Longwall partial extraction has been proven to be an effective method of subsidence damage control. The design of such layouts has to ensure that the tilt and strain values are acceptably small, first, over the alternating panel and pillar sequence and second, in the vicinity of the extremities of such a sequence when it approaches and then undermines the structures to be protected (edge effects). The first requirement is fulfilled by keeping the pillar center below a specific fraction of the mining depth. The second criterion is met by specifying a small enough span for the panels to have acceptably low edge effects. The latter of these requirements may be implemented in either a 'One-pass' or a 'two-pass' system.
The distortion of ground surface by underground coal mining has been the cause of costly damages to structures and has resulted in controversy in many countries of the world. Many proposals have been made during the last few decades to alleviate this problem. The intention in this paper is to focus attention on an effective method of subsidence control, which was proposed by British mining engineers in the 1950s. The system is known either as "longwall partial extraction" or "panel and pillar" mining. The method has been described in a number of articles (e.g. Orchard 1964, Wardell 1969) and in essence it consists of parallel panels separated by abutment pillars of appropriate dimensions. In the panels some form of total extraction is practiced. If the span of the panels is less than a critical value, British engineers suggest 1/4 of the mining depth for this 'critical span', then such mining results in a characteristically shallow, flat-bottomed, virtually slope and strain free subsidence basin on the surface. The method has been applied in coal, potash and iron-ore mining, at depths ranging from about 50 m to 1,000 m and in seams up to 3.5 m in thickness. In spite of the demonstrated success of the method, no attempt, apart from those appearing in a few isolated publications (Salamon 1962, 1974), seem to have been made to explain the deformation mechanism involved. An attempt will be made in the sequel to fill this gap with the view of formulating some basic principles for the design of such mining layouts. The objective is to design panel and pillar layouts which will induce surface tilts and/or strains smaller than the values tolerated by the structure to be protected and, at the same time, lead to the maximum coal extraction which is compatible with safety.
Two mechanisms need to be considered when attempting to elucidate the reasons for the success of longwall partial extraction as a method for subsidence control. These are, first, the development of subsidence induced by a narrow long panel and, second, the interaction between several of such panels. A simple linear laminated model of the stratified coal measures, Salamon (1961, 1991a) have been employed to perform the investigation.
