Groundwater flow through fractured rocks and seepage control in geotechnical engineering: Theories and practices

Groundwater flow through fractured rocks has been recognized as an important issue in many geotechnical engineering practices.Several key aspects of fundamental mechanisms,numerical modeling and engineering applications of flow in fractured rocks are discussed.First,the microscopic mechanisms of fluid flow in fractured rocks,especially under the complex conditions of non-Darcian flow,multiphase flow,rock dissolution,and particle transport,have been revealed through a com-bined effort of visualized experiments and theoretical analysis.Then,laboratory and field methods of characterizing hydraulic properties(e.g.intrinsic permeability,inertial permeability,and unsaturated flow parameters)of fractured rocks in different flow regimes have been proposed.Subsequently,high-performance numerical simulation approaches for large-scale modeling of groundwater flow in frac-tured rocks and aquifers have been developed.Numerical procedures for optimization design of seepage control systems in various settings have also been proposed.Mechanisms of coupled hydro-mechanical processes and control of flow-induced deformation have been discussed.Finally,three case studies are presented to illustrate the applications of the improved theoretical understanding,characterization methods,modeling approaches,and seepage and deformation control strategies to geotechnical engi-neering projects.

Monitoring models for base flow effect and daily variation of dam seepage elements considering time lag effect

Affected by external environmental factors and evolution of dam performance, dam seepage behavior shows nonlinear time-varying characteristics. In this study, to predict and evaluate the long-term development trend and short-term fluctuation of the dam seepage behavior, two monitoring models were developed, one for the base flow effect and one for daily variation of dam seepage elements. In the first model, to avoid the influence of the time lag effect on the evaluation of seepage variation with the time effect component of seepage elements, the base values of the seepage element and the reservoir water level were extracted using the wavelet multi-resolution analysis method, and the time effect component was separated by the established base flow effect monitoring model. For the development of the daily variation monitoring model for dam seepage elements, all the previous factors, of which the measured time series prior to the dam seepage element monitoring time may have certain influence on the monitored results, were considered. Those factors that were positively correlated with the analyzed seepage element were initially considered to be the support vector machine(SVM) model input factors, and then the SVM kernel function-based sensitivity analysis was performed to optimize the input factor set and establish the optimized daily variation SVM model. The efficiency and rationality of the two models were verified by case studies of the water level of two piezometric tubes buried under the slope of a concrete gravity dam.Sensitivity analysis of the optimized SVM model shows that the influences of the daily variation of the upstream reservoir water level and rainfall on the daily variation of piezometric tube water level are processes subject to normal distribution.

Seepage simulation of high concrete-faced rockfill dams based on generalized equivalent continuum model

This research focused on the three-dimensional(3 D) seepage field simulation of a high concrete-faced rockfill dam(CFRD) under complex hydraulic conditions. A generalized equivalent continuum model of fractured rock mass was used for equivalent continuous seepage field analysis based on the improved node virtual flow method. Using a high CFRD as an example, the generalized equivalent continuum range was determined, and a finite element model was established based on the terrain and geological conditions, as well as structural face characteristics of the dam area. The equivalent seepage coefficients of different material zones or positions in the dam foundation were calculated with the Snow model or inverse analysis. Then, the 3 D seepage field in the dam area was calculated under the normal water storage conditions, and the corresponding water head distribution, seepage flow, seepage gradient, and seepage characteristics in the dam area were analyzed. The results show that the generalized equivalent continuum model can effectively simulate overall seepage patterns of the CFRD under complex hydraulic conditions and provide a reference for seepage analysis of similar CFRDs.

当前渗流力学理论遇到的挑战与对策思考

本文详细阐述了渗流力学经典理论——达西定律的发展历程及其适用条件,并从Navier-Stokes(N-S)方程推导出了达西定律在多孔介质中的毛细管渗流和裂缝渗流中的数学表达式。文章指出了当前达西定律应用中存在的8大问题,并综合分析了渗流力学理论在油气田开发中的主要挑战。针对这些挑战,本文提出了一系列对策和思考。文章强调指出:构建多尺度、多物理场耦合模型并借助AI科学计算是揭示油气储层复杂真实流动机制,填补目前理论空白的必由之路。建议指出:进一步发展核磁共振、电镜扫描及智能数据与图像处理等高精度实验技术,以直观展现流体在储层中的流动行为和过程。最后,建议综合运用实验研究、新理论模型建立和AI科学研究方法(AI for Science),突破油气渗流力学理论中目前遇到的挑战。研究成果可为我国高校、科研机构和研究者开展石油科学理论研究和课题立项提供重要参考,同时可为我国油气资源可持续进行科学和技术战略规划提供强有力的技术支撑。

