Mechanism And Engineering Countermeasures Of Highway Foundation Settlement And Slope Instability In The Northeast Frozen Soil Region

In the frozen soil region of Northeast China,precipitation and seasonal thawing-freezing can reduce the mechanical strength of the slope soil,resulting in thawing-freezing instability of roadside slopes;The thawing of permafrost can cause strength damage to natural foundation soil and insufficient bearing capacity of the foundation,leading to the uneven settlement of subgrade.These two disasters interact and influence each other,and have become the main problems faced by highway construction,operation,and maintenance in the northeast frozen soil region,which have not yet been completely solved.The main reason is the unclear understanding of the occurrence mechanism of the two disasters,and then the poor treatment.At present,the research results of subgrade disasters in frozen soil regions are mainly concentrated in the Qinghai-Tibet Plateau region.However,due to the influence of geographical differentiation,the environmental geological conditions in the northeast region are different from those in the Qinghai-Tibet Plateau,and the existing research results do not apply to the northeast frozen soil region.Therefore,studying the two major disasters faced by highways in the frozen soil region of Northeast China is of great significance to improve the quality of highway construction,ensure the durability of highways and reduce the cost of highway construction,operation,and maintenance.Subgrade failure caused by slope instability in frozen soil regions is very common,and permafrost thawing will not only lead to slope instability but also lead to uneven subgrade settlement.To solve the problem of road failure in the northeast permafrost region,two typical roadside slopes(roadside mountain slope and cutting slope)are selected.Using geophysical exploration,field measurement,sensor monitoring,laboratory test,and numerical simulation,the instability mechanism is studied and corresponding treatment measures are proposed.At the same time,the influence of slope instability on the thermal-mechanical stability of the subgrades is explained;In addition,based on the existing research results of subgrade treatment measures in frozen soil regions,the water-heat variation laws and deformation mechanism of the block stones-soil layer system was studied through indoor tests,and two engineering measures are put forward to prevent uneven settlement of subgrades.The main contents and research results of this article are as follows:(1)Instability mechanism and driving force transformation of the roadside mountain slope under groundwater supply conditionTaking an unstable mountain slope within the scope of the Bei’an-Heihe Highway as the research object,based on field investigation,borehole detection,field measurement,sensor monitoring,indoor test,Google satellite image,photogrammetry,high-density resistivity method,and ground penetrating radar data,the formation mechanism and driving force transformation reason of the landslide are studied,and the hydro-thermo-mechanical coupling model of frozen soil based on the damage constitutive model of warm frozen soil is established,The sliding process of the landslide is reconstructed,and the pore water pressure distribution,effective stress change,and deformation of the slope during the sliding process are analyzed.The results show that the groundwater supply generated by the permafrost thawing and precipitation infiltration reduces the mechanical strength of the soil near the rear edge of the landslide and increases its weight,providing a dynamic condition for the landslide to start.In warm seasons,the melting of the segregation ices in the seasonally frozen layer and the precipitation infiltration reduces the anti-sliding force of the slope and provides the deformation conditions for the start of the landslide.The joint action of the two causes the landslide.The front edge of the landslide continues to slide forward in warm seasons after the deformation of the rear edge of the landslide stops.This is because the increased water content caused by precipitation infiltration and seasonal freezing and thawing reduces the anti-sliding force of the transverse ridges near the front edge,providing power for its sliding under its gravity.(2)Protection and reinforcement mechanism of turf and shrubs(1:1 Amorpha fruticosa Linn to Lespedeza)on cutting slopeTaking the cutting slopes protected by turf and shrubs on the Tongjiang-Sanya Highway as the research objects,through in-situ monitoring,field and laboratory tests,the moisture and temperature changes of the shallow soil of the two slopes and the influence of moisture on the strength of the slope soil were analyzed,and the protection and reinforcement mechanism of turf and shrubs on the cutting slopes were studied.The results show that there are two reasons for the slope instability of turf protection: first,precipitation infiltration increases the water content and weight of the shallow soil mass of the slope(mainly the slope toe)and reduces its mechanical strength,thus providing potential power for the initial sliding of the slope;Second,the thawing of frozen soil in the seasonally frozen layer reduces the mechanical strength of the soil mass and provides deformation conditions for the formation of the sliding surface.Shrub-reinforced slopes remain stable for two reasons: first,the strong transpiration of shrubs reduces the moisture content of the shallow soil mass and reduces the weight of the soil mass,thus ensuring that the mechanical strength of the soil mass is not weakened;Secondly,the shrub root-soil complex(reinforcement)increases the mechanical strength of the shallow soil.(3)Build a hydro-thermo-mechanical coupling model of frozen soil based on plant physiological characteristics,and analyze the protection and reinforcement mechanism of turf and shrubs on cutting slopeTo analyze the protection and reinforcement mechanism of turf and shrubs on the cutting slope,a hydro-thermo-mechanical coupling model of frozen soil based on plant physiological characteristics was established to simulate the temperature,moisture,and deformation of the turf and shrub slopes.The results show that the slope protected by turf does not lose stability only under the action of precipitation,and the slope slides only when the precipitation infiltration leads to the melting of the segregation ice in the seasonally frozen layer.The position of the sliding surface of the slope protected by turf is determined by the precipitation infiltration depth and the thawing depth of the seasonally frozen layer.During the initial thawing of frozen soil in the seasonally frozen layer,the precipitation infiltration depth and thawing depth of frozen soil are small,the landslide mass has a small volume and slow sliding.With the increase of precipitation infiltration depth and thawing depth of frozen soil,the sliding rate of the slope protected by turf increases continuously.When the thickest segregation ice melts,the sliding rate reaches the maximum.For the slope reinforced with shrubs,because the moisture content of the shallow soil mass is close to the optimum moisture content,the mechanical strength of the soil mass is not weakened,and the root-soil complex(reinforcement)increases the mechanical strength of the shallow soil mass,so the slope only has a small range of seasonal freeze-thaw fluctuations,without failure.