Experimental Study On Bearing Capacity Of Energy Pile Under Heat Exchange

Energy piles place the heat exchange pipes of conventional ground source heat pumps in the building pile foundation,and use the temperature difference between the circulating heat transfer medium in the pipes and the rock mass around the piles to realize the heat exchange between the soil around the piles.The energy pile is not only the heat exchanger of ground source heat pump,but also the load bearing structure of the upper building load.It has the advantages of small area,good heat transfer effect and low cost.However,when the temperature cycle of the pile body changes,the pile-soil is affected by the cyclic additional shear strain,which will also affect the interface properties of the pile-soil,causing changes in the frictional resistance of the pile,affecting the axial bearing capacity of the single pile and the settlement of the pile top,and even causing destruction of pile base.In this thesis,through indoor experiments,the effect of temperature field changes on the mechanical properties of soil was studied.At the same time,the stress variation law of pile-soil interface and the variation law of bearing capacity under the heat exchange of energy piles were studied through outdoor experiments.The main research contents are as follows:(1)Study the test site conditions and geophysical parameters and thermal conductivity of the soil.A geotechnical survey was conducted on the Taoyuan canteen project of the Jiulonghu Campus of the Southeast University on the test site,and the basic physical and mechanical parameters and hydrogeological conditions of the rock and soil were obtained.At the same time,in the laboratory,the transient physical heat source technology(TPS)was used to test the thermal properties of the site and obtain the thermal properties of the layered soil.(2)To develop a temperature-controlled triaxial apparatus that performs triaxial tests under variable temperature conditions.Different confining pressure and temperature loads are applied to the layered soil samples to study the effects of confining pressure and temperature on the deformation characteristics of the samples,and the effect of temperature on sample strength characteristics.The results show that the shear strength of soft clay has a strong dependence on temperature.Within the range of test confining pressure,the shear strength indexes c and φ of soft clay have a significant relationship with temperature.That is to say,as the temperature increased,the force of soft clay c reduced and the internal friction angle φ increased.(3)The strain gauges are prefabricated into the energy pipe pile wall,and the high-strain testing of the energy pile under heat exchange is performed to obtain the ultimate bearing capacity of the pile and the distribution of soil resistance along the pile.The results showed that the lateral friction resistance of soft soil increased by 47.8%-222.2% after the temperature of the pile increased by 37°C,the friction resistance of hard soil layer changed little,the resistance of pile tip increased by 7.4%,and the bearing capacity of single pile increased by 13.8%.After standing for 112 days,the piles recovered to the initial temperature of the test,the friction resistance of the soft soil layer remained basically unchanged,the friction resistance of the hard soil increased by 20%-28%,the resistance of the pile end increased by 26.1%.However,the bearing capacity of the single pile after the increase did not decrease with the temperature of the pile,but increased by 11.0% before standing.