Long Term Settlements in Soft Clays

Doktorsavhandling, 2003

When structures are founded on deep layers of lightly overconsolidated, soft clay large settlement often occur during a long period of time after completion of the structure. Therefore it is of major importance to be able to calculate and predict the expected settlements with a satisfactory degree of accuracy, as this has a great impact on the design of the foundation and the possible need for soil improvements. The objective of the present study was to improve the accuracy of the methods for predicting settlements over time in deep layers of soft clays with a greater accuracy, with particular attention to loads that results in an effective stress around the preconsolidation pressure. In order to accomplish this, the empirical base was extended regarding creep behaviour and the creep parameter. The study also included testing of samples from deep layers, depths between 25 and 70 m below the ground surface, of clays. A number of laboratory test were conducted in order to document and study the behaviour under various testing conditions, thereby providing a better understanding and knowledge of the consolidation process. Furthermore, a number of test sites were examined, where the soil conditions could be documented in detail and where the settlements had been monitored over a long period of time. These test sites constitute a vital base for verifying the settlement calculation model. This work constitutes the base for developing a computer code for modelling both laboratory tests, such as incremental loading tests, and full-scale field conditions. During this work the parameter models, with emphasis on the compression modulus and the creep parameter, were studied and improved. The model implemented in the computer code is able to predict the stress - strain behaviour and the pore pressure dissipation with respect to time under various conditions with a satisfactory degree of accuracy. For full-scale settlement calculations compared with case records the most significant results were, firstly, that the agreement was apparently very good and, secondly, that the contribution of calculated creep effects to settlements could be equal or greater than the calculated settlements excluding creep effects.