Freeze-thaw geological disasters may also occur when spring comes and flowers bloom, so two areas should pay special attention!

If you think landslides only occur during the rainy season, you are wrong! In northern and northwestern my country, the peak season for landslides is from March to May when the flowers are blooming in the spring!

Most of the landslides in this period are freeze-thaw geological disasters. Unlike geological disasters induced by rainfall and human engineering activities, freeze-thaw geological disasters often have no obvious inducing factors, but occur suddenly with the warming of the climate. They are highly latent and sudden, and often occur in the early morning or midnight. The losses caused are often greater than those of ordinary landslides. For example, the Sale Mountain landslide in Dongxiang Autonomous County, Gansu Province, which occurred on March 7, 1983, caused significant economic losses and casualties.

The most recent major freeze-thaw geological disaster occurred in Zizhou County, Yulin City, Shaanxi Province. At 1:30 a.m. on March 10, 2010, 90,000 cubic meters of loess poured down from Shigou, Shuanghuyu Village, Zizhou County, burying 44 people and killing 27. Because these landslides often occur in early spring after the temperature has warmed up for several consecutive days, they are called freeze-thaw geological disasters.

In addition to Qinghai, Gansu and Shaanxi, freeze-thaw geological disasters also occur from time to time in the loess-covered Shanxi Loess Plateau, causing a large number of casualties. For example, a landslide occurred in Zaoling Township, Ning County, Linfen, Shanxi on March 15, 2019, killing 20 people and injuring 13 people. Two residential buildings, a hospital inpatient building, a small bathing center and supporting structures collapsed, causing direct economic losses of 7.1348 million yuan.

Spring freeze-thaw geological disasters in Shanxi since 2005

(The chart is based on the following two documents: Distribution characteristics of loess in Shanxi and landslide (collapse) disasters; Research on green coal mining strategy in Hedong Coalfield based on resource and geological environment characteristics)

Different from the physical damage caused by freeze-thaw in the shallow frozen layer of perennial frozen areas, freeze-thaw geological action is triggered by the change of the physical form of water in the strata, thereby changing the flow field of groundwater. The specific process is that freeze-thaw causes the enrichment and dispersion of groundwater in the slope body. The enrichment causes the softening of the loess slope soil, and the dispersion brings about a sudden change in the static water pressure and dynamic water pressure of the loess, which in turn affects the stability of the slope.

Schematic diagram of groundwater changes in loess slopes under the action of surface frozen soil

(Image source: Zhang Maosheng, Li Tonglu. Research on the factors causing loess landslide and its formation mechanism [J]. Journal of Engineering Geology. 2011, (4): 530-540.)

It is generally believed that the mechanism types of loess landslides induced by freeze-thaw are divided into two types: frozen water stagnation effect type and thaw intensity reduction type. The latter is more common and is common from March to May every year when the seasonal frozen soil thaws. In spring, with the rapid rise in temperature, the ice and snow melt, the soil thaws, and freeze-thaw geological disasters begin to enter a high incidence period.

In terms of disaster effects, although the thickness of seasonal frozen soil is not large, generally about 1~2.5m, it often causes large-scale catastrophic landslides. This is because after repeated freezing and thawing, the mechanical strength of loess decreases sharply, which can easily induce large landslides and cause major disasters. For example, the Shigou landslide in Zizhou County, Shaanxi mentioned above belongs to this type. In early spring, the temperature warms up, and the melting of seasonal frozen soil causes the loess high-cut slope, which is already in a critical state, to become unstable and slide, thus inducing disasters. The "3.15" landslide in Xiangning, Shanxi, was identified as the freezing and thawing action that reduced the weak structural surface in the original terrain layer, causing the slope to be sheared out along the weak structural surface under the action of gravity.

Through observations of groundwater at frequent locations of frozen-thaw loess landslides, such as the edge of Heifangtai in Gansu, my country, scientists have found that the retention and sudden discharge of groundwater in the soil caused by freezing and thawing may be the direct cause of many landslide disasters. Unfortunately, research on the mechanism of such landslide disasters is still insufficient. With the proposal of the strategy of ecological protection and high-quality development in the Yellow River Basin, we look forward to more in-depth research in the future to solve the mystery of loess frozen-thaw geological disasters.

Like all geological disasters, the prevention of freeze-thaw geological disasters first requires advance inspections and identification of dangerous areas. The loess slopes in northern Shaanxi and the high-cut slopes of the Loess Plateau in western Shanxi are high-risk areas and high-incidence periods for freeze-thaw geological disasters from March to May each year. In particular, newly built immigrant relocation areas and concentrated residential resettlement areas in the gully areas of the Loess Plateau should be key inspection areas. The second is to do a good job of monitoring, early warning and risk avoidance and transfer. The Fengyugou landslide in Lin County, Shanxi Province on May 2, 2020 was caused by the immediate organization of transfer after the cracks in the mountain were discovered during the inspection, which timely avoided casualties.