The aim of the project is to investigate the rock mass parameters at a site in the vicinity of Matjiesfontein (Western Cape Province) so as to ascertain the suitability for the construction of a vault in a rock face. This vault will host a permanent LaCoste and Romberg ultra-sensitive gravimeter as well as a long period seismograph. The site is close to the contact between the Dwyka Formation of the Karoo Supergroup and the Witteberg Group, both part of the Cape Supergroup. A site survey, joint analysis, core logging and laboratory testing on rock specimens formed part of the geotechnical investigation. Laboratory tests included unconfined compression strength and point load tests. The data obtained from the fieldwork and laboratory analysis were used to determine the rock quality designation value of the rock, which was subsequently combined with the other parameters to obtain a rock mass rating. All data were utilised to develop a model for best practice construction (e.g. excavated vault in solid rock versus a concrete structure against rock face), as well as to develop a preliminary design for the vault. The results of the geotechnical investigation indicate that the site is suitable for the proposed project.