Unsaturated soil mechanics: Pitfalls in fundamental principles and engineering relevance

The last two to three decades have seen significant advances in the mechanics of unsaturated soils. It is now widely recognized that the fundamental principles in soil mechanics should cover both saturated and unsaturated soils. Nevertheless, there is still a great deal of uncertainties amongst the geotechnical community about how well-established soil mechanics principles for saturated soils can be extended to unsaturated soils. There is sometimes even uncertainty about the necessity of such extension in engineering practice. This paper discusses some common pitfalls related to the fundamental principles that govern the volume change, shear strength and hydro-mechanical behaviour of unsaturated soils. It also discusses the relevance of these principles in engineering practice, in attempt to answer some of the critical questions that are often raised in the geotechnical community.