The Study Of Thermal Conductivity Of MICP-treated Graded Sands

Nowadays,the main energy structure of China and world is mainly based on non-renewable fossil fuels.Therefore,all countries have begun to explore the development and utilization of renewable clean energy.In this context,geothermal energy as a natural eco-friendly energy has attracted attention from all parties.In the field of geotechnical engineering,energy piles and ground source heat pumps are the main geothermal energy stuctures.Among them,the exchange efficiency of geothermal energy is greatly affected by the heat transfer performance of the surrounding soil,that is,the thermal conductivity of the soil.In the process of exploring the thermal conductivity of soil,some scholars have found that microbially induced calcite precipitation(MICP)method,as an eco-friendly technology,has an effect on improving the thermal conductivity of soil.Based on the MICP,this paper uses the transient plane heat source method to study the thermal conductivity of sand with different gradations,and further combines the particle size and fines content to explore the mechanism of MCP on the thermal conductivity of soil.First of all,this paper studies the thermal conductivity of Fujian standard sands with different gradations,and explores the Ca CO₃ content and thermal conductivity of standard sands under different MICP treatment cycles,and obtains its relationship with gradation,void ratio,and Ca CO₃ content.It is concluded that MICP can improve the thermal conductivity of soil up to about 3 times.Finally,an empirical model for predicting standard sands with different gradations,void ratios,and Ca CO₃ content was proposed.Then,this paper uses glass beads with approximately the same shape parameters and gradation as the research object to explore the effect of MCP on its thermal conductivity improvement under different particle sizes.The test results show that the smaller the particle size,the smaller thermal conductivity of the glass beads,but MICP has the best effect on improving the thermal conductivity of small-size glass beads.It is also concluded that under the same void ratio,glass beads with large particle size can precipitate more CaCO₃crystals.Finally,in this paper,CaCO₃ powder is mixed into the coarse standard sand with poor gradation to study the improvement effect of fines content on the thermal conductivity.The specimens with different fines content were treated with MICP to study the effect of improving the thermal conductivity.This chapter uses the sketelon void ratio and the global void ratio to further study the effect of fines content on the thermal conductivity improvement.At the same time,the thermal conductivity of specimens under different fines content and MICP treatment degree is compared to explore the effect of MICP on thermal conductivity.In general,this article first analyzes and discusses the influence of soil properties on the thermal conductivity,and then further explores the influence of these properties on the MICP according to the regularity of the Ca CO₃ content and thermal conductivity of the MICP-treated specimens.The contact area between sand particles and the coordinate number of particles are essencial parameters to affect the mechanism of MICP and thermal conductivity,and the improvement effect of MICP on thermal conductivity depends on the pore distribution inside the sand during the MICP treatment.