| The South-to-North Water Diversion Project is a large-scale water conservancy hub project organized and constructed by China,and is currently the largest water conservancy project in the world.It aims to allocate water resources more evenly between the northern and southern regions by rationalizing the integration of water resources across regions,providing great development opportunities for economic and social development,as well as people’s production and living in both regions.The study area of this article is in the Nanyang section of the middle route of the South-to-North Water Diversion Project.The main channel passes through a section of over 100 kilometers of expansive soil,which has the characteristics of water absorption and expansion,as well as shrinkage upon water loss.Due to the expansive and contractive nature of the soil,the existence of cracks in the soil mass has a significant impact on the engineering properties.On one hand,the presence of cracks destroys the integrity of the soil mass,greatly weakening its strength.On the other hand,due to the existence of cracks,rainwater can directly penetrate the deep soil mass during rainfall,and the deep soil mass will also be directly affected by evaporation during droughts.As a result of various factors,cracks in the soil mass will gradually develop deeper,leading to canal slope damage during construction and daily maintenance.Cement modification of the expansive soil surface is an effective slope treatment method.By mixing a certain amount of cement into the expansive soil,the potential for soil expansion can be reduced,the shear strength of the soil can be increased,and its water stability can be improved.Cement modification can effectively slow down or stop the development of cracks and,at the same time,provide moisture protection for the underlying expansive soil,reducing the depth of the atmospheric influence on the canal slope.This article focuses on the expansive soil in the Nanyang area of the first section of the South-to-North Water Diversion Project.Through various research methods such as field investigation,laboratory experiments,and numerical simulation,the nonsaturated hydraulic parameters of the cement-modified expansive soil,infiltration laws,and the development rules of cracks in the expansive soil before and after modification were studied.The distribution of water content in the deep soil after rainfall infiltration and drying of expansive soil at different degrees of cement modification,and the development of cracks in depth were obtained,followed by a comprehensive discussion on the variation law of the depth of atmospheric influence.The specific conclusions are as follows:(1)The greater the matrix suction of the expansive soil,the smaller the corresponding volumetric water content.At the same matrix suction,the water content of the cement-modified expansive soil is lower and decreases with the increase of cement mixing amount.(2)The development degree of cracks in the cement-modified expansive soil is significantly lower than that in the unmodified expansive soil.With the gradual increase of cement mixing amount,the development rate and maximum crack ratio of cracks will gradually decrease.This is because the cement particles mixed into the soil have a bonding effect with the original soil particles,which enhances the cohesion between the soil particles.At the same time,the cement particles and expansive soil particles have a gelation effect,and the particles cluster together,reducing the original expansive and contractive nature.(3)Unmodified expansive soil has a faster infiltration rate,while the permeability of expansive soil modified by cement decreases significantly,leading to a slower water seepage rate.With an increasing amount of cement added,the anti-seepage and moisture retention effects on the underlying soil continuously improve.When external water supply occurs,the depth of water infiltration gradually decreases,and during evaporation,the depth of water loss also gradually decreases.Overall,there is a clear relationship between the degree of cement modification and anti-seepage and moisture retention.Effective improvement of the anti-seepage and moisture retention performance of the channel slope can be achieved when the degree of modification is6% and 8%.(4)After different degrees of cement modification were applied to the surface expansive soil of the channel slope,it was found that cracks developed within the modified layer,with a lower crack rate observed in layers with higher modification degrees.Furthermore,the crack rate gradually decreased with increasing depth.When the modification degree was 6% and 8%,the degree and depth of crack development were the lowest,and the underlying natural soil layer was not affected by atmospheric changes.The reduction in crack rate effectively played a role in preventing seepage and maintaining moisture in the channel slope.(5)In practical engineering,it is recommended to apply 6% cement modification to the surface layer of the expansive soil channel slope.This can significantly reduce the development of surface cracks,reduce the depth at which the soil is affected by atmospheric conditions,and provide a moisturizing effect to the underlying soil layer.This is beneficial for the stability of the expansive soil channel slope.However,it is important to pay attention to the failure of the modification thickness during maintenance to prevent further crack development.Therefore,the modification of the surface layer of the expansive soil channel slope is an effective prevention and control measure in practical engineering.The water permeability of the cement-modified expansive soil is greatly reduced,effectively preventing large-scale infiltration and evaporation of water,and the depth at which atmospheric conditions affect the soil is significantly reduced.At the same time,the degree of crack development after dry-wet cycles is also reduced,effectively stabilizing the stability of the channel slope.This thesis includes 41 figures,15 tables,and 83 references. |
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