Experimental Study Of Evaporation And Crack Characteristics In Cohesive Soils At High Temperatures

Being a typical kind of engineering rock soils,cohesive soils have a complex mechanism of the thermo-hydro-mechanical coupling,thereby becoming a hot topic in many engineering fields including energy,rock soil,geography,environment,and so on.Based on previous studies,in this paper,an extensive theoretical and experimental investigation on the evaporation and cracking characteristics of different cohesive soils under high temperature was performed.The main work include as follows:i)Evaporation characteristics:Taking two kinds of cohesive soils from Guiping(Guangxi)and Langfang(Hebei)as examples,the effects of the evaporation temperature,thickness,salt content and clay content on the evaporation rate were tested.Results showed that the evaporation process of cohesive soils under high temperatures can be mainly divided into two stages according to the evaporation rate,namely the constant rate stage and the falling rate stage.The former had a longer duration than the latter.Under the present experimental conditions,when the evaporation temperature was increased by 10?,the evaporation rate was increased by 1.2 to 1.5 times while the evaporation time was decreased by 1.32 times.Within the same the evaporation time,the thicker the soil layers were,the higher water contents were.The evaporation rate was almost not affected by the variations of the soil layer thickness.With the increase of the initial salt concentration,the vapour pressure on the soil surfaces tended to be decreased,thereby reducing the evaporation rate and even causing the crease of evaporation.The evaporation rate of silt and soils with less aggregate was higher than that of aggregate-rich soils.Based on the experimental results,a semi-empirical prediction model was developed,which can better describe the evaporation rate of bare cohesive soil under different temperatures.ii)Cracking characteristicsBased on a series of digital image processing techniques,a multi-index evaluation system was built for the quantitative analysis of cracking network images during the evaporation of cohesive soils.Taking three kinds of cohesive soils from Langfang(Hebei),Guiping(Guangxi)and Liangping(Sichuan)as examples,the microstructure and phase compositions were analyzed by means of SEM and X-ray diffraction.The cracking experiments of three cohesive soils with or without the additive of salt were conducted,and the onset,expansion and formation process of cracking was observed.The effects of the evaporation temperature and thickness on the cracking index were analyzed quantitatively,and the effects of the saline concentration on the cracking behavior were discussed.Results showed that the cracking began from the surface layer of the cohesive soils.When the matrix suction of soils was greater than the air entry value,the initial cracks was prone to form from the defect positions on the surface of soils.As the evaporation temperature increased,there was a decrease in the network knots and the number and length of cracks,but an increase in the area,length and width of crack zones.As the evaporation thickness increased,there was a decrease in the network knots,the number and total length of cracks,and number and average area of crack zones,but an increase in the average length,average width and cracking ratio of cracks,and the area of crack zones.In addition,as the saline concentration increased,cohesive soils suffered less surface cracks,and the fractal dimension tended to be decreased ranging from 1.65 to 1.20.The degree of cracking of cohesive soils increased with their plasticity index,with the order as follows: Langfang clay > Guiping red clay > Liangping clay.