ANALYSIS OF MEASURED AND PREDICTED LAND SURFACE SUBSIDENCES CAUSED BY RETREAT MINING

This article presents the impact of the retreat mining (i.e., involving an intended collapse of the excavation roof, subsequent to extraction) on the subsidence of the ground surface. The analysis was carried out for two areas of coal underground mining located in the Upper Silesian Coal Basin (Górnośląskie Zagłębie Węglowe). The assessment of the influence of exploitation on the ground surface was based on the results of geodetic measurements performed over a long period of time, whereas the land deformation prediction was made with the use of the EDNOPN program. The calculated and the predicted values were further compared, and the parameters of theory were determined. The results discussed in this paper have been shown by way of diagrams. The observed differences in the processes of vertical displacement were used in the analysis which took into account the degree to which the rock mass had been disturbed during the previous excavations, as well as the type of incumbent rock in the area under study.

Effects of undrained strength anisotropy on surface subsidences induced by the construction of shallow tunnels

The implementation of an anisotropic soil model that allows consideration of the variation of undrained strength due to anisotropy is described. This analysis is then used to identify the significance of strength anisotropy on the prediction of deformation behaviour of a shallow tunnel. It is found that, for the case of an unlined tunnel, attention should be given to the effect of strength anisotropy particularly for a soil possessing type "K" anisotropy (i.e., where the smallest undrained strength occurs at an angle θ other than the vertical or the horizontal, and the smallest value usually occurs at θ = 45°). On the other hand, for a lined tunnel, the effect of strength anisotropy upon the surface settlement profile will depend upon the size of a so-called "gap" parameter. The gap represents the net effect of loss of ground and some "workmanship" factors in a plane strain finite element analysis. Increasing the gap eases the restrictions imposed by the tunnel lining upon possible soil deformations, thereby increasing the effect of strength anisotropy. However, it is shown that for a lined tunnel with a moderate value of gap, detailed consideration of strength anisotropy may not be necessary. Key words: strength anisotropy, tunnelling, predictions, surface subsidences, finite element method, soil model, soft clays, vane strength.