A comparison of methods for measuring defect orientation from boreholes in weak rock

The identification and characterisation of defects within a rock mass is a valuable tool when analysing potential and pre-existing slope instability as well as when assessing the stability and support requirements for tunnelling and underground structures. A good knowledge of rock defect data is extremely important to enable accurate modelling and for the robust design of appropriate stabilisation and support options.

Two recent projects in Auckland independently used a range of in-situ tools to identify and characterise rock defects encountered in boreholes drilled in extremely weak to weak rock of the East Coast Bays Formation. Both projects discovered problems with these technologies. This paper provides an overview of the methods used and data obtained, discusses the advantages and limitations of each method, and provides recommendations for future use. Although this paper is specific to geotechnical issues in weak rock, many of the findings will be applicable to other geological environments and purposes.