【作者】 李博勇；

【导师】 周创兵； 陈益峰；

【作者基本信息】 武汉大学 ， 水工结构工程， 2021， 博士

【摘要】 地下水渗流是能源开发利用、地质灾害防治和自然环境保护等领域的关键问题之一。随着我国200-300 m级高坝工程、200-800 m级高水头抽水蓄能电站以及岩溶地区引调水工程的建设发展,坝基岩体及洞室围岩承受的渗透压力和水力梯度不断增大,岩体渗流的惯性效应显著增强,传统基于线性达西定律的岩体渗流分析理论已不能满足工程实践需求,研究岩体渗流的非达西特性及其参数确定方法,对大型水利工程的安全建设和高效运行具有重要的理论意义和应用价值。本文以高水头水电工程为研究背景,以裂隙岩体为主要研究对象,以发展岩体非达西渗流参数的确定方法为研究目标,采用理论分析和数值模拟相结合的研究手段,深入开展了岩体裂隙非达西渗流的宏细观机制、基于现场试验的岩体非达西渗流参数确定方法以及非达西渗流理论在工程中的应用等研究。主要研究工作及创新性成果如下:(1)裂隙是岩体渗流的主要通道和基本单元,研究裂隙渗流的基本规律是确定岩体渗流参数的前提。通过直接求解N-S方程,模拟了吻合裂隙和错动裂隙的渗流过程,阐明了裂隙非达西渗流的宏观规律和细观机制,并发现了裂隙非达西渗流特性对流动方向的依赖现象。研究结果表明,等效水力开度b_h和非达西惯性系数β_c是影响裂隙非达西渗流特性的关键参数,裂隙粗糙度和剪切位移对非达西效应具有显著的促进作用;涡旋区的形成和演化是裂隙产生非达西渗流现象的内在机制,当水流方向发生变化时,因惯性效应产生的涡旋区位置和演化特征存在差异,导致裂隙的宏观非达西渗流特性也产生差异。水流方向不改变裂隙的固有渗透特性,但可导致非达西惯性系数β_c发生变化;裂隙渗流对流动方向的依赖性与裂隙的相对粗糙度、剪切位移以及裂隙尺寸密切相关,随着裂隙尺寸的增大,流动方向的影响逐渐减弱并最终消失。(2)抽水试验是确定含水层渗流参数的重要手段,在断层破碎带、裂隙密集带以及侧向隔水的含水层中,流向抽水井的水流呈线状流,且极易随抽水流量的增大而发展为非达西流态。通过理论分析,研究了侧向隔水条件下将二维井流问题近似为一维线状流问题的适用条件,提出了Forchheimer方程的一般化线性近似策略,建立了侧向隔水含水层中的非达西线状流解析模型,给出了线状流条件下确定岩体非达西渗流参数的方法,并应用于长河坝基坑覆盖层中的抽水试验数据分析。对解析模型的数值模拟验证表明,只要抽水试验持续适当的时间,解析模型就具有足够的精度,且Forchheimer方程的最优线性近似策略,恰好为当前两种常用近似策略的均值。对解析模型的应用研究表明,在抽水试验数据解析中忽略非达西效应将低估地层的渗透系数,并高估地层的储水率;在长河坝基坑抽水试验中,降深曲线明显存在初期一维流、中期过渡流和后期二维流3个阶段,导致含水层流动维度发生变化的原因与覆盖层下伏的基岩弱风化岩体的侧向补给有关,从而阐明了基坑施工期涌水的成因。由初期一维流和后期二维流降深数据,分别实现了覆盖层和基岩弱风化岩体渗流参数的辨识。(3)钻孔压水试验是确定岩体渗流参数最重要的原位试验技术,现有的压水试验解析模型主要针对达西流态和竖直钻孔。基于Forchheimer方程,采用镜像法和流速积分方法,建立了考虑钻孔倾角、地下水位以及非达西渗流效应的一般化压水试验解析模型,通过有限元数值模拟验证了解析模型的合理性,并给出了岩体发生水力劈裂之后的解析解。在竖直孔和达西流条件下,该模型退化为Zangar(1953)公式;当钻孔试段与地下水位之间的距离c超过试段长度L的2倍时(即c/L>2),该模型近似退化为Chen(2015b)模型(即规程推荐公式),据此提出了现有压水试验解析模型的选用原则。将解析模型应用于琼中抽水蓄能电站倾斜孔的高压压水试验数据分析,表明在高压压水试验过程中,岩体渗流出现显著的非达西流态,忽略非达西效应将显著低估岩体的渗透系数K;当岩体发生水力劈裂之后,岩体的渗透系数K将明显增大而非达西系数β将明显减小。Chen模型因不考虑钻孔倾角和地下水位的影响,将高估渗透系数K而低估非达西系数β。此外,不同倾向钻孔孔段渗透系数的差别还反映了岩体渗透性的非均质性和各向异性。(4)非达西渗流对工程安全的影响以及如何对非达西渗流进行控制是高水头水电工程防渗设计的关键技术问题。在合理确定岩体非达西渗流参数的基础上,采用基于Forchheimer方程的有限元数值模拟方法,对阳江抽水蓄能电站高压隧洞区渗流场进行了模拟分析,重点研究了非达西渗流效应和固结灌浆对高压隧洞围岩渗流场分布特征、渗透稳定性和渗流量的影响,并据此对固结灌浆进行了优化设计,结果表明,当渗流呈非达西流态时,洞周压力水头的削减幅度增大,内水外渗量增幅变缓,并导致灌浆区外侧岩体的渗透坡降显著增大,进而对隧洞围岩及断层的渗透稳定性产生不利影响;固结灌浆厚度达到6 m时,有效减小了非达西流的扩展范围,并抑制了非达西流的发展程度,将灌浆区外侧围岩及断层的渗流控制在达西流态,因而极大降低了发生渗透失稳和水力劈裂的风险。更多还原

