Influence of creep and drying/wetting on the initial stress state of clays

Abstract

The coefficient of lateral earth pressure at rest (K0) is an important parameter in any geotechnical problem since it provides information on the initial stress state, which governs the response of the soil to the proceeding stress changes. A proper determination of the stresses in any geotechnical problem is required to be able to predict the soil behaviour. Any change in the conditions the soil is subject to can alter the value of K0, how this value changes for conventional stress histories such as loading and unloading is defined well. However, the influence of unconventional stress histories such as creep or drying/wetting cycles is not thoroughly understood. Since every soil is subjected to natural environmental stresses, resulting in creep and unsaturated conditions, it is interesting to look at their influences of the soil stress state expressed by K0.

The goal of this work is to determine if it is possible to better understand what is happening to K0 during creep and under unsaturated conditions and if the prediction of K0 can be improved by accounting for these phenomena, with the focus being on clays. Literature showed that for saturated samples, the value of K0 increases with time during creep. For unsaturated conditions it was found that K0 decreases with an increase of suction.

In order to see if it is possible to improve the prediction of K0, a model needed to be constructed. The starting point of this model was a saturated, elastoplastic model based on the SANICLAY model. The first step in extending this model was to include viscosity which was done by adopting Perzyna’s overstress approach. The model was validated to experimental data on OostVaardersPlassen (OVP) clay obtained from literature and the validation showed that the model was satisfactory in predicting the soil behaviour. Accounting for unsaturated conditions was done by adopting the average soil skeleton approach. Implementation was initially done in the original elastoplastic model. Again, the model was validated to experimental data obtained from the literature, this time unsaturated loading/unloading tests on London clay (LC) were used. The results showed that the model prediction was accurate up to suctions up to 600 kPa. The final step in the model development was to include both Perzyna’s and the average soil skeleton stress approach in the basic model giving an unsaturated elasto viscoplastic model version. Unsaturated creep tests on London clay were used to validate the model but the results showed that an uncoupled stress-suction approach gave inaccurate predictions. The viscous nucleus in Perzyna’s approach was changed to become suction dependent and the results showed that the experimental data could be reproduced reasonably well.

The unsaturated elasto viscoplastic model was then used to analyse K0 during the unsaturated creep tests. The results showed that the model predicted a decrease in K0 with time for low loads and high suctions. For higher loads and low to moderate suctions, the model predicted an initial increase followed by a decrease. For all cases it was found that the value of K0 decreased with suction. The role of anisotropy on the model prediction was analysed by predicting the change in K0 using an isotropic version of the model. This version also predicted a decrease at low loads and high suctions but an initial increase was no longer predicted to decrease. The decrease of K0 with suction was still observed. No experimental data was available to confirm either of the findings, comparing the results with the literature study showed that the decrease of K0 with suction was previously observed. The decrease with time on the other hand was not found in previous work. However, the creep tests in previous works were performed on saturated samples and in general for a shorter time period which could show different results.

It is concluded that by accounting for creep and unsaturated conditions, the qualitative prediction of the soil behaviour can be improved. By accounting for coupled behaviour through an unsaturated viscous nucleus, the currently available unsaturated and time dependent experimental deformation data can be simulated accurately. The prediction of the change in stress state, due to these natural phenomena, is likely to be improved as well since the outcome of the model matches findings from the literature. However, due to the lack of experimental unsaturated time dependent data stating the change in K0, no conclusions can be drawn on the importance of anisotropy and the quantitative model performance. What this work does offer is a good modelling tool to support future experiments or investigations into unsaturated creep behaviour.