Study On The Thermo-mechanical Response And Design Method Of PHC Energy Piles

The energy pile,as an innovative technology for building energy saving,has received more and more interest in recent years.The basic concept is to facilitate the heat exchange between the upper building and the soils adjacent to the piles by incorporating heat exchange pipes into the piles.In this way,shallow geothermal energy is utilized to heat or cool the building,which saves energy and reduces carbon emissions.The energy piles not only bear the mechanical load of the superstructure but also carry the thermal load simultaneously.Therefore,the estimation of heat exchange capacity and the analysis of thermo-mechanical response have become the main concerns of scholars across the world.Under the financial support of the National Natural Science Foundation of China（Grant No.51578146）,the National construction of high-level university public graduate project（201906090230）and the Postgraduate Research&Practice Innovation Program of Jiangsu Province（Grant No.KYCX18＿0106）,the mechanism of the thermo-mechanical response and the design method of energy piles are investigated in this study.Theoretical analysis,numerical simulation,in-situ testing,laboratory experiments,and full-scale experiments are combined to investigate the in-situ thermal properties testing method,thermal influences on the mechanical responses of in-situ soils and soil-structure interfaces,and the mechanism of the thermo-mechanical response of the energy pile.Furthermore,a thermo-mechanical analysis method and design procedure are established based on in-situ parameters from piezocone penetration test（CPTU）.The main works and conclusions are as follows:（1）Based on the analytical solution of instantaneous heat dissipation along a line source,the cone penetration test（CPT）was modified with a heating module into a thermal cone penetration test（T-CPT）equipment,and the according method for measuring the in-situ thermal conductivity was proposed.The feasibility of this method in soils with different thermal conductivities was verified by parametric analysis through numerical simulations,and the testing methods were proposed accordingly.This method was also verified by a preliminary in-situ trial test.（2）An experiment base for the in-situ thermal response test（TRT）on a PHC energy pile was established.The temperature variation in the pile and the surrounding soils were investigated in a layered stratum.The comprehensive thermal conductivity derived from the TRT test was higher than that from laboratory tests,and this was caused by the selection of fitting duration,the end effect of heat transfer,and high heating power.Furthermore,a numerical model was established and verified by the temperature monitored in the field.Then a parametric study of the TRT test was performed and the testing and the data processing methods for TRT on the energy piles with different aspect ratios were proposed accordingly.（3）The in-situ CPTU tests were used to reveal the variations of mechanical behaviors of soil around an energy pile at different temperature levels and after thermal cycles.The results show that the average tip resistance（q_t）of the normally consolidated clay（NC）layer with lower strength decreased by 47%to 0.23 MPa after heating to TH level（ΔT=14.4°C）,while the average q_t loss for the overconsolidated（OC）soil layers was 0.55 MPa accounting for a reduction of 15%.After the thermal cycle,the in-situ shear strength of the NC layer increased by 11～24%,while the strength of the OC layers returned to the initial level.After re-heating to the TH level,all soils strength decreased but were still higher than that of the first time indicating that the thermal cycle could attenuate the strength degradation caused by heating.According to the changes of q_t,u₂,and the critical state soil mechanics,the reduction of soil strengths may be due to the excess pore pressure during heating,and the recovery or increase of strength was caused by the thermal consolidation of the soils.（4）A series of temperature-controlled direct shear tests were used to reveal the effects of heating/cooling,thermal cycles,and the combination of initial shear stress on the shearing characteristics of sand and clay interfaces.The results showed that in the range of 2～38°C,the strength parameters of the interface were not affected by any of the above factors.The volumetric plastic thermal strains may be generated during temperature cycles,depending on the soil type,the stress state,and the thermal load direction.Nevertheless,such strains had little effect on the subsequent drained shearing process.During the undrained shearing after heating cycles,the stress paths of the clay interfaces showed typical overconsolidation characteristics and the undrained shear strength increased.The combination of initial shear stress and temperature cycles caused additional shear displacement at the interface,but it gradually stabilized as thermal cycles proceeded.The effects of temperature on the cyclic shear strengthes of the interfaces were insignificant for practical purposes,but the significant deterioration of the interface strength under cyclic constant normal stiffness（CNS）shearing indicated the thermal elongation/contraction of energy piles during the operation may cause a local reduction in shaft resistances.（5）The in-situ heating tests,high strain dynamic tests,and static load tests on the pre-stressed high-strength concrete（PHC）energy pile were performed to analyze the pile axial stresses,shaft frictions,and pile-soil relative displacement distributions.The load transfer mechanism and bearing characteristics of the energy pile were also elaborated.The results showed that the shallower soils generated negative friction resistances,and the deeper soils produced positive friction resistances when the pile temperature increased;The pile expanded from the null point to both ends,and the farther from the null point,the greater the friction resistance generated,resulting in the nonlinear distribution of the axial force along the pile;The position of the null point is related to the restraints from surrounding soils,and pile ends and is located closer to the side with grater constraints;The magnitude of additional axial stresses of the pile was influenced by the strength of the surrounding soils,the pile type,and the pile construction method.Due to the tight contact between the soil and the pile tip and the hollow structure of the PHC pile,the additional axial force was relatively large and reached 52%of the design load after the temperature increase of 20°C;The dynamic test showed that the bearing capacity of the energy pile increased by 20%after the temperature cycle,which was mainly caused by the increase of the undrained shear strength of the soils;The static load test after heating indicated,the resistances of the soils below the null point were firstly mobilized and approached the limit values,while the shaft resistances above the null point were restrained due to the existing negative friction.However,the load-settlement pattern of the energy pile was not significantly different from that of the ordinary pile,so the ultimate bearing capacity from the pile side was considered unchanged.（6）The finite difference method was adopted to establish the theoretical load transfer analysis method for the thermo-mechanical response of the energy pile,which was able to consider the load-unloading characteristics of the soils.The parameters of load transfer curves for pile-soil interface and pile base were determined by in-situ tests.The mechanical responses of the PHC energy pile were then calculated,and the reliability of this method was verified.There is no requirement of assuming the position of null point and thus this method has less iterations and good portability.According to this method,the influences of soil strength,stiffness,and aspect ratio of pile body on the maximum tensile stresses in an energy pile were evaluated.Finally,the critical points of this study are summarized,and a design procedure and method of the energy pile based on CPTU are proposed.