Soil Desiccation Cracking Monitoring Using High-density Electrical Resistivity Tomography

In recent years,due to global climate change,extreme droughts and floods have occurred frequently.Under extreme arid climate conditions,the engineering soil is prone to desiccate,shrink and crack,which seriously damages the engineering properties of the soil,and is one of the causes of many engineering geological disasters and accidents.However,because traditional monitoring techniques are difficult to meet the monitoring needs of actual projects,accurately monitoring the development of soil shrinkage crack is a challenging task.Using high density electrical resistivity inversion technology and digital image processing technology,three experiments were systematically carried out during the evaporation and cracking process of soil,including an indoor three-dimensional monitoring experiment,an in-situ monitoring test of natural cracks and an in-situ monitoring experiments of artificial cracks in the field.Resistance,inversion resistivity,surface crack rate and water content in the three-dimensional space were analyzed in-depth.It effectively validates the feasibility of high density electrical resistivity inversion technology,and analyses the main causes of existing problems.Moreover,it also provides an important basis for improving accuracy and expanding application scenarios.Using high-density resistivity inversion tomography(ERT)and digital image processing techniques,this paper focuses on the development of desiccation and cracks in soils.By carrying out indoor and in-situ monitoring tests,the resistance,inversion resistivity,surface crack ratio,water content and other data in the three-dimensional space were calculated and analyzed respectively.All of them were used to verify the feasibility of ERT for soil shrinkage crack monitoring,and analyze the existing deficiencies of the technology.This paper provides an important basis for further improving the monitoring plan,enhancing the monitoring accuracy and expanding the application field in the future.The main insights gained in this paper are as follows:(1)The resistance has a very clear correlation with the generation and development of the cracks.Near the crack-generation region,the resistance time-history curves have a significant inflection point when the crack is suddenly generated,which can be used to monitor the crack generation process.Away from the crack-generation region,the resistance time-history curves do not have any obvious inflection point;therefore,the inflection point mainly reflects the process of the gradual extension of the crack.This is also the basis for inverting 3D images of soil shrinkage cracks from high-density resistivity data.(2)The high-density electrical resistivity inversion tomography can monitor the soil desiccation cracking process.In the indoor model test,the inversion resistivity can predict the occurrence of cracks,but it is also affected by stress conditions andsoil inhomogeneity.In the development and morphological construction of the main cracks,the inversion tomography has a good effect,and can clearly show the 3D dynamic development process and morphology of the cracks.As to other secondary cracks,the inversion results are deviated from the real cracks.Due to the field conditions and the ERT resolution limits,the in-situ test is so complicated that the results are not ideal.However,for simpler artificial simulated cracks,the monitoring aim can be achieved very well.(3)In the surface of the soil with obvious desiccation cracks,the inversion resistivity is dominated by the surface crack rate,but the inversion resistivity is significantly correlated with the water content at the depth at which the crack cannotextend to.in the indoor model test,soil The correlation coefficient between the surface crack rate and the surface averaged inversion resistivity is 0.85.In the in-situ test,the monitoring results of the natural soil shrinkage cracking process show that the trend of inversion resistivity and the water content change are completely consistent,which reflects the difference of the underlying soil structure and water content to a greater extent,but the effect of the crack is less obvious.(4)The high-density electrical resistivity inversion tomography can effectively realize the monitoring on the depth of soil cracks.In the in-situ test,by setting artificial simulated cracks to different depths,the monitoring results show that the abnormal depth of resistivity obtained by ERT is consistent with the real artificial crack depth.It indicates that ERT can reflect the development and morphological characteristics of the cracks on the profile.