To design an efficient ground source heat pump (GSHP) system, it is important to accurately measure the thermophysical parameters of the geotechnical layer. In the current study, a borehole is tested in detail using a combined thermal response test system (CTRTS) based on a distributed optical fiber temperature sensor (DOFTS) and a laboratory test. Real-time monitoring of the stratum temperature according to depth and operation time and the geothermal profile and thermal conductivity of each stratum are obtained. The results show that the undisturbed ground temperature is 10.0 °C, and the formation temperature field within 130 m can be divided into variable temperature formation, constant temperature formation (9.13 °C), and warming formation (geothermal gradient is 3.0 °C/100 m). The comprehensive thermal conductivity of the region is 1.862 W/m·K. From top to bottom, the average thermal conductivity of silty clay, mudstone, argillaceous siltstone, and mudstone is 1.631 W/m·K, 1.888 W/m·K, 1.862 W/m·K, and 2.144 W/m·K, respectively. By comparing the measurement results, the accuracy and effectiveness of the CTRTS are verified. Therefore, it is recommended to use the thermal conductivity obtained by the CTRTS to optimize the design of the borehole heat exchanger (BHE). This study provides a case for establishing a standard distributed thermal response test (DTRT).

Ground source heat pump (GSHP) systems have been widely used in the field of shallow geothermal heating and cooling because of their high thermal efficiency and environmental friendliness. A borehole heat exchanger (BHE) is the key part of a ground source heat pump system, and its performance and investment cost have a direct and significant impact on the performance and cost of the whole system. The ground temperature gradient, air temperature, seepage flow rate, and injection flow rate affect the heat exchange performance of BHEs, but most of the research on BHEs lacks field test verification. Therefore, this study relied on the results of a field thermal response test (TRT) based on a distributed optical fiber temperature sensor (DOFTS) and site hydrological, geological, and geothermal data to establish a corrected numerical model of buried pipe heat transfer and carry out the heat transfer performance analysis of a buried pipe in the heating season. The results showed that the ground temperature gradient of the test site was about 3.0 °C/100 m, and the temperature of the constant-temperature layer was about 9.17 °C. Increasing the air temperature could improve the heat transfer performance. The temperature of the surrounding rock and soil mass of the single pipe spread uniformly, and the closer it was to the buried pipe, the lower the temperature. When there is groundwater seepage, the seepage carries the cold energy generated by a buried pipe’s heat transfer through heat convection to form a plume zone, which can effectively alleviate the phenomenon of cold accumulation. With an increase in seepage velocity, the heat transfer of the buried pipe increases nonlinearly. The heat transfer performance can be improved by appropriately reducing the temperature and velocity of the injected fluid. Selecting a backfill material with higher thermal conductivity than the ground body can improve the heat transfer performance. These research results can provide support for the optimization of the heat transfer performance of a buried tube heat exchanger.

Geothermal energy exhibits considerable development potential in space heating. Shallow geothermal energy stored in the soil in the form of low-grade energy is mainly extracted via the ground source heat pump (GSHP) system. GSHP systems use the subsoil as a heat source, typically involving a vertical borehole heat exchanger (BHE) to extract heat from the formation. Accurate measurement of the thermal properties of the formation is very important for the design of BHEs. At present, the most common and effective method to measure the thermal conductivity of the formation in the field is the thermal response test (TRT). However, the test conditions (heat load, test time) during the thermal response test can impact the test results. Therefore, in this study, a borehole with a depth of 130 m was evaluated in the field. The TRT module and the distributed thermal response test (DTRT) module based on distributed optical fiber temperature sensor (DOFTS) technology were used to monitor the test with different working conditions in real-time. In the field tests, geothermal conditions and the evolution of the formation temperature with time and depth were determined. Based on the test results under different heat loads and test times, the influence of the test conditions on the thermal conductivity results was analyzed and described. A constant temperature zone was located at a depth from 25 m to 50 m, and an increasing temperature zone was located at a depth from 50 m to 130 m, with a geothermal gradient of 3 °C/100 m. The results showed that the heat load slightly influenced the thermal conductivity test results. At the initial stage of the test, the temperature significantly increased from 0 to 12 h. After reaching the quasi-stable state, the test time slightly influenced the thermal conductivity test results. The characteristics of the formation thermal recovery stage after the test stage were studied. The heat load decreased, which could shorten the time for the formation to recover the initial temperature. The results could provide a basis for the optimization of thermal response test conditions.

A similarity experiment study is conducted to investigate the evolution characteristics of temperature field and frozen wall closure judgment criteria for the inclined shaft, under inclined holes freezing condition using an adjustable-angle freeze sinking analogous instrument. The changes of temperature and frozen wall thickness with the freezing time in the inclined shaft section, are recorded in real time, and the water pressure of axial and radial hydrological holes are measured. The results demonstrate that the soil temperature change can be divided into three stages under the conditions of salt-water temperature of -32°C and flow rate of 3.18 m/s: rapid cooling before reaching the freezing point, slow cooling in the process of freezing latent heat release, and accelerated cooling and stabilization after freezing latent heat release. The distance from the freezing pipe is the main factor affecting the freezing wall temperature. Affected by the arched arrangement of frozen pipes, the freezing speed in the position of arch crown and arch baseline is slower than the position of wall corner and base plate. A new judgment criterion of frozen wall closure based on the water pressure of the axial hydrological pipe is recommended. In all, this paper has an enlightenment significance for understanding the evolution of the soil temperature field, and predicting the frozen wall closure time in applications of the inclined holes freezing method for the inclined shaft.

The asymmetric temperature field is one of the main factors inducing the cracking of the inner shaft lining during freezing-shaft sinking. The evolution equation for the thermophysical and mechanical properties of shaft lining concrete during construction period is the basis for revealing the cracking mechanism. In this study, several experiments were conducted to reveal the evolution of the temperature field, thermal conductivity, specific heat capacity, compressive strength, tensile strength, and elastic modulus of shaft lining concrete with age and lining thickness within the first 7 d after pouring. Results show that the shaft lining concrete temperature curve after pouring can be divided into five stages: induction, slow heating, rapid heating, rapid cooling, and slow cooling. Thermal conductivity and specific heat capacity reached the maximum on Day 1 and gradually decreased with an increase in age. The compressive strength, tensile strength and elastic modulus significantly increased with age. With an increase in thickness, the shaft lining concrete at the same age improved its three mechanical parameters. Finally, the evolution equation for these thermophysical and mechanical parameters with age within the first 7 d after pouring was fitted based on experimental data. This study is expected to provide a thermophysical and mechanical basis for studying the cracking mechanism of the inner shaft lining.

Studying the additional force in topsoil containing multiple aquifers can have a significant impact on predicting shaft wall fracture and guaranteeing shaft safety, as the aquifer number increases as drainage occurs. In this study, a numerical model calculating the effect of drainage on additional force in topsoil, containing multiple aquifers, was established on the basis of several practical engineering cases. The changes in the stress displacement of the shaft wall was analyzed during three different stages of shaft construction using typical parameters, and the effects of the various factors on the additional force variation under different water level gap conditions, depending on whether the drainage was synchronized or unsynchronized, were studied. The results indicate that the increment in the additional force, with an increasing water level gap in the central aquifer, is obviously larger than that in the bottom aquifer, and the difference in the maximum additional force between these two aquifers is approximately 0.6 MPa. The increasing number of central aquifers results in a higher increment in this force, which reaches 12 MPa with an increasing number of central aquifers. Meanwhile, a threshold value (about 0.6~0.7 H) exists for the depth of a central aquifer in terms of its effect on the additional force.