Bulking factors and extents of caved zones in weathered overburden of shallow abandoned underground workings

A comparison between the bulking factor and height of the caved zone for different weathered subsurface rocks over shallow abandoned mines and for strong rocks at intermediate and large depths of longwall mining is performed. In this study, vertical boreholes drilled from the ground surface have been used to detect the height of caving in weathered (W) and very weathered (VW) rock mass overlying abandoned underground workings at a shallow depth, and then to calculate the bulking factor. It is found that key parameters that control the bulking factor (k) and the ratio between the heights of caving and mining (H/h) are different for W and VW rocks. In W rocks, such parameters are the rock strength of immediate roof and rock porosity, whereas those for VW rocks are rock quality designation (RQD, %) and standard linear fracture intensity (P10). It is argued that H/h in VW rocks increases according to linear-logarithmic law with an increasing ratio P10/RQD in the immediate roof and P10 in four rock layers over the immediate roof. Observations show that VW rocks exhibit smaller values of k=1.06-1.165 and larger H/h = 6.07-15.6 compared to W rocks having 1.09< k<1.24 and 4.1 < H/h <11.25.