Model Test Study Of The Groundwater On Single Pile’s Mechanical Properties In Permafrost

In order to Promote the development of economy and natural resources in permafrost,lots of transportation Infrastructures,such as road and rail, have been constructed.It is been used widely because of pile’s excellent mechanical properties in permafrost. Pile foundation that built in permafrost not only faces the frost heave and thawing settlement;At the same time,the groundwater has a worse difference on the mechanical property of pile foundation in permafrost.The scale of groundwater increases with the increase of permafrost temperature,the change of climate and increase of human activity in frozen soil. Thus its harm on pile foundation can be neglected any more in permafrost.In addition,combined with the characteristic and nature of groundwater,it is feasible to simulate the presence of groundwater by the way of built-in heat source approximately.So This paper mainly bases on the indoor experiment of concrete single pile-frozen soil model.By burying heat source in model and setting up the different test conditions,the paper carries out the model test study of the heat source of groundwater on single pile’s mechanical properties in permafrost.According to the different research purposes,different loading way of the test have been set up.To study the influence of groundwater on the load transfer of pile- soil system,the experiment use the hierarchical load way by changing the state of heat source and maintaining-1.5℃ through the low temperature test chamber.The result shows that:the heat source of pile-side makes frictional resistance less in the great depth;to maintain the stability of pilesoil system,the pile tip resistance has been exploited.So the pile shaft force becomes smaller from the top of pile to end in great depth.The frozen soil in pile end is melted and the bearing capacity is smaller because of the heat resource of pile end.As a result,the pile shaft force becomes smaller;meanwhile, the pile-soil freezing stress has been exploited.The existence of groundwater also makes the pile-soil relative displacement and pile end displacement increased seriously.By maintaining the temperature-1.5 ℃ and hierarchical load, the paper study the influence of groundwater on the pile-soil rheological effect and pile ultimate bearing capacity.The results show that:it can be used by logarithmic function that curves of pile top displacements varying with time under different load levels;the increase of load level makes the difference of pile top displacement largen when it becomes stable;the heat resource also makes the time longer when pile top displacement becomes stable.The rheological effect of frozen soil leads to the decrease of ultimate bearing capacity of pile foundation; it is enhanced that the influence of rheological effect on the mechanical property of pile foundation because of the presence of groundwater.In addition,when the low temperature test chamber stops working and there is no heat resource,the pile appears balance in a short time under less load;the heat resource makes frozen soil maintain stable in incipient stage,and failure occurs soon with the passage of time.So in actual working condition of permafrost, once climatic warming,it will have a worse difference on pile foundation when the groundwater appears in permafrost.Meanwhile,through studying the influence of rheological effect on load transfer, the result shows that:due to the presence of heat resource,the pile-soil freezing stress decreases with time apparently.The heat resource makes pile end frozen soil melt gradually,and the effect of melting has a continuous development with time because of its presence; when there is no heat resource,the pile end displacement still increases due to the rheological effect in the late load;for the reason that,the groundwater brings about pile end resistance decreased,and not enough to provide the carrying capacity for pile;ultimately,it leads to pile foundation appearing destruction.It can clearly be seen, the groundwater in permafrost not only makes the carrying capacity lower, but also lets the rheological effect of frozen soil speed up and results in the subsidence of pile end placement becoming intensify.This will make a bad difference on safe operation of the train or car.So it is meaningful for the paper to carry out the study of groundwater on mechanical property; on the one hand it perfects the theory of rheological effect,on the other hand it can provide guidance of pile foundation for the future in frozen soil.