Technological monitoring of subgrade construction on high-temperature permafrost

Three stages of complex technological monitoring for the increase of high-temperature-permafrost soil bearing capacity are described. The feasibility of process monitoring to improve the targeted strength properties of subgrade bases on frozen soils is demonstrated. The rationale for the necessity of predictive modeling of freeze-thaw actions during the subgrade construction period is provided.

To the issue of stabilization of permafrost soil subgrade

This paper summarizes an analysis of consequences of railway subgrade construction and maintenance solutions in northern areas of the Russian Far East. An idea of the natural long-term stabilization of the subgrade-base geotechnical system is presented. Proposals to improve the decision-making of construction and engineering solutions are formulated.

Numerical analysis for permafrost temperature field in the short term of permafrost subgrade filling

Purpose-It is of great significance to study the influence of subgrade filling on permafrost temperature field in permafrost area for the smooth construction and safe operation of railway.Design/methodology/approach-The paper builds up the model for the hydrothermal coupling calculation of permafrost using finite element software COMSOL to study how permafrost temperature field changes in the short term after subgrade filling,on which basis it proposes the method of calculation for the concave distortion of freezing front in the subgrade-covered area.Findings-The results show that the freezing front below the subgrade center sinks due to the thermal effect of subgrade filling,which will trigger hydrothermal erosion in case of sufficient moisture inflows,leading to the thawing settlement or the cracking of the subgrade,etc.The heat output of soil will be hindered the most in case of July filling,in which case the sinking and the distortion of the freezing front is found to be the most severe,which the recovery of the permafrost temperature field,the slowest,constituting the most unfavorable working condition.The concave distortion of the freezing front in the subgrade area increases with the increase in temperature difference between the filler and ground surface,the subgrade height,the subgrade width and the volumetric thermal capacity of filler,while decreases with the increase of the thermal conductivity of filler.Therefore,the filler chose for engineering project shall be of small volumetric thermal capacity,low initial temperature and high thermal conductivity whenever possible.Originality/value-The concave distortion of the freezing front under different working conditions at different times after filling can be calculated using the method proposed.

Analysis of temperature field characteristics based on subgrade site measurements of Harbin-Qiqihar High-speed Railway in a deep seasonal frozen soil region

Recent years have seen a large number of high-speed railways built and will be built in seasonal frozen soil regions ot China. Although high-speed railways are characterized by being fast, comfortable and safe, higher standards for defor- mation of the railways＇ frozen subgrade are required. Meanwhile, changes in subgrade soil temperatures are the main factors affecting the deformation of frozen subgrade. Therefore, this paper selected typical test subgrade sections of the Harbin-Qiqihar Line, a special line for passenger transport built in the deep seasonal frozen soil regions of China, to monitor field temperatures. Also, the temperature changing laws of railways＇ subgrade in this region was analyzed by using testing data, the aim of which is to provide a technical support for future design and construction of buildings and structures in a deep seasonal frozen soil region.

Analysis of railway subgrade frost heave deformation based on GPS

In order to analyze the connection between the railway subgrade frost heave deformation and temperature variation, five GPS stations＇ data were used to monitor the deformation on a certain section of railway subgrade in northeast China. GAMIT software is used to process the data, providing daily solution, daytime solution and nighttime solution. Vertical trends of these five stations were analyzed to investigate frost heave effect on railway subgrade deformation. The results show that the temperature difference between daytime and night induces stations, significant vertical displacement, and the temperature difference between seasons causes settlement of station which appears linear trend.

Systematization of features and requirements for geological survey of railroad subgrades functioning in cold regions

The operation of a railway track in cold regions results in the premature deformation of subgrade soils caused by significant temperature fluctuations and ecological imbalance.Identification and calculation of the thawing degree of permafrost soils,frost heaving of clays,and groundwater flooding require careful engineering and geological surveying.The paper describes the unique,long-standing experience of the university scientists connected with maintaining the Russian EastSiberian and Trans-Baikal Railways'facilities.Specific features of and requirements for the surveying,depending on the geological and climatic conditions,are identified.

A method used to determine the upper thermal boundary of subgrade based on boundary layer theory

In the numerical simulation of long-term subgrade temperature fields, the daily variation of soil temperature at a certain depth h is negligible. Such phenomenon is called the ＂boundary layer theory.＂ Depth h is defined as the boundary layer thickness and the soil temperature at h is approximately equal to a temperature increment plus the average atmosphere temperature. In the past, the boundary layer thickness and temperature increment were usually extracted from monitored data in the field. In this paper, a method is proposed to determinate the boundary layer thickness and temperature incre- ment. Based on the typical designs of highway or railway, the theoretical solution of boundary layer thickness is inferred and listed. Further, the empirical equation and design chart for determining the temperature increment are given in which the following factors are addressed, including solar radiation, equivalent thermal diffusivity and convective heat-transfer coefficient. Using these equations or design charts, the boundary layer thickness and temperature increment can be easily determined and used in the simulation of long-term subgrade temperature fields. Finally, an example is conducted and used to verify the method. The result shows that the proposed method for determining the upper thermal boundary of subgrade is accurate and practical.

