Knowledge of earthen heritage deterioration in dry areas of China: salinity effect on the formation of cracked surface crust

Abstract

Numerous important earthen heritage sites in northwestern China have survived until today because of the arid and semi-arid climate. However, most of them suffer from various types and degrees of deterioration, with many questions not answered. Scaling off is one of the main types of deterioration widely observed at the Site of Yar City. To clarify the underlying mechanisms of cracked surface crust formation, previous experimental achievements are reviewed in the framework of unsaturated soil mechanics in this paper. A series of laboratory tests on the properties of local soil during desiccation, including water retention, volumetric shrinkage, cracking and tensile strength, are presented and analyzed, with a particular emphasis on salinity effect. Results show that suction is the key factor controlling the soil behavior. With the decrease in water content, all suction components increased gradually. The presence of NaCl led to a large increase in total suction, but a negligible change in matric suction. The soil shrinkage characteristic curve, which can be divided into three zones, was not affected by NaCl. The suction-loading curve indicates that the change in void ratio was governed by matric suction. In addition, most of the drying shrinkage of soil took place in saturated and near-saturated conditions. According to the quantitative analysis of final crack patterns, the highly fragmented surface morphology tended to be induced in thinner, less salinized soil specimens. The soil tensile strength also increased during drying, which was further enhanced by the cementation of salt crystals between particles. These preliminary results can provide some insights in understanding the on-site deterioration of earthen heritage sites.