Many cavities remain at shallow depths of mining areas where coal and other minerals have been excavated. The strata above the cavity consist mainly of soft rocks deposited in the Tertiary Period and of weathered or fractured sandstone and shale.
Due to the effect of groundwater or surface loading, the roofs of cavities deteriorate and collapse, or the pillars may crush with time and the cavities may eventually migrate to the surface and cause sudden cave-ins.
In the Chikuho district in Kyusyu, old Coal workings have been responsible for about 50 – 60 cave-ins every year. Engineering structures constructed over such Workings are susceptible to sudden collapse and related damages. It is therefore important to take this phenomenon into consideration when undertaking construction projects such as railroads, highways, hospitals, schools, power plants etc. in areas where old workings are at shallow depths.
In this report, the authors have investigated the practical data of cave-ins Which have occurred in Chikuho and Ube districts and have made theoretical studies on the effects of water and loading by the finite element method. Finally, some preventive measures against cave-ins have also been considered.
Fig. 1 shows the location where cave-in has occurred from 1974 to 1980 and the outcrop of coal in Tagawa district. Cave-in has occurred along the outcrop and the depth from the cavity to surface is within about 30 m. In this district, many mines were worked at very shallow depths by the pillar and stall mining method from the beginning of the Meiji era till after the second world war in the Showa era. Many shallow cavities of unrecorded workings still maintain their original dimensions. Table 1 shows the relationship between the depth.
(Figure in full paper)
The authors have studied the mechanism of cave-in theoretically by the incremental load procedure of the finite element method, on the assumption that the ground is an elasto - plastic material which has a bilinear stress strain relationship. And we applied the Mohr - Coulomb condition of failure, where the tensile strength of the ground is half the cohesion.
This figure leads to the following deductions: when y = 1.0 t/m3, failure occurs at the center or corner of cavity's roof and the plastic zone rapidly develops to the surface as the unit weight increases; and finally, when y = 2.3 t/m3, the plastic zone develops perfectly to the surface. At this moment, the ground surface falls in, having the inverse funnel shape.
In Fig. 4 the relationship between the depth of the cavity and cohesion with parameter of angle of internal friction is shown when 2b = 10 m and y = 2.5 t/m3. of the angle of internal friction can be neglected.
Where ε: coefficient of convergence If Eg. (6) was not satisfied, the piezometric head at the outer boundaries are uniformly changed, up or down, automatically. That is, if Qm>Qp the piezometric head will take down and Qm<Qp the piezometric head will rise.
