Research in Cold and Arid Regions最新文献

A numerical simulation platform that analyzes the variation of the slope factor of safety with time instantaneously is proposed based on heat conduction theory to study the law of stability development of permafrost slopes during thawing. This platform considers ice-water phase change, elastoplastic constitutive behavior and strength reduction in thawing permafrost and can evaluate the factor of safety of permafrost slopes with different slope angles and water contents. Results indicate that under different slope angles and water contents, the evolution of the factor of safety with time displays two stages: nonlinearly decreasing at first and then essentially remaining constant. During the decreasing stage, the plastic slip line overlaps with the thawing front. In this stage, the self-weight of the post-thawed permafrost layer increases continuously while the shear strength of the frozen-thaw interface keeps unchanged. This is the main reason leading to the decrease in the factor of safety. In the second stage, the thawing depth increases continuously while the position of the plastic slip line remains unchanged, resulting in a constant safety factor stage.

To prevent the thawing of ice-rich permafrost, it is suggested that gas should be transported in a chilled state (below the freezing temperature) in pipelines buried in permafrost. However, frost heave occurs when water migrates towards the chilled pipeline and ice lenses grow underneath the pipe. This might endanger the integrity of the pipeline and the environment as well. Therefore, innovative frost heave mitigation measures are required when designing the pipeline, especially those sections in discontinuous permafrost or near the compressor stations. The ground temperature field in response to the operation of a proposed chilled gas pipeline traversing permafrost regions in Alaska was simulated by a pipe-soil thermal interaction geothermal model. Frost heave mitigation measures, including insulation around the pipe, flat slab insulation under the pipe, and heating cables combined with slab insulation, were evaluated for chilled pipeline operation in seasonally varying ambient temperatures. The numerical results show that the minimum temperature of the observation point at 2.5 m below the pipe bottom increases by 17%, 29%, and 48% when the thermal conductivity of the outer insulation layer is 0.1, 0.05, and 0.02 W/(m·K), respectively. For flat slab insulation, the thermal field is less sensitive to varying slab thicknesses than to varying thermal conductivity, implying the thermal conductivity, not the thickness, is the crucial factor. Additionally, the heat flow could be redirected from vertical to horizontal by flat slab insulation. The electrical heating cables could be regarded as a new heat source to balance the heat removal rate of the soil around the chilled pipe. The minimum temperature of the observation point at 1.1 m below the bottom of the pipe increases from −15.2 °C to −3.0, 1.5, and 7.5 °C, corresponding to the heating cable power of 20, 30, and 40 W, respectively, with the power of 30 W deemed appropriate for the study case. It is concluded that heating cables in combination with insulation slabs could be adopted to regulate the temperature field around the chilled pipeline efficiently and economically. The advantages of this combination include redirecting the heat flow and eliminating frost in the soil underlying the pipe. These approaches could be considered for applications in gas pipeline projects in arctic and alpine/high-plateau permafrost regions.

In perennially frozen or seasonally frozen soil regions, freeze-thaw cycling adversely impacts the mechanical properties of rock mass, resulting in landslides, rock erosion, and other geological disasters. The microscopic damage evolution law of loaded sandstone under the freeze-thaw cycle is analyzed by conducting Nuclear Magnetic Resonance (NMR) and uniaxial compression acoustic emission (AE) experiments. The experimental results have shown that: (1) Freeze-thaw cycling increases sandstone's internal pores, enlarges the pore size, and modifies the original pore distribution. (2) The damage due to freeze-thaw cycling is positively correlated with the initial damage to the rock, and the damage on the rock surface is more severe than inside the rock sample. (3) Freeze-thaw cycling negatively impacts the mechanical properties of sandstone, and the elastic deformation stage of sandstone gradually decreases as the number of freeze-thaw cycles increases and gradually transitions from brittle failure to ductile failure. (4) The characteristic parameters of AE ringing count and accumulated energy can reveal the severity of freeze-thaw damage and the dynamic evolution process, and the damage development rate exhibits abrupt changes at critical moments. After five freeze-thaw cycles, the damage development rate rises suddenly, as manifested by a sharp increase in the frequency and energy of AE events. High-energy AE events frequently occur during the rapid expansion period of damage, which can be adopted as an essential reference for damage propagation and aggravation.

