Effect of mineralogical composition related to profile depth on index and strength properties of regolith soil

Abstract

While chemical weathering is the most significant geochemical process that affects the engineering behavior of rocks, the fact that authigenic soils present different values of their physical and mechanical properties depending on the depth is also directly related to weathering. Although it is known that such variation is applicable for regolith soils with thickness reaching tens of meters, significant differences can be observed through the depth of residual soils with the thickness of only a few meters. Ignoring such aspects especially in sites with a high risk of landslide leads to many engineering problems. Therefore, Trabzon Province, which has the highest potential of landslide in Turkey, has been selected as the pilot area within the scope of this study. The variations in engineering properties of regolith soils, which have a depth of up to 15 m and are weathered products of volcanic rocks, have been researched as a function of geochemical properties. For this purpose, the data collected from borehole applications, geophysical surveys, in situ cone penetration tests (CPT), and standard penetration test (SPT) as well as laboratory experiments have been evaluated; and it is observed that physico-mechanical properties are directly based on geochemical variations. While the zones with low SiO2 and high Al2O3 and Fe2O3 values are characterized with high content of clay and cohesion, depths with low S wave velocity correspond to areas with the highest chemical alteration index values and the highest degree of weathering. And high liquid limit (LL) values correspond to areas with high illite content as well as high Al2O3 and Fe2O3 values. At these depths, high friction ratio values are observed in CPT results. Obtained data indicate that physico-mechanical properties vary as a function of the depth based on geochemical properties, and this suggests that the soil in projects with regolith soils needs to be separated into different layers and analyzed.