A numerical case study was conducted to analyse the mechanism of large deformation of weak rocks around excavations. Typical results of in situ measurements are summarized. The numerical investigation using the proposed finite element model is described. Conclusions are drawn with descriptions of their practical implications in rock engineering.
Since 1978, a number of laboratory tests and in situ measurements have been conducted by British Coal and Nottingham University for analysis of rock behaviour in and around the Warwickshire Thick Seam at Coventry Colliery, U.K. (Mallory, 1982; Baxter,1987). The seam has been excavated using the longwall mining method at a depth of 750 m ~ 800 m. Although it is not at a significantly great depth, the weak rocks around the mining roadways and longwall faces have generally exhibited a large and time - dependent deformation, which is due to the relatively high in situ stress level comparing with its strength. In order to obtain a more complete knowledge of strata deformation mechanics, which mainly includes prediction of the extent of the strata movement, their interactions with support systems, and the effects of the progressive excavation and construction of the longwall face and gate roadways, a numerical study using the finite element method has been conducted based on the results of in situ measurements and laboratory tests. The in situ investigations involve the measurements of roadway closure, floor heave, pack load and closure, roof bed separation and support forces, etc. Typical results from the existing literature are summarised in the following section. The numerical investigation scheme is then described. Some of the computed results from the numerical analysis are presented in section 4 with comparison of the in situ measurements. At the end, several conclusions from this case study are drawn.
The thickness of the coal ceams at Coventry Colliery vary from 5.5 m ~ 7.3 m. The longwall face is about 3.5 m in height and 230 m in length. The Coal Seams are overlain by layers of mudstone and sandstone and below the coal floor are situated bands of weak seat earth mudstone. Coal was left in both floor and roof to help in stabilising the weak strata in the excavation areas. Fig.l illustrates a typical geological section of the thick seam which is abstracted from the general strata conditions in the colliery. The representative strength parameters of the strata are also shown in the figure. The roadway structure details and typical in situ measurements are shown in Fig. 2. The results obtained from in situ measurements show that vertical roadway closure was mud greater than the horizontal closure. In some of the sections, the maximum vertical closure was up to 2.5 m. Fig. 3 and Fig. 4 show plots of roadway deformation versus distance of face advance. The pack load and deformation versus face advance are shown in Fig. 5. A number of other measurement data are also available, such as the roof bed separation, change of support leg angle, leg penetration and arch support load, etc.
