As shown in Fig. 1, a frozen rock slope is characterized by fractured rock mass and ice coexisting naturally or caused by human intervention. As a result of the periodic temperature changes, the rock mass of the slope is subjected to a reciprocating freeze-thaw cycle, which results in a different stability mode and failure mechanism than that of an ordinary slope. This section aims to analyze the influence of the freeze-thaw cycle on the stability of fractured rock mass slopes and the types of failure they are prone to from the following perspectives.
2.1 Stability Factors of Frozen Rock Slope
The stability of frozen rock slope is mainly affected by the water content, freeze-thaw action and variable-value temperature load in addition to the conventional factors such as rainfall, earthquake and external load [20, 21]. According to the geographical location of frozen rock slopes, they can be divided into seasonal rock slopes (Class I) and multi-year frozen rock slopes (Class II).
The first type of frozen rock slope is mainly affected by rainfall and periodic temperature. Precipitation enters the rock fissure, and the gradual freezing of pore water leads to the expansion of Rock joint fracture propagation. After spring thawing, the bond strength inside the rock mass will decrease significantly, and the fissure ice will dissolve into the water and further migrate, and the fissure will continue to expand[1], which may lead to slope collapse and failure under the long-term freeze-thaw cycle.
The second type of frozen rock slope is formed by engineering construction in permafrost area, which is not only affected by the factors mentioned in the first type of frozen rock slope but also by the permafrost. The excavation exposes the permanent frozen rock which is not affected by the freezing and thawing cycle, so it not only changes the stress field of the original stratum, but also changes the temperature field and moisture field of the stratum at a certain depth, causing the permafrost to move down, as shown in Fig. 2.
Slope stability must be considered when engineering rock slopes in cold regions, especially when considering the changes in rock properties as well as the cumulative effect (damage) of freeze-thaw cycles [20]. Because the open-pit mine engineering construction is a long-term process, the stability of open-pit mine slope in cold areas, along with the conventional slope stability factors and the influence of periodic air temperature, is also significantly affected by the engineering construction (construction dynamic load, etc.), making stability analysis of frozen rock slope factors more complex.
2.2 Failure Mechanism of Frozen Rock Slope
Cold areas are primarily prone to slope failure due to cyclic freezing and thawing caused by periodic temperature changes[23]. During the winter, the slope is covered with ice and snow. Snow and ice melt as spring approaches or the Development of global warming[24], infiltrating the lower rock mass. While the upper soil body is melting at this point, the lower soil body is still frozen, which means that moisture cannot continue to infiltrate the lower soil body. As a result, the soil body at the upper end of the slope is saturated or supersaturated, reducing its shear strength to a great extent. Consequently, the freeze-thaw interface becomes a sliding surface that continuously creeps and deforms under repeated day and night air temperature and gravity action, resulting in the shallow slump of the slope[1], as shown in Fig. 3.
There is a significant difference between the failure mechanism of fractured frozen rock slopes and that of fractured frozen soil slopes. As a result of fractures in the frozen rock slope, the failure mechanism is more complex. Water inside a fractured rock mass will transform into ice when its temperature drops to the freezing point. Considering that the volume of the water phase will expand by 9% when it is converted to ice[25], the resulting volume expansion will exert pressure on the fractured rock mass (frost heaving force) causing fractures to expand (Fig. 4)[26–27]. As the temperature increases, the melted moisture will infiltrate into the newly formed cracks. Water will freeze again when the temperature falls. This will result in damage to the rock mass when the temperature falls again. As a result, the strength of the rock mass will continue to decrease over time. As can be seen, the failure of frozen rock slopes is primarily due to the periodic changes in air temperature that continuously damage the rock mass of the slope as a result of the reciprocating freeze-thaw cycle. In addition, the seasonal change and the temperature difference between the day and night cycle will also have an important influence on the mechanical properties of rock mass (especially the rock mass with low strength and high water content).
Furthermore, some frozen rock slopes have loose overburden at a certain depth on the surface, which requires that the failure mechanism of frozen rock slopes also consider frozen soil slopes and their mutual influence.
2.3 Types of Frozen Rock Slope Failure
Different from in the normal temperatures. Slope instability in cold regions is primarily caused by the freeze-thaw cycle, rather than rainfall or other external loads.[24] As a result of engineering studies and system theory research, people have been able to identify the type of failure of frozen soil slopes. Among the instability types of frozen soil slopes, McRoberts and Morgenstern[28] distinguished between mud flow, landslides, collapse and additionally distinguished. According to Niu et al. (2016)[29] unstable slopes in cold regions can be divided into normal frozen landslides, normal melting landslides, and freeze-thaw landslides depending on their causes, while the disease pattern of loess cutting slopes has also been classified as massive spalling, layered spalling, flaking, middle-layer fragmented spalling, and surface crust spalling. Shan et al.[30] obtained that landslides are seasonal, progressive and low-angle
Few studies have been conducted on the classification of failure types in frozen rock slopes. In his study, Chen[31] distinguished three types of freeze-thaw slump of frozen rock slopes: toppling collapse, sliding collapse, and falling rocks. As a result of its comprehensive frozen soil slope failure type division method and actual frozen rock slope failure, this paper categorizes frozen rock slope failure types according to their deformation failure mechanisms:
(1) Freeze-thaw collapse of loose body
The upper part of a rock slope that is covered with loose material has similar properties to that of a frozen soil slope, or is damaged by freeze-thaw action along the rock-soil interface, which is common in open-pit mine slopes in the Qinghai-Tibet Plateau, as shown in Fig. 5(a).
(2) Freeze-thaw toppling failures
It is common for tiltling damage to occur when the frozen rock slope in the cold area has a steep slope[32], severe weathering, and groundwater recharge. As a result of the huge frost heaving force generated by the ice-water phase change in winter, tiltling damage may occur as a result. There is also the possibility of tiltling damage occurring during other seasons due to the action of freeze-thaw cycles, as illustrated in Fig. 5(b).
(3) Thermal melt collapse failure
Freeze-thaw damage caused by freeze-thaw cycle can significantly reduce rock strength. There is a particular tendency for this phenomenon to be evident when there is a high water content. The failure of frozen rock slopes is a result of long-term freeze-thaw action, and it usually occurs during the spring thaw season. Frost heaving in winter increases the degree of weathering, causing the ice phase to change to water after the temperature rises, reducing the bond strength between the rock masses, and causing the collapse damage shown in Fig. 5(c).
(4) Deep instability caused by meltwater infiltration and frost heave splitting
As shown in Fig. 5(c), frozen rock slopes in cold regions may undergo deep unstable failures along the upper and lower limits of structural planes or permanent frozen layers in addition to the above three types of unstable failures. Engineering disturbances and frost heaving may also result in deep unstable failures along these lines such as Fig. 5(d).