Experimental Study On Load Capacity Of Highway Subgrade Loess

Loess is a kind of quaternary period deposit in the arid and semi-arid areas of northwest China,its special distortion characteristic and water sensitivity seriously affect the running of loess road construction.So apprehending and searching for the nature and development rules of subgrade and pavement disease become the strategic issues.Therefore,besides loess strength and deformation under static and cyclic load as well as water penetration were investigated,the seepage and disintegration properties were also discussed.Details as follows.The main factors influencing subgrade diseases were summarized, the depth of roadbed work area and vehicles wheel load were calculated,which were based on the observation and analysis of a large number of literature references.By the triaxial shear tests of disturbed and compacted loess,the variation laws of shear strength index changed with water content and dry density were analyzed,the variation law of loess sample's volume was also studied by CD test. The results shows:the cohesion of compacted loess gradually decreases as the water content is increased,the variation law of internal friction angle of loess is much the same as the cohesion.The variation trend of cohesion and internal friction angle is increscent as the dry density is increased. Both of cohesion and internal friction angle have a significant negative linear correlation with the ratio of water content to dry density.When the loess sample is at high water content level, the volume of samples are all minished and exhibiting shearing contraction.And the volume of samples appear shear dilatancy when the loess sample is at low water content level or under low confining pressure,as we narrow these phenomenon down to the issue of highway engineering, this is reflected in the subgrade soil that the shallow subgrade soil and exposed slope pattern will be squeezed lateral slip deformation.It directly raises two serious questions:the settlement problems of highway foundation in the case of lateral extrusion and the design problems of the width of embankment of fill subgrade.Experiments of uniaxial cyclic loading on compacted loess samples were carried out on general triaxial shear equipment,and the the variation law of the deformation under cyclic loading/unloading and shear strength before and after cyclic loading/unloading in accompany with different compaction degree and water content were studied.Regardless of the quantity of water content and cyclic loading, the axial deformations of compacted loess samples characterize a synthesis of elastic and plastic deformation under the action of cyclic loading.The residual (plastic) deformations of those samples are larger in the first few load cycles than after several roles',and the stress-strain curves of loading and unloading gradualy close to each other with the increase of the number of cycles at low stress levels,in this case the residual deformation decreased and stabilized,and the loess samples appear elastic deformation only.Moreover, the residual deformation grows continuously with the increase of the number of cycles at high stress levels.As the moisture content or the load value increases, the deformation of soil characterized by the cumulative growth of the plastic strain'increase and elastic deformation'decrease,and the total deformation decrease on the whole.It can clearly be seen that it is vital significant to strictly control water immersion and overweight vehicles in roadbed construction and other foundation projects.Comparison of shear strength before and after cyclic loading/unloading shows: The stress-strain curves are basically softing model under cycle and static loading at low confining pressure, the strength is increases with the increase of static consolidation loading while decrease with the increase of cyclic loading,and the deformation caused by cyclic loading is much larger than that caused by the static consolidation loading.The stress-strain curves are hardening model under cycle and static loading at high confining pressure,and the shear strength has a slight increase under static loading,while the shear strength is significantly reduced with the increase of cyclic loading/unloading. The shear strength is diminished when both of compaction degree and water content are in a low state,and the compaction degree has a significantly impact on the residual strength while has little effect on the peak strength of the loess sample when it's compaction degree is fairly high.Through the triaxial test and analysis of compacted loess wetting deformation by the way of taking loess roadbed as simulation object,the wetting distortion characteristics and possible reasons were tentatively discussed. The results showed that: The required times of wetting deformations re-steady after stabilization by consolidation are related with initial water content and compaction degree, applied confining pressure and principal stress difference as well as wetting waterhead.The soil samples wetting distortion process curves and the wetting axial strain and body strain also have the explicit relations with the above conditions.For sake of gaining a deep understanding of the water immersion characteristics of compacted roadbed loess,recognizing the relation between roadbed damages and disintegration, the experiments were performed with the application of triaxial permeability test and disintegration test,also a comparative analysis of the disintegration properties between pre and post-shear failure in different water contents was conducted.And obtained some results about the qualitative relationship involving the permeability coefficient of compacted loess and compactness or confining pressure as well as the relation relation between disintegration properties and compactness or moisture content.Finally,some suggestions for practical engineering protection were put forward.