Engineering Works Affected by Soft Rocks

Weak rock is usually defined as the intact rock presenting strength bellow about 25 MPa. Lately, more and more engineering projects are being built on sites dominated by soft rocks, as hard rocks might be absent in the region or the best geological sites have been already utilized. Many authors include in the category of soft rocks those rock masses that although presenting initially strengths above that upper limit, may be transformed into soft rocks by accelerated weathering, or behave as a weak rock mass with strong deformation due to high degree of jointing, high ground pressures, high temperatures, etc. Since soft rocks are usually less studied, there is a lack of knowledge about their properties and little confidence in their use. As a result, it is usual to adopt very conservative parameters for them, resulting commonly in additional economical expenses. The paper presents three cases (a dam foundation, a slope and an underground opening) to exemplify the influence of soft rocks in engineering works.