EFFECTS OF WEATHERING ON THE MECHANICAL STRENGTH OF LAYERED BASALTS

Heterogeneous rock masses, for which material properties such as permeability and weathering potential vary, may confound our expectations for the structure of the weathered profile near the Earth’s surface. We evaluate the effect of weathering intensity on the strength of layered rock sequences using both geophysical and geotechnical methods at seven sites on the Kohala peninsula, Hawai’i. Here we exploit a dramatic gradient in mean annual precipitation across these sites (MAP 500 – 3000 mm/yr) as a proxy for variable weathering intensity in a landscape developed on a Pleistocene age, layered basalt sequence. In dry sites (MAP < 1000 mm/yr), rock strength increases rapidly and monotonically with depth. However, in wet sites (MAP >1000 mm/yr), we observe deeply weathered horizons with soil-like strength properties, interspersed within layers of much less-weathered basalt that retains rock-like strength properties. The presence of soft layers corresponds with observed low S-wave velocity horizons and a decrease in the average P-wave velocity. While lower seismic wave speeds suggest mechanical weakening, we propose that preservation of strong layers within the weathered sequence maintains the bulk rock mass strength on the wet side of the island despite the development of some weak zones. At the hillslope scale, mechanical layering of strong and weak layers produces anisotropic slope behavior that contrasts sharply with the predicted behavior of monotonically decreasing profiles of weathering. This anisotropy is expressed as slope stability that varies strongly depending on the orientation of layering relative to hillslope angle. Such an interpretation for Kohala may explain the steep hillslopes of the amphitheater-headed canyons, where the sub-horizontal orientation of strong/weak horizons allows hillslopes on the wet side of the island to remain equally as strong as their drier counterparts. These results emphasize the need to understand the weathering-related evolution of the entire rock-strength profile. Strength anisotropy at the hillslope scale may be an important feature of critical zone architecture in many geologic settings such as sedimentary sequences, metamorphic terrains and highly-tectonized environments where rock fracturing may produce spatial variations in weathering potential.