Accounting for the solar radiation in thermal regime prediction for railway subgrade in cold regions

This paper presents a comparative analysis of simulation processes of seasonal freezing-thawing of railway subgrade and permafrost degradation, with and without accounting for solar radiation. Also, the effect of sun screens to reduce the degradation of subgrade permafrost under different climatic conditions is numerically substantiated. And finally, the temperature criterion of the origination of permafrost is illustrated.

Subgrade-reinforcement techniques for the dangerously deforming sections of railway lines in the north of the Russian Far East

This paper lists difficult railway construction conditions that are typical for the northern areas of the Russian Far East.Permafrost-bound issues of railway and highway construction and operation are described.Some of the authors'developments for subgrade stabilization in the foregoing conditions are given as examples.

岛状多年冻土区高速铁路路基热状态及工程措施效果分析

为揭示岛状多年冻土区高速铁路路基热状态,提出合理有效的制冷结构,在新建哈尔滨至伊春高速铁路某车站试验段开展现场监测,获得岛状多年冻土的地温数据;基于实测地温数据,采用冻土水热耦合理论,对将在试验段实施的两侧双排普通热棒路基、两侧双排+中心单排全季热棒路基、两侧单排+基底横向通铺全季热棒路基3种制冷路基结构进行数值模拟,对比了3种制冷路基结构的地温分布特征及对下伏岛状多年冻土的降温效果。研究结果表明:铁力地区年均气温和降水呈增大趋势,天然场地岛状多年冻土地温在-0.3℃左右,属于高温极不稳定多年冻土。3种制冷路基结构中,两侧单排+基底横向通铺全季热棒对岛状多年冻土保护及降温效果最优,两侧双排普通热棒最差。普通热棒路基的多年冻土上限呈“两侧凸,中间凹”形态,抬升不明显;全季热棒路基的多年冻土上限呈“上凸缓斜平顶”形态,抬升显著。研究成果可对多年冻土区高速铁路路基建设和结构优化提供技术支撑。

探地雷达在多年冻土区特殊结构路基服役状态评价中的应用

选取某高原高速公路(片块石路基、热棒路基和通风管路基)进行无损检测,运用探地雷达技术识别多年冻土区特殊结构路基类型、病害及下部冻土分布情况。结果表明:片块石路基通风层处波形杂乱呈区域化分布,热棒路基具备固定间距窄波幅月牙双曲线特征,通风管路基具备固定间距宽幅双曲线特征;对路基病害进行识别,发现检测段片块石路基部分孔隙被堵塞、热棒路基部分热棒失效、通风管路基存在不均匀沉降;通过路基下部多年冻土人为上限的识别及对比以往勘察数据,显示检测段特殊结构路基下方多年冻土均出现退化现象,多年冻土上限下移。

冻结层上水渗流对斜坡稳定性影响分析

为了研究冻土斜坡路基中冻结层上水渗流对斜坡稳定性的影响,本文基于强度折减法、莫尔—库仑屈服准则和达西定律建立数值模拟模型,对路基修筑前后、缓坡陡坡工况下的冻结层上水渗流场和土体变形进行了模拟计算。结果表明:路基修筑改变了渗流路径和渗流速度。受路基传热的影响,路基下多年冻土产生融化,冻土上限有所下降并形成融化槽。冻土上限界面是潜在的滑移面,斜坡坡度越大,多年冻土上限融化下降的深度越大,产生塑性变形而滑动失稳的可能性越大。建议做好路基排水,避免水分在路基坡脚聚集,以提高路基稳定性。

青藏铁路多年冻土区不同结构路基长期稳定性分析

在高温、高含冰多年冻土区,气候变暖导致的冻土升温造成了青藏铁路部分路基断面出现明显的沉降变形。为了解气候升温背景下不同结构路基的稳定性变化过程,本研究选取了普通路基、U型块石结构路基和块石护坡路基进行分析。通过分析2006—2018年的地温和变形监测数据,研究这三种类型路基的稳定性以及多年冻土地温和路基变形的关系。研究表明,这三个断面中U型块石结构路基地温稳定性最好,块石护坡路基浅层地温稳定性优于普通路基,但其深层多年冻土的地温稳定性较差。对路基变形监测数据的分析表明,三种路基的变形稳定性从高至低分别为U型块石结构路基、块石护坡路基和普通路基。此外,通过分析含冰量与地温变化数据发现,对于减小路基变形而言,控制浅层多年冻土的升温和融化相比于控制深层地温更有效。

冻融循环作用下路基冻胀性能试验研究

为了探究冻融循环作用下路基土的冻胀性能变化规律,以西部某高速公路冻害路段典型断面为依托工程,取现场土样制作路基模型,开展室内冻胀试验,分析冻融循环下路基温度、位移、含水量及含盐量的变化规律。研究认为,不同深度、不同水平距离处的温度变化规律相近,均呈现余弦曲线变化,约滞后环境温度1/4周期;距路基顶部越近,以及距中心线越远处(即越接近冷端)温度波动越明显;在温度到达最低点或最高点后约1/2个周期,冻胀量或融沉量达到最大值;温度对路基水分迁移有较大影响,温度较低时,路基内部水分向上部冷端迁移,导致上部土体含水量增大并冻结;温度较高时,水分几乎不发生迁移。

