary to hold down the costs occasioned by mining damage which average, for example, 6-8% of the per-ton cost of coal production and to ensure the workability or reserves even under built-up areas. Mining subsidence engineering thus has a positive contribution to make to securing a competitive domestic supply of raw materials. A Statement to the U.S. House Committee on Interior and Insular Affairs Morris K. Udall, Chairman Oversight Hearing on the Surface Mining Control and Reclamation Act of 1977 held August 3, 1987 by Mrs. Peggy Clark Chair of CAWLM R.D. 5 Box 195 Indiana, Pa. 15701 CAWLM is a group of rural land owners with Concern About Water Loss due to underground Mining, subsidence and other matters related to surface effects. * Bracketed portions will be omitted for brevity in the oral presentation. Mr. Chairman: My name is Mrs. Peggy Clark, here today to represent a group of rural residents who live over deep mines in several counties of southwestern Pennsylvania. Thank you for this opportunity. In an age when acceptable living standards include a dependable quality and quantity of water for health, convenience and efficiency- we report there are people in coal areas who must carry drinking water into their homes, who flush their toilets with rainwater, (when it rains), must drive to the laundromat or YMCA or intown relatives to do laundry and take showers. We sometimes must spend hours and shift schedules, and use a lot of energy, somewhat as third world citizens must devote their time and energy to get essential water. Our community and church involvements, our jobs are curtailed with one more major problem, this loss of water supply as a result of underground mining. We are suddenly faced with decreased property values. and in the case of farmers and businesses, the ability to earn a living is affected. Subsided buildings, occurring more frequently with wider use of full extraction mining methods add to the problems. I cannot spell out to you, in this short time, all of the hardship, inconvenience, expense and hassle. Here is a little taste of it:: a. the pregnant woman who received two five gallon jugs from her coal company, so she could cross a major highway, walk uphill to the buffalo (water tank) which arrived while her husband was at work, to receive her daily supply. Normally that amount of water would be used in three toilet flushes. sed in b. a dairy farmer who lost his well water, then his springs. He had to spend hours trucking water in to his cows while their milk production plummeted from lack of enough water. He worried about sanitary conditions with less "scrubbing down". c the family with five school children who finally built an outhouse, to handle their needs and get them to school on time. d.people constantly making choices. If the children wash their hair tonight, the dishes wait till tomorrow. The cleanest kid is first to go into the bathwater. e.Running the washer may take two days to get through the last cycle. That is especially hard with dirty workclothes, large families and sick peple with lots of bed linens. page one |