Determination of in-situ stresses in Lesotho basalt under low cover using hydraulic fracturing

The in-situ testing technique of hydraulic fracturing in boreholes is commonly used to determine the minimum in-situ stresses in rock masses. This paper describes an extensive programme of investigations using this technique which was used to select appropriate linings over a 2.5km long section of low cover in a water transfer tunnel in Lesotho. The work was carried out in four phases. The first phase was undertaken at the pre-construction stage and consisted of testing done in vertical boreholes sunk from the surface. The second phase of testing was undertaken in boreholes drilled from the tunnel. The third and fourth phases were undertaken in the same way and were done in order to refine the minimum stress profile along this section of the tunnel. The paper gives a detailed description of the techniques used and the system developed to rate the reliability of the test results.