The unanticipated windowing of the frozen curtain and the random windowing points always are the primary bottlenecks to threaten the artificial ground freezing (AGF). However, the obtained studies on detecting approaches of location of freezing front were lack. A cross-borehole scale-based technique for accurately determining the location of freezing front was urgently needed. In the present study, a focused direct current (DC) resistivity-based locating method of freezing front was developed on the basis of the principle of current focusing. The configuration of electrodes and circuits and its detailed implementation process were initially presented. Then, a practical correlation between the measured cross-borehole resistivity and the location of freezing front was established. The focused DC resistivity-based locating method was subsequently used in a field test. The detected results were evaluated and validated by the comparisons with the actual freezing states observed at the excavation face. It is demonstrated that the newly-developed method can well reflect the dynamics of the freezing front and is expected to be an effective approach to detect the windowing of the frozen curtain during the formation process and maintenance stage.

Frost heave occurs frequently in high-latitude or high-altitude areas in many parts of the world, and it is a common challenge for geotechnical engineers in road construction. In order to solve the excessive frost heave induced frost damage to geotechnical engineering in cold regions, frost heave susceptible soil from the Tibetan Plateau (the seasonal frozen area) is modified by nonionic polyacrylamide (NPAM) and then used for frost heave testing. Moreover, cohesion test, permeability test, scanning electron microscopy (SEM) test, and Fourier transform infrared spectroscopy (FTIR) analysis are carried out on the NPAM modified soil to gain more insight into the frost heave mitigation mechanism. The impact of NPAM on soil frost heave and its physicochemical mechanism for frost heave mitigation are subsequently investigated. Our results demonstrate that NPAM is physically adsorbed on the surface of soil particles through hydrogen bonding. The increase of NPAM content does not only increase the viscosity of the solution, but it also enhances the aggregation of fine soil particles. In addition, the permeability of the modified soil decreases with increasing NPAM content, thus implying an increase in the water migration resistance. NPAM can be effectively used to reduce the frost heave susceptibility of soil. Our results show that 0.3 wt% of NPAM can significantly convert a strong frost heave soil to a weak frost heave soil. Our results reveal the mechanism of frost heave mitigation using NPAM and provides engineers and researchers with a new physicochemical method for the treatment of frost heave susceptible soil in railway and road construction in cold regions.

Tensile strength is one of the important parameters of frozen soil mechanics, which is widely used in engineering design of frozen soil and theoretical research of frost heave. At present, the research on tensile strength of frozen soil mainly focuses on low-temperature frozen soil, while that of warm frozen soil is less studied, thus limiting scientific evaluation of the tensile strength characteristics of warm frozen soil. In order to enrich the knowledge of tensile strength of warm frozen soil, the tensile strength experiments of silt clay from the Tibetan Plateau are carried out under various temperatures higher than-2 degrees C using the hydraulic fracturing method (HFM). Our results show that HFM can enable a reliable evaluation of the tensile strength of warm frozen soil, and the relationship between the tensile strength of warm frozen soil and negative temperature can be described by a power function. Compared with the results of uniaxial tension method (UTM) and radial splitting method (RSM), the feasibility of HFM can be regarded as an indirect tensile method in determining the tensile strength of frozen soil, and the suggestion for correcting the test results of indirect tensile method is given. In addition, the mechanism behind the influence of initial water content and temperature on tensile strength of warm frozen soil is analysed. Our results provide a new approach for researchers to analyse the tensile strength of warm frozen soil, thus can benefit the development of frost heave theory, and can reveal the influence of initial water content and temperature on the tensile strength of warm frozen soil.

