Development Of Model Test Material Of Strongly Collapsible Loess And Study On Bearing Behaviors Of Pile In Loess Subgrade

With the great development of infrastructure,more and more construction projects are increasingly built in loess area.Especially in the strongly collapsible loess areas,much more engineering problems have put forward a series of challengs to both scientific research and engineering practice.The proper selection of similar material is very crucial to the correct simulation of engineering prototype.The model test material of strongly collapsible loess is the bottleneck of similar materials model research in geomechanics,which has limlited the application of model tests in loess area.In this thesis.the similarity theory and state of the art of geomechanical model similar material was briefly introduced.The mechanism and characteristics of hydro-collapsibity were also analyzed.Using different sampling methods,a model test material for strongly collapsible loess was successfully developed after a lot of compounding tests.This similar material was firstly mixed with barite powder and sand as non-cohensive material,and industrial salt,gypsum and bentonite as cohensive material.Then water was added to mix them evenly.After wetted for 24 hours,the samples were produced by airfall method.At last,the samples were maintained at constant temperature of 50℃, which simulates the deposition method and formation conditions of aeolian loess,to obtain the targeted model test material.From oedometer collapse tests,this manmade material had a similar collapsibility with natural loess,and also produced significant collapse when wetted. By shear test,the shear strength parameters of similar material were much close to natural loess.Since the density of the similar material can be adjusted widely,and it can be bought conveniently and cheap,the model test material devolped by the above method would be an ideal model test material for strongly collapsible loess.A pile foundation model test was carried out by using aluminum tube to simulate pile,and the developed model test material to simulate loess with careful control of the density and water content.The axial force and side resistance of the model pile were measured by strain gauges,the additional stress in the soil and tip resistance of the pile were measured by pressure cells,and the collapsible deformation of soil above the pile and the settlement of pile bottom were meseared by displacement sensors.Then the characteristics of bearing capacity of pile foundation,negative skin friction and the settlement patterns were discussed.The bearing behaviors,settlement performances and negative skin friction of the model pile were simulated using finite element software ANSYS based on the modulus reduction method.The influence of variations in the coefficient of hydro-collapsibility was studied.The related problems associated with pile foundation were analysed and compared respectively,such as the load transfer mechanism and patterns,distribution and variation of negative skin friction,settlement deformation characteristics et al,based on the results from model test and numerical simulation.The conclusions of the above researches can be summarized as follows:The model test material using airfall method can properly simulate the collapsibility of natural aeolian loess.The artiflcal loess samples were very similar to natural loess in collapsibility,physical properties and mechanical behaviors,which would make the test result for repetitive and controlled. It can also make it possible for the individual examination of the different parameters that control the bonding in loess.Therefore,the method and test results can provide a good reference to model similar materials in geomechanics.Significant axial force and settlemeent were produced in the pile because of negative skin friction,which can reduce the bearing capacity of pile foundation thanks to the collapse of loess.Although the negative skin friction varied with the type,thickness,and settlement of collapsible soil layer on specific site,it would be safer to regard the negative friction as the negative load added on the pile,and it would also be more conservative to select the higher value of negative skin friction in strongly collapsible loess subgrade.The stress field caused by gravity in the model could be simulated by modulus reduction method which considered gravity as primary stress to load but yielding no displacement field. Then the parameters were reduced to eliminate the influences of gravity on numerical simulation. Thus,it can better simulate the self weight collapse deformation of loess and its effect on bearing characteristics of pile.