Research On The Effect Of Geothermal Energy Extraction On Thermo-active Diaphragm Wall Structures And Geological Environment

Shallow geothermal energy(SGE),recognized as one of sustainable energy sources,is widely developed and utilized in different modes,including traditional ground-coupled heat pump system,water source heat pump system,as well as the thermo-active ground structures heat pump system in which heat exchange tubes are directly buried inside the underground concrete structures.During the utilization of shallow geothermal energy,the external energy will have heat exchange with the underground structure,the surrounding rock/soil/water in a direct or indirect way.As a result,it will cause effects on the underground structure and geological environment inevitably.Widely acknowledged as the advantage of thermo-active ground structures buried in concrete is,various experiments data of energy pile indicate that this mode will induce lateral friction resistance,vertical displacements and the axial stress of the pile,which raises great concerns about the safety performance of heat exchange structures.Although most researchers believe that these changes in mechanical properties do not have adverse consequences for the building,these factors must be taken into account during the design stage.Compared with energy piles,the researches of thermo-active diaphragm walls are very insufficient,especially the experimental research on the mechanical properties of the wall under heat transfer conditions.Meanwhile,there is a mania phenomenon in domestic shallow geothermal energy utilization projects.The quality of the system is completely ignored,resulting in poor engineering quality and bad effects on the geological environment.Therefore,it is urgent and significant to conduct research on the impact of geothermal energy extraction on the underground structures and geological environment.Based on the status quo of R&D of thermo-active diaphragm wall,this paper carried out some researches as follows:1)Large-scale model experiment of thermo-active diaphragm wall.The model test involves different heat transfer conditions and different constraints.The temperature change of the model,wall strain and normal stress of soil/wall interface were analyzed under the heat transfer conditions;2)The changing mechanism of mechanical properties of the wall was analyzed using numerical simulation method.Afterwards,the boundary conditions and initial conditions of the model were changed,and the mechanical behavior changes of the wall under different temperature rise conditions and constraints were analyzed;3)Numerical simulation on the mechanical behavior of the thermo-active diaphragm wall was done under excavation conditions.The simulation focuses on the influence of the wall depth changes,different heat transfer conditions,different buried pipe forms and the different thermal expansion coefficients of rock and soil nearby;4)The impacts of shallow geothermal energy development on geological environment were studied through experimental research,numerical simulation and on-site monitoring,and the results obtained can provide technical reference for the Nanjing local technical standards.The main results obtained are as follows:1)The temperature field,the stress/strain state in the wall and the normal stress of the wall-soil interface were obtained through the model test of thermo-active diaphragm wall under non-excavation condition.It was found that the temperature rise will generate large thermal stress inside the wall.However,unlike the energy pile,this stress is on the one hand due to the increase of the side frictional resistance caused by the temperature rise,and on the other hand is the reason of the differential deformation caused by the inhomogeneous distribution of the temperature inside the wall.2)The numerical simulation results which based on the model test indicate that due to the asymmetric distribution of the heat exchange pipes in the wall,the thermal expansion causes the wall bending,resulting in different stress/strain responses on both sides of the wall.At the same time,the bending deformation of the wall also explains the different variation of the normal stress of the wall-soil interface between the left and right sides of the wall in the laboratory test and the significant stress/strain changes in the wall during the first few hours.3)The numerical simulation results under non-excavation conditions also show that the stress-strain behavior of the wall mainly depends on the constraints of the top of the wall.Although the temperature rise can increase the normal stress of the wall-soil interface,it will be less important when it compared with the initial normal stress value.4)The numerical simulation results under excavation conditions show that the thermal load is an important factor in the change of wall mechanical properties,which is mainly reflected in its horizontal displacement of the wall,vertical stress in the wall and bending moment of the wall;The distribution of bilateral heat sources can reduce the influence.The influence of different thermal expansion coefficients of rock and soil on the mechanical properties of the wall is mainly reflected in the horizontal displacement of the wall and the normal stress of the wall-soil contact surface,but with less influence on the vertical stress in the wall and the bending moment of the wall.5)It was found that in the long-term operation of the shallow geothermal energy utilization system,thermal accumulation may occur,which will affect the geological environment and the energy efficiency of the system itself.The research process and related results provide a technical reference for the Nanjing local technical standards of "Specification of geological environmental monitoring in shallow geothermal energy exploitation".