Seasonally frozen ground regions occupy approximately 55% of the exposed land surface in the Northern Hemisphere, and frost heave is the common global problem in seasonally frozen soil areas. Frost heave induces uneven deformation of ground and damages railways, road paving, and buildings. How to mitigate frost heave is the most important technical issue in this field that has provoked great interest. Here, using freezing experiments, we investigate the effect of anionic polyacrylamide (APAM) polymer on frost susceptible soil. The results demonstrate a so-far undocumented inhibition of frost heave by APAM in freezing soil, namely APAM (tested at concentrations from 0.0 wt% to 0.60 wt%) slows down the frost heave by a factor of up to 2.1 (since 0.60 wt% APAM can decrease frost heave from 8.56 mm to 4.14 mm in comparison to the control experiment). Moreover, it can be observed that the maximum water content near the frozen fringe decreased from 53.4% to 31.4% as the APAM content increased from 0.0 wt% to 0.60 wt%, implying a mitigated ice lens growth. Hydrogen bonding between APAM and soil particles triggers an adsorption mechanism that accumulates soil particles, and thus can potentially inhibit the separation and growth of the ice lens. Moreover, the residue of APAM due to hydrogen bonding-induced adsorption in the pores of granular media may narrow seepage channels (capillary barriers) and provide an unfavourable condition for water migration. The use of APAM can also increase the viscosity of the solution, which causes a greater water migration resistance. This research provides new insights into APAM-influenced frost heave (introducing APAM into the soil can induce bridging adsorption between APAM polymer segments and a particle surface), can enable engineers and researchers to utilise chemical improvement design and to consider suitable actions (e.g., by injecting APAM solution into a frost susceptible soil or using APAM-modified soil to replace the frost susceptible soil) to prevent frost heave from having a negative impact on traffic roads and buildings in cold regions.

人工冻结法用于地下工程建设时,过量的冻胀变形及冻胀力将抬升地层并导致构筑物的破坏及失效,提出冻胀控制方法及揭示冰透镜体生长抑制机理是推进人工冻结技术发展的重要途径.从冰透镜体生长速率和冰饱和度与渗透率的特定数学模型出发,得到了冻结缘厚度演化对冰透镜体生长速率的影响.研究指出,冰透镜体的生长对冻结缘结构具有强烈的依赖性,冻结锋面向冷端移动诱导的冻结缘结构退化将直接抑制冰透镜体的生长.通过研究冻结缘厚度对冰透镜体生长的影响机制,提出了基于冻结锋面移动控制的人工地层冻结技术思路,发展了相应的冻结控制系统及试验平台,该技术思路可有效激活冰透镜体的间歇性生长过程并实现对冻胀的控制.研究指出,减小设计冻结深度及提高冷端冻结温度均能有效抑制冻结土体中冰透镜体的生长及减缓冻胀.

Heavy metal pollution of soil has become one of the most common hazards in human development. The artificial freezing method, especially the progressive freezing method, can reduce heavy metal pollutants in the soil and promises to be an effective in-situ treatment of contaminated sites. This study analyzes the freezing purification mechanism of heavy metal contaminants in saturated sand and identifies three main factors that impact the effects of purification: freezing rate, initial concentration, and diffusion coefficient. Moreover, one-dimensional freezing tests are carried out by different freezing modes. The experimental results show that the heavy metal chromium could only be removed effectively with a slow freezing rate. By optimizing the freezing mode and freezing rate, a long section of soil was frozen and purified, with the maximum purification rate reaching 65.8%. This study shows that it is feasible to treat contaminated saturated sand by a gradual-cooling freezing method.

全面掌握冻土抗拉强度研究现状是进一步深化研究的基础.首先,分类介绍了目前可用于冻土抗拉强度测试的各典型方法,详细阐述了不同测试方法的试验条件、试样形式和受力机理,对比列举了典型抗拉强度测试方法优缺点.其次,归纳总结了基于不同试验方法已进行的研究工作和不足.然后,全面分析了温度、含水量、加载(变形)速率、土质及试样尺寸等影响因素对冻土抗拉强度变化规律影响的最新研究进展.最后,提出发展并完善冻土抗拉强度研究方法和体系,增加高温冻土抗拉强度测试研究,从而获得更加准确模拟冻土张拉破坏行为的展望.指出应结合冻土微细观结构和数字图像技术研究手段,深入揭示冻土抗拉强度形成内因和张拉破坏机制.阐述以多影响因素试验为基础,探寻更为完善的冻土抗拉强度预测方法.同时,拓展冻土抗拉强度的现场原位测试研究,加强室内外双轨并行式研究思路.通过对国内外研究现状及发展趋势的分析,为冻土抗拉强度试验研究、冻胀理论模型完善、寒区岩土工程设计和人工冻结加固工程等提供参考和指导.

