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1、混凝土重力坝基础流体力学行为分析摘要：一个在新的和现有的混凝土重力坝的滑动稳定性评价的关键要求是对孔隙压力和基础关节和剪切强度不连续分布的预测。本文列出评价建立在岩石节理上的混凝土重力坝流体力学行为的方法。该方法包括通过水库典型周期建立一个观察大坝行为的数据库，并用离散元法（DEM）数值模式模拟该行为。一旦模型进行验证，包括岩性主要参数的变化，地应力，和联合几何共同的特点都要纳入分析。斯威土地，Albigna大坝坐落在花岗岩上，进行了一个典型的水库周期的特定地点的模拟，来评估岩基上的水流体系的性质和评价滑动面相对于其他大坝岩界面的发展的潜力。目前大坝基础内的各种不同几何的岩石的滑动因素，是用德
2、国马克也评价模型与常规的分析方法的。裂纹扩展模式和相应扬压力和抗滑安全系数的估计沿坝岩接口与数字高程模型进行了比较得出，由目前在工程实践中使用的简化程序。结果发现，在岩石节理，估计裂缝发展后的基础隆起从目前所得到的设计准则过于保守以及导致的安全性过低，不符合观察到的行为因素。关键词：流体力学，岩石节理，流量，水库设计。简介：评估抗滑混凝土重力坝的安全要求的理解是，岩基和他们上面的结构是一个互动的系统，其行为是通过具体的材料和岩石基础的力学性能和液压控制。大约一个世纪前，Boozy大坝的失败提示工程师开始考虑由内部产生渗漏大坝坝基系统的扬压力的影响，并探讨如何尽量减少其影响。今天，随着现代计算资
3、源和更多的先例，确定沿断面孔隙压力分布，以及评估相关的压力和评估安全系数仍然是最具挑战性的。我们认为，观察和监测以及映射对大型水坝的行为和充分的仪表可以是我们更好地理解在混凝土重力坝基础上的缝张开度，裂纹扩展，和孔隙压力的发展。图.1流体力学行为：（一）机械;（二）液压。本文介绍了在过去20个来自Albigna大坝，瑞士，多年收集的水库运行周期行为的代表的监测数据，描述了一系列的数值分析结果及评估了其基础流体力学行为。比较了数值模拟和实际行为在实地的监测结果。在此基础上比较了一系列的结论得出了基本孔隙压力在节理岩体的影响可以考虑在其他工程项目，认为那里的岩石节理流体力学行为应予以考虑。这些项目
4、包括压力管道，危险废物处置，以及对流动行为的控制断面沿岩石地质遏制依赖的其他情形。流体力学的行为自然对先进设备，机械和个别岩石节理的水力特性的概要。一个对岩石联合流体力学行为的更详细的描述中可以在阿尔瓦雷斯（1997年）和阿尔瓦雷斯（1995年）和在实验室调查和数值模拟模型进行了乌鸦和Gale（1985），Gentier（1987年），江崎等人（1992），和其他人中发现。该水力行为的联合可以表示为非线性应用之间的有效正应力双曲线关系，,并联合， 在装卸，重大的联合封发生在低有效正应力的地方。该单位的压力关闭规模迅速下降，但是，随着应力水平增加。双曲线的定义是由初始切线刚度定义，并联合最大的渐
5、近结束，。这种关系也是非线性，迟滞的卸载条件，直到成为有效正应力为零（图1a）。和的价值观通过对实验数据的回归分析来估计的。对于自然和花岗岩裂隙，这些参数都是相互关联的下列限制范围之间的阿尔瓦雷斯等。 （1995年）：这里的单位是M pa/m， 的单位是m粗糙关节展览最大规模的联合最高和最低的封闭初始关节僵硬，关节光滑而有最低和最大的岩石的共同特点是液压行为之间的线性关系液压孔径，它控制流动规模，关闭和机械联合，用于水平应力。液压孔绘制相应的联合与关闭（图1b），以获取拦截线，起始水力孔径，边坡系数和耦合，而“刻画了联合流体力学行为，i. e，两者在液压机械孔径由于孔径的变化变化的关系，鉴于其
6、中是剩余的水力孔径对于给定的岩石节理，两者之间是有粗糙度及耦合系数的关系，因为f的分布和沿关节面流道曲折而定。对于理想的平行板，以在整个关节面单流道，f= 1.0.对于集中流道蜿蜒穿过关节面，f1.0。因此，用经典的立方定律表示通过岩石节理流率：其中Q是流量; 是水的单位重量; 是沿岩石节理头部下降;是水（11.005ps）的动力粘度; 是联合液压孔径而G是形状因子，由水流几何而定。直流地下G=W/L（其中W和L是宽度和长度，分别联合），为不同径向流，G =2/ln(re/),其中和re分别为内外圆柱面半径。裂隙岩体渗透性随深度变化另外，岩体等效渗透，公里，可以以同样的形式作为修改后的定律，或
7、在液压口径计算，同样的形式占关节间距，S:在裂隙岩体渗透性的变化，由于覆盖层和围应力，计算。 1 - 3。岩体的渗透性，K，理论的深度关系的结果高达1000米，采用当量。 5载于图2。孔的液压随覆盖减少强调在岩体渗透性，随深度的增加，从 cm/s到附近的水面在600厘米深度/秒 - 1000米的结果估计岩体渗透性得到假设f= 1.0，=和= 10，这是在实验室测试中取得的值与（阿尔瓦雷斯等al.1995）相似，巴西在这一测试中描述位置的花岗岩编队部分。覆盖层讲估计使用的是26.0 kN/m3单位重量。在这种情况下，它的假设是横向和纵向应力大致相同（土压力系数Ko = 1.0），这也被认为将在巴
8、西的测试位置的火成岩地层的代表，但其他价值在原位强调可以预计，如对高e.g., for Ko1.0，垂直节理将有较大的渗透率。在深露天矿在巴西花岗岩开采项目获得的场渗透率测量在图2中绘制与理论的关系比较。联合间距从钻孔岩心观察值都在数米范围内，从而产生了一个5米间距是常数的计算假设。阿霍的价值在300 -1000m范围被用来确定公里= f的理论关系（z）的，其中Z是深度，以实地测量和比较这两个钻孔测量值相对渗透率在100至200米深处的高，可能表明的一个区或剪切节理岩带更多的存在。所测岩石渗透率稳步下降，在深度的增加，然而，它们的值与对应的岩体渗透性的理论与模型估计趋势良好。典型液压孔径400
9、 -500m的和后关节僵硬= 10V的双曲线关系，与三菱商事和= 似乎同意这些结晶岩体观测场行为良好。图.2.裂隙岩体渗透性随深度的关系。虽然真正的流体力学节理岩体的行为是需要考虑具体的地点和地质因素，该方法提供了一个框架，但在设计阶段，其中岩石资料尚未提供大规模渗透。Hydromechanical analysis of flow behavior in concrete gravity dam foundationsAbstract: A key requirement in the evaluation of sliding stability of new and existing c
10、oncrete gravity dams is the prediction of the distribution of pore pressure and shear strength in foundation joints and discontinuities. This paper presents a methodology for evaluating the hydromechanical behavior of concrete gravity dams founded on jointed rock. The methodology consisted of creati
