Abstract

Sensitive clay is the type of clay, which loses its shear strength when it is subjected to cyclic loading. High-rise buildings, towers, bridges etc., founded on sensitive clays and subjected to overturning moment are usually suffer from a steady reduction of the bearing capacity of their foundations and accordingly the safety factor. Cyclic loading of foundation on sensitive clay during the undrained period may lead to quick clay condition and catastrophic failure of the structure.
In the literature, governing parameters are listed as: cyclic deviator stress, pore water pressure, axial strain, pre-consolidation pressure, confining stress, initial degree of saturation, water content, liquidity index and the number of cycles. The present study has introduced the governing parameters in two categories; namely physical and mechanical as a function of sensitivity number of the clay material. A well planned experimental investigation was conducted to examine the effect of these governing parameters during the undrained and the drained periods of sensitive clay subjected to static or cyclic loadings. The soil samples, known as “Champlain clay” were obtained from the city of Rigaud, Quebec (Canada). Consolidation tests, static and cyclic undrained and drained triaxial tests were performed on representative samples of this clay.
Tests were conducted to identify the role of the key parameters governing this complex behavior during the drained and the undrained periods. The study examined individually the effect of cyclic loading, deviator stress, frequency, pre-consolidation pressure/OCR, and the confining pressure during the drained and undrained conditions. Absence or presence of the matric suction in fully saturated or partially saturated clay, effect of sensitivity number and liquidity index were also examined.
Based on the results of the present experimental investigation, a hypothetical model was introduced to explain the process of shear strength reduction for the case of static and cyclic loading of sensitive clay subjected to cycling loading. The model was capable to define the term remolding agent or degree of remolding, the reduction in shear strength due to remolding. The increase in the water content is identified as the most critical or intrinsic shear strength for sensitive clay.
The present study used the “Modified Cam Clay Model” to predict the factor of safety for a foundation on sensitive clay subjected to cyclic loading as function of the physical and mechanical parameters. A design procedure is developed to determine the safe zone for the undrained and drained responses, within which a combination of the cyclic deviator stress and the number of cycles for a given soil/loading/site conditions can achieve a quasi-elastic resilient state without reaching failure. The proposed design procedure is applicable to all regions around the world, where sensitive clays can be found. Furthermore, this procedure can be adopted to examine the conditions of existing foundations built on sensitive clay at any time during its lifespan.