Based on the Kangding Tunnel No. 2 project, this study analyzes the heat exchange between air and the rock mass surrounding the tunnel under wind flow by the finite difference method. The influence of factors on the temperature field of a tunnel in cold regions, including ventilation and initial conditions, is investigated. The results show that: 1) The lower the air temperature, the greater the wind speed, the larger the rock mass temperature influence circle and the greater the frozen depth; 2) When the wind speed is less than 3 m/s, its change has an obvious impact on the rock mass temperature; 3) For every drop of 5 °C in air temperature, the frozen depth increases by about 5 m, indicating that the air temperature is an essential factor affecting the rock mass temperature regime; 4) The higher the initial rock mass temperature is, the smaller the influence circle on the rock mass is. And to a certain extent, it determines the temperature distribution in the rock mass within a specific range from the wall surface.

Permafrost is widely distributed in China and around the world. In permafrost regions, soil frost heave and thawing are severe and frequent, and can destabilize pile foundations. To this end, a finite element model of a single pile in frozen soil is established to investigate the frost heave and frost jacking response to ensure its safety in the Qinghai-Tibet Plateau. Firstly, a hydro-thermal coupling model of a single pile in frozen soil is established based on coupling parameters and initial and boundary conditions. Then the temperature and moisture distributions are analyzed through the established coupling model. A hydro-thermo-mechanical coupling model is developed by importing the ice content and temperature results. Simulation results indicate that the amount of frost heave is greater at locations closer to the ground surface, and the displacement is smaller for frozen soil that is closer to the side of the pile. The results on frost jacking behavior of piles from this study can serve as a reference for the design, construction and maintenance of foundations.

In order to accurately analyze vibration characteristics and site effects of loess hills under moving load of a high-speed train, four types of loess hill models under railway viaduct was established by ABAQUS of finite element analysis software by field test. The dynamic response and stability of loess hills under two different vibration sources under high-speed train load were studied by using two-dimensional equivalent linear response time-history analysis, and the influence of the mechanical parameters of loess on the vibration of different types of loess hill was analyzed. Results show that there are obvious differences between peak displacement cloud maps of loess hills under the railway viaduct under gravity and train load action. We analyzed the influence of the change of elastic modulus on vibration propagation of soil of foundation and loess knoll, and found that the change of elastic modulus of soil in different position of foundation has more effect on vibration propagation than that of loess knoll soil. At the same time, the vertical acceleration cloud maps of the four types of loess hills are obviously different.

Based on the engineering background of the contact channel between Shangyang and Gushan of Fuzhou Metro Line 2 undercrossing the existing tunnel line, the freezing temperature field of the contact channel, the displacement field of the existing tunnel line and the contact channel with different net distances and horizontal angles are analyzed by ANSYS finite element software and field measurement method. The obtained results indicate that during the freezing period, the temperature drops at different measuring holes are almost the same. The temperature near the bottom freezing tube drops faster than that far from the tube. It is found that the bilateral freezing technique improves the formation of the freezing wall in the intersection area. In this case, the intersection time of the cross-section is 7 days faster than that of the adjacent ordinary section. The change curve of the displacement of the surface uplift in different freezing periods with the distance from the center of the channel is “M” shaped. The maximum uplift displacement at 12 m from channel center is 25 mm. The vertical displacement of the measuring point located above the central axis of the connecting channel is large. The farther the point from the central axis, the smaller the corresponding vertical displacement. When the horizontal angle between the existing tunnel and the connecting channel is less than 60°, the existing vertical displacement of the tunnel changes rapidly with the horizontal angle, reaching 0.17 mm/°. Meanwhile, when the net distance is less than 6.1 m, the change rate of the vertical displacement of the tunnel is up to 2.4 mm/m.

