Thermal-mechanical Analyses On Structural Responses And Heat Transfer Enhancement Of Energy Piles

As a novel form of foundation,the energy pile,not only,provides an environmentally protective way for energy using,but also,ensures sufficient load capacity for buildings.By the utilization of the buried thermal transmission pipes and the temperature difference between the ground and the building,energy pile absorbs the thermal energy in the ground when the temperature of the pile is lower than that in the ground,and disturb the energy vice versa.At this very moment,the development of the geotechnical research regarding the energy pile is far behind that of its fast-growing applications.Partially,the demand for the theory,the design,and the construction method are in need.This paper stems from such geotechnical consideration for energy piles,and mainly focuses on two topics.One is the load capacity of energy piles during the heating and cooling process;the other is the enhancing method for the thermal transmission of energy piles:1.For the first object,the thermal-mechanical coupling behavior of energy piles was studied through the interaction between the pile and the ground.Particular focus was put on the nonlinear relationship of load transmission between the pile and the soil.In-situ experiments were first carried out to analyze the axial force and the mobilized shear force on the interface under cyclical heating and cooling conditions.Influences of the top loading were also investigated.Subsequently,a series of model tests were conducted to study such behavior under more loading conditions.The results show that the cyclic heating and cooling condition have a significant impact on the load capacity of energy piles.Such influence also depends on the strength of surrounding soil,the top loading and top constraint of the pile,the strength of the soil where the bottom of the pile stands,the frequency and the intensity of temperature changes.Based on the experimental data,a novel relationship of the pile-soil interaction was developed to modify the backbone curve.Different from previous models such as the elastoplastic model and the hyperbolic model,a piecewise nonlinear relationship and the Massing's rule was introduced in the newly developed relationship.The results predicted by the new load-transmission model and that of experiments show a great consistency,which examines its validity.The improved load-transmission model was then implemented in the numerical simulations for analyzing the influence of various types of soil,thermal-mechanical loading conditions,and pile structures.2.For the second object of enhancing method of heat transmissions,the heat distributions in the pile and ground were studied,and the key influencing factors for heat transmissions were analyzed.Through the numerical model developed in the first part of this paper,influences of buried forms of pipes,pile diameter,pile material and pile spacing on the heat-transmission efficiency were studied.Furthermore,the additional impact of intermittence-working condition was analyzed.The results indicate that for improvement of heat-transmission efficiency,the buried types of the thermal-transmission pipe,the diameter and the spacing of the piles are the most influential factors among others.Due to the intermittence-working condition of energy piles,the residual temperature has remained in the pile structure for the non-working period,which should be considered in the design method.Based on the heat transmission behavior of energy piles,enhancements were made by adding graphite into the concrete.Through experiments and simulations,the mechanical and thermal responses of such mixture were analyzed.Results demonstrate that the developed mixture could considerably enhance the thermal-transmission capacity of energy piles,and,at the same time,maintains the chemical stability.