Use of LBPs to estimate VBPs as observed from an investigation of physical model bimrocks

Abstract

Heterogeneous geological mixes with stronger blocks in a weaker matrix originate from several geological processes. Depending on the matrix cementation and block-matrix contact strengths, these mixes are bimrocks or bimsoils. For excavations, slopes, dams, tunnels, and open-pit mines, bimrock and bimsoil geomechanical characteristics are challenging to measure. The volumetric block proportion (VBP) is one of the most important properties. Given the usual geotechnical methods of 1D drilling explorations and 2D outcrop mapping, site-scale VBP estimate is problematic. Herein, 1D Linear Block Proportions (LBPs) were used to estimate VBPs in cubic physical models with nominal VBPs of 20, 30, 40, 50, and 60%. The cubic models were cut into slabs, and the LBPs were measured along scan lines on each slab to simulate boreholes bored into bimrock masses. Stereologically, the 25 models’ VBPs were believed to match their LBPs. These varied substantially from the target nominal VBPs for each model. The Monte Carlo-type approach was used to analyze the uncertainties (errors) between LBPs assumed to be VBPs and true VBPs. The results showed that a VBP can be estimated for any length of drilling by applying the uncertainty factor to the LBP measurement. The optimal drilling lengths and drilling patterns for a bimrock/bimsoil were also investigated. The results showed that selecting these two parameters for drilling is a function of VBP and the desired error in estimation. Graphs are presented for selecting a drilling program and desired uncertainty at the site scale, which will be helpful for geopractitioners.