Ground in winter as a heat source can increase the coefficient of performance of the heat pump. However, cost of digging the ground is high. Thermal properties of the soil affect the length of boreholes. The saturated soil of Kerman has a unique potential for establishment of geothermal boreholes. In this paper, the effects of Kerman's saturated soil with freezing and thawing, on lengths of a double U-tube borehole with two independent circuits and a parallel double U-tube borehole have been investigated. Besides, effects of the saturated soil on the power consumption of a solar assisted ground-coupled heat pump system and a conventional ground-coupled heat pump system have been investigated numerically. The results show that the double U-tube borehole with two independent circuits in Kerman's soil comes out 28.6% shorter in length compared with a parallel double U-tube borehole in a conventional soil. Besides, the power consumption of the solar assisted ground-coupled heat pump system in Kerman's soil decreases by 6.1% in comparison with a conventional ground-coupled heat pump in the standard soil. Moreover, it has been shown that the length of the boreholes in the lower thermal conductivity of the soil can be reduced by as much as 35.4%, 40 m. (C) 2019 Elsevier Ltd. All rights reserved.