Freeze-thaw hazard is one of the main problems in cold regions engineering and artificial ground freezing engineering. To mitigate freeze-thaw hazards, it is essential to investigate the effects of freeze-thaw on soils engineering properties. This paper summarizes the effects of freeze-thaw on the physical and mechanical properties of soils reported in recent studies. The differences of freeze-thaw conditions between freezing shaft sinking and cold regions engineering are discussed. Based on the technological characteristics of freezing shaft sinking in deep alluvium, we further attempt to identify key research needs regarding the freeze-thaw effects on the engineering properties of deep soils.
Transport structures built throughout the period from 1960 to 1980 in permafrost regions based on the principle of permafrost preservation are subject to deformations. In many cases, the reason is a gradual change in temperature and their subgrade condition within the active zone due to the structures' technogenic impact. Design solutions for the fifty-year-old structures fail to ensure in all cases their reliable operation at the present time. The greatest danger to the reliable operation of railway lines in cold regions is uneven deformations of bridges, which are barrier places. Therefore, the solution to this problem is urgent especially due to the necessity of increase carrying capacity. The purpose of this study is to increase reliability of bridge operation in cold regions through strengthening the subgrade by reinforcement with injection of solidifying solutions. The problem of uneven deformations due to permafrost degradation is considered using the example of a railway bridge located in the northern line of the Krasnoyarsk railway. Deformations of the bridge abutments began immediately after the construction was completed and the bridge was open for traffic-since 1977. Permafrost degradation was developing more actively straight under the abutments due to higher thermal conductivity of the piles concrete. Notably, thawing intensity of frozen soils under the bridge abutments is uneven due to its orientation to the cardinal points. The analysis of archive materials and results of the geodetic survey made it possible to systematize the features of augmenting deformations of each abutment over time. The engineering-geological survey with drilling wells near the abutments ensured determination of soil characteristics, both in the frozen and thawed states. Thermometric wells were arranged to measure temperatures. The analysis and systematization of the data obtained allowed us to develop geotechnical models for each abutment of the bridge. The peculiarity of these models is allowance for changes in the strength and deformation characteristics of the soil calculated layers depending on changes in temperature and the soil condition. Thus, different calculated geological elements with the corresponding strength and deformation characteristics were identified in the soil layers of the same origin. The analysis of the systematized geodetic data allowed us to confirm adequacy of the developed geotechnical models. Studies carried out using geotechnical models made it possible to predict improvement of physical and mechanical characteristics of the subgrade to prevent further growth deformations of the bridge abutments. The method of reinforcement by injection is proposed. Injecting a solution under pressure leads to strengthening of weakened thawed soils and improving their physical and mechanical properties. This research theoretically substantiates and develops the geotechnical models of the reinforced pier footing of bridge abutments by injection of solidifying solutions. The models take into account the reinforcement parameters and elements for the case in question. The influence of reinforcement on the change in physical and mechanical properties of the soil mass is determined.
In the presents work, the authors have carried out a field study of moisture transfer processes in highway subgrades in a cold region during freezing and laboratory experiments on freezing samples of silicified soils used as a capillary barrier. The study showed that the creation of a capillary barrier from an injected solution, blocking the access of groundwater to the freezing zone, will reduce the amount of heaving to permissible values. Based on the results of laboratory studies, an exponential dependence of the relative deformation due to frost heaving on the relative height of soil silicatization in the freezing zone has been established. For creating a capillary barrier in the already existing subgrade, the authors have proposed designs of injection-silicified soils and substantiated the use of such soils.