考虑基质渗透性的粗糙单裂隙非达西流动特性研究

粗糙单裂隙及其周围岩石基质是组成复杂岩体裂隙网络的基本单元,了解其中发生的流体流动行为具有重要意义。通过直接求解Navier-Stokes(N-S)方程开展三维粗糙单裂隙和裂隙-孔隙双重介质模型的渗流模拟工作,研究了裂隙表面粗糙度、匹配性和基质渗透性对粗糙单裂隙中发生的非达西流动的影响规律。结果表明:Forchheimer方程可以准确描述双重介质模型中粗糙单裂隙流体流量与压力梯度间的非线性关系;表面粗糙度和不匹配性的增大会加剧裂隙内部开度分布的不均匀性,增大裂隙的流动阻力而促进非达西流动的发生;可渗透基质会使得裂隙过流能力增强,通过粗糙单裂隙的流量最多可增大约14%,这会抑制非达西流动的发生;基质渗透性对裂隙过流能力增长的影响程度与表面粗糙度和不匹配性呈正相关。

煤层气扩散理论及控制因素研究进展

煤层气扩散特征研究对提高煤层气产量和减少矿井瓦斯灾害具有重要意义。基于国内外学者在煤层气扩散领域取得的成果,从煤层气扩散模型、扩散实验和扩散控制因素等3方面进行系统研究。结果表明:基于Fick定律的单孔扩散模型不能描述吸附/解吸扩散全过程,双孔扩散模型、多孔扩散模型和基于时间变化的动态扩散模型能更好解释扩散特征;测压法和质量法用来获取吸附扩散系数,体积法用来获取解吸扩散系数,选择合适的扩散模型拟合实验数据是获取扩散系数的关键;煤层气扩散特征主要受控于煤阶、孔隙结构、煤岩水分、温度、压力和粒径。探寻扩散系数随时间变化规律,定量不同孔径扩散模型,明确扩散渗流联合机制,是该研究领域的发展趋势。

Debris Flows Introduced in Landslide Deposits under Rainfall Conditions:The Case of Wenjiagou Gully

Debris flows often occur in landslide deposits during heavy rainstorms.Debris flows are initiated by surface water runoff and unsaturated seepage under rainfall conditions.A physical model based on an infinitely long,uniform and void-rich sediment layer was applied to analyze the triggering of debris-flow introduced in landslide deposits.To determine the initiation condition for rainfall-induced debris flows,we conducted a surface water runoff and saturated-unsaturated seepage numerical program to model rainfall infiltration and runoff on a slope.This program was combined with physical modeling and stability analysis to make certain the initiation condition for rainfall-introduced debris flows.Taking the landslide deposits at Wenjiagou gully as an example,the initiation conditions for debris flow were computed.The results show that increase height of surface-water runoff and the decrease of saturated sediment shear strength of are the main reasons for triggering debris-flows under heavy rainfall conditions.The debris-flow triggering is affected by the depth of surface-water runoff,the slope saturation and shear strength of the sediment.

AN UNSTEADY SEEPAGE FLOW MODEL OF VISCO-ELASTIC POLYMER SOLUTION

With the consideration of the visco―elasticity, the adsorption effect andthe variation of rheological parameters, a seepage flow model of visco-elastic polymer solutions wasestablished. The model was numerically treated with the finite difference method. Then curves ofBottom Hole Pressure (BHP) and formation pressure were drawn. The influences of the relaxation time,the injection rate, the permeability reduction coefficient, the consistency coefficient and thepower-law exponent of the injected fluid on pressure performance were analyzed. This study showsthat it is necessary to consider the visco-elasticity of non-Newtonian fluid in analyzing ofpressure performance in the polymer flooding.

基于MODFLOW的地下水渗流与地面沉降耦合模型工程应用一例

随着我国基础建设的迅猛发展,特别是城市轨道交通工程大规模建设,出现了大量的深基坑工程,如果基坑开挖深度超过该区的地下水位则需要首先采取地下水的控制措施,工程降水是地下工程施工中常采用的地下水控制方法,大量抽取地下水必将导致地面沉降的发生,本文以天津站交通枢纽工程后广场工程为依托,以水文地质试验为基础,辅以孔隙水压力监测及变形监测,应用MODFLOW软件,建立基坑地下水渗流模型与地面沉降的数学耦合模型,并根据现场试验资料,反演识别了模型中的有关参数,对降水引起的水位变化进行模拟预测,对可能产生的地面沉降进行定量评估,以更好的指导基坑工程降水。

A 3D basin modeling study of the factors controlling gas hydrate accumulation in the Shenhu Area of the South China Sea

