Design and construct 2.5m finished diameter shaft 260m deep. Upper 108m of strata consists of Tertiary deposits of saturated sand, clay and lignite overlain by fresh basalt with comprehensive strengths of over 300 MPa and weathered basalt near the surface. The Tertiary was drilled using 3.15m diameter cutter head to provide clearance for a 2.7m diameter externally reinforced intermediate steel liner extending into the Permian sandstone at 113m depth. The remainder of the shaft is drilled at 2.65m diameter to the production coal seam level 257m total depth from surface. A heavy wall 2.2m diameter external ring reinforced steel liner was installed from the top of the coal to one metre above surface grade.
The entire construction sequence of drilling and lining the shaft is completed while the hole is filled with drilling fluid. The drilling fluid includes additives intended to stabilise the excavation by inhibiting the clay and shale from swelling and subsequent deterioration while protecting the sand from erosion until the steel liners have been installed and grouted.
Peabody, North Goonyella Coal Mine