## 4 Applications of the TPCT
### Highways  
[PERSON] et al. [49] monitored an integrated temperature control system consisting of hairpin TPCTs (Figure 5) and ventilated shoulders on the Alaska Highway, which is a high-temperature permafrost region that is particularly sensitive to temperature, and the construction of the highway would disrupt the original geothermal equilibrium of the ground air, which was found to be functioning well after one year of operation through the on-site monitoring data. In China, TPCT was firstly applied to damaged highway culverts. Nearly 2000 straight and diagonally inserted TPCTs were used to mitigate severe longitudinal cracks and melt subsidence in the Chumar River section of the Qinghai-Tibet Highway from 2002 to 2004 [61]. [PERSON] et al. [51] analyzed the characteristics of the ground temperatures in the same horizontal and vertical directions based on the observations of the TPCT test section of the Qinghai-Tibet Highway from 2004 to 2012. Through indirect calculations, it was found that the TPCT transfers about 1200 MJ to 1500 MJ of heat per year, transferring more energy in the first few years, and gradually stabilizing the heat transferred by the TPCT over time. [PERSON] et al. [52] found that the TPCT could effectively cool the submerged multiyear permafrost of the roadbed, and that the optimal cooling effect of the TPCT was located in the middle of its evaporation section. [PERSON] et al. [61] analyzed the ground temperature characteristics, temperature field morphology, and the freezing and thawing process of TPCT roadbed based on nearly 11 years of field monitoring data of the TPCT roadbed test project on Qinghai-Tibet Highway, estimated the horizontal heat gain and loss condition near the TPCT under the influence of the roadbed's yin and yang slope effect, and concluded that the long-term cooling effect of the bi-lateral TPCT roadbed was stronger than that of the unilateral TPCT roadbed, and that the diagonally inserted TPCT roadbed was stronger than that of the straight-inserted TPCT roadbed. Figure 5: The harpin TPCT with the sensor [49]. High-grade highways have the structural characteristics of \"wide, thick and black\" and strong heat absorption, heat storage, and heat gathering effects, which pose a greater threat to the stability of perennial permafrost roadbeds. Many scholars have carried out a series of research on the thermal stability of high-grade wide roadbeds [7]. [PERSON] et al. [38] and [PERSON] et al. [62] designed an L-shaped TPCT-block gravel berm-insulated panel composite roadbed in order to achieve the purpose of maintaining the thermal stability of a wide high-grade highway in a permafrost zone while protecting the underlying permafrost. In order to verify the cooling effect of the new TPCT composite roadbed, an indoor model test was carried out, and the test results preliminarily proved the effective cooling ability of this roadbed, and it was found that the air temperature remained negative in most parts of the roadbed after the end of the positive temperature period, remaining around \(-0.4\)\({}^{\circ}\)C. [PERSON] et al. [39] conducted a wide test roadbed in the Beiluhe area of the Qinghai-Tibetan Plateau monitoring and comparatively analyzed an L-shaped TPCT-insulation board-block gravel berm composite roadbed and an unprotected roadbed as a control. The longitudinal length of the L-shaped TPCT-insulation board-block gravel berm composite roadbed was 16.5 m, the top surface width was 15.9 m, and the asphalt pavement width was 11.5 m, with four lanes in both directions.