(4)Water and heat change laws and their influence mechanism on deformation in the block stones-soil layer systemTo solve the problem of uneven settlement of permafrost subgrade,the water and heat change laws in the block stoned-soil layer system and their influence mechanism on deformation are studied,which provides theoretical support for the deformation analysis of the prevention measures for uneven settlement of filling block-stone subgrade and replacement block-stone subgrade.The freeze-thaw cycle tests of three kinds of crushed rocks-clay samples(crushedrock instead of block-stone in the laboratory tests)under two conditions of water supplement and no water supplement were designed.Sample 1: clay column with a height of 12 cm;Sample2: the upper part is crushed-rock 1 with a height of 5 cm(particle size range of 0.5 – 3 mm),and the lower part is clay with a height of 7 cm;Sample 3: the upper part is crushed-rock 2 with a height of 5 cm(particle size range of 1 – 5.5 mm),and the lower part is clay with a height of 7cm.The test results show that during the freeze-thaw cycle,the low thermal conductivity of the upper crushed-rock layer can reduce the heat transfer efficiency of the sample,thus reducing the temperature variation amplitude of the lower clay layer.Increasing the particle size of crushed rock can reduce the water content that migrates upward in the unfrozen zone,and also reduce the sample temperature and moisture change amplitude,thus improving its thermal stability.Water refill or not does not change the water and heat change laws in the sample but can affect the temperature and moisture change amplitude.The water supplement at the bottom can increase the heat absorption intensity and upward water migration of the sample.The greater the water migration,the greater the impact on heat transfer,but the two are not linear.The existence of a crushed-rock layer can reduce the accumulated settlement and frost heave on the top of the sample.The larger the crushed-rock particle size,the smaller the accumulated settlement and frost heave.(5)Analysis of the deformation mechanism and optimum filling height of the filling blockstone subgradeBased on the study of the water and heat change laws in the block stones-soil layer system under the action of freezing and thawing and their influence on the deformation,it is proposed to reduce the uneven settlement of the foundation by using the large-particle block-stone(20-40 cm in diameter)as the subgrade filler.To study the long-term stability of this measure and determine the optimal filling height of the block-stone,based on the theory of heat and mass transfer,a hydro-thermo-mechanical coupling model of unsaturated porous media considering the influence of air convection is established,and the geometric model and boundary conditions of the subgrade are determined according to the boreholes and ground temperature monitoring data,The hydrothermal and deformation conditions of the filling block-stone subgrade with five filling block-stone heights(the filling block-stone heights are 5.0,4.0,3.0,2.0 m,respectively)in 20 service years are calculated,and its mechanical stability is evaluated by two indicators of uneven settlement and transverse deformation difference.The results show that the increase of the height of the filling block-stone layer(>3.0 m)can raise the permafrost table to above the natural surface,thus effectively protecting the underlying permafrost,and at the same time can reduce the water content of the deep foundation moving to the active layer and the consolidation deformation of the natural foundation.This measure can improve the thermal-mechanical stability of the subgrade by improving the water and heat conditions of the foundation.(6)Influence of different foundation replacement materials on water,heat,and deformation of the permafrost subgradeBased on the design concept of allowing permafrost thawing,a new subgrade structure(replacement block-stone subgrade structure)is proposed to prevent uneven settlement of subgrade by replacing the natural foundation permafrost with large grain size block stones,overlaying with thermo-resistant and water-resistant soil layer and normally filling soil above the natural surface.Using numerical simulation,the water,heat,and deformation conditions of the replacement block-stone subgrade,replacement breccia subgrade,and replacement gravel subgrade in 20 service years are compared and analyzed.In addition,the influence of the two freezing forms of water in the active layer in winter(dispersed ice crystal and continuous ice lens)on hydrothermal migration in these subgrades are discussed,the deformation is corrected and the thermal-mechanical stability of the three subgrades is evaluated.The results show that the replacement block-stone layer can effectively reduce the total heat entering the deep foundation in warm seasons and the water content migrating from the melting interlayer to the surfaces,thus reducing the thawing rate of the permafrost and the consolidation deformation of the natural foundation.The replacement block-stone subgrade structure can not only reduce the accumulated settlement and frost heave at the top of the subgrade but also reduce the transverse deformation and longitudinal cracks,thus effectively improving the thermal-mechanical stability of the subgrade.On the contrary,the deformation of the replacement breccia subgrade structure and replacement gravel subgrade structure is too large,and these two structures are not suitable in warm permafrost regions.Through the research on the deformation mechanism of the roadside mountain slope,cutting slope,and uneven settlement of subgrades in the northeast permafrost region,the corresponding engineering countermeasures are put forward,and the actual effects are verified through engineering practices.The research provides an important basis and practical reference for solving the problems of roadside mountain slope instability,cutting slope deformation,and uneven settlement of subgrades in the northeast permafrost region.