Study On Integrative Technology Of Treatment Of Subgrade On Black Cotton Soil And Pavement Structure

The construction of highway engineering is frequently faced with problems brought about by black cotton soil(BCS)in East Africa.The BCS has the characteristic of water swelling and water loss shrinkage,which can result in the crack of subgrade and pavement constructed on it.Therefore,it is necessary to deeply study the engineering characteristic and treatment technology of BCS and the pavement structure combination.In order to analyze the physical and chemical properties and strength characteristic of the BCS,the grain size analysis,mineral composition analysis,Atterberg limits,free swelling ratio tests,cation exchange capacity,the standard Proctor test and California bearing ratio(CBR)tests were conducted.The test results indicate that BCS is high in clay content,liquid limit,plasticity index,free swelling ratio,montmorillonite content,as well as the number of exchangeable cations,and low in CBR value.The high montmorillonite content is easy to absorb more exchangeable cations,which leads to the higher expansiveness of BCS.In order to analyze the initial cracking behavior of highway embankment in the regions of BCS soil,the change of peaks of tensile stress and its location on the top of embankment for a typical highway embankment section was simulated by Abaqus.The simulation results indicate that when the evaporation intensity is larger,the peak of tensile stress on the embankment top increases at a faster rate following the soil surface evaporation,and its location is closer to the shoulder.The thicker BCS layer helps the peaks of tensile stress to reach the critical tensile stress fast.The higher the embankment,the smaller peak of tensile stress occurred on the embankment top,and its location is more far away from the shoulder.The peak of tensile stress?crack extent and depth gradually decrease following the increase in the treatment deepness of BCS foundation,but the influence of treatment deepness on initial crack of embankment has large differences under different BCS layer thickness and embankment height.Therefore,the different foundation treatment deepness should be adopted under different BCS layer thickness and embankment height.The chemical and physical treatment technology of BCS embankment is proposed for its engineering characteristic.The influence of lime and volcanic ash on the engineering characteristic of BCS is analyzed by Atterberg limits,CBR tests and more.The deformation characteristics of highway embankment under different physical treatment technology are analyzed by numerical simulation.The results show that the lime and the combination of lime and VA significantly enhanced the engineering characteristic of BCS.Based on the code for design of highway subgrades,the BCS can be used as the highway embankment filling at the addition of 6%lime or 3%lime and 15%volcanic ash.When the side slopes of BCS embankment are enveloped with nonexpansive clay,the deformation of highway embankment decrease following the increase in the treatment deepness of BCS foundation and the width of nonexpansive clay,a design method for increasing the stiffness of BCS subgrade can be used to determine its top sealing thickness based on deflection equivalence.When the BCS foundation is treated with the geomembrane,the treatment deepness can be determined by the active zone of BCS.The surveillance of test road verifies the effectiveness of different physical treatment technology of BCS embankment.In order to analyze the adaptability of different pavement structures versus treated BCS embankment,the deformation and additional stress of pavement structure constructed on treated BCS embankment was simulated by Abaqus.The simulation results indicate that the influence of change in the thickness and modulus of the subbase on deformation of the pavement surface is small,but it significantly influences the additional stress of pavement structure.When the sandwich structure is constructed on the treated BCS embankment,the thickness and modulus of the subbase should be increased appropriately,but when the semi-rigid base pavement structure is constructed,the conventional modulus and design thickness of subbase can be adopted.Known from the grey incidence decision-making method,the sandwich structure of unbound graded aggregate paved fatigue-resistant layer is more adaptable to differential deformation of treated BCS embankment,compared to the semi-rigid base pavement structure.