Study On Mechanical Properties And Desiccation Cracking Behavior Of Jute/PVA Fiber Reinforced Cemented Soil

Deep cement-soil mixing retaining wall has been widely used as retaining structure in some foundation pit engineering.Deep cement-soil mixing retaining wall is composed of brittle materials and its tensile strength is much lower than its compressive strength.Due to the increasing excavation depth and limited construction space in urban areas,the pile length and wall thickness need to be increased which is not satisfied the constrcution space.Thus,the inclusion of fiber can not only improve the flexural and post-cracking performance of soil–cement column under the condition of limited wall thickness,but also increase the residual flexural strength capacity after crack occurs,which can transfer the brittle failure pattern to ductile one so as to ensure the serviceability and stability of the structure.Moreover,drying shrinkage due to temperature stress may lead to desiccation cracks,which may result in leakage and collapse.The addition of fibers can restrain the initiation and propagation of desiccation cracks.In this thesis,the mechanical properties and desiccation cracking bebavior of Jute and PVA fiber reinforced cemented soil are systematically studied by means of laboratory test and theoretical analysis,the main work and achievements of this paper are as follows:1.The compressive and tensile performances of fiber reinforced cemented soil were investigated by unconfined compressive strength test and uniaxial tensile strength test.The inclusion of fiber increases the compressive strength of cemented soil and the corresponding peak strain,but has an insignificantly effect on the limit of proportion compressive strength.Moreover,the fiber addition greatly improves the tensile strength of cemented soil at early curing days,while it is not observed improvement in tensile strength in long term.The addition of jute and PVA fiber significantly improve the compressive and tensile toughness of cemented soil.Higher compressive and tensile toughness indicates better performance of energy absorption capacity during the compressive and tensile failure process.2.The effect of jute fiber and PVA fiber on the flexural performance,post-cracking performance and fracture toughness of cemented soil were studied by conducting a three point bending test of pre-cut notch beams.The results indicate that the fiber addition significantly improves the peak flexural strength of cemented soil.The residual flexural strength of fiber reinforced cemented was much higher than the plain cemented soil,and the residual flexural strength and fracture energy were significantly improved with increasing fiber content.The fiber reinforcement greatly improves the brittle fracture failure pattern of plain cemented soil even if with lower content of fiber due to the fiber-bridging effect.The incorporation of fiber significantly improves the unstable fracture toughness of the cemented soil,but has little effect on the initial fracture toughness.3.The fiber bridging stress-crack mouth opening width model of the fiber reinforced cemented soil can be obtained on the basis of the interface parameters between fiber and cemented soil matrix which is determined by the single fiber pull-out test,in addition with the fiber orientation distribution probability density function.It was found that the prediction results of the model on PVA fiber specimens were better than that of jute fiber specimens.4.The measurement of the strain and crack field of specimens was carried out by the Digital Image Correlation(DIC)method,and the effects of two types of fibers on the desiccation shrinkage cracks in the early stage of cemented soil were studied.The results show that the addition of fibers in cemented soil can significantly reduce the total area and length of desiccation shrinkage cracks in cemented soil,and the fiber inclusion alter the cracking position,the propagation direction of cracks.Morover,the fiber addition alleviate the stress concentration along the crack.5.On the basis of macro mechanical tests,the mechanism of fiber reinforcement in cemented soil and the interface between fiber and matrix were studied by microscopic tests including XRD,thermogravimetric analysis,SEM tests.It is proved that in the hardening process of fiber and cemented soil mixture,the bonding effect between fibers and cement-soil mixture is mainly through hydration products.The intersecting network model of fiber reninforced cemented soil is proposed to reveal the reinforcement mechanism by the impact indicators of nodal density and strength in the intersecting network.