Laboratory Procedure to Calibrate Non-Stressed Borehole Size

ABSTRACT: Measurement of the pre-excavation 3D stresses is of great importance for mining, petroleum and other underground rock engineering. An alternative method by differential-direction drilling (called D3 method) without overcoring was recently developed based on back-analysis of the measured borehole diametrical deformation. One of the key input parameters in this back-analysis method is the borehole convergence. To calculate the convergence, the initially-drilled borehole diameter before deformation is required. The rock mass in the ground is always under stress and a drilled hole size is the ultimate result after deformation. Additionally, the size of a borehole typically deviates slightly from the nominal drill bit size. Hence, the first step is to determine the actual borehole size under zero stress to provide a base for comparison in practical implementation. This paper presents the laboratory experiment setup and the procedure for drilling and measurements. Boreholes are drilled in non-stressed rock blocks using a drill press and customized drill bits. The borehole size is measured with a borehole deformation gauge and calibrated with a precisely machined aluminum tube. The relationship between the borehole size and the nominal core bit size is thus established. A calibration procedure for determining the non-stressed borehole size is ultimately established through laboratory experiments. 1. INTRODUCTION Pre-excavation stresses refer to the stresses existing within a rock mass before the initiation of any excavation or mining activity. It influences the stability of underground structures and excavations (Hoek and Brown, 1980; Amadei and Stephansson, 1997; Brady and Brown, 2004). Incorporating the pre-excavation stress analysis into engineering practices in mining, petroleum, and underground rock engineering is essential for ensuring safety, optimizing design, maximizing resource recovery, and managing risks, ultimately contributing to the sustainable development of these industries. The pre-existing field stresses are in three dimensions, which can be described by six independent components. It can also be represented by three orthogonal principal stresses: the maximum, the intermediate and the minimum normal stresses (Zou, 2020). Unlike one-dimensional or two-dimensional stress analyses in different directions, which oversimplifies the stress distribution, stress analysis in three dimensions provides a more comprehensive understanding of how stresses act within the rock mass. This understanding is crucial for accurate engineering design and risk assessment.