Located in the eastern margin of the Tibetan Plateau, the Gonghe-Yushu high-grade highway was the first of its kind in plateau permafrost regions. Most of the road sections along the high-grade highway are unstable or extremely unstable warm permafrost with an average annual ground temperature above -1 °C, which is vulnerable to global warming and human engineering activities. This paper describes permafrost characteristics, roadbed design, and operation of the Gonghe-Yushu high-grade highway in detail. It is found that thaw settlement of warm and ice-rich permafrost is the main cause of subgrade subsidence in permafrost sections of this highway due to insufficient permafrost survey and drainage design. It is recommended that the interception and drainage system's design be optimized, and the permafrost upper limit and the variation of ground temperature be further investigated to provide essential data for the treatment of highway distress. It should be emphasized that protecting permafrost soil environment and optimized engineering design are crucial to successful high-grade highway engineering in permafrost regions.
The meso-structure of sandstone has a significant effect on its mechanical properties under external loads. In this paper, by taking two types of sandstone with different grain sizes as the study objects, the effects of grain size and freeze-thaw cycles on tensile strength and damage mode of sandstone are analyzed using a combination of laboratory tests, theoretical analysis, and numerical calculation. The Brazilian splitting tests are carried out on sandstone samples subjected to freeze-thaw cycles. The results show that: (1) The Brazilian splitting mode of the fine-grained sandstone is dominated by the central fracture, whereas that of the coarse-grained sandstone is controlled by a noncentral fracture. (2) The freeze-thaw cycles aggravate the initial damage of sandstone, and the cumulative freeze-thaw damage has a greater impact on the Brazilian splitting damage mode of the coarse-grained sandstone than on the fine-grained sandstone. (3) The numerical analysis software RFPA2D system can simulate the Brazilian splitting failure process of the two types of sandstone with varying grain sizes under different freeze-thaw cycles. It is shown to be an effective method to reveal the tensile failure process and deterioration mechanism of sandstone under freeze-thaw cycling. (4) The formation mechanisms of the two splitting modes are analyzed according to the energy principle. The energy release of coarse-grained sandstone forms a noncentral splitting mode along the rock sample internal weak structural plane, whereas the fine-grained sandstone sample's energy accumulates in the rock sample center and releases it instantaneously at its center, showing the failure mode of central splitting. In addition, based on damage mechanics theory, the damage evolution equation of sandstone subjected to freeze-thaw cycles under tension is established, and the influence of energy release and dissipation on the sandstone's tensile properties is quantitatively analyzed.
The moisture content of a road subgrade in cold regions will increase after freeze-thaw cycles, resulting in subgrade strength and stiffness losses. Electroosmosis is widely used in treating saturated soft soils to decrease the moisture content. The induced moisture migration during electroosmosis in unsaturated soil is much more complex than that of saturated soil because of a series of nonlinear changes in soil properties. This study first uses an exponential function to characterize the relationship between electroosmotic permeability and saturation degree. Then, a one-dimensional model is developed to simulate the electroosmosis-induced moisture migration in unsaturated soil. Simulation results show that electroosmosis reduces the saturation degree of the unsaturated soil, indicating that it can be applied to subgrade dewatering. Key parameters such as soil pore size distribution coefficient, air entry value, and effective voltage significantly affect moisture migration. Electroosmotic properties of unsaturated soils are extremely important to the efficiency of electroosmosis.
In cold regions, the widened subgrade could produce uneven frost heave that is detrimental to the pavement. This study investigates the differential frost heave characteristics in a widened subgrade. The field monitoring system mainly consists of temperature, moisture, and displacement sensors and distributed optical fiber cables for strain measurement. The monitoring results show that the cooling period in the subgrade is longer than the warming period. Water content in the subgrade changes significantly within 0-2 m below the subgrade surface but stabilizes within 2-5 m. The maximum frost heave occurs from February to March. In comparison, the existing subgrade has a longer freezing period and larger heave value, caused by the higher density and water content inside. Water in the existing subgrade migrates into the new one after widening, leading to frost heave reduction in the existing subgrade. Simultaneously, the traffic loads result in the consolidation of the new subgrade, thus reducing the heave value in the second year. In the third year, the water supply from the existing subgrade facilitates the frost heave in the new subgrade. The tensile strain distributions obtained by the distributed optical fiber cables show that the maximum differential frost heave occurs at the joint between the existing and new subgrades. The differential frost heave gradually stabilizes after three years. Finally, an improved frost heave prediction model is developed based on the segregation potential concept and monitoring results.
