A problem arising in sequential open-cast mining and underground working is considered. Displacement of earth's 'surface in the process of underground mining presents a danger for the shaft sunk near the quarry edge. Model studies have been made into evaluating the methods of shaft protecting and forecasting a possible depth of mining work. The technique of modelling is described. The effect of the quarry upon the subsidence process is evaluated qualitatively and quatitatively.
A valuable ore deposit is represeis- ted by a bed-like orebody with dif use boundaries within the metamorphic rock mass of complex structure (Fig. 1). The massif of enclosing rocks and the orebody are represented by metamorphosed. ultrabasic rocks: amphiobolites, serpentine- tes, talc and coaly-flinty schists. The massif has numerous fractures, parallel to the general rock occurence. A characteristic thing is the tectonic boudinage occuring in more fragile varieties of rocks enclosed within more plastic talk schists. The strength properties,of rock samples of laboratory, dimensione are given in table 1.
(Table in full paper)
The rocks are permeated with several systems of fractures, the system parallel to the general mode of occurrence being prevalent of all.(Figure in full paper)
The composition, the tectonic and fracture structure of rock cause the low strength properties of the massif; the distinct anisotropy being seen due to low strength of rocks in the planes of rock occurrence. It does not seem possible to give even an approximate estimation of the massif strength basing on the laboratory studies.
The upper section of the rock body has been worked out as an open pit. Eastward of the quarry a shaft was sunk for underground development of the deposit up to the proposed depth of the +0 level. The deposit has been developed up to the +116 m level with the overlying rocks caving completely.
The caving of rocks into the worked space caused the formation of a slump hole with the angle of caving γ= 80°. In the quarry slope beyond the caving funnel formed the steps at the lines of layers' slipping along the contacts. At that stage no instrumental observations of the displacement process were being made.
Subsequently, however, the change of the situation made it possible to plan the development of the deeper sections of the deposit below the +0 level with a lower, content of the useful component.
In order to minimize the effect of mining work upon the hanging wall rocks, where the pit shaft is located, the further ore excavation is followed by backfilling. The volume of hardening cementous filling amounts to 95 per cent of the extracted mineral volume, the remaining voids and compressibility of filling amount to 5 per cent of it. By the present moment the deposit has been worked out in this way to the level of +68 m.
From the time the filling system was introduced the permanent observations have been made of the deformation of the massif; the location of observation points is shown in Fig. 1.