| As a special soil mass,expansive soil accounts for about one third of China’s total territory,and widely distributes in the southwest China,northwest China,northeast China,middle and lower reaches of the Yellow River,middle reaches of the Yangtze River and southeast coastal areas in China.With the further development of China’s national economy and the continuous promotion of infrastructure construction,it is inevitable to perform civil engineering construction in these areas.In infrastructure construction projects,expansive soil often poses a potential risk to geotechnical structures due to the special engineering characteristics,and it is widely concerned by scholars and engineers.Expansive soil is a typical unsaturated soil with multiple fissures,strong expansion and contraction,and obvious strength delay under the action of dry and wet cycles.The expansive soil is famous for its expansive characteristic because of obvious reciprocal expansion and contraction.It is also called fissured soil because of its obvious fissure development.The main chemical components of expansive soil include silicon dioxide,aluminum oxide and iron oxide,with a total content of 81%～86%,plus a small amount of potassium oxide and magnesium oxide.The main mineral composition is quartz,followed by feldspar and mica.The clay particle size composition of expansive soil is40%～49%,and some soils even reach 70%.The content of colloidal particles with particle size less than 0.002 mm is 24%～37%,which belongs to highly plastic clay.The appearance of most expansive soil is grey,brown,red,yellow and black.Expansive soil generally has the property of expansion and contraction because there are a lot of hydrophilic minerals in soil mass.The hydrophilic minerals usually exist in the form of surface to surface contact or edge to surface contact.The micropores and microcracks between the particle structures form an excellent water seepage path.The strength of swelling and shrinking of expansive soil depends on the content of hydrophilic minerals mainly composed of montmorillonite in the soil.When the external water penetrates into the pores between clay minerals,the hydrophilic clay minerals,mainly montmorillonite,will form a thick hydration membrane,which will lead to the increase of the spacing between particles,and the macroscopic performance is water absorption expansion.When the expansive soil loses water,the original hydration film formed by water absorption and expansion will be greatly reduced,resulting in the reduction of the spacing between particles,which is shown as water loss shrinkage.In addition,due to the rapid increase of the overall volume of the soil after water absorption and expansion and uneven shrinkage during water loss,it is very easy to cause the soil to crack and form a crack structure.Although the degree of expansion and contraction of expansive soil is different,the engineering property of "water absorption expansion and water loss contraction" of expansive soil has caused great potential harm to engineering construction.Therefore,expansive soil is classified as a typical special soil with unfavorable geology.Physicochemical improvement method is widely used in the treatment of expansive soil subgrade.The improved method has obvious advantages in the cost control of expansive soil treatment and a large amount reduction of excavated spoil,which is conducive to environmental protection.The expansive soil improvement methods can be divided into physical improvement methods,chemical improvement methods and other improvement methods.Take lime to improve expansive soil as an example.The divalent calcium and magnesium ions in lime are easily adsorbed in potassium and sodium plasmas in the expansive soil particles.The combination of expansive soil and divalent calcium and magnesium ions will lead to the thinning of the hydration film,which will cause the expansive soil to disperse and disintegrate.The hydrophilicity,expansibility and plasticity index of the expansive soil will decrease,thus realizing the improvement of the expansive soil.Deeply analyze the expansive soil improvement technology using lime as additive,further explore the working mechanism of lime additive,and clarify the specific proportion parameters and treatment process of lime improved expansive soil,which is conducive to the promotion and application of lime improved expansive soil method in practical projects.On the basis of the existing research results,relying on the solid project of expansive soil slope on the left side of K2+035 ～ K2+120 section of the 4970 special line in Dangyang City,Yichang City,Hubei Province,the paper conducted indoor geotechnical test research on the physical and mechanical properties of expansive soil by selecting typical soil samples on site,and discussed the reinforcement effect of lime improved expansive soil treatment methods.Based on the summary and analysis of the existing classification methods of expansive soil expansion grade,the reasonable classification method of expansive soil was selected,and the expansive grade of Dangyang expansive soil sample was divided into weak expansive soil.In combination with the engineering application of lime improvement method,a range of 5%～8% lime was designed,and the corresponding amount of expansive soil sample was made to explore the change of physical and mechanical properties of expansive soil mixed with lime,providing guidance for the engineering application of lime improved expansive soil method.On this basis,in order to further study the evaluation method of the treatment performance of lime improved expansive soil,three-dimensional simulation analysis was carried out by using FLAC3 D software,and the rationality of numerical simulation was verified by combining the indoor soil test results,and the treatment performance of lime improved expansive soil was evaluated.The main research contents of this paper are as follows:(1)Based on physical engineering,soil mass identification was carried out according to the basic characteristics of natural expansive soil.On this basis,typical expansive soil samples were collected and carried out microstructure test,direct shear test,unconfined compressive strength test,unconfined expansion