A three‐phase thermo‐hydro‐mechanical finite element model for freezing soils

Artificial ground freezing (AGF) is a commonly used technique in geotechnical engineering for ground improvement such as ground water control and temporary excavation support during tunnel construction in soft soils. The main potential problem connected with this technique is that it may produce heave and settlement at the ground surface, which may cause damage to the surface infrastructure. Additionally, the freezing process and the energy needed to obtain a stable frozen ground may be significantly influenced by seepage flow. Evidently, safe design and execution of AGF require a reliable prediction of the coupled thermo‐hydro‐mechanical behavior of freezing soils. With the theory of poromechanics, a three‐phase finite element soil model is proposed, considering solid particles, liquid water, and crystal ice as separate phases and mixture temperature, liquid pressure, and solid displacement as the primary field variables. In addition to the volume expansion of water transforming into ice, the contribution of the micro‐cryo‐suction mechanism to the frost heave phenomenon is described in the model using the theory of premelting dynamics. Through fundamental physical laws and corresponding state relations, the model captures various couplings among the phase transition, the liquid transport within the pore space, and the accompanying mechanical deformation. The verification and validation of the model are accomplished by means of selected analyses. An application example is related to AGF during tunnel excavation, investigating the influence of seepage flow on the freezing process and the time required to establish a closed supporting frozen arch. Copyright © 2013 John Wiley & Sons, Ltd.     

Modeling heat and mass transfer during ground freezing subjected to high seepage velocities

Natural or induced groundwater flow may negatively influence the performance of artificial ground freezing: high water flow velocities can prevent frozen conditions from developing. Reliable models that take into consideration hydraulic mechanisms are then needed to predict the ground freezing development. For forty years, numerous thermo-hydraulic coupled numerical models have been developed. Some of these models have been validated against experimental data but only one has been tested under high water flow velocity conditions. This paper describes a coupled thermo-hydraulic numerical model completely thermodynamically consistent and designed to simulate artificial ground freezing of a saturated and non-deformable porous medium under seepage flow conditions. On some points, less restrictive assumptions than the ones usually used in the literature are considered. As for the constant-porosity assumption, its validity is verified. The model appears to be well validated against analytical solutions and a three-dimensional ground freezing experiment under high seepage flow velocity conditions. It is used to highlight key thermo-hydraulic mechanisms associated with phase change in a porous medium.     

低温裂隙岩体水-热耦合模型研究及数值分析

裂隙渗流会引起裂隙周围岩体中的温度场变化,在低温岩体中其影响更为明显;此外,裂隙水与周围低温岩石介质发生热交换会引起裂隙中的水冰相变过程发生,而裂隙水冻结将阻碍裂隙渗流,引起裂隙渗流场的变化。因此,低温下的裂隙岩体水-热相互作用是一个强耦合过程。考虑裂隙中的水冰相变过程和渗流作用,建立了低温冻结条件下裂隙岩体水-热耦合模型;以冻结法施工为例,考察了低温冻结过程中裂隙水渗流对裂隙冻结交圈的影响。研究结果表明：由于裂隙渗流的存在,距裂隙较远处岩石先冻结,裂隙冻结所需时间远大于周围岩石;裂隙宽度和裂隙水压力差都会影响冻结交圈时间,裂隙越宽、水压力差越大,裂隙冻结需要时间越长;随着冻结时间的推进,裂隙水渗流速度逐渐降低,当裂隙冻结后裂隙渗流停止。最后通过构建随机裂隙网络模型,利用所建立的水-热耦合模型考察了裂隙网络渗流对冻结交圈的影响,说明了在冻结法施工中考虑裂隙的重要性。     

地下水封油库围岩地下水渗流量计算

为准确地计算特大型地下水封储油岩洞库地下水的渗流量,根据现场调查、测试与监测以及室内外物理力学试验,采用了三维多孔连续介质流-固耦合有限差分数值模拟计算分析软件模拟地下水渗流场。提出了一种适用于特大型地下水封储油岩洞库地下水渗流量计算的合理方法。经与法国专家经验法、大岛洋志经验式、《铁路工程水文地质勘测规范》（TB10049－96）等方法的计算结果进行比较,结果比较接近。该计算方法适用于试验数据和资料匮乏的可行性研究阶段的特大型地下水封储油岩洞库围岩地下水渗流量或涌水量的计算;考虑地下水流-固耦合分析时,单洞室的涌水量比只考虑地下水流动作分析时稍小;考虑流-固耦合作用比不考虑流-固耦合作用时围岩地下水的最大渗流速度稍小。     

