Study On The Strength Characteristics Of Frozen Coarse-grained Soil

With the increase of the scale of infrastructure construction in cold regions and the development of urban underground space in China,frozen coarse-grained soil is often encountered during the construction process.The strength of frozen soil is the key parameter in the construction of frozen soil engineering.Due to the influence of different soil types,water content,temperature and deformation rate,the strength law of frozen coarse-grained soil is difficult to grasp.This also leads to restrictions on the use and promotion of emerging technologies represented by the freezing construction on coarse-grained soil layers.In order to ensure the stability and safety of frozen soil engineering,it is very important to study the strength characteristics of frozen coarse-grained soil.Based on the current research methods of the strength of frozen soil and conventional coarse-grained soil,a test instrument and a test program that can be used to study the strength of frozen coarse-grained soil is designed in this paper.The coarse-grained soil material in the Sanlitun area of Beijing is taken as the research object,and the unconfined compressive strength characteristics of frozen coarse-grained soil are studied.Through the unconfined compression test,the variation law of the strength of frozen coarse-grained soil under the influence of temperature,strain rate and gravel content is systematically studied.The test results were compared with the classical formulas in the in the literature,and the relationship between temperature and strain rate and the parameters in the formula is analyzed.From the perspective of the gravel-sand-ice structure,the influence mechanism of the gravel content on the strength of frozen coarse-grained soil is analyzed.The main research results obtained are as follows:(1)A test instrument used for the research of the strength of frozen coarse-grained soil is developed.According to the characteristics of frozen coarse-grained soil,three parts of the instrument,including temperature control unit,loading unit and numerical data acquisition unit,have been improved and upgraded,and a practical solution for strength test of frozen coarsegrained soil is given.(2)The effects of temperature and strain rate on the uniaxial compressive strength(UCS)and failure characteristics of frozen coarse soil is studied systematically through the uniaxial compression test.It is found that the stress-strain relationship of the frozen coarse-grained soil samples under uniaxial compression is mostly strain softening,and the inclination angle of the failure surface of the samples is approximately between 55° and 58°.The UCS of frozen coarsegrained soil samples increases with the decrease of temperature and the increase of strain rate.A functional relationship that can describe the change of frozen coarse-grained soil UCS with temperature and strain rate is established The relationship between UCS and temperature can be described by the linear function or the power function,both of which have good fitting degree.From the point of view of simplicity and practicability,the linear formula is recommended;the relationship between UCS and strain rate can be described by the power function.(3)Under the research conditions,the influence of gravel content on the UCS of soil samples is not obvious,and there is no consistent relationship between them.In general,UCS decreases with the increase of gravel content,but there is no strict negative correlation between them.At high temperature and high strain rate,the effect of gravel content on compressive strength is obvious.At low temperature with low strain rate,the effect of gravel content on compressive strength is small.