rate test,uniaxial compression test,free expansion rate test,etc.to obtain the main physical and mechanical properties of expansive soil.The comparative analysis test of lime improved expansive soil was carried out.The basic physical and mechanical properties and expansion characteristics of expansive soil with different lime content,such as free expansion rate,no-load expansion rate,loaded expansion rate,maximum dry density,optimal moisture content,compaction characteristics,shear strength characteristics,etc.were discussed.The service performance and availability of lime additive improved expansion were explored.The preliminary mixing range of lime modified expansive soil was determined.The research work includes:(a)The structural characteristics of expansive soil in Yanwumiao railway project were obtained by XRD,SEM and other soil tests.The influence of water content and overlying load on the shear strength of expansive soil was studied,and the physical and mechanical standards for soil treatment were provided.(b)A series of tests were carried out on undisturbed expansive soil and improved expansive soil,and parameters such as direct shear strength,unconfined compressive strength(UCS),unconfined expansion rate,uniaxial compression,and free expansion rate were obtained.The compaction test of expansive soil was carried out to obtain the main compaction characteristics of expansive soil.With the increase of overburden thickness,the trend of soil strain softening decreased rapidly.The envelope of soil shear strength was not a straight line,so power function can be used to fit the shear strength curve.(c)The expansive soil improvement tests with 5%,6%,7% and 8% lime content were carried out,which significantly reduced the liquid limit and plasticity index of expansive soil.Compared with plain expansive soil,lime improved expansive soil can improve its performance.Lime improved expansive soil had higher elastic modulus,shear strength and better soil stability,showing better engineering performance,and it can be used as subgrade material.(2)Based on the laboratory test of unconfined compressive strength of Yanwumiao expansive soil,further analysis was carried out by using the threedimensional numerical simulation software FLAC3 D,and the corresponding numerical model was constructed to determine the model parameters during the loading process.The numerical simulation method could effectively reflect the stress rising stage before the failure of the model and the rapid stress reduction state under the brittle failure state.M-C model was selected as the constitutive model of the improved expansive soil,and the parameters of the calculation model were reasonably assigned based on the geotechnical test results.The improved expansive soil samples with 7% and 8% lime content were selected for the simulation of unconfined compressive strength test.The variation of soil physical and mechanical parameters was used to simulate the difference of lime content,and the unconfined compressive strength of expansive soil samples under different loading speeds was analyzed.Comparing the indoor test results and simulation results of unconfined compressive strength of Yanwumiao expansive soil,the reasonable values of model parameters in the loading process was determined,and a simulation analysis method for the road performance evaluation of lime improved expansive soil was formed.The research results provided a reference for the road performance evaluation of lime improved expansive soil.Based on the research work carried out in this paper,the following conclusions were drawn:(1)According to the test results of basic physical and mechanical parameters of Dangyang expansive soil,the clay content of plain soil sample was 30%,and the liquid limit was 45%.It can be inferred that the soil sample was a cohesive soil with low liquid limit.According to XRD test results,Dangyang expansive plain soil sample contained 60% clay minerals,mainly composed of illite and montmorillonite.According to the test results of free expansion rate,the average free expansion rate of plain soil sample was 47.3%.Based on the comprehensive analysis of test measurement indicators,Dangyang expansive soil was judged as weak expansive soil.(2)The direct shear test of expansive soil sample was carried out.With the increase of the overburden load,the strain softening trend of the soil rapidly decreased,and the shear strength envelope of the soil was not a straight line.Power function can be used to better match the shear strength curve of expansive soil.In slope engineering,the determination of shear strength of expansive soil should pay attention to the overlying load of soil.(3)The no-load expansion rate test of expansive soil was carried out,and the process of uncharged expansion of soil was divided into four stages: rapid expansion,transitional expansion,moderate deformation,and stable expansion.With the increase of the initial water content of soil sample,the water absorption rate and loaded expansion rate of soil also decreased.When there was no overlying load,there was a linear relationship between the change of soil moisture content and the no-load expansion rate.(4)The test results of lime improved expansive soil showed that with the increase of lime content,the plastic limit,liquid limit,plastic index,maximum dry density and optimum moisture content of soil basically showed an increasing trend,while the free expansion rate showed a decreasing trend.The increase of lime content caused the soil to become harder and stronger,so when the peak load required for soil failure increased,and the strain at failure also decreased.The compaction characteristics of the treated expansive soil depended on the plastic properties of the original soil.For highly plastic expansive soil,the maximum dry density was reduced and the optimum water content was increased because of of the addition of lime.For non high plastic soil,the compaction characteristics of lime treated soil basically remained unchanged.