In: Stagg KG, Zienkiewicz OC (eds) Rock mechanics in engineering practice. Imperial College Press, London, p 554ĭeere DU (1968) Chapter 1: geological considerations. Balkema, Rotterdam, pp 65–71Ĭosgrove J, Hudson J (2016) Structural geology and rock engineering. In: AGI (ed) International symposium on indurated soils and soft rocks. Keywordsīosio JJ, Kanji MA (1998) Soft rocks of the Rio de la Plata basin. 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.

So harder rocks may be transformed into sot rocks or “altered” 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). 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. 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.

One may think that geological affiliation and environmental conditions of formations are the main hint to understand the mechanical properties of the rocks.

Soft country rock