【Abstract】 The fluid flow in fractured rocks is of great concern in numerous industrial and scientific fields including energy exploitation,geological disaster prevention and environmental protection.In high dams(200-300 m),high-head(200-800 m)pumped-storage hydropower stations and water diversion projects in karst areas,the water pressure and hydraulic gradient in the surrounding rocks of the dam foundations and diversion tunnels are so large that the inertial effect of flow can not be neglected,and the traditional seepage analysis theory based on Darcy’s law can no longer meet the requirements of engineering practice.To fill this gap,this thesis is aimed to develop the methods of identifying hydraulic properties for non-Darcian flow in fractured rocks through theoretical analysis and numerical simulation.We focused on macro-and meso-scopic mechanisms of non-Darcian flow in rock fractures,identification of non-Darcian hydraulic properties of fractured rocks based on field tests,and the applications of non-Darcian theory in engineering practice.The main achievements are listed as follows:(1)Fractures are the main flow paths in rocks,and the hydraulic properties of rocks depend on flow characteristics in fractures.In this study,numerical simulations by directly solving Navier-Stokes equation were conducted in fractures with various roughness and degrees of matching,the results show that the non-Darcian behaviours of fractures internally depend on effective hydraulic aperture b_h and non-Darcian inertial coefficientβ_c.Roughness and shear displacement would enhance the degree of flow nonlinearity.The occurrence of non-Darcian flow is mesoscopically ascribed to the inertia-enlarged recirculation zones.The shape and location of recirculation zones for opposing direction are different,which results in dependence of macroscopic flow behaviours of fractures on the flow direction.The inertial coefficientβ_cwould change with flow direction but b_hwould not.The effect of directionality on non-Darcian flow depends on the fracture geometry including the relative roughness,shear displacement and length,and this effect gradually decreases and finally vanishes with the increasing fracture length.(2)Pumping test is an important tool for estimating the hydraulic properties of aquifers.The water flow towards a pumping well in fractured zones and aquifers laterally bounded by no-flow barriers frequently has a linear pattern and tends to be non-Darcian with the increasing pumping rate.In this study,a general linearization approximation strategy was suggested for the Forchheimer equation,and an analytical solution was proposed by using Laplace transform for non-Darcian flow with linear pattern.Numerical simulations using the finite volume method prove that the linearization approximation performs best when it takes the mean of two commonly-used strategies,and the analytical model is sufficiently accurate at late times for observation wells located moderately far from the source,the methodology for hydraulic properties estimation of linear aquifers was further proposed.This methodology was applied to data interpretation of the pumping tests at the Changheba dam foundation bounded by two cut-off walls,the results show that the hydraulic conductivity will be underestimated while the specific storage coefficient overestimated if the non-Darcian effect is neglected.The drawdown curves can be divided into 1D flow,transitional flow and 2D flow stages as a result of lateral flow through weathered bedrocks at late times.The aquifer parameters interpreted with the 1D flow model from the early-time data and with the 2D radial flow model from the late-time data well represent the aquifer properties of the unconsolidated deposits and the weathered rocks,respectively.(3)Packer test is the most important field test for characterizing the permeability of rocks.The current interpretive models for packer test mainly apply for Darcian flow and vertical borehole.In this study,a Forchheimer’s law-based analytical model incorporating the effects of groundwater level and borehole inclination was presented by image method and velocity integration,which was then validated by numerical simulations,and an interpretive model for the hydraulic fracturing stage was further established.The proposed model can be immediately reduced to Zangar’s(1953)equation for vertical borehole under laminar flow condition and approximately reduced to Chen’s(2015b)model when the distance(c)between the test interval and the groundwater level is larger than twice the length(L)of interval.The applicability of these models was then summarized.The proposed model was applied to data interpretation of packer tests performed in fractured sedimentary rocks in a pumped-storage power station in Qiongzhong County.The results show that significant non-Darcian flow occurred in the tests,and the negligence of flow nonlinearity would yield a significate underestimation of hydraulic conductivity.The hydraulic conductivity K increases while the non-Darcian coefficientβdecreases with the increasing pressure in hydraulic fracturing stage.Chen’s model without considering the groundwater level and borehole inclination would overestimate K but underestimateβ.Furthermore,the discrepancy of the estimated parameters from the boreholes with different inclination angles demonstrates the anisotropy and heterogeneity of the tested rocks.(4)The effect of non-Darcian flow and how to control this unfavourable effect are key technological issues for seepage control design in high-head hydropower projects.In this study,we focused on the concrete-lined hydraulic tunnel in Yangjiang Pumped Storage Power Station,Guangdong Province.On the basis of determining the hydraulic properties reasonably,the non-Darcian flow behaviours in the surrounding rocks were investigated with Forchheimer’s law-based numerical simulations,the results show that in non-Darcian regime,the reduction of the pressure head around the tunnel will increase,the increase of leakage will slow down,and the gradient of the rock outside the grouting area will increase significantly,which is not conducive to the seepage safety of the tunnel.The high-pressure grouting(6 m in thickness)is effective in reducing the degree of non-Darcian flow and controlling the groundwater flow in the regime of Darcian flow in the fault zones and fracture network,which significantly reduces the risk of seepage failure or hydraulic fracturing.更多还原