Impacts of reference strain rate and strain level dependency on rate effects in fine grained soils

Viscous rate effects in fine-grained soils may occur in many engineering applications (e.g. dynamically penetrated anchors, sampling, rapid load testing of piles or high-speed soil characterisation), where high strain rates are induced during shear. Often it is difficult to correct for this enhancement of soil strength as fundamental understanding and quantification of rate effects is an area where further research is required. This is at least in part due to limitations of soil characterisation equipment and instrumentation. With the aim of tackling some of these issues a high-speed triaxial testing system was developed incorporating lubricated end platens, which was used to test kaolin at strain rates from 0·1 to 180 000%/h. This allowed observation of behaviour from fully drained, partially drained and undrained conditions, through to viscous effects. Results were used to investigate how rate effects are influenced by strain and consolidation level and how rate normalisation parameters are affected. This was then used to improve a commonly adopted rate characterisation approach such that it performed better across a wider range of strain rates and can be used for more consistent evaluation of rate effects between studies.