岩体水力学基础(五)——岩体渗流场与应力场耦合的裂隙网络模型

岩体是由裂隙和其间的岩块组成。当岩块处于近乎隔水状态时，岩体中的渗流可看作为裂隙网络流。本文通过分析岩体裂隙网络渗流场、应力场以及它们之间的相互力学关系，提出了岩体渗流场与应力场耦合的裂隙网络模型及数值计算方法。     

Subway construction is Stuttgart under protection of a frozen soil roof

Experiences gained on the building of the City Railway turning loop, Stuttgart, Section 12, are described, where the new Austrian tunneling method is used in connection with the freezing technique. The Metro-tunnel lies in leached-out gypsum marl and unleached zones, respectively.

In a section of this tunnel within the leached-out gypsum marl the excavation was protected by a frozen soil roof in order to keep away any water seepage which could be dangerous for the excavation itself and for the buildings superimposed as well The drilling for the freeze pipes, the installation and operation of the freezing system and the tunnel driving including the erection of the final support are described. A point of special interest is the application of the shotcrete method as the shotcrete has to adhere to the frozen soil and has to harden sufficiently before the hardening process is interrupted by frost penetration.     

岩体三维非稳定渗流模型及数值模拟

地下水是导致地质灾害发生的重要因素之一。岩体本身的复杂性及地下水在岩体内随时间而变化造成地下水在岩体内运动机理的复杂性。文章采用数值模拟方法探索地下水在岩体内的运动规律,了解其致灾机理。以岩体结构力学为基础,运用现代分形理论,建立主干裂隙分形网络;根据裂隙发育规模与工程尺度关系将岩体看作拟连续介质与块裂介质混合介质,并根据两类介质接触处水头相等及节点流量相等建立合理的三维非稳定渗流模型;给出渗流模型的有限元解法,开发出相应的有限元软件;给出算例,计算结果体现了主干裂隙在渗流中的强导水作用及网络状裂隙的贮水功能与渗流滞后效应;强调根据裂隙发育规模与工程尺度关系确定合理渗流模型的重要性。     

Elasto‐plastic seepage‐induced stresses due to tunneling

When tunneling is carried out beneath the groundwater table, hydraulic boundary is altered, resulting in seepage entering into the tunnel. The development of flow into the tunnel induces seepage stresses in the ground and the lining is subjected to additional loads. This can often cause fine particles to move, which clog the filter resulting in the long‐term hydraulic deterioration of the drainage system. However, the effect of seepage force is generally not considered in the analysis of tunnel. While several elastic solutions have been proposed by assuming seepage in an elastic medium, stress solutions have not been considered for the seepage force in a porous elasto‐plastic medium. This paper documents a study that investigates the stress behavior, caused by seepage, of a tunnel in an elasto‐plastic ground and its effects on the tunnel and ground. New elasto‐plastic solutions that adopt the Mohr–Coulomb failure criterion are proposed for a circular tunnel under radial flow conditions. A simple solution based on the hydraulic gradient obtained from a numerical parametric study is also proposed for practical use. It should be noted that the simple equation is useful for acquiring additional insight into a problem on a tunnel under drainage, because only a minimal computational effort is needed and considerable economic benefits can be gained by using it in the preliminary stage of tunnel design. The proposed equations were partly validated by numerical analysis, and their applicability is illustrated and discussed using an example problem. Comments on the tunnel analysis are also provided. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermal and rheological computations for artificially frozen ground construction

Design considerations for frozen retaining structures are summarized including geology and groundwater conditions; ground deformations due to freezing; the strength and deformation of the frozen earth retaining structures; the thermal assumption and methods of computation; and the refrigeration system. Brief case histories of typical construction projects that illustrate the use of mechanical refrigeration and liquid nitrogen for freezing are presented. Emphasis is placed on the constitutive equation for creep deformation of frozen earth structures and the thermal calculations for predicting times to freeze a structure as well as estimating the energy and power required for different stages of freezing.     

