The diamond pipe at Ghaghoo is overlain by approximately 70m of poorly consolidated Kalahari sequence deposits consisting of fine grained aeolian sand particles which are cemented weakly by a clay (smectite) binder. In places the sequence is more strongly cemented with calcite (calcrete) or silica (silcrete). The thickness and geotechnical characteristics of this sequence presented a challenge with regard to accessing the pipe.
An innovative decline shaft construction method using and Open Face Tunnelling Shield (OFTS) with installation of a segmental concrete lining for long term support was selected. While this method has been used extensively in international tunnelling projects, development usually has been close to horizontal. Little experience existed of development on an 8° (1 in 7.1) gradient.
Development of the box cut commenced during July 2011. An artificial portal was created, approximately 25m below surface, to minimise the cost and risk associated with box cut development.
Construction of the 6m diameter OFTS on site commenced in November 2011 and excavation of the decline commenced in December 2011. A total of 758, 600mm wide concrete rings were installed in the Kalahari deposits and weathered basalt before conventional development continued in basalt.
This paper briefly presents aspects of the design philosophy and method, construction and monitoring processes involved in excavating the box cut, constructing the portal and sinking the decline shaft.
