# Study on the Effect of Insulation Materials on the Temperature Field of Piles in Ice-Rich Areas
## 1 Introduction  
Permafrost is widely distributed throughout China [1]. In these areas, bored piles are used for the construction of highways, railroads, and power lines because of their strong horizontal and vertical bearing capacity, ease of construction, and wide range of applications [2]. However, the construction of conventional bored piles melts the permafrost around them. At the same time, the lower environmental temperature in the permafrost region has an extremely negative impact on the strength development of concrete in the early age of the pile body [3]. This, in turn, reduces the strength of the pile body, resulting in an insufficient bearing capacity of the pile and ultimately endangering its structural integrity. Therefore, in ice-rich environments, it is important to reduce thermal disturbances to the construction on the perimeter of the pile permafrost and increase the pile concrete maintenance temperature to ensure the formation of early concrete strength for construction projects in permafrost regions. Many scholars have researched the early thermal stability of concrete bored piles in permafrost areas and their strength growth under negative-temperature conditions. The exothermic heat of the hydration of concrete has a more significant effect on the refreezing process, with a reduction in the total heat of hydration, the maximum temperature rise, and refreezing time. The higher the entry temperature in the concrete filling, the higher the maximum temperature rise and the longer the refreezing time [4]; therefore, it isrecommended that the temperature of concrete in the mold should be as low as possible when the conditions permit, preferably controlled at 2-9 \({}^{\circ}\)C. The freezing time of a 1.2 m diameter test pile is 1.14 times longer than that of a 1.0 m diameter test pile at the same entry temperature. The refreezing time of the frozen soil around the pile is also related to the initial conditions of the frozen soil, where a lower frozen soil temperature results in a shorter refreezing time [5]. To avoid temperature cracks caused by the large temperature difference between the inside and outside of newly-cast mass concrete, physical temperature control methods are often used. For example, cold pipes can be laid inside before casting, or insulation materials can be wrapped outside to reduce the temperature difference between the inside and outside of mass concrete [6]. [PERSON] et al. experimentally found that the compressive strength of concrete increased as the product of temperature and time increased [7]. [PERSON] [8] conducted a more systematic study on the temperature field of a bored pile bank and pile-side soil in ice-rich areas based on field tests and heat conduction theory [8]. The subsurface temperature measured in the ice-rich soil layer at the time of boring was within the range of \(-\)2.0 to \(-\)3.0 \({}^{\circ}\)C. The paper noted that the 28-d compressive strength of concrete at 0 \({}^{\circ}\)C maintenance only reached 75.2% of that at 20 \({}^{\circ}\)C. As the temperature decreased, the strength loss rate of concrete rose [9; 10; 11]. Many researchers have mainly focused on adding antifreeze agents and other measures to ensure the concrete strength of piles in negative-temperature environments. They have also studied changes in the temperature field of concrete piles. However, measures to change the pile concrete temperature by adding insulation to reduce heat exchange between the concrete and surrounding ice-rich frozen soil have not been studied. In this paper, relying on the construction project of the bored pile of the Jingmo Highway Bridge, the temperature field of a test pile with double sheaths filled with polyurethane insulation and a test pile without polyurethane insulation was measured. Based on the pile-soil heat conduction theory, the temperature disturbance law of the pile body was calculated by finite element software with an insulation layer applied around the pile.