考虑渗流条件下开挖面失稳离心试验研究

当土压平衡盾构穿越高水位地层（如穿越江河）时,地下水与土舱之间的高水压差会产生过大的渗透力,导致开挖面失稳。为了研究渗流条件下开挖面失稳问题,开发了一套隧道离心模型试验装置,主要包括刚性模型箱、模型盾构、开挖面伺服加载系统、水位控制系统、储水箱。针对饱和砂质粉土地层,开展了一系列不同水位高度的稳态渗流开挖面失稳模型试验。结果显示,开挖面失稳过程中随着开挖面位移的增加,有效支护压力迅速下降;在达到最小值 之后缓慢回升并趋于稳定;极限有效支护压力 与水头压力 呈线性关系。     

冻结层上水的分布及工程影响研究现状与展望

作为一种多年冻土区的特殊水文地质现象,冻结层上水（或多年冻土层上水）的分布受局地因素的控制,且随活动层的季节性冻融而变化,影响地表水和地下水循环以及多年冻土环境中的水热平衡。多年冻土将冻结层上水限制在一个狭窄的空间内,在暖季冻结层上水侧向和竖向的渗流传热将加剧多年冻土的退化,也会对上覆工程构筑物的稳定运营造成极大威胁。目前关于冻结层上水的研究主要集中在分布特征、变化规律、流量计算、渗流模拟、水热耦合等方面。研究发现：在全球升温背景下,多年冻土退化速率加剧,随着冻土厚度变薄和融区出现,冻结层上水的流量及其与地下水的交换量均发生变化,除了影响局地水文特征外,还与工程病害密切相关,如坡脚积水、路基沉降以及路面裂缝等。以区域分布特征为出发点,对冻结层上水的研究现状进行了归纳和总结,并对其工程影响有关的渗流传热理论研究成果进行了梳理,对今后需要进行深入研究的方向进行了展望。这有助于全面理解冻结层上水在冻土区水文过程中的功能,为相关研究提供了进一步的理论参考。     

深基坑降水三维变参数非稳定渗流与地面沉降耦合模型

为了精确模拟预测松散沉积层中深基坑降水引起的地下水渗流场和地面沉降的变化特征,考虑土体孔隙度、渗透系数、储水率随地下水位下降发生的动态变化,建立了深基坑降水三维变参数非稳定渗流与太沙基一维固结理论的地面沉降耦合模型,并采用有限元数值分析方法对模型进行求解。以南京地铁三号线浦珠路站深基坑降水为例进行模拟计算。结果表明：采用15口坑内抽水井,抽水井过滤器埋深为22.0～37.0 m,基坑围护连续墙底部埋深至41.5 m为最优降水方案;不仅使基坑内地下水位满足开挖要求,又使基坑外地面沉降在控制范围内。经验证,所建立的模型合理,计算结果可靠,研究理论用于模拟预测此类地区深基坑降水引起的地下水流场变化具有较高的可信度。     

Groundwater dewatering optimization in the Shengli no. 1 open-pit coalmine, Inner Mongolia, China

Impacted by groundwater, faults, lithology, and other factors, the slope of the Shengli no. 1 open-pit coalmine has been in a state of instability. Among these factors, groundwater in the quaternary aquifer is considered primary. Thus, an optimum design for dewatering well pumping rates is presented in this paper. A two-dimensional groundwater simulation model is built to characterize the groundwater flow of the study area. A steady-state model was applied to the observed data (head and discharge) to verify and calibrate the groundwater model. The pilot point method, with a regularization option provided by parameter estimation, was used to identify the hydraulic conductivity field. Afterward, a groundwater optimization model is integrated with the calibrated simulation model to realize groundwater dewatering optimization in the studied open-pit coalmine, and an optimization method called modified Pareto dominance-based real-coded genetic algorithm is adopted. Taking into account the safety of the mine, slope and dewatering wells, seepage discharge is added to objective function and the maximum aquifer saturated thickness is set as the constraint condition in the optimization model. The results indicate that the dewatering optimization procedure developed in this paper can serve as a useful template and framework for solving mining related water problems.     

绥化市红兴水库渗漏分析及渗控方案模拟预测

为解决水库建成后渗漏较为严重的问题,依据黑龙江省绥化市红兴水库初期蓄水失败后补充的地质勘探资料,分析水库渗漏的主要原因是库区内的粘土层分布不连续。采用三维数值模拟计算,建立红兴水库区地下水流概念模型及其数学模型,运用FEFLOW 软件的剖分功能对研究区进行了刻画;在此基础上,对无防渗方案和拟修4种防渗墙方案的防渗效果进行了模拟预测和对比。结果表明：有坝段防渗对渗漏总量的控制具有十分明显的作用,可以使水库渗漏总量减少约70％;渗漏量及渗透比降基本随防渗长度的增大呈线性趋势减小;南岸东侧防渗1 000 m、西侧防渗1 700 m时,为防渗效果最好的方案。     

