The Interface Characteristics Of Reinforcement Soil And The Treatment Of Granular Material Wrapping At The Junction Of New And Old Subgrade

As a commonly used reinforcement material in highway reconstruction and expansion,geogrid plays an important role in controlling the differential settlement of old and new roadbeds.The interfacial friction characteristic between geogrid and soil is important for quantifying the reinforcement effect.However,in the hot and humid southern regions reconstruction and widening of expressways,the j oint surface laid by the grid is relatively loose with a high moisture content,resulting in a weak interface strength.In this case,it is difficult to give full play to the strength of the grid,and then reduce the differential settlement control effect of the new and old roadbed.There are few studies investigating the characteristics of geogrid-soil interface at present.It is of theoretical significance and engineering application value to research it.On the basis of the literature surveys and the investigation of the reconstruction and expansion project of the G40 Shanghai-Shanxi Expressway from Hefei to Dagudian,some onsite excavation tests on the slopes of the old road were carried out in this thesis,and the variation laws of compaction and moisture content at different layers of the old road were analyzed.The basic physical properties of the subgrade fillers for the widened subgrades had been tested,which is helpful to guide the on-site construction.Then,the test parameters were determined by the pull-out tests according to the engineering conditions and the filler properties,and the effect of the normal stress,grid types,filler moisture contents and compaction degrees on the interaction characteristics of the reinforcement-soil interface was analyzed.The uniaxial and biaxial grids were sorted by the significant degree of influence under different factors,which provides a guidance for the design and construction of geogrid reinforcement widened subgrade.Finally,in view of the looseness of the old road joint and the high moisture content,it is proposed to set a layer of solid waste layer wrapping grille on the old road joint surface,its treatment effect with sand wrapping,ash mixing and other measures were compared,and the digital image correlation(DIC)method were adopted.The DIC method determined the reinforcement influence zone of geogrid interacting with the solid waste wrapping layer and subgrade,and the particle rotation and displacement were analyzed,the reliability and accuracy of the reinforcement influence zone determined by the DIC observation were validated.The main conclusions are listed below:(1)The old subgrade had a higher moisture content than the new one.With the increase of the subgrade depth,the moisture content increased,but the compaction degree decreased.The moisture content of the new subgrade filler became significantly high,which can meet the requirements of the code after adding 4%ash in zone 94 and 6%ash in zone 96.(2)The apparent friction coefficient between reinforcement and soil decreases with the increase of water content,and the greater the decrease is with the increase of water content.With the increase of the compaction degree;When the normal stress increases,the frictional coefficient decreases,but the shear stress increases.The significant degree of the factors was type of the grid>compaction degree>moisture content>normal stress.Therefore,it is suggested to adopt bidirectional grid with transverse ribs in engineering reinforcement to improve the compactness of subgrade soil and reduce its water content at the same time,and the grilles at the bottom of the subgrade were able to play a better reinforcing role.The elasticexponential hardening model was more suitable for describing the mechanical behavior of the reinforcement-soil interface than the ideal elastic-plastic one and the bilinear one.Among the influencing factors of geogrid-soil interface characteristics,the geogrid type has a significant impact,and the significant degree of the uniaxial and biaxial grid factors was:moisture content>normal stress>compaction degree.For the uniaxial geogrid,the moisture content had a more important influence than the compaction degree,and they showed the comparable effect for the two-way grille.To improve the compaction degree of the subgrade soil and reduce its moisture content;it is recommended to use the two-way grille with transverse ribs during engineering reinforcement Therefore,the grilles at the bottom of the subgrade were able to play a better reinforcing role;the elastic-exponential hardening model was more suitable for describing the mechanical behavior of the reinforcement-soil interface than the ideal elasticplastic one and the bilinear one;(3)Among the measures to improve the interface strength of reinforcement soil of old road steps,the effect was found to be in the order:solid waste C>solid waste B>sand>solid waste A>ash mixing 6%>ash mixing 4%,and it increased with the increase of the particle size of the wrapped solid waste.In the DIC testing,the results indicated that the reinforcement effect zone decreased with the increase of the normal stress,and the effect thickness above the grid was greater than one below the grid,and the rotating construction solid waste particles are mainly concentrated within 3cm below and 5cm above the grille.The recommended thickness of the solid waste laying was 20cm for the middle layer of subgrade,and 15cm and 20cm for the bottom and upper one.