Rate Effect Behaviour of Different Clays from High Speed Triaxial Element Testing

Abstract

The rate effect response of three natural clays has been studied using high-speed triaxial element testing at axial strain rates up to 100,000 %/hr. Strain rate effects lead to increasing soil shear strength in fine grained soils under undrained conditions and are important for accurate predictions in a number of geotechnical applications. These include Rapid Load Testing (RLT) of piles, Free-Falling Penetrometers (FFP) and Deep Penetrating Anchors (DPA), as well as in the numerical modelling of high strain-rate activities in general. The triaxial testing used a number of techniques to improve the accuracy of the results including lubricated end platens to minimise strain localisations and a new mid-height pore pressure sensor capable of measuring rapid pore pressure changes.

Each of the three clays investigated were tested over a range of initial mean effective stresses to consider the impact of soil state and it was identified that rate effects increase with moisture content. These rate effects were shown to lead to increasing soil strength, reducing pore pressures and expanding yield envelopes. The overconsolidation ratio (OCR) was found to have no impact on the observed rate effects.

Hall effect local strain sensors were used to allow rate effects on the deviator strength to be measured throughout the strain range, from pre-yield up to the point of localisation. The rate effects were found to be highly strain dependent, with negligible rate effects observed before yield and the point of maximum rate effect occurring before 1 % shear strain. A new model for the variation of rate effects with strain across the entire measured strain range (up to the point of localisation) has been developed, which can also account for the impact of soil state and the soil properties.

The triaxial testing was combined with in-depth characterisation of the three soils, allowing the properties influencing rate effects to be determined. It was found that for the clays tested, the Specific Surface Area of the clay determined from Methylene Blue spot testing (SSA-MB) provided a useful rate effect indicator, with the observed rate effects increasing as the specific surface area reduces. The gradient of the critical state line (CSL) in q-p’ space, M, derived at an axial strain rate of 1 %/hr was found to be another possible indicator, as SSA and M are inter-related. Additionally, M is more widely available than SSA in geotechnical studies.

Date of Award	2019
Original language	English
Supervisor	Michael Brown (Supervisor)