Surface subsidence characteristics of grout injection into overburden: case study of Yuandian No. 2 coalmine, China

A study on the surface subsidence characteristics is essential for evaluating the effects of subsidence control technologies and improving such technologies. One such technology is grout injection into overburden (GIO), which is used in coal mining areas. In the past, the subsidence reduction ratio-a single index-was often used to evaluate the final amount of subsidence control achieved by GIO; however, little research has been conducted on the dynamic process of surface subsidence. In this study, the surface subsidence characteristics of GIO on the side of a stopping line (SSL) of a longwall were examined through in situ monitoring, and the characteristics of areas with grouting were compared with those without. The final maximum subsidence, horizontal displacement, and subsidence rate decreased considerably (70.2, 80.4, and 77.5 %, respectively) with the use of GIO. However, 4.6 % of the mining height subsided at a certain surface point on the SSL before GIO controlled subsidence at that point. Compared with the duration of the active period without grouting, that with grouting and the corresponding subsidence decreased considerably (44.2 and 87.1 %, respectively). Generally, it is quite difficult to control the surface subsidence when GIO is implemented on the SSL of a longwall because the surface and subsurface are already affected by the coal extraction, without grouting on the side of the open-off cut. Thus, the success of GIO technology in the case study described in this paper demonstrates its effectiveness in controlling the surface subsidence in coal mining areas.