Verifying the ground treatment as proposed by the Secondary Permeability Index during dam foundation grouting

Abstract

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. The calculated SPI class, together with information obtained from detailed mapping of the foundation rock mass, is compared to the mix, the take, and the success of each of the primary production grout boreholes. Based on the success of the grout, a degree of jointing is assumed, which is evaluated and validated against the RQD values obtained from the exploratory investigation boreholes. Overall, the degree of jointing inferred from the success of the grout mix showed that most of the compared boreholes validated the ground treatment as suggested by the SPI. Successful thin mixes coincide with minor fault zones with or without dolerite intrusions, as well as very closely jointed gabbro bedrock, whilst unsuccessful thin mixes are associated with the contacts of dolerite intrusions, which are generally highly weathered and fractured. This is highlighted by the success of thick mixes when applied to these intrusions. Unsuccessful thick mixes are mostly attributed to major fault zones.