Experimental Study On Temperature Threshold For Warm Frozen Soils In Terms Of Mechanical Properties

The development of energy and the rapid increase of engineering constructions in cold regions promote the research of frozen soil engineering.Warm frozen soils are a kind of special material which are commonly encountered in geotechnical engineering in cold regions.Because of its high content of unfrozen water,their mechanical properties are different from the cold frozen soils,which are rather similar to those of unfrozen soils.Recently,people are paying more and more attentions to this kind of special geological material,and the mechanical properties of warm frozen soils have become one of the important topics in frozen soil mechanics and engineering.Previous studies defined the warm frozen soil regions according to annual ground temperature which is a geographical concept.However,with respect to geotechnical engineering,it would be more reasonable and useful to define the temperature threshold according to the mechanical properties.In this paper,confined and triaxial compression tests were carried out on frozen soil samples,and multiple mechanical indexes were used to define the temperature threshold of warm frozen soils.Three soils,i.e.,frozen saturated ISO standard sand,silty sand and clay were taken as study objects.Mechanical properties such as cohesion,modulus and compressibility index were obtained.Based on the analyses of the changing tendency of these mechanical parameters along with temperature,it is attempted to propose a definition of warm frozen soils from the point view of mechanical properties.The research results are as follows:(1)For frozen saturated ISO standard sand,it was found that the coefficient of compressibility,compressibility index,cohesion and modulus has an abrupt change between-1.0? to-0.5?.Therefore,-1.0? may be defined as the temperature threshold for warm frozen standard sand.For frozen saturated silty sand,the analysis showed that the compressibility index increases as the temperature rises gradually,with a significant change at the temperature range of-2.0? to-0.5?.Considering that the stress-strain curve is relatively close when the temperature is-1.0? and-2.0?,it is suggested that the temperature threshold for frozen silty sand be between-1.0? to-2.0?.For frozen saturated clay,its mechanical properties have a significant change at the temperature range of-5.0? to-2.0?,and the properties are relatively stable with a high compressibility from-2.0? to-0.5?.Considering the compressibility index increases as the temperature rises rapidly at the temperature range of-3.0? to-2.0?,-3.0 ? can be defined as the temperature boundary for frozen saturated clay.(2)From standard sand to silty sand to clay,the temperature threshold of warm frozen soils decreases gradually.This is because when the content of fine grains is higher,the electric molecular attraction between particles increases,and the adsorbed water becomes thicker.While the freezing temperature is lower,the temperature boundary of warm frozen soils goes down.(3)This study provides a methodology for defining the threshold according to the mechanical indexes of frozen soils.The research is of great significance for frozen soil mechanics and engineering.