PUC固化路基融化夹层强度及耐冻融性能研究

多年冻土退化导致路基融化夹层(SSLT)数量增加、范围扩大、埋深加剧,其含水量高、压缩性大、承载力低的特点,威胁多年冻土区基础设施安全运行。因此,开展融化夹层的处置研究工作,对寒区基础设施的维护和修复至关重要。为此,本文提出借鉴软土加固技术对其进行加固,采用亲水性聚氨酯(WPU)及改性水泥基地聚物材料(PUC)为固化剂,研究对SSLT的加固效果。结果表明,WPU固化SSLT试样呈弹性特征,压缩强度随WPU掺量增加而增大;当SSLT含水量分别为35%、40%和50%时,WPU最佳掺量分别为6%、10%和15%;冻融循环降低固化土无侧限抗压强度,冻融循环15次,掺入WPU和未掺入WPU固化土强度损失率分别为43.11%和49.38%;微观分析表明,PUC固化剂固化SSLT强度的增加,一部分是WPU充填、包裹、胶结土颗粒增加土体密实度,一部分来自水泥水化过程。本文为寒区老旧路基改造及提高其安全稳定运营提供一种新的解决思路。

考虑相变作用的冻土路基应力与变形分析模型

基于冻土路基温度场的控制方程,考虑水分转化为冰的相变作用对土体瞬时变形和蠕变变形的影响,建立路基应力和变形的二维数值方程,并通过1月份的路基冻胀力学模型,分析冻胀带内水分相变引起的路基应力和变形的分布规律。研究发现,在冻胀区域一定的情况下,冻胀率的大小决定了路基表面应力和变形的极值大小;竖向位移的最大值在坡脚处产生,并向路基中部和左侧边界逐渐递减;随着冻胀率的增加,路基表面裂缝有从坡脚向路中发展的趋势;路基表面产生最大拉应力的位置与最大竖向位移的位置基本吻合;路中所承受的拉应力主要发生在水分集聚的相变带范围内。结果表明,相变作用是引起路基发生冻胀病害的直接因素,分析路基应力与变形的分布规律是研究多年冻土路基破坏机理的有效方法。

多年冻土地区路基临界高度研究

根据青藏公路的路况调查结果和大量的计算结果 ,对冻土路基中的温度分布和变形特点进行分析 ,将临界高度的概念拓宽为保证路基处于安全状态的填土高度 ,提出了冻土路基的临界高度应有下和上两个值 ,并得到了确定砂砾路面路基和沥青路面路基 ,下临界高度和上临界高度的计算方法。计算结果发现 ,由于融土核的发育状况与路基高度关系密切 ,增加路基高度并不能使上限上升 ,计算还发现 ,采用低吸热性的面层材料以及分期铺筑沥青路面面层 。

青藏公路多年冻土段路基病害分布规律

多年冻土道路多出现路基沉陷变形和路基路面开裂破坏这两类病害 .根据青藏公路大量调查资料 ,对路基病害与多年冻土类型及其稳定程度、工程历史、路基高度、路基边坡坡向的关系进行了分析 .结果表明 ,多年冻土稳定性差、工程完工时间长、位于路基阳坡面的沉陷和纵裂均较严重 。

高原冻土区公路路基病害及工程对策

本文分析了青藏公路60多年来的历次改建及病害资料,以道路病害率为依据将青藏公路分为稳定区、基本稳定区、不稳定区和极不稳定区;选取了134 km典型病害路段,分析年平均地温、冻土上限退化速率、含冰量与道路病害关系,确定各因素影响下道路的平均使用年限;研究热棒、片块石、保温板、通风管路基处治效果,以及各处治措施实施后引发的新工程病害,分析新生病害的时间效应,并介绍了弥散式通风路基、单向导热板路基和路基路面一体化散热结构等适用于大尺度冻土路基的新型稳定技术。研究结果表明:年平均地温、冻土上限退化速率、含冰量等因素与道路使用寿命均为负相关关系,在各类处治措施中,热棒、片块石、保温板、通风管加片块石对病害预防的有效率较高。

无动力热棒冷却冻土路基研究

基于热棒工作原理及已有的研究成果,依托青藏公路整治改建工程实施大规模热棒试验工程。介绍青藏公路多年冻土区公路路基热棒试验情况,通过现场设置观测装置观测地温变化,分析研究热棒的工作状态、冷却路基效果及对冻土热稳定性的影响。1年观测结果表明,进入大气负温期后,热棒开始工作,路基及基底土体冷储量显著增加,且总增加量随冻结期增长;热棒影响范围内,距离热棒越远,冷量增长期越滞后且增幅越小,符合一般规律;路基工程中使用热棒保护冻土和改善路基热稳定性是有效的。