Civil Engineering April 2021 | Vol 29 No 3

22 April 2021 Civil Engineering In recent years, technological advancements in the field of unsaturated soil mechanics have helped improve understanding in this field for both researchers and engineering practitioners. Gaspar & Jacobsz (2019) pre- sented a short overview of some key prin- ciples of unsaturated soil mechanics and highlighted how measurement techniques (both in the laboratory and in-situ) are becoming increasingly available in South Africa. The purpose of this article is to pre- sent a review of the two years following that publication to illustrate the uptake of these developments in engineering practice. LABORATORY TESTING The relationship between soil suction and water content, referred to as the soil water retention curve (SWRC), is often regarded as the most fundamental relationship gov- erning the behaviour of unsaturated soils (Fredlund, 2018). Whereas conventional soil characterisation methods have long utilised, at the very least, grading curves and Atterberg limits, any application of unsaturated soil mechanics necessitates knowledge of the SWRC. Gaspar & Jacobsz (2019) described some developments at the University of Pretoria (UP) which facilitated the measurement of the SWRC with con- tinuous measurement of changes in water content, suction, and sample volume. Since that publication, the university has received numerous requests from various consulting firms to measure SWRCs. It has become increasingly evident that the information contained in the SWRC is particularly useful in modelling seepage problems and to assess surface infiltration and evaporation for water balance purposes. Figure 1 illustrates the results of some typical SWRCs measured at UP. Included in this figure are measurements on two railway formation materials labelled “granular layer” and “compacted fill”, as well as a measurement on a clayey sand. The measurement on the clayey sand contains a portion which was measured continuously using UP’s in-house tensi- ometer, as well as discrete measurements in the high suction range performed using a dewpoint hygrometer. Since 2019, substantial improve- ments have been made to the laboratory equipment used to measure SWRCs. The relationship between soil suction and water content is not unique, and often exhibits hysteretic behaviour depending on whether a wetting or drying path is followed. Despite this knowledge, drying curves are generally measured regardless Application of unsaturated soil mechanics – two years on Dr Tiago Gaspar Postdoctoral Research Associate Durham University (formerly of University of Pretoria) tiago.gaspar@durham.ac.uk Rick Vandoorne PhD candidate University of Pretoria vandoorne.r@gmail.com Prof. Schalk Jacobsz Pr Eng Associate Professor University of Pretoria sw.jacobsz@up.ac.za Prof. Gerhard Heymann Pr Eng Professor University of Pretoria gerhard.heymann@up.ac.za Prof. Hannes Gräbe Pr Eng Associate Professor University of Pretoria hannes.grabe@up.ac.za Degree of saturation (%) 100 90 80 70 60 50 40 30 20 10 0 Suction (kPa) 10 000 1 000 100 10 1 0.1 Granular layer Compacted fill Clayey sand (continuous) Clayey sand (discrete) Figure 1 Typical SWRCs measured at UP unsaturated soils laboratory Since 2019, substantial improvements have been made to the laboratory equipment used to measure SWRCs