11、ng a database of observed dam behavior throughout typical cycles of reservoir filling and simulating this behavior with a distinct element method (DEM) numerical model. Once the model is validated, variations of key parameters including litho logy, in situ stress, joint geometry, and joint character
12、istics can be incorporated in the analysis. A site-specific simulation of a typical reservoir cycle was carried out for Albigna Dam, Switzer land, founded on granitic rock, to assess the nature of the flow regime in the rock foundations and to evaluate the potential for sliding surfaces other than t
13、he damrock interface to develop. The factor of safety against sliding of various rock wedges of differing geometry present within the dam foundations was also evaluated using the DEM model and conventional analytical procedures. Estimates of crack propagation patterns and corresponding uplift pressu
14、res and factors of safety against sliding along the damrock interface obtained with the DEM were also compared with those from simplified procedures currently used in engineering practice. It was found that in a jointed rock, foundation uplift estimates after crack development obtained from present
15、design guidelines can be too conservative and result in factors of safety that are too low and do not correspond to the observed behavior. Key words: Hydromechanical, jointed rock, flow, dam design.Introduction: Evaluating the safety of concrete gravity dams against sliding requires an understanding
16、 that rock foundations and the structure above them are an interactive system whose behavior is controlled by the mechanical and hydraulic properties of concrete materials and rock foundations. About a century ago, the failure of Boozy Dam prompted dam engineers to start considering the effect of up
17、lift pressures generated by seepage within the damfoundation system and to explore ways to minimize its effect. Today, with modern computational resources and much more precedent, it is still most challenging to determine the pore-pressure distribution along foundation discontinuities to assess pert
18、inent stresses and evaluate factors of safety. It is our opinion that observing and monitoring the behavior of large dams on well mapped and adequately instrumented foundations can bring important insights for a better understanding of factors controlling joint opening, crack propagation, and pore-p
19、ressure development in foundations of concrete gravity dams.Fig.1.Hydromechanical behavior of natural joints :(a) mechanical;(b)hydraulic.This paper presents behavior representative of cycles of reservoir operation in the last 20 years collected from monitored data of Albigna Dam, Switzerland, and a
20、lso describes the results of a series of numerical analyses carried out to assess the hydromechanical behavior of its foundations. Comparisons are made between results of numerical modeling and the actual behavior monitored in the field. Based on these comparisons, a series of conclusions are drawn
