Study On Mechanism Of Soil Desiccation Cracking Based On Fracture Mechanics

Desiccation cracking of soils is a common natural phenomenon,which can greatly weaken the engineering properties of soils and is the direct cause of many geotechnical,hydraulic and geological engineering problems.Although previous studies have been carried out from different perspectives,and many important results have been achieved,due to the complexity of the soil itself and the diversity of influencing factors,there is no unified understanding of the mechanism of the development of dry shrinkage cracks,especially the mechanical mechanism contained in the phenomenon of cracks.Therefore,it is of great significance to explore the occurrence,development law and related mechanical mechanism of soil shrinkage cracking for guiding engineering geology disaster prevention and mitigation in arid areas and making rational and effective use of clay materials.In this paper,the application of fracture mechanics theory in this subject is systematically summarized and summarized based on the work of soil shrinkage cracking in recent years at home and abroad.Taking Xiashu soil in Nanjing area as the research object,strictly controlling the moisture content of the sample,simulating the evaporation of soil under natural conditions,three-point bending fracture test was carried out to investigate the influence of moisture content and loading rate on the fracture toughness of soil,and to test the law of crack tip opening angle changing with time in the process of fracture failure.In addition,real-time photographs of samples were taken by camera to record the whole process of crack generation,development and fracture.At the same time,the stress and strain state of soil in the process of fracture was analyzed by combining DIC(digital image correlation)technology.Finally,the soil dry shrinkage cracking test is carried out,the crack network is analyzed quantitatively,and the mechanical mechanism in the process of soil dry shrinkage cracking is discussed in combination with fracture mechanics.The main achievements of this paper are as follows:(1)Fracture process of soil can be divided into three stages:(?)load rising stage,that is,with the increase of specimen displacement,the load increases continuously until the ultimate strength of soil is reached;(?)overall failure stage,with the increase of displacement,the specimen gradually demonstrates failure after the maximum load(fracture toughness),micro-cracks begin to appear at the pre-crack,and the width of micro-cracks begins to appear.The depth and depth expand rapidly under the action of load until fracture,and the load decreases rapidly with the increase of displacement after reaching the peak value;(?)In the failure stage,the crack has penetrated the whole sample,and the soil has been completely broken.With the increase of displacement,the load tends to zero.(2)Fracture toughness reflects the ability of soil to resist crack propagation and is an important mechanical index of soil.With the decrease of water content,the fracture toughness of soils increases gradually,and the ability of resisting crack propagation of soils increases gradually.On the contrary,with the increase of soil moisture content,the fracture toughness of soil decreases gradually,and the ability of resisting crack propagation of soil decreases gradually.(3)When the soil moisture content is lower than the critical water content(about 15%),the fracture toughness increases gradually with the increase of loading rate;when the soil moisture content is higher than the critical water content(about 15%),the loading rate is in the range of 0.5-2.0 mm/min,and the fracture toughness is almost not affected by loading rate.(4)DIC technology can track the state of soil surface during the occurrence,development and destruction of soil cracks in real time.It can effectively and rapidly acquire the displacement field and strain field of soil surface,and visually display the dynamic process of crack development.As a high-precision dynamic and non-contact monitoring method,it has a unique advantage over traditional soil deformation measurement methods in the study of soil shrinkage cracking characteristics,and has great application value.(5)When the water content of soil is less than 17%(saturated water content),the opening angle of crack tip decreases with the increase of water content.When the water content is higher than 17%,the opening angle of the crack tip hardly changes with the change of water content.When the loading rate is in the range of 0.5-2.0 mm/min,it has little effect on the opening angle of the crack tip.(6)The generation of soil fracture and the appearance of dry shrinkage cracking are caused by the imbalance of stress or energy in the soil.The mechanical mechanism of dry shrinkage cracking can be reasonably explained by fracture mechanics.The development rate of dry shrinkage cracks decreases with the increase of fracture toughness.