Geotechnical proficiency testing for field-based soil plasticity and expansivity assessment: Do we need it?

The intention of this study was to assess whether proficiency testing, as outlined in ISO/IEC 17043:2010, could be beneficial for field assessment of selected soil properties, similar to the laboratory ‘interlab’ system. The geotechnical profession is unique amongst engineering disciplines, as project-specific specialised quantitative testing is not always used to establish material properties used for analysis and design. For the numerous smaller projects, plasticity, for example, is typically assessed on-site through experience as opposed to laboratory testing. This experience is specific to each consultant and, in conjunction with no requirements for inter-consultant comparison, presents the position where industry-wide consistency is unknown.

The study undertook a proficiency test on soil plasticity logging between various geotechnical consultants using the procedure outlined in ISO/IEC 17043:2010 section 4.4. Participating consultants received a uniform soil sample for logging by at least two staff members. The participating companies and engineers were kept anonymous throughout the study. The participants were also asked to interpret the soil class according to AS 2870:2011. Results were collated and compared to a laboratory tested control sample to assess logging and interpretation consistency.

The implications of any variance between participants and laboratory tests were also explored. Whilst plasticity was the parameter chosen for investigation, the results of the exercise provide an insight into whether proficiency testing by a regulatory body or professional organisation would benefit the industry and its customers. Processes that reduce the inconsistency and variance in the field based assessment of soil performance characteristics could reduce the risk of excessive conservatism or the risk of underestimating plasticity and underspecifying foundations.