Great advancement has been made on natural gas hydrates exploration and test production in the northern South China Sea.However,there remains a lot of key questions yet to be resolved,particularly about the mechanisms and the controls of gas hydrates enrichment.Numerical simulaution would play signficant role in addressing these questions.This study focused on the gas hydrate exploration in the Shenhu Area,Northern South China Sea.Based on the newly obtained borehole and multichannel reflection seismic data,the authors conducted an integrated 3D basin modeling study on gas hydrate.The results indicate that the Shenhu Area has favorable conditions for gas hydrate accumulation,such as temperature,pressure,hydrocarbon source,and tectonic setting.Gas hydrates are most concentrated in the Late Miocene strata,particularly in the structual highs between the Baiyun Sag and the Liwan Sag,and area to the south of it.It also proved the existence of overpressure in the main sag of source rocks,which was subject to compaction disequilibrium and hydrocarbon generation.It also shown that the regional fault activity is not conducive to gas hydrate accumulation due to excess gas seepage.The authors conjecture that fault activity may slightly weaken overpressure for the positive effect of hydrocarbon expulsion and areas lacking regional fault activity have better potential.

渗流对潮沟沟壁崩塌影响的三维物理模型试验研究

潮沟在海岸动力作用下因沟壁崩塌而频繁地迁移摆动,显著影响潮滩-潮沟系统的稳定性。现有研究多关注涨潮时期潮沟内部的水流冲刷,落潮期间的渗流侵蚀过程却少有涉及,亟需研究渗流对潮沟沟壁稳定性的影响机制。本研究通过物理模型试验探究渗流对潮沟沟壁崩塌的影响,从沟壁崩塌类型、沟壁线变化过程和沟壁后退速率3个方面开展分析。研究结果表明:沟壁破坏类型与渗流水头相关,渗流水头的增加导致破坏类型由悬臂破坏转变为张拉破坏;沟壁后退速率与渗流水头之间存在正相关关系。本研究揭示了渗流作用下潮沟沟壁崩塌的力学机理,阐明了落潮不同阶段沟壁的崩塌过程,对研究潮沟演变有重要意义。

尾矿库不同坝体形态渗流稳定性分析

为研究不同坝体形态对尾矿坝渗流稳定性的影响,分析上游式尾矿坝子坝向上游平移的安全距离,以新疆某尾矿库为工程实例,采用Geo-Studio软件Geo-Seep/W渗流分析模块饱和-非饱和流Van Genuchten模型进行尾矿坝渗流场分析。通过模拟分析尾矿库现状各工况下渗流稳定性,并与勘察成果进行对比,渗流计算结果与勘察资料相吻合。同时,对不同坝体形态的渗流进行对比分析,当坝体形态发生改变时,尾矿堆积材料相对位置发生改变,从而对渗流产生较大影响。研究结果表明:尾矿堆积坝标高在940.0 m时,向上游平移距离不宜超过80 m,否则应增设排水系统,以降低坡脚位置浸润线。

陕北Q2黄土入渗模型优化及边坡稳定性分析

采用Brooks&Corey土水特征曲线公式对Green-Ampt模型中入渗剖面的非饱和过渡区进行计算,得到优化的Green-Ampt公式;引入湿润层重分布比例函数的概念,采用Seep/W模拟非饱和土壤水分性质数据库(UNSODA)中256个样本10种类型土的降雨入渗过程,得到湿润层重分布函数的解析表达式;基于本文改进的Green-Ampt模型、湿润层重分布函数及Frdlund抗剪强度公式,推导出改进的边坡稳定计算公式;以陕北延安地区樊庄Q2黄土高边坡为算例与前人修正过的Green-Ampt模型解结果进行对比,验证新模型的适用性,经验证本文改进的Green-Ampt模型适用性好。

基于GAMLSS-GLO及随机森林算法的土石坝渗流监测模型

根据土石坝渗流原型观测,厘清水位、降水等因素的影响,模拟大坝渗流的真实状态,合理评估其渗流监测结果,是土石坝安全监控亟待解决的关键问题。基于此,根据数理统计原理,采用随机森林算法构建无降水条件下渗流量与上下游水位的回归模型;考虑渗流量受前期累积降水的综合影响,引入广义可加模型(GAMLSS-GLO),模拟降水影响下土石坝渗流监测值的波动区间,并将其与渗流-水位回归模型叠加,预测土石坝渗流监测的可靠区间;最后,将该方法应用于糯扎渡心墙堆石坝的渗流监测。结果表明:所提模型方法对渗流的水位、降水响应表现出良好的适用性,显著提高了渗流模拟预测质量。同时求解了渗流量置信区间,有利于土石坝的运行工况判断及安全监控。

COUPLING EFFECT OF SEEPAGE FLOW AND RIVER FLOW ON THE BANK FAILURE

On the basis of the generalized physical model of the riverbank, the experiments were conducted to study the mechanisms of riverbank failure under the coupling effect of seepage flow and river flow. The experimental setup was specially designed, as well as test point location, parameters and procedures, and the main influencing factors were analyzed affecting riverbank failure based on the failure types, the variations of pore water pressure and soil displacement. The results indicated that the coupling effect has different influences on the bank failure in three aspects： the failure type, the process and the extent. In addition, the river flow played a more important role than the seepage flow in the coupling effect on the bank failure.

