Voids and significant zones of core loss were encountered during routine geotechnical investigations carried out for an iron ore project located in the Pilbara Region of Western Australia. The geotechnical investigations were concentrated within proposed foundation areas of the major plant site infrastructure. Early drilling results showed that the presence of voids appeared to be fairly extensive across the site but locally highly variable in size and distribution. Initially the voids were assumed to be associated with solution features within calcrete deposits as had been encountered during previous investigations at a nearby project. However, geological assessment of the borehole data did not show any evidence of calcrete or other carbonate rocks, but did show strong evidence that the voids were in fact associated with a ferricrete deposit that extended across the site.
The occurrence of significant voids within ferricrete deposits was not generally known within the geotechnical community at that time. Subsequent investigations including a review of the mining company geological database, additional cored borehole drilling and downhole camera surveys were undertaken in order to gain a better understanding of the foundation conditions.
A series of meetings and risk assessment workshops was carried out in order to assess possible options to progress the project within the extremely tight time constraints involved. The resultant solution involved grouting of voids underneath the more critical/vulnerable items of infrastructure. This approach was accepted by the proponents and a programme of foundation grouting was completed underneath the primary and secondary crushers. Recorded grout takes were highly variable and unpredictable across the foundation areas as had been anticipated from the investigation data. Significant grout takes were recorded at several locations.