Wenqiang Li

To investigate the characteristics of water and salt migration (WSM) in saline soils subjected to climate conditions characterized by large temperature differences. In this paper, salted soil highway roadbeds in Uzbekistan were selected as the background for the study, and the salted soils in the region of the site were remodeled through a field study and based on field data. Subsequently, utilizing the assembled laboratory test apparatus, a model test of the WSM on the remodeled saline soil was carried out to analyze the behavior of the WSM in response to large temperature differences, the resulting soil deformation, and the influence of varying cold-end temperatures and alternating cooling and heating times on the WSM. The findings indicated that the temperature profile with in the soil column (SC) exhibits a “wavy” pattern as a result of thermal conductivity. During the freezing phase, water migrates from the lower section to the upper section in response to the temperature gradient and soil-water potential. Conversely, during the melting phase, water diffuses and migrates from the upper section downward in the form of steam molecules due to the action of evaporation. This process culminates in a “C” shape distribution of the curve of the water content inside the SC. During the freezing process, salt migrates from the bottom to the top along with the liquid water. Conversely, during the melting process, salt accumulates at the top of the SC accumulates. This phenomenon results in an “S” distribution of the salt content inside the SC. At the end of the freeze-thaw cycle (FTC), the deformation of the saline soil increased with the decrease of the cold end temperature, and the alternating time between heat and cold did not produce significant changes in the deformation of the saline soil. The moisture content at the upper end of the SC increases with decreasing cold-end temperature. The increase in the number of FTCs exacerbates the salinization of the soil in a certain part of the interior of the SC. The findings of this research may offer a theoretical foundation and technical assistance for the design of subgrade structures in saline soils subjected to climates characterized by large temperature differences.

Influenced by factors such as the freeze–thaw cycle and water–salt migration, road construction in Uzbekistan’s highway project areas is prone to dissolution and subsidence, salt swelling, corrosion, and other engineering diseases. To investigate how various factors impact saline soils in Uzbekistan’s monsoon freezing zone, we conducted analyses of stress–strain curves, failure strength, and shear strength parameters of these soils through freeze–thaw (F-T) cycle tests and unconsolidated and undrained (UU) triaxial shear tests. The findings indicate that with the increase of salt content, the average reduction in the failure strength of saline soil was 15.8%, 6.3%, and 5.7%; with the increase of water content, the average reduction in cohesion was 10.8%, 44.1%, and 32.6%; and the internal friction angle increased with the increase of the number of F-T cycles and decreased with the increase of freezing temperature. Ultimately, we defined the rates of failure strength deterioration and cohesion damage in saline soil due to various factors, analyzing the destructive impacts of these factors. The results demonstrate a strong correlation between the curves of failure strength deterioration and cohesion damage ratios, indicating that the significant degradation of saline soil due to salt is primarily influenced by F-T cycles, with the extent of damage closely linked to water content.

Reasonable excavation step footage and lining support timing are highly important for improving tunnel construction efficiency and ensuring construction safety. Taking the Huanxian No. 1 Tunnel of the Xi-Yin railway as the basis of this study, a 3D numerical model was established using MIDAS GTS NX290 finite element software. This model was used to investigate the deformation and force characteristics of the tunnel-surrounding rock and support structures under three different excavation footages and four different lining construction timings; the numerical results were then compared with the on-site monitoring results. This research aimed to determine reasonable excavation parameters for the three-bench seven-step excavation method used in shallowly buried loess tunnels. The results revealed positive correlations between the excavation step footage and surface subsidence, crown subsidence, and clearance convergence. An excavation footage of 3 m could balance construction efficiency and safety effectively. Keeping the secondary lining construction time unchanged, the early closure of the initial support was beneficial for reducing the force on the secondary lining. Keeping the early closure time of the initial support unchanged, the early construction of the secondary lining would lead to an increase in the force on the secondary lining. The initial support of the tunnel is recommended to be closed as early as possible, and the construction of the secondary lining should be shifted by 21 m behind the upper step palm surface. By comparing the on-site monitoring data with the numerical simulation results, similar trends were observed, providing reference and guidance for the subsequent construction of large-section tunnels in shallowly buried loess formations.

In order to solve the problem of engineering structure safety caused by temperature crack of mass concrete in tunnel, the construction measures and monitoring data of mass concrete in Yangcheng Xihu tunnel are analyzed. First of all, six construction measures were put forward to suppress temperature cracks, including pouring in layers, controlling vibration quality, controlling the temperature of concrete entering mold, controlling mold removal time, concrete curing and controlling support removal time. Then, the High Performance Dual-Grating Temperature-Strain Sensor was used to monitor the temperature and strain of construction site structure intelligently. Based on the data analysis of the temperature and strain of the floor, side wall and top slab, it is pointed out that the temperature of the floor, side wall and top slab increases sharply first and then decreases slowly due to the release of a large amount of hydration heat during the hardening process of concrete. Due to the poor thermal conductivity of concrete, the hydration heat generated in the middle center of the floor, side wall and top slab can not be transferred to the surface of concrete in time, so the temperature is generally "high in the center and low on both sides". Due to the effect of thermal expansion and cold contraction of concrete, the strain variation trend of the floor, side wall and top slab is similar to that of temperature, showing a sharp increase at first, and then a slow decrease. Based on the mechanism of temperature crack, the variation law of temperature and strain during mass concrete pouring is analyzed, which provides reference for subsequent engineering construction.