This paper investigates the influence of the deviation in freeze pipe installation on the development of the frozen wall in long cross passages by numerical simulation with ANSYS software. The study case is from the artificial ground freezing project along the Fuzhou Metro Line 2 between Ziyang Station and Wuliting Station. Two freeze-pipe configurations, i.e., one with perfectly aligned pipes without any deviation from design and another with randomly distributed deviation, are included for comparison. The effect of the random deviation in the freeze pipes on frozen wall interconnection time, the thickness of the frozen wall and the development of the temperature field is explored. For the characteristic section of the numerical model at a depth of 25 m, it is found that the frozen wall interconnection time under the random deviation case and no deviation case is 24 days and 18 days, respectively. The difference in the thickness of the thinnest frozen wall segment between the random deviation and no deviation cases is the largest in the early freezing stage (up to 0.75 m), which decreases with time to 0.31 m in the late freezing stage. The effects of random deviation are more significant in the early freezing stage and diminish as the freezing time increases.

Damage caused by frost heave leads to costly maintenance in cold regions, like Hokkaido, Japan. Therefore, the study of the frost mechanism with experimental and numerical methods has been of great interest. Numerous models have been developed to describe the freezing process of saturated soil, which differs from the partially saturated conditions in the field. In fact, most subsurface soils are unsaturated. The freezing process of partially saturated soils is more complex than saturated soils, as the governing equations show strongly nonlinear characteristics. This study proposes a thermo-hydro-mechanical coupled model considering the heat transfer, water infiltration, and deformation of partially saturated soil to reproduce the freezing process of partially saturated frost susceptible soils distributed in Hokkaido. This model better considers the water-ice phase change and soil freezing characteristic curve (SFCC) during freezing under field conditions. The results from the multiphysics simulations agree well with the frost heave and water migration data from frost heave tests of Touryo soil and Fujinomori soil. In addition, this study discussed the influence of the various factors on frost heave amount, including temperature gradients, overburden pressures, water supply conditions, cooling rates, and initial saturation. The simulation results indicate that the frost heave ratio is proportional to the initial degree of saturation and is inversely proportional to the cooling rate and overburden pressure.

Moreover, simulation under the open system generates much more frost heave than under the closed system. Finally, the main features of the proposed model are revealed by simulating a closed-system frost heave test. The simulation results indicate that the proposed model adequately captures the coupling characteristics of water and ice redistribution, temperature development, hydraulic conductivity, and suction in the freezing process. Together with the decreased hydraulic conductivity, the increased suction controls the water flow in the freezing zone. The inflow water driven by cryogenic suction gradient feeds the ice formation, leads to a rapid increase in total water content, expanding the voids that exceed the initial porosity and contributing to the frost heave.

Portable in-situ devices have been used for characterizing low accessible field, such as the railway subgrade. In this study, the automated cone penetrometer (ACP) was designed for the application on the railway subgrade. ACP is composed of the cone tip, driving rod, and hydraulic hammer system. The hydraulic motor lifts and drops the 294.3 N hammer from a height of 200 mm such that the potential energy of 58.9 N m impacts the driving rod. The N-value (N_ACP) from the ACP test was compared with the dynamic cone penetration index (DCPI) from the dynamic cone penetrometer (DCP) test. The test results show that the N_ACP and DCPI profiles show opposite trend owing to the inverse concept of the unit. From the correlation of DCPI and N_ACP, the limitation of DCPI reveals owing the minimum manually measured value of 1 mm/blow. Additionally, the evaluation of the deflection modulus (E_FWD) using N_ACP is more efficient than that using DCPI. Based on the result of this study, we suggest that ACP can be used for strength and stiffness evaluation of railway subgrade rapidly and reliably.