渠基土在冻融循环作用下的变形和应力变化特征

受季节性气候变化和昼夜交替的影响，处于寒区的地表浅层土体不可避免地会发生冻融循环作用。冻结过程引起土体的膨胀变形，融化过程引起土体的压缩沉降变形。同时冻融交替变化会诱发渠基土的结构与物理力学性质发生显著改变，从而危害工程设施的服役性。土体所处的应力环境是影响冻融过程中土体变形发展的关键因素。为了研究不同上覆荷载条件下冻融循环过程对寒区渠基土变形与冻胀应力发展特性的影响，开展了一系列冻融循环试验。结果表明：在上覆荷载为10 kPa时，冻融循环会使土体产生膨胀变形；当上覆荷载为50 kPa或100 kPa时，冻融循环会使土体产生非常明显的固结沉降，且上覆荷载越大，沉降量也会越大。随着冻融循环次数的增加，土体在其所处的应力环境下逐渐形成相对稳定的固结结构，单次冻融过程中产生的冻胀量与融化固结量趋于相等，即冻融稳定系数趋于1。在不同上覆荷载条件下固结稳定后，保持试样两端约束的位移不变，发现土体冻融过程中产生的最大竖向冻胀应力随冻融循环次数的增加不断衰减，且冻胀应力的发展与孔隙水压力的变化具有一致性。因此，通过对恒定上覆荷载条件下冻融过程中正冻与正融界面附近孔隙水压力分布的研究，可揭示冻融过程中土体变形发展的内应力机理。

Deformation and stress variation characteristics of canal foundation soils under freeze-thaw cycles

Due to the seasonal change of climate and day/night cycle， it is inevitably that the shallow soil is influenced by freeze-thaw cycles in cold regions. The freezing process will cause the soils to expand， and the thawing process will lead the soil to settle. At the same time， the freeze-thaw cycles can induce significant changes in the physical and mechanical property and structure of frozen soils， which will seriously threat the serviceability of some structures. The stress field of soil is one of the key factors that affects the deformation of soil during freezing-thawing process. In order to research the impact of freezing-thawing cycles on the deformation and frost heave stress characteristics of foundation soil in cold regions under different overburden pressure， the two groups frozen-thaw cycles tests are conducted. The first group tests were conducted under the constant overburden pressure in the process of freezing-thawing. In order to investigate the variation of frost heave stress in the frozen-thaw cycles process， another group were carried out under constant displacement defined. It is discovered that the freeze-thaw cycle can lead the soil expand under the 10 kPa overburden pressure， and it is compressed when the stress is 50 kPa or 100 kPa. The larger the stress is， the large settlement deformation is. The soil gradually forms a stable structure under the special stress field with increasing freeze-thaw cycles， which leads to the amount of frost heaving equal to that one of thawing settlement， namely， the coefficient of freeze-thaw stabilization is near to 1. With increasing the number of freeze-thaw cycles under the constant displacement the maximum vertical frost heave stress decreases continuously. From the measured results， it is observed that the developing trend of frost heave stress is similar with that of pore water pressure. The results indicated that frost heave stress increasing in the freezing process will compress the soils structure， which lead to the pore of the soil smaller， and it shows the pore water pressure rising. The pore of the soil will partly resilient in the thawing process， which shows the pore water decreasing. According to the principle of effective stress in frozen soil， the internal microcosmic stress mechanism of soil deformation in freezing-thawing process can be revealed by studying the distribution of pore water pressure near to the freezing-thawing interface. At the same time， the principle of frost heave stress generation in freezing process was explained under microscopic scale. Finally， through analyzing the distribution of pore water pressure under constant overburden pressure， the maximum value of frost heave stress was determined under different unfrozen water content of frozen fringe. In engineering practice， especially for water supply channels， frost heave stress is the main reason for the failure of lining structure. Therefore， the determination of the maximum frost heave stress in the process of freeze-thaw cycle is of great significance to propose a reasonable design code.