The Geology of Soft Rocks

Abstract

One may think that geological affiliation and environmental conditions of formations are the main hint to understand the mechanical properties of the rocks. It is expected that the input of highly energetic pressure–temperature (PT) conditions should impose the formation of harder rocks in relation to those related to the origin of soft rocks. Although reasonable, this is not a general rule since exceptions are found specially during the formation of chemical sedimentary rocks and by cementation effects by silica/iron rich oxides that may turn soft sands into very hard rocks. However, the transformation of hard rocks into soft is imposed by the inverse physical–chemical equilibrium conditions to which high PT minerals are easily transformed through weathering or when subjected to hydration under lower conditions of pressure and temperature (retro metamorphism, for example). So harder rocks may be transformed into sot rocks or “altered” rocks. For practical reasons soft rocks which are differentiated from soils due to engineering reasons through a UCS of 5 MPa approximately will no more be considered soft when reaching a uniaxial compression strength about that of a strong concrete, that is, approximately 27.5 MPa and a modulus ratio above 200:1.