The Secondary Permeability Index (SPI) is a permeability-based rock mass classification, which when complemented with the degree of jointing can be employed as an approximation to the ground treatment design. However, when the grout mix and success of the grout operation are known, a back-analysis can be conducted to infer the degree of jointing. The aim of this paper is to back-analyse and verify the ground treatment as proposed by the SPI from water pressure tests conducted in primary production grout boreholes, at the De Hoop Dam in South Africa.