Alteration in Material Properties of an Intact Rock due to Drilling and Bolting Operations

Rock material properties play an important role in assessing rock mass behaviour. Passive rock bolts stabilize a rock mass by restricting relative displacements; however, they may reduce rock material properties. While assessing properties of rock mass reinforced by rock bolts, the alteration caused in rock material properties due to drilling and bolting operation should be well understood. The present work makes an attempt to study the alteration in rock material properties due to drilling and bolting operations. The study has been conducted through physical tests on natural rock specimens in the laboratory. Virgin cylindrical specimens of intact rock were tested under uniaxial compression. Drilling was done through specimens and fully grouted bolts were installed. These specimens were also tested under uniaxial compression. It is observed that the drilling has substantial effect on the uniaxial compressive strength and modulus. A simple problem of tunnel is analysed which demonstrates the effect of alteration in material properties on the strength of rock mass around the tunnel in the field condition. 1. Introduction Rock mass encountered in projects like tunnels and caverns generally consists of intact rock blocks separated by discontinuities (joints). Two important ingredients in stability analysis of rock mass are the rock material properties and the joint characteristics [1–4]. In many situations, the severity of jointing makes rock mass so incompetent that it becomes necessary to reinforce it. Rock bolts are extensively used for enhancing rock mass stability. Installation of bolts reduces relative displacement between rock blocks (Figure 1) and enhances the rock mass stability. Though there is overall improvement in stability of the rock mass, the rock material properties themselves alter due to drilling and bolting operations. Since rock material properties are important in assessing rock mass properties, the effect of alteration in material properties should be well understood by the designer. The drilling and bolting operations may affect the properties of intact rock material in the following manner.(i)The vibrations induced during drilling by machine may create weak zones around the drill hole.(ii)The deduction of rock material for creating drill holes may induce weakness in the rock block.As the hole is drilled through intact rock blocks, zone of weakness will develop around the hole. The grout and bolt filled in this hole will compensate to some extent the loss of strength, but it will not be equal to that which was gained through