Tensile strength during drying of remoulded and compacted clay: The role of fabric and water retention

The paper presents an experimental investigation on the tensile strength of an unsaturated clay, both in remoulded and compacted states. To complete the experimental study, a new apparatus was designed with the purpose of determining direct tensile stress under controlled-displacement or controlled-force. The design exploits a direct shear test box, which was adapted to the scope of this investigation. Three different hydro-mechanical paths were followed before the tensile strength was determined: a) drying paths on remoulded samples; b) drying paths after compaction at different initial water contents; and c) dynamic compaction at different water contents, hence at different suctions and degrees of saturation. A microstructural investigation was also performed to better understand the experimental results and obtain the most relevant physical parameters. The preparation methods induced different initial fabrics, which dominated the water retention properties, as well as the tensile strength behaviour. A recent model for water retention, including the effects of multimodal fabric, was calibrated on available data and used to predict the suction of the samples along the drying paths, where direct information was not available. A simple model for tensile strength was proposed for its evolution, which included the role of multimodal fabric and their different water storage capabilities, the evolution of dry density and the initial water content (at the beginning of the tensile test path). For the compacted sample, typically showing a multimodal pore size distribution, the tensile strength was found to be a function of the state variables describing the macropore network. Dry density increases on drying appeared in all the cases to dominate the tensile response, Water exchange and suction increase were well correlated to the tensile response evolution.