Geomechanical and geochemical changes during early stages of weathering of Karamu Basalt, New Zealand

Variations in geochemical and geomechanical characteristics were measured through a basalt weathering sequence exposed at an abandoned quarry at Karamu, near Hamilton, New Zealand. Geochemical measurements show that at the early stages of weathering (fresh to slightly weathered) reductions in the concentrations CaO and MgO are rapid, although they are not accompanied by major mineralogical change; during later stages of weathering secondary clay minerals form following well-described mineralogical changes. Fresh basalt is strong (uniaxial compressive strength ∼262 MN m−2; point load index 5.59) with well-developed, closely spaced, columnar jointing. Slightly weathered material is distinguished in the field only by slight discolouration along joint planes, but laboratory tests show a dramatic loss of strength in slightly weathered rock (point load index 0.41; CBR 43%). A causal relationship between the loss of cations and loss of strength in the early stages of weathering is suggested. This stage of weathering is diffusion controlled whereby cations are lost from the constituent primary minerals and replaced by H+. This process disrupts the lattice structure and causes a marked loss of strength. Implications are that slightly weathered rock, which appears competent in the field, is likely to have much lower strength and durability than assumed from field evidence.