Extensive frost heave problems deriving from 2D freezing occurred behind retaining structures. A novel test apparatus was thus developed to observe frost heave behaviors governed by 2D growth of an ice lens. The results obtained showed that the moving characteristics of the freezing front, the frost heave process, and the change of water distribution were similar with that found in 1D freezing. However, the ice lens-dependent frost heave in 2D freezing accounted for less than 75% of that in 1D freezing due to the strong interactions between the frozen zone, the unfrozen zone, and the neighboring constraints. The geometry of an ice lens in 2D freezing within soil specimen was an arc. The relationship between geometry of ice lens and its location coordinate satisfied a parabolic function. The growth rate of ice lens perpendicular to thermal gradient was usually restrained in 1D freezing, whereas it made up nearly 100% of that parallel to thermal gradient in 2D freezing. The observations on anisotropic growth of ice lens in 2D freezing have significances to develop a frost heave model for coupled heat and mass transfer.

为探究粉土与结构面抗剪强度规律以及更好地模拟结构面粗糙效果,在不锈钢试块表面设计了不同规格、数目凹槽.通过凹槽数量的变化表征接触面粗糙程度的变化.利用应变控制式直剪仪对粉土-钢板接触面进行了剪切试验,定量分析了温度、接触面凹槽规格和数量对接触面抗剪强度及其参数的影响.结果表明:在试验研究范围内,随着温度的降低土面相互作用强度显著增长,剪切强度参数增加.常温试验条件下,光滑结构面对于土面作用强度有一定的衰减作用,土面抗剪强度随着法向应力增加而增大.不同规格凹槽面对于土面作用抗剪强度参数的影响存在差异性,黏聚力和内摩擦角均随着相应凹槽数量的增加而呈现出不同程度的增长趋势.同时,当作用面法向应力较低时,随着凹槽数量的增加,剪切面土体表现出一定的剪胀性,应变软化现象显著.

为分析冷冻温度和含水量对于砾石土抗剪强度参数的影响,通过室内试验对南宁地铁联络通道砾石土层进行冻结状态下的三轴剪切强度分析,研究围压、冷冻温度以及含水量对于其强度演变的影响,分别得到几个特征围压下的砾石土冻结强度与冷冻温度及含水量的关系.试验结果表明:砾石土的三轴剪切强度随着冷冻温度的降低而升高,温度效应明显;同时,含水量变化对于其剪切强度影响也十分显著,在试验研究范围内冻结砾石土偏应力峰值与含水量成一定的正相关性,随着含水量增加,冻结冰晶体含量随之升高进而引起土体胶结能力增大,相应的强度有所提升.该三轴剪切强度符合Mohr-Column准则,黏聚力与内摩擦角随着冷冻温度的降低而增大,随着含水量的增加而增加.同时,冷冻温度对于砾石土三轴剪切强度参数的影响受土体含水量变化影响显著.

