Using instrumented bulldozers to map spatial variation in the strength of regolith for bauxite mine floor rehabilitation

The high strength of some regolith types can limit the growth of rehabilitated jarrah forest following bauxite mining in southwest Australia. Ripping mine floors to a depth of 1.5 m alleviates high strength zones and improves root exploration of substrates. Understanding the variability of regolith strength at the mine pit scale may be useful for improving site-specific ripping and reducing rehabilitation costs. Regolith strength maps were developed based on real-time, field measurement of the hydraulic fluid pressure in the tilt cylinders of Komatsu 475 and Caterpillar D11R bulldozers operating at an average speed of 0.8 ms−1 with standard tip, single shank deep-ripping tines. The strength maps rank regolith into strength classes and show positions of low-, medium-, high- and extreme strength zones in the floors of former opencast bauxite mines. Maps were evaluated using strength measurements on excavated regolith profiles revealing a reliable relationship between bulldozer-mapped regolith strength and actual regolith strength. Weighted unconfined compression strength for mine floor materials within a regolith profile can be grouped as follows: saprolite <4000 kN/m2; quartz-rich, sandy clay (Zm) and silty clay (Zp) 1000–4000 kN/m2; ferruginous/gibbsitic (cemented) material (Zh) 4000–8000 kN/m2; and granite or dolerite rock and hard saprock 5000–14,000 kN/m2. Ripper hydraulic pressure was linearly related to the weighted unconfined compression strength (kN/m2) of classified regolith profiles (r2 = +0.47). The instrumented bulldozer mapping technique can partly distinguish between classified regolith types, particularly granite and granitic saprock (>75 bar) and dolerite and doleritic saprock (25–75 bar). Some regolith types including: quartz-rich, sandy clay; silty clay; and soft saprolite have low bulldozer-measured strength (25 bar) and are indistinguishable by the bulldozer. Regolith strength maps may improve the targeting of secondary contour ripping to parts of a mine floor where it is most-needed.