The research shows that the selection of pavement type is very important for the thermal stability of high-grade highway embankment in permafrost regions because of the different solar absorption rates between asphalt concrete and asphalt concrete pavement. In this paper, the common embankment of high-grade highway in permafrost regions is selected as the research object to study the influence of asphalt concrete and cement concrete pavement on the embankment temperature, freeze-thaw cycle process and the change law of the permafrost table, which provides a basis for the use of reasonable pavement materials in permafrost regions.
Engineering activities in the salt lake region continue to increase where fresh water resources are scarce. This paper investigates the physical properties of saline soils during mixing with brine. Fine-grained saline soils with salt content varying from 2.6% to 78.5% were collected along Qarhan-Golmud Highway (QGH) and Sebei-Qarhan Highway (SQH) on the Qinghai-Tibet Plateau to conduct laboratory physical properties tests. Liquid plastic limit tests were conducted. Results show that liquid plastic limit parameters will decrease with an increase of salt content ranging from 2.6% to 78.5%, and the relationship between them is linear. After considering the content ratio of chloride and sulfate, results show that liquid plastic limit parameters will decrease with an increase of the ratio of chloride to sulfate ranging from 0.7% to 7.0%; liquid plastic limit parameters enter a stable period at the ratio of chloride to sulfate ranging from 7.0% to 37.4%; liquid plastic limit parameters enter a decline period at the ratio of chloride to sulfate ranging from 37.4% to 77.2%. After brine and fresh water are separately mixed into saline soil, the optimal moisture content of the soil samples after the brine action is lower than the saline soil under the action of fresh water, and the maximum dry density of the soil sample is higher than that under the action of fresh water. At the same time, these changing laws show a certain correlation with the chloride ion content and the ratio of chloride to sulfate in saline soils. The results are of significance for engineering activities in salt lake regions with extensive saline soil distribution.
Cyclic triaxial tests are conducted to analyze the evolution of strength parameters and energy dissipation of thawing silty clay under different stress paths.The effects of freezing temperature, thawing temperature and confining pressures on the stress-strain and strength characteristics of soil samples are studied through monotonic loading and cyclic loading tests by using high- and low-temperature triaxial apparatus. The variation of the total work, elastic deformation energy, dissipated energy, energy dissipation rate, residual strain, and damage variable during loading and unloading are discussed. The experimental results show that the samples have higher strain tolerance under high confining pressure, low freezing temperature, and low thawing temperature, and the same other conditions. The soil sample state and failure pattern can be judged by using the energy parameters measured in the experiment.
The soil-rock mixture, a collection of soil particles and rock blocks, is inherently heterogeneous and anisotropic due to significant particle size and material strength differences. This study conducts triaxial tests on soil-rock mixture samples of various compactness subjected to varying freeze-thaw cycles. A mesoscopic simulation is carried out by particle flow code (PFC) to analyze the effects of freeze-thaw cycles on the mechanical properties of soil and rock particles. The results show that the mechanical properties of the soil-rock mixture under freeze-thaw cycles are greatly affected by the initial compaction. In general, when the degree of compaction is higher, the influence of freeze-thaw cycles on the soil-rock mixture is greater. The stress-strain curves of the samples with different compactness demonstrate strain-softening behavior. The freeze-thaw cycles greatly influence the failure strength of the samples with a higher degree of compaction but have little impact on the samples with a lower degree of compaction. On the microscopic level, during freeze-thaw cycles, the pore volume in the highly compacted sample is too small to accommodate the volume expansion from ice crystal formation, causing significant strength loss among the soil and rock particles and deterioration of the macroscopic properties of the soil-rock mixture.
