Vertical Load Transfer Mechanism And Bearing Characteristics Of Single Energy Pile In Sand

Energy pile is an innovative technology that incorporates the heat exchangers embedded in concrete to exchange heat between the subsurface ground and the building above to capture the shallow geothermal energy.During heating or cooling operations,the energy piles will be subjected to thermal and mechanical loads simultaneously.However,varying temperatures may cause expansion or contraction of surrounding soils around the pile,influencing its thermo-mechanical behaviour.Nevertheless,the current research mainly focused on the thermal response of energy piles,while much less on thermo-mechanical coupling response.Based on the model tests,numerical simulations,and theoretical analysis,vertical load transfer mechanism and bearing characteristics of single energy pile under multiple heating-cooling cycles were investigated systematically.The influence of different types of heat exchanger configuration was also emphatically analysed.In addition,calculation method of ultimate bearing capacity for a single energy pile was preliminarily discussed.The main scopes and results of this study are as follows:(1)Model tests with and without a vertical load while considering different soil compaction degrees were conducted.The mechanism of pile tip resistance and horizontal soil presure affected by temperature was revealed.The distribution of thermal strain,stress,and mobilished side shear stress and curve trend of pile head displacement were obtained.The result indicates that the vertical load has no influences on the horizontal soil presure.However,the distribution of thermal stress after heating will be influenced.In addition,unrecoverable settlement will be produced after natural recovery under a vertical load.The result also shows that the constraint of the surrounding soil on the pile expansion will be more considerable under higher compaction degree.(2)The numerical simulations on the thermo-mechanical behavior of energy piles influenced by pile tip stiffness,types of pipe configurations and the surrounding temperature were conducted.The position of null point and distribution of thermal stress influenced by the pile head stiffness restraint and pile tip soil stiffness restraint were then evaluated.Moreover,comparative analyses of pile head angular distortion for W-shaped and U-shaped piles were also conducted.In view of this,a method for the purpose of reducing uneven displacement of pile head was proposed.Furthermore,the heat convection with ambient environment was analysed to investigate its effect on the pile temperature and pile head displacement.The result shows that the null point upwards and downwards as pile head stiffness restraint and pile tip soil stiffness restraint increase respectively.Simultaneously,the thermal stress will increse with the increasing of stiffness restraint,but it decreases and increases with the depth for these two cases,respectively.Under the same temperature increment,the angular distortion of U-shaped pile is two times as large as that of W-shaped.The environment temperature has significant influences on the pile temperataure but less impact on the pile head displacement.And the influenced zone may reach 17%L depth considering the effect of environment temperature.(3)Model tests aimed at investigating the thermo-mechanical behavior of energy piles with different pipe configurations were carried out.On the basis of that,comparative analysis including pile and soil temperature,pile tip resistance,horizontal soil pressure,thermal stress,and pile head displacement for U-shaped,spiral and W-shaped heat exchanger piles were performed.Furthermore,the effects of thermal stress and pile head displacement on the safety of structure were also investigated.The result shows that the W-shaped heat exchanger pile is subjected to the greatest variation,followed by the spiral and U-shaped pile.Moreover,for the W-shaped pile the tension stress and settlement produced during cooling have great influences on the safety of structure.An improved energy pile was also designed.Considering both heat transfer performance and mechanical response,the new improved energy pile had a slight influence on the safety of structure and was applicable.(4)Investigation on the long time thermo-mechanical behavior of energy piles was conducted.The mechanism of thermal storage with multiple heating-cooling cycles was revealed.Besides,the accumulated thermal stress and curve trend of long time pile head displacement were also evaluated.Furthermore,model tests were carried out to investigate the ultimate bearing capacity of energy piles.The pile load-displacement relationships were obtained under different pile temperatures and heating-cooling cycles.The ultimate bearing capacities under one,three and five heating-cooling cycles were estimated,respectively.Further analyses of influential factors including the dry and saturated soil conditions,soil densities,types of pipe configurations were performed.The result shows that the ultimate bearing capacity increase as pile temperature increases.The change of ultimate bearing capacity after one heating-cooling cycle is marginal.However,a significant reduction of bearing capacity was observed after multiple heating-cooling cycles.In addition,with greater soil relative densities,the ultimate capacity of a pile exchanging larger amount of heat varies more obviously.(5)The influential factors related to shaft friction and tip resistance and calculation method of ultimate shaft friction based on ? method were examined.In combination with the field tests and model tests,the influential factors ? and ? were mainly adopted to reflect the mobilisation of the shaft friction and pile tip resistance,respectively.The range and average ? and ? values were then proposed.On the basis of these findings,the influential factors of piles under different bearing conditions were concluded.In addition,the influence of temperature was investigated.Adapted from the conventional ? values suggested by design specifications,a new ? value was back-calculated through field tests and was recommended in this study.