Making unsaturated soil mechanics accessible for engineers: preliminary hydraulic-mechanical characterisation and stability assessment

The paper presents an accessible approach for preliminary hydraulic and mechanical characterisation of unsaturated soils and stability analysis of geo-structures under unsaturated conditions. The approach is ‘accessible’ in the sense that the laboratory investigation and geotechnical analyses can be completed in a relatively short time and at a reasonable cost. The overall philosophy behind this approach is that laboratory testing should not be eliminated but kept to a minimum and that any simplification introduced should be conservative. The paper illustrates that shear strength can be characterised conservatively by performing constant water content tests with no facilities to control/monitor suction and that water retention behaviour can be predicted successfully by performing a single water retention measurement. The paper also demonstrates that stability analyses can be carried out within the classical framework of limit analysis. We show that a preliminary analysis of the influence of infiltrating rainwater on suction can be carried out using a simple and conservative method that makes use of solutions available in ‘saturated’ geotechnical textbooks. Such an approach may be used to assess when it is worthwhile or necessary to conduct further time consuming and costly unsaturated experimental testing, field monitoring and numerical analyses when considering the stability of geostructures.