多含水层渗流系统电导示踪模型

在利用钻孔测定含水层水文地质参数时,如遇到多含水层情况,需使用栓塞系统对各含水层进行隔离探测,由于现场操作较为不便且钻孔条件要求高,故提出多含水层渗流系统电导示踪模型。在不使用栓塞系统的情况下,考虑含水层涌水或吸水等不同情况,通过测定抽水条件下各含水层的电导率,建立多含水层渗流系统电导示踪模型确定含水层渗透性参数。当钻孔中只存在涌水层时,直接利用电导率曲线面积与溶质质量的比例关系求解出流量;当钻孔中同时出现涌水层和吸水层时,根据溶质质量曲线的斜率可较准确地确定吸水层的位置,再由斜率的变化计算出含水层的流量。基于某工程实例采取3种方法求解各含水层的渗透系数、静水头、渗流速度、导水系数等水文地质参数,结果表明,采用电导示踪模型计算结果与同位素示踪测试及注水试验测试结果吻合较好,且电导示踪模型方法现场操作更为简便,可极大地提高含水层水文地质参数的测定效率。

高密度电阻率法在土质边坡渗流试验中的应用

渗流对边坡稳定性具有重要影响,为了解高密度电阻率法在渗流试验中的探测效果,建立边坡模型,并对模型边坡渗流裂隙进行探测,通过高密度电阻率法反演数据与降雨入渗时边坡渗流裂隙变化的对比,得到高密度电阻率法在渗流试验中探测的可行性.以0.62 mm/min的降雨强度进行2次集中降雨,间隔时间为12 h.首次集中降雨时,雨水下渗较为均匀,随着渗流的进行,边坡表面出现细微裂隙,同时高密度电阻率法反演图像中显示边坡内部出现视电阻率较周围高的异常体,并且范围逐渐扩大;第2次集中降雨时,由于裂隙附近的细微颗粒被带入裂隙,导致裂隙重新闭合,随着降雨继续,裂隙再次沿原有裂隙裂开,如此反复,边坡裂隙从无到有并经历“裂开—闭合—裂开—闭合”的循环,导致裂隙逐渐变宽.通过高密度电阻率法探测,其反演视电阻率结果与边坡实际裂隙变化情况吻合,说明高密度电阻率法能有效地应用于渗流试验中,并能发挥良好的探测效果.

运行期土石坝渗流量分析对大坝安全影响评价

土石坝渗流量监控可及时掌握水库安全运行性态,提早发现主体工程的异常与隐患,提升工程的安全管理与应急预案水平,是水库安全管理的重要手段之一。采用实测渗流量、有限元模型及渗漏量统计回归模型等进行评价水库运行期的大坝安全。结果表明:有限元模型计算大坝总渗流量与统计回归模型下的渗流量数值接近,结合实测渗流量可知,大坝坝体渗漏量整体趋于减小,大坝渗流稳定运行正常。分析结论可为水库运行期大坝安全评价提供技术支撑。

低渗透岩石非线性渗流规律研究

通过大量的实验研究表明,流体在低渗透岩石中的渗流不同于在中高渗透岩石中的渗流,主要表现在低渗透岩石由于孔隙微细,固液界面作用明显,渗流曲线呈现明显的非达西渗流特征。通过理论分析,建立了描述低渗透岩石非线性渗流的运动方程,给出了确定出现非达西渗流临界条件的方法,最后利用实验数值验证了所建立的运动方程的正确性。

润湿性对模拟原油微尺度流动和渗流的影响

考察了流体在不同润湿性条件下在低渗透多孔介质微米量级孔喉中流动的渗流特性,实验研究了模拟原油(航空煤油与胜利油田原油以5∶1的体积比配制)在内径为14.9μm、5.03μm、2.05μm的亲水和憎水微圆管中的流动特性。结果表明,模拟原油在5.03μm和2.05μm亲水和憎水微管中的流动均出现了明显的微尺度效应,流动规律均偏离经典理论,且模拟原油在亲水微管中的流量高于其在憎水微管中的值。根据微管实验数据,利用毛管束模型分析了润湿性对模拟原油在多孔介质中渗流特性的影响。在考虑微尺度效应的条件下,模拟原油在低渗透多孔介质中的流动为非线性渗流,润湿性显著影响。在其他条件相同时,模拟原油在亲水模型中的视渗透率大于其在憎水模型中的值。