Ground collapse and caving mechanisms in strata overlying sublevel caving mines: a case study

Mechanisms responsible for the caving of the strata overlying Chengchao Iron Mine were investigated by analyzing in situ deformation monitoring results and the distinctive engineering geological conditions. The results indicated that waste rocks caved from the overlying host strata and the large-scale sliding rocks at the footwall near the goaf continuously flow towards the caved zone when surrounding rocks of goaf are relatively broken. As a result, a large air gap does not generally form above the caved zone. In this case, the strata overlying the goaf do not globally cave upwards to the surface on a large scale. Instead, they cave in a direction in which the rock is weak and soft and thus form locally caved pipelines within the overlying strata. That is, pipe caving occurs in the overlying strata which results in collapse sinkholes in the ground surface that have a punctate distribution. The continuous flow of large-scale sliding rock masses towards the caved zone can effectively slow down the occurrence of pipe caving. More importantly, rocks surrounding a caved pipeline will cave continuously with excavation of orebodies. However, this caving tends to decrease or completely stop after a certain period of time. This is mainly due to a change in the location of the underground orebody being excavated or because the flow in the caving channel has become blocked. Pipe caving has become a caving mode that is typically encountered in strata overlying the metal ore in mines excavated through sublevel caving with no sill pillars.