二维非恒定渗流的有限元并行计算

建立了二维非恒定渗流的有限元并行计算模型,在windows操作系统下实现了基于消息传递的二维渗流的有限元并行计算。模型采用广义极小残余算法(GMRES)对方程组进行并行迭代求解,通过分析数据执行时的相关性和检验算法结构的固有串行性,将原有串行算法中的算法元直接并行化。对溪洛渡上游围堰的渗流分析进行了并行数值模拟,并针对水位骤降情况下非恒定渗流进行了并行计算,证明了模型的合理性。对模型进行了加速比测定,可以看出并行计算的效率随着问题规模的增加而逐渐提高。     

Numerical simulation of groundwater flowing to horizontal seepage wells under a river

A horizontal seepage well, consisting of an interconnected vertical well, galleries, chambers and small-diameter radiating bores, is used to acquire relatively clean water that has been filtered through natural alluvial deposits in a riverbed. It has wide application, especially in arid and semi-arid areas. The lack of calculation formulae or models for horizontal seepage wells, up until now, has resulted in several false applications. Based on the analysis of groundwater flow characteristics, it has been concluded that several flow regimes coexist and hydraulic head loss exists in the horizontal seepage well. To avoid the difficulty of confirming the flux or head distribution in such a complex system, the model boundary of the whole horizontal seepage well has been moved to that of just the vertical well, and the well-aquifer system was treated as a heterogeneous medium, where the horizontal seepage well itself is a highly permeability medium. A mathematical model has been developed, based on the coupled seepage-pipe flow, by the introduction of equivalent hydraulic conductivity according to different flow regimes. Then a three-dimensional finite difference numerical model, based on the mathematical model, was developed and applied to a horizontal seepage well in China. The numerical model verified the groundwater flow characteristics of the horizontal seepage well. An erratum to this article can be found at     

Modelling the cutoff behavior of underground structure in multi-aquifer-aquitard groundwater system

The quaternary deposit of Shanghai is composed of an alternated multi-aquifer-aquitard system (MAAS) consisting of a sequence of aquitards laid over aquifers one by one. In the MAAS, any drawdown of groundwater head in an aquifer may cause consolidation of the overburden aquitard. When underground structures penetrate those aquifers, groundwater seepage path changes and drawdown occurs at the side characterized by the lower hydraulic potential along the flow direction (hereafter refers as to the lower side). This drawdown may cause additional subsidence at the lower side and unbalanced load between the two sides of the underground structure. In order to evaluate the cutoff effect of an underground structure on groundwater seepage in a MAAS representative of the underground of the city of Shanghai, a numerical analysis based on a groundwater flow model has been carried out. The simulated results have shown that underground structures which cut off groundwater flow locally change both magnitude and direction of the flow velocity field. The induced changes in the groundwater field are highly sensitive to the penetration depth and width of the underground structure. Design recommendations for underground structures in aquifers belonging to a MAAS are also presented, which has not yet been considered in the engineering practice of Shanghai.     

连续墙周围的地下水渗流特征及数值模拟

本文以上海市彭越浦泵站降水工程为例,分析了连续墙周围地下水的渗流特征:上部地下水不连续,底部地下水连续,呈三维流态。根据场地的工程地质与水文地质条件,提出了粘弹性越流含水层组三维修流数学模型,并进行了数值模拟。计算的结果基本上反映了连续墙周围地下水三维流特征。     

土压式盾构施工中地下水出渗机理研究

针对土压平衡式盾构机在高水头强透水地层掘进工程中，排土器出口易出现地下水渗出的现象进行了理论研究。通过建立地下水在盾构机内的渗流计算模型，提出了判别地下水渗出发生与否及其出渗量大小的方法，阐明了盾构施工中发生地下水出渗的机理。利用这一模型计算了出渗发生时地下水流失情况，对影响出渗发生的各相关参数进行了分析，为施工中克服和避免这一现象提供了理论依据。     

渗流场反分析及其在多孔介质中的应用

本文讨论了复杂地质环境中渗流场的反分析问题，利用地下水流数值模拟结合最优化技术，通过对地下渗流场的模拟和寻优，求得描述地下含水介质非稳定渗流场的重要参位──渗透系数和给水度的模型解，并应用于各向同性的多孔介质渗流场反分析实例中。