Experimental Study On Cracks And Water Characteristics Of Laterite Under Dry-wet Cycle

In practical projects such as slopes,highways and building foundations,most of the soil encountered is unsaturated soil,and the solid,liquid and gas phases in the unsaturated soil change all the time under the influence of the natural environment,which makes its engineering properties become complex and difficult to grasp.As a main distribution area of red soil in China,red soil is widely distributed in Jiangxi Province.The change of laterite saturation has a great impact on the stability of slope,roadbed and other projects,especially in the natural environment after the dry-wet cycle of rainfall and evaporation,the soil will crack,which is more likely to cause safety risks under the action of rainfall.Therefore,it is necessary to study the cracking and unsaturated properties of soil under dry-wet cycle,and the study of soil-water characteristic curve and permeability coefficient is the key to the study of unsaturated properties.In this paper,taking Jiangxi laterite as the research object,the crack test of unsaturated laterite under dry-wet cycle was carried out,and the soil-water characteristic curve test of steady-state and dynamic multi-step overflow method was carried out by pressure plate instrument.the permeability characteristics of unsaturated laterite were studied by dynamic multi-step overflow method,soil-water characteristic curve prediction method and wetting front advance method based on soil column seepage test.Through the finite element numerical simulation,the permeability coefficients obtained by the three methods are used as parameters in the simulation of soil column seepage test,and the differences between simulation and test results are compared.The main contents and conclusions are as follows:(1)the law of crack development of samples with different compaction degree under dry-wet cycle is studied.the four quantitative indexes of surface crack ratio,total crack length,average crack width and absolute shrinkage all decrease with the increase of compactness.it is found that high pressure compactness has an inhibitory effect on the development of cracks.(2)Based on the pressure plate instrument,the soil-water characteristic curves of non-dry-wet cycling samples with different compaction degrees and cracked samples produced by dry-wet cycling were obtained.By comparing the test data,it is found that the difference between the soil-water characteristic curve measured by the dynamic multi-step overflow method and the steady-state multi-step overflow method is very small,and the dynamic multi-step overflow method can greatly save the test time.The dynamic multi-step overflow method can be used instead of the steady-state multi-step overflow method to obtain the soil-water characteristic curve.(3)The influence of dry-wet cycle and compaction degree on soil-water characteristic curve is analyzed.Under the same matrix suction,the volume moisture content of the dry-wet cycle sample is lower than that of the non-dry-wet cycle sample;under the same matrix suction increment,the slope of the soil-water characteristic curve of the dry-wet cycle sample is steeper than that of the non-dry-wet cycle sample;the lower the compaction degree of the sample is,the steeper the slope of the soil-water characteristic curve is.(4)Based on the dynamic multi-step overflow test of pressure plate instrument,the permeability coefficient of unsaturated laterite is studied by theoretical analysis,and the unsaturated permeability coefficient is predicted based on soil-water characteristic curve.The effects of dry-wet cycle and compaction degree on unsaturated permeability coefficient are analyzed.Under the same matrix suction,the permeability coefficient of dry-wet cycle is larger than that of non-dry-wet cycle.The greater the degree of compaction,the smaller the permeability coefficient of the sample under the same matrix suction.(5)based on the soil column method,the permeability coefficient is studied,and the wetting front advance method is used to record the change of the volume water content of the monitoring point during the arrival time of the wetting front,and the permeability coefficient under a certain moisture content can be calculated by theoretical analysis.this method can obtain the unsaturated permeability coefficient directly.(6)The trend of the relationship between permeability coefficient and saturation calculated by dynamic multi-step overflow method,soil-water characteristic curve prediction method and wetting front advance method based on soil column test is similar,but under the same saturation,the permeability coefficient calculated by the wetting front advance method is about an order of magnitude larger than that calculated by the dynamic multi-step overflow method.Both CCG and VGM model based on soil-water characteristic curve can predict the permeability coefficient calculated by dynamic multi-step overflow method,but the prediction result of CCG model is closer.(7)The soil column seepage test of unsaturated laterite is simulated,and four kinds of unsaturated permeability coefficients calculated by three methods are substituted into the finite element software for numerical simulation,and the differences between simulation and test results are compared.The results show that the numerical simulation can better restore the actual seepage,but based on the permeability coefficient calculated by the wetting front advance method in the soil column test,the numerical simulation results are most consistent with the actual seepage.