During soil freezing, the frozen strength of the soil-structure interface measured as shear strength is critical to the evaluation of the anti-heave performance of the structure. Hence, this paper presents an experimental study of the shear stress response, shear strength and shear strength indexes of a soil-structure interface under frozen conditions by utilising an improved roughness algorithm. This study also deeply analysed the effect of roughness, temperature, moisture content, and vertical stress on the evolution of mechanical properties. The results show that the shear stress-displacement characteristics of the soil-structure interface displays strain-hardening at room temperature and strain-softening at negative temperatures. The strain-softening characteristic, in particular, is more pronounced with a decrease in negative temperature. The shear strength, cohesive force, and inner friction angle present a trend of linear growth with an increase in roughness under different values of vertical stress, temperature, and moisture content. Under the same values of vertical stress and roughness, the reduction in temperature and increase in moisture content significantly improve the shear strength of the soil-structure interface. Compared with its composition of interface friction at room temperature, the shear strength in this case is mainly derived from the contribution of cohesive force under frozen conditions, i.e., the cementation of ice crystals.

基于地铁工程中人工地层冻结法应用的工程背景,以粉质黏土地层为研究对象,通过室内试验研究冷冻温度、含水量和围压等因素对人工冻结粉质黏土强度和冻胀变形演化规律的影响.结果表明:冷冻温度、含水量和围压显著影响人工冻结粉质黏土的应力应变发展趋势和破坏模式,三轴剪切强度、弹性模量与冷冻温度、含水量和围压的相关性因三者的耦合作用而呈现不同的演化趋势;单向冻结模式下,温度场达到恒定温度梯度后,人工冻结粉质黏土达到了最大冻胀变形量;冻胀率与冷冻温度之间存在较好的线性回归关系,且外界水源补给条件下粉质黏土冻胀率远大于封闭不补水条件;冷冻温度、含水量和地层埋深是确保人工地层冻结技术中冻结壁达到设计强度、控制地表变形以及防止结构物破裂、渗水或漏泥等问题的3个关键指标.

Cement-stabilized macadam (CSM) is being used in high-speed railway (HSR) subgrade in seasonal frozen regions in China. This study conducted a series of tests to determine the characteristics of HSR CSM subgrade in cold regions. In particular, the influence of cement content and grain-size composition on the compaction effect, frost heave property, permeability, freeze–thaw durability, and shrinkage property of macadam subgrade were analysed. The results show that the compaction quality indexes of CSM subgrade satisfy and even exceed the compaction quality standards for HSR subgrade. Removal of soil particles smaller than a certain size weakens the frost heave sensitivity, improves permeability, and reduces the unconfined compressive strength (UCS) and moisture stability of CSM. Cement addition effectively compensates for the reduction in UCS; however, it increases the shrinkage deformation of CSM. The temperature and dry shrinkage coefficient of CSM increase with cement content. The shrinkage properties of CSM subgrade are the least favourable in the temperature range −10°C to 10°C. The UCS first decreases with increase in the number of freeze–thaw cycles and then becomes stable after 10cycles. In the engineering practice of HSR subgrade in seasonal frozen regions, in a subgrade where permeability is not a necessary characteristic, CSM with 3% fines content and 3% cement addition is suitable, and in a subgrade where permeability is a necessary characteristic, CSM with grains larger than 0.5mm and 3% cement addition is suitable.

Silty clay is often used in roadbed stuffing.The influence of freezing-thawing cycles on the microstructure leads directly to the change of subgrade strength.For the silty clay in north-eastern regions,the scanning electron microscope (SEM),mercury intrusion porosimetry (MIP) and unconfined compressive strength tests after freeze-thaw cycling are conducted.Based on fractal theory,threedimensional fractal dimensions of soil particle and pore are calculated,and the relation formula between fractal dimensions and mechanical strength is created.The results indicate that with the increase of freezing and thawing cycles,soil is rearranged and its integrity is destroyed.Meanwhile,the pore volume tends to fluctuate and increase.With the increase of the water content,the pore content of pore size in 5 ～20 μm gradually decreases,while,that of pore size in 0.3 ～ 5 pm increases.In the progress of freeze-thaw cycling,the changing characteristics of soil microstructure are well reflected by fractal dimension.The egression equations constructed by fractal dimension and unconfined compressive strength show that the bigger the fractal dimension,the higher the strength.