(5)The test of free expansion rate of plain soil sample of Dangyang expansive soil and lime improved expansive soil showed that the results of free expansion rate of many groups of plain soil samples were quite different,and the free expansion rate of expansive soil sample after lime treatment showed a trend of linear decline first and then maintaining constant with the increase of lime content.The analysis and test results showed that the montmorillonite content of expansive soil was linearly related to the lime content,and the addition of lime could effectively control the content of expansive mineral montmorillonite in the soil,thus fundamentally inhibiting the expansion.With the increase of lime content,the montmorillonite content also decreased linearly until it reached a stable value.For expansive soil samples without lime treatment,the montmorillonite content of the samples was different due to sampling differences,so the problem of large difference in free expansion rate was easy to occur.However,the content of montmorillonite of expansive soil treated with lime decreases with the continuous increase of lime content,but it could reach a relatively stable value in the end.Therefore,the free expansion rate of treated expansive soil samples showed a trend of decreasing first and then stabilizing.(6)Based on the direct shear test,the shear strength test of lime treated expansive soil sample showed that after adding lime admixture,the expansive property of expansive soil decreased rapidly,and the shear strength was significantly improved.With the increase of ash content,the shear strength parameters of soil,cohesion c and internal friction angle φ increased accordingly.The analysis mainly considered that the coarse particle composition of expansive soil improved by lime increases,and the internal friction angle increased significantly.The increase of lime content greatly reduced the expansion potential of expansive soil and increased the strength characteristics of expansive soil.(7)The test results of unconfined compressive strength of improved expansive soil with lime content of 5%～8% showed that when the lime content was 5%,the unconfined compressive strength of expansive soil samples solidified for 7 days reached 986 k Pa;The unconfined compressive strength of expansive soil increased with the increase of lime ratio and curing time.(8)The no-load expansion rate test of lime improved expansive soil showed that the lime mixing ratio of the designed sample was 5%～8%,compared with the untreated expansive soil,the free expansion rate decreased to 12.7%,11.7%,13.7% and 9% respectively,indicating that lime can control the expansion of soil mass.The expansive soil samples with 5%,6%,7% and 8% lime ratio were made.The design curing period was 7 days.The moisture content of the samples was controlled to be 15.3%,17.8%,20.3% and 22.8% respectively.The measured no-load expansion rates of soil samples with different lime ratios were 2.14%,1.84%,0.9875% and 0.58% respectively.With the increase of lime ratio,the no-load expansion rate of expansive soil continued to decline.The test results proved the effectiveness of lime in improving expansive soil.(9)The simulation results of unconfined compression tests of expansive soil modified with 7% and 8% lime at different loading speeds showed that the stress of expansive soil modified with 7% and 8% lime increased with the increase of strain(the corresponding strain in the data acquisition stage is 0.4%,0.8%,1.2%,1.6% and 2% respectively).It can be seen that with the increase of lime content,the compressive strength of the specimen also increased,and the axial strain corresponding to the peak stress point was about 2%.When the specimen strain changes between 0.4% and 2%,the compressive stress of the specimen also increased continuously.When the load exceeded the limit stress,the model was damaged,the strain increased rapidly,and the stress at the top of the test also decreased.The simulation results were similar to the test results of lime modified expansive soil.(10)According to the simulation and test results of expansive soil improved by 7% and 8% lime,the unconfined compressive strength of soil samples increased significantly with the increase of lime content.When the strain of expansive soil was 2%,the maximum stress peak was about 700 k Pa,which was close to the experimental results.The simulation results showed that the unconfined compressive strength of expansive soil can be significantly improved by adding lime.The unconfined compressive strength of expansive soil increased significantly with the increase of the content of modified materials.Based on the above research results,the following suggestions and prospects are proposed for the treatment technology of expansive soil:(1)The research work of this paper mainly focuses on the chemical treatment method of expansive soil improvement,and the treatment agent is lime.For the treatment of expansive soil,more extensive treatment methods can be studied,such as the physical and chemical improvement methods using bacteria,enzymes,blast furnace slag,geo polymers and other materials as additives,and the advantages and disadvantages of controlling expansive soil expansion performance can be compared and analyzed to provide guidance for the comparison and selection of improvement schemes.(2)Under the condition of controlling temperature and humidity,a limited number of indoor soil tests were conducted on representative soil samples in this paper.The rationality of the results and the significance of engineering guidance were affected by the nature and size effect of soil samples.Therefore,large-scale field tests can be further carried out as an expansion study in the future work,considering the comprehensive impact of seasonal changes in soil moisture content and temperature,and comparing the field test results with laboratory test data.(3)Limited by test conditions and funds,only one typical soil sample was selected for the study of lime improved expansive soil in this paper,and the universal significance of the research object needs to be further verified.In the future work,expansive soil samples from other regions can be selected to carry out similar experimental studies on lime improved expansive soil,so as to compare and analyze research work in this paper with the expanded research results,and further clarify the scope of application of lime improved expansive soil method. |
PDF Full Text Request