An experimental study on the cyclic behavior of natural loess

Instability problems have been observed along the Northern High Speed Line in France. It was suspected that these problems are related to the behavior of loess situated along the line, as collapse, fatigue and liquefaction under the cyclic load induced by the high speed train. In order to investigate the soil behavior in the laboratory corresponding to the above mechanisms, a cyclic triaxial cell was adapted in order to study the behavior of unsaturated soil. This development consists of using a differential pressure transducer for the measurement of the soil volume change in a triaxial double-wall cell. Cyclic triaxial tests were performed on saturated and unsaturated undisturbed soil specimens. The results from tests on initially saturated specimens showed that the soil taken at 2.2 m depth has the lowest resistance to cyclic loading, in relation to its highest porosity and lowest clay fraction. This soil was then studied at unsaturated state with various initial water contents. Unsaturated soil specimens were first subjected to cyclic loadings to decrease their volume. These cyclic loadings were stopped when the soil was considered to become saturated. Afterwards, the back-pressure tubing was saturated with de-aired water and cycles were applied under undrained condition. Significant effect of initial water content was evidenced: the lower the initial water content, the higher the cyclic resistance. This can be explained by the densification of the soil during the initial cyclic loadings.