*4.1. Risk Section and Deformation Law*

Affected by the thermal thawing and frost heaving of the active layer of permafrost, there are roadbed deformation and frozen soil collapse in WB and TZ of the two sections, as shown in Figures 5a and 5a', respectively, representing the original amplitude map and annual average deformation of WB section in 2020. The deformation is mostly concentrated in the valley area in the north and appears to be lesser in the flat area in the south. Figure 5b,b' is the original amplitude map and annual average deformation map of TZ section. As it passes through the inland river, it is greatly affected by runoff and thermal melting, resulting in melting collapse in many places in the southern section of the area.

## 4.1.1. Deformation from WangKun Station to Budongquan Station

The deformation section between WangKun station and Budongquan station is located in the valley area, and its terrain, deformation and geological structure, is shown in Figure 6.

Affected by global warming and human activities, the phenomenon of frost heaving and thawing settlement of permafrost active layer in this area is obvious. Because the terrain of this section is high on both sides and low in the middle, when the shear force reaches a certain threshold due to the thawing settlement of frozen soil, a shallow landslide will occur at the front of the mountains on both sides. At the same time, because this section is located in the valley, there is a large amount of glacier melt water and precipitation, and the surface and underground runoff is more abundant. The hydrothermal effect will continuously thicken the frozen soil active layer, aggravate the frost heave and thaw settlement, and further promote the collapse on both sides of this section. It can also be clearly seen in Figure 6d that melting collapse occurs in R1 and R2 areas along both sides of the railway section. The center latitude and longitude of R1 is 35.66984◦ N, 94.05412◦ E; the average annual deformation rate is −62.91 mm/a; and the collapse area reaches 0.36 km2. The deformation on both sides of the railway at R2 is uneven, with roadbed lifting on the left and thawing collapse of frozen soil on the right. The annual deformation is 48.13 mm/a and −158.46 mm/a, respectively. The existence of large-scale melt collapse body and the difference in roadbed deformation affect the normal operation of railway. In addition, since the railway section in flat areas is basically free of deformation, it can be inferred that the influence of human activities of railway operation on frozen soil deformation is smaller than that of topography and hydrothermal. In other words, topography and hydrothermal action are the main causes of railway deformation in this section.

Considering that geological factors may also affect it, we compared the geological distribution Figure 6c with the deformation distribution Figure 6d. It was determined that the surface deformation of this section is mainly concentrated between the two faults, and the deformation is weak in the north of the North fault. At the same time, there are Qp2gl, Qp3gl, TB3, P1QQ3 and other strata between the two fault layers. The severely deformed strata are mainly middle Pleistocene ice deposit Qp2gl and late Pleistocene ice deposit Qp3gl, which are mainly ice water accumulation. The moraine is composed of boulders, gravel, sand and clay. The soil is soft, sensitive to hydrothermal changes, and prone to hot melting and frost heaving. Therefore, the roadbed deformation in frozen soil section may be affected by thawing settlement of frozen soil, fault, and lithology.

Due to the complex physical movement along the slope of this section, more information (movement rate, direction, and trend) about terrain (gradient and direction), geology (mantle composition and surface coverage), hydrology (surface and underground runoff and ice melting of permafrost) is needed to monitor the evolution of active layer of permafrost so as to better grasp the deformation inducement and law of this area.

#### 4.1.2. Deformation from Tanggula Station to Za'gya Zangbo Station

The section with serious roadbed deformation from Tanggula station to Zajiazangbu station is selected. The whole section shows a downward trend, with uneven deformation on both east and west sides. The total length of the section is approximately 620 m, and the central latitude and longitude are 32.90665◦ N, 91.52807◦ E. This section passes through Za'gya Zangbo, the longest inland river in Tibet. The runoff is supplemented by the water melted by ice and snow. As many sections of runoff pass through this section, the river has a great impact on the thermal thawing of permafrost, resulting in changes in the hydrothermal status of the active layer above the permafrost [65]. Coupled with the joint impact of railway operation activities, the phenomenon of frost heaving and thawing settlement of the frozen soil layer is obvious.

The red arrow shows the connection between the north and south sections. It can be observed that the deformation of the south section is larger than that of the north section as a whole. In Figure 7a, the annual average deformation phase at P3 is smaller than that of P1 and P2. The water activity and temperature distribution of railway roadbed in frozen soil area are the key factors affecting roadbed frost heaving and thawing settlement. In unsaturated state, the higher the water content, the greater the frost heaving amount of soil with the same density, the greater the corresponding frost heaving and thawing settlement rate; that is, water supply is the fundamental reason for frost heaving and thawing settlement of frozen soil [6,66]. There are two branches of runoff in the south, which have a connecting trend, and the runoff at the arrow is expanding, which has a great impact on the frozen soil. The frost heaving and thawing settlement process of frozen soil active layer is more intense, resulting in uneven roadbed deformation on the north and south sides.

**Figure 5.** (**a**,**a'**) are the original amplitude map and annual average deformation map of WB deformation section, respectively (the annual average deformation of the surface after being affected by the thermal thawing and frost heaving of permafrost active layer), (**b**,**b'**) are the original amplitude map and annual average deformation map of TZ deformation section, respectively, In the Figure, the red solid dot indicates the roadbed deformation or serious deformation area threatening the roadbed, and the white box indicates the deformation risk section.

In the south section, the deformation on the left and right sides is uneven and the difference is obvious, which poses a threat to the railway operation. As shown in P1 and P2 in Figure 7c, the cumulative deformation is 113.54 mm and 54.76 mm, respectively, and the annual average deformation is −112.768 mm/a and −52.084 mm/a, respectively. The uneven deformation may be affected by the thickness difference in the active layer of permafrost. The thermal melting of frozen soil in summer leads to different collapse degrees on both sides of the railway, and it cannot be completely frozen in winter, resulting in greater and greater differences on both sides of the roadbed and affecting the stability of the roadbed.

**Figure 6.** (**a**) shows the DEM of the railway section; (**b**) is the optical image of Google Earth; (**c**) is the distribution of surface deformation at unfrozen spring station; (**d**) is a geological distribution map, and the fault zone is shown in red dotted line.
