Effects of flow structure in lavas from Sierra de Guadalupe, Northern Mexico City, on point load index and rock mass quality evaluation

摘要

Sierra de Guadalupe in northern Mexico City urban zone is a volcanic complex of stratovolcanoes and domes. These domes are mainly of dacitic and rhyolitic composition exploited as quarries at least during the first half of the last century, now abandoned. As part of a geomechanical study for landslide hazards, this study focused in lavas with flow structures (or banding) since these structures defines the size of the rock block and landslide mechanism. We characterized the flow structures and performed point load tests in irregular blocks and calculated the Point Load Index, parallel (Is//) and perpendicular (Is~±) to banding in order to calculate the Anisotropy Index (Ia). The results show that Sierra de Guadalupe lava flows banding is generated by elongated zonation of rich ferromagnesian and rich silicate minerals. Bands can be from few millimeters to 1 cm wide. The anisotropy index ranges from Ia = 0.9-4.9. In rock masses affected by this flow structure, Rock Mass Rating ( RMR) changes depending on the direction of measuring discontinuity spacing and Is, as parallel (RMR//) or perpendicular (RMR~±) to the flow. Differential weathering in lavas with flow banding can form a thin and continuous bedding-like structure where detritus flows (or "block streams") are common. Where this structure is less defined and flow planes are more discontinuous, commonly they are the base of large rock blocks in cantilever to collapse as rock falls or toppling.