Formation Law Of The Landslide And Its Effect On The Subgrade Stability In Permafrost Degradation Region

Highway often has to traverse many topographic, tectonic and geologic units, and influenced by all kinds of geological disasters. Bei’an to Heihe Freeway intersects the Lesser Khingan Mountains in Sunwu County; the area is located in the southern margin of China high latitude permafrost region, and with typical characteristics of periglacial geomorphology. In recent years, the landslides phenomena are increasing and threaten the subgrade stability and operation safety in that road area; in addition, the distribution and morphological characteristics, movement law of the landslides have significant difference from the landslids in other region. In order to explore the distribution characteristics, development mechanism, triggering factors, failure mode, movement law and influence factors of the landslides in permafrost region, study the effective landslide exploration technology, rock and soil testing and slope stability analysis method, ensure the highway engineering quality and operation safety, enrich and improve the high-grade highway construction technology results of unfavorable geological section in cold region, protect the geological environment, human life and property security in cold region. Took the Bei-Hei Freeway expansion engineering K177+300-K179+200 section road area which traverses Lesser Khingan Mountains as study area, carried out a systematic research on the landslides in that road area.Started from analysis the environmental geology condition of the study area, then combined with the regional permafrost distribution characteristics and a long sequence of temperature change data, carried out study on the relationship between the regional climate change, permafrost degradation and shallow landslide formation. The results showed that:the air temperature has increased and the permafrost has degraded year by year caused by climate change in the study area. To the shallow low-angle landslides which have attached permafrost developed in the substratum distributed in the valley upstream hillside area, their formation and development is affected by freezing thawing action and permafrost degradation. With the climate change process evolving in the area, the upper limit of the permafrost will decline, the landslide depth will deepen, and the risk will increase.Through many-times high density resistivity exploration in the landslides area, obtained the resistivity distribution of underground space in different periods, combined with the geological survey results, site conditions and monitoring data, carried out study on the resistivity distribution law and influence factors of rock and soil. The results showed that:the Tertiary Sunwu formation pebbly sandstone presents high resistivity; the Cretaceous Nenjiang formation mudstone presents low, medium and high resistivity, increases with its weathering degree increased; the Cretaceous Nenjiang formation siltstone presents low and medium resistivity, decreases with its weathering degree increased. The resistivity of rock and soil decreases with its water content increased, increases with its water frozen, and increases with its structure destroyed, and decreases with its compactness increased. In the process of rocky landslide dislocating, the sliding zone rock is sheared, the structure appears damage, and the resistivity increases significantly; in the process of soil landslide forming, water content of the sliding zone soil increases and reaches saturated water content, the resistivity decreases obviously.Combined with the meteorological data of meteorological station, the site condition and the ground temperature, ground surface displacement, rock and soil horizontal displacement, water content and pore water pressure monitoring data, carried out study on the deformation law, the instability trigger and influence factors of the landslides. The results showed that:the K177+550 landslide is a intermittent sliding rotary type landslide; continuous infiltration of the successive and low intensity precipitation and seepage of the permafrost thawing water is the main reason leading to the slope losing stability. The K178+530 landslide is a intermittent sliding translational type landslide; rapid infiltration of the concentrated and strong precipitation and seepage of the permafrost thawing water is the main reason leading to the slope losing stability. The change of regional temperature and atmospheric precipitation have a close correlation with the ground temperature, rock and soil water content, groundwater level and landslide movement. With the landslide potential energy decreasing and the regional permafrost degrading which results in moistuer supplement capacity for rock and soil decreasing, the movement velocity of landslides decreases year by year.Adopted finite element method, based on the strata distribution of the landslides and the physical and mechanical indexes of the rock and soil which obtained from indoor test, established the numerical analysis model, calculated the saturation and deformation distribution of rock and soil and safety factor of the slope at different times, carried out study on the formation and instability reason of the landslides. The results showed that:to the K177+550 landslide, water infiltrates through surface strongest permeability pebbly sandstone and residual soil, the shallow unsaturated pore water is blocked by mudstone in the vertical infiltration process, forms Tertiary pore phreatic water at the bottom of pebbly sandstone layer and forms Quaternary pore phreatic water in the ancient landslide tailing edge clinosol soil layer, the stronger permeability shallow and sliding zone mudstone is soaked by pore phreatic water and reaches saturated sate, results in Cretaceous pore phreatic water forming. With the external water supply, vertical infiltration of the shallow unsaturated pore water, vertical and lateral infiltration of the deep saturated pore water, phreatic water level rises, saturation of the deep rock and soil increases. The strength and anti-deformability of the sliding zone mudstone gradually decrease with its saturation increases, leading to the horizontal displacement and plastic strain of the slope increase gradually; until the rear sliding zone mudstone basically reaches saturation state, the plastic zone runs through to the ground surface, the landslide rear residual soil loses stability and squeezes the front rock and soil. To the K178+530 landslide, water infiltrates through surface strongest permeability pebbly sandstone and residual soil, the substratum of the pebbly sandstone is the stronger permeability mudstone, the shallow unsaturated pore water is blocked by the poor permeability siltstone and silt clay in the vertical infiltration process, forms Cretaceous pore phreatic water in the mudstone layer and forms Quaternary pore phreatic water in the residual soil layer. With the external water supply, vertical infiltration of the shallow unsaturated pore water, vertical and lateral infiltration of the deep saturated pore water, phreatic water level rises, saturation of the deep rock and soil increases. The strength and anti-deformability of the bottom residual soil rapidly decrease with its saturation increases, leading to the horizontal displacement and plastic strain of the slope increase rapidly; until half of the rear residual soil reaches saturation state, the plastic zone runs through to the ground surface, the landslide middle and rear residual soil loses stability and squeezes the front soil.