## 4 Applications of the TPCT
### Railways  
In 1978, TPCTs were used on the Hudson Bay Railway in Canada to stop the degradation of perennial permafrost and thus stabilize the roadbed. Through five years of monitoring data, it was found that it was feasible for TPCTs to control the thawing and sinking of roadbeds in high-temperature discontinuous perennial permafrost zones [63]. TPCTs are mainly inserted vertically or at a certain angle diagonally in the shoulder or foot of slope position in the construction of the Qinghai-Tibet Railway (Figure 5(b)), and their application mileage exceeds 34 km, and many TPCTs are used in the operation and  
Figure 6: Application of TPCTs in Permafrost regions. maintenance process of the Qinghai-Tibet Railway. Depending on the height of the railway roadbed, the length of TPCTs is divided into 7 m, 9 m, and 12 m. [PERSON] et al. [64] found that the upper limit of perennial permafrost was consistently lifted after the use of composite measures of TPCTs and thermal insulation boards in the roadbed of the Qinghai-Tibet Railway. [PERSON] et al. [53] added block gravel slopes and TPCT composite reinforcement measures to the high temperature and high ice content section of the Qinghai-Tibet Railway. The results showed that the upper limit of perennial permafrost was further lifted for the roadbed without an insulation board, and the yin-yang slope effect was weakened; the upper limit of perennial permafrost was lifted to the vicinity of the insulation board for the roadbed with the insulation board, and the thawed interlayer disappeared after two freeze-thaw cycles. [PERSON] et al. [48] found that the composite roadbed structure of inline TPCT-block gravel berms could solve the roadbed diseases of Qinghai-Tibet Railway and mitigate the settlement of the embankment. In addition, the TPCT is also used in the transition section of the Qinghai-Tibet Railway to alleviate the phenomenon of \"jumping car at the bridge\". Along the railway line of Qaidam-Muli, high-temperature swampy permafrost is widely developed, and the high-temperature, high ice-content permafrost section is more than 35 km. According to the meteorological observation data along the railway line of Qaidam-Muli, the low average air temperature in winter and the negative temperature in summer that promotes the work of TPCTs, coupled with the high average annual wind speed, provide favorable climatic conditions for the work of TPCTs [65]. Therefore, about 20,391 TPCTs were used in a 20 km long permafrost subgrade with a longitudinal and transverse spacing of 3.0 m, and the design parameters were strengthened on the sunny slopes [66]. By comparing the upper limit of permafrost in the TPCT section with that in the non-TPCT section, the TPCTs can effectively reduce the road base temperature after one year of construction, and the upper limit of permafrost was found to be elevated by 1.5 m relative to the natural borehole ground temperature. In addition, [PERSON] et al. [67] observed the ground temperature of the TPCT section in Chaiwu Railway, and took the ground temperature drop of 0.5 \({}^{\circ}\)C in the middle of the TPCT evaporation section as the criterion for determining the effective cooling range of the TPCT, and used the annual minimum temperature and the annual average temperature as the basis for division, and found that the effective cooling radius of the TPCT was 4.5 m and 2.4 m. Therefore, the design spacing of TPCT of Chaiwu railway is in the range of TPCT cooling, and the TPCT roadbed can effectively cool the frozen soil roadbed and improve stability. In order to rectify the freezing and thawing disaster of the roadbed in the perennial permafrost area of the Yalin Line in the northeast region, TPCTs and thermal insulation materials were used, and the upper limit of man-made permafrost was found to be significantly raised after rectification. It is worth mentioning that TPCTs are also used in seasonal permafrost zone to eliminate frost heave, and [PERSON] et al.