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Feng L.,Hebei University of Technology | Wang X.,Hebei University of Technology | Wang H.,Hebei University of Technology | Zuo L.,Hydrogeology Survey Institute of Hebei Province | And 4 more authors.
Electronic Journal of Geotechnical Engineering

The ground thermal conductivity is an important parameter for geotechnical and geothermal engineering applications. Generally, a group of in-situ thermal response tests (TRTs) are used to determine the effective thermal conductivity of the ground surrounding the borehole heat exchangers (BHEs). However, TRTs are affected easily by the complex geological conditions and different evaluation methods, which frequently lead to a growing uncertainty of test results. In the present work, two heating- and cooling-based TRTs were conducted and compared in the Hebei Plain, China. Test results showed that the thermal conductivity obtained by the heating and cooling-based TRTs was 8 to 10 percent higher than the average value directly measured from soil samples. A stable power supply was crucial for the accuracy improvement of TRTs. In terms of the hydro-geological, geomorphologic and climate conditions in the Hebei Plain, the heating-based TRT instead of the cooling-based TRT was first recommended, because there was a relatively small temperature difference between the average fluid temperature through the BHE and the surrounding soil during the cooling-based TRT. © 2015 ejge. Source

Wang H.,Hebei University of Technology | Feng L.,Hebei University of Technology | Wang Y.,Hebei University of Technology | Zuo L.,Hydrogeology Survey Institute of Hebei Province | And 4 more authors.
Electronic Journal of Geotechnical Engineering

The natural or undisturbed ground temperature is of great importance for the design of many geotechnical and geothermal systems. The ground temperature is inevitable to be disturbed by the drilling process or in-situ thermal response tests (TRTs) if conducted. This paper was performed to understand how long it took for a borehole to recovery its natural ground temperature after being disturbed by drilling and heat injection during TRTs. Five survey boreholes were drilled in the Hebei Plain, China, and the variations of the ground temperature in each borehole were recorded until reaching completely steady. Test results showed that for conductive type boreholes, the whole recovery period after the heat injection disturbances ranged from 31 to 45 days. The ground temperature recovered quickly during the first 10 days. If the design requirements are not very strict, a reduced recovery period of 10 to 15 days is also acceptable. For convective type boreholes, the ground temperature recovered rapidly due to the enhanced effect of heat diffusion by groundwater flow. If only drilling disturbances were imposed, a safe recovery period of 7 days was recommended. The order of importance affecting the ground temperature recovery was determined as follows: heat injection disturbances > groundwater flow > drilling disturbances. The present results can provide useful references on the design of practical engineering systems under similar hydro-geological conditions. © 2015 ejge. Source

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