Regolith strength, water retention, and implications for ripping and plant root growth in bauxite mine restoration

The lateritic bauxite in the Darling Range of Western Australia is approximately 4-6 in deep and is composed of caprock (duricrust) and underlying friable bauxite developed from highly weathered granite and dolerite. This paper investigates the impact of mine floor ripping operations on materials exposed after mining in relation to strength, water retention, and plant root growth. Deep ripping has been shown to create a structured rootzone and increase both recharge and plant available water. Material classified as quartz rich (Zm) lacks structural features for plant root growth, yet the material requires the least force to rip. These materials require deep pre-ripping followed by contour or multi-tine ripping for successful restoration. Materials that contain 50% or more coarse fragments (gravels, cobbles, and stones) increase the force required for pre-ripping operations but only require contour or multi-tine ripping; doleritic clay with a high coarse fragment content may be readily colonized by roots without pre-ripping. Localized compression (smearing) of materials may occur during ripping due to the pre-ripping tine working deeper than its critical depth of 1.0-1.5 in and/or ripping of materials at high moisture content. A relationship exists between the ratio of reactive silica to total silica and the clay content of regolith, both properties being determined from pre-mining borehole samples. This ratio can be used to identify future mine floor materials from borehole data prior to mining and to target ripping operations to appropriate regolith types to enable the most effective and economical restoration practice.