21、regarding basic pore-pressure buildup mechanisms in jointed rock masses with implications that may be considered in other engineering projects, where the hydromechanical behavior of jointed rock should be considered. Such projects include pressure tunnels, hazardous waste disposal, and other situati
22、ons dependent on geologic containment controlled by flow behavior along rock discontinuities.Hydromechanical behavior of natural joints A brief summary of the state-of-the-art of mechanical and hydraulic behavior of individual rock joints is presented here. A more detailed description of rock joint
23、Hydromechanical behavior can be found in Alvarez(1997)and Alvarez et al.(1995)and in investigations in laboratory and numerical model simulations carried out by Raven and Gale (1985), Gentier (1987),Esaki et al.(1992),and others.The mechanical behavior of the joint can be represented by a nonlinear
24、hyperbolic relationship between the applied effective normal stress,， and joint closure, During loading, significant joint closure takes place at low effective normal stresses. The magnitude of the closure per unit of stress decreases rapidly, however, as the stress level increases. The hyperbola is
25、 defined by the initial tangent stiffness, and the asymptote maximum joint closure, . This relationship is also nonlinear and hysteretic for the unloading condition until effective normal stresses become zero (Fig.1a).The values of and are estimated by regression analysis on experimental data. For n
26、atural and induced fractures in granite, these parameters are interrelated and range between the following limits Alvarez et al. (1995):Where is in M pa/m and is in mRough joints exhibit the largest joint maximum closure and the lowest initial joint stiffness, whereas smooth joints have the lowest a
27、nd the largest The hydraulic behavior of the rock joint is characterized by the linear relationship between hydraulic aperture, which controls the magnitude of flow, and mechanical joint closure, , which depends on stress levels. Hydraulic apertures are plotted versus their corresponding joint closu
28、re (Fig.1b)to obtain the line intercept, ,initial hydraulic aperture, and the coupled slope coefficient, ,which characterizes the hydromechanical behavior of the joint ,i. e., the relationship between changes in hydraulic aperture due to changes in mechanical aperture, given byWhere is the residual
29、hydraulic aperture.For a given rock joint, there is a relationship between roughness and the coupled coefficient, because f depends on the distribution and tortuosity of flow channels along the joint surface. For ideal parallel plates, with a single flow channel along the entire joint surface, f=1.0
30、.For concentrated flow channels meandering across the joint surface, f1.0.Hence, the classic cubic law expresses flow rate through a rock joint:Where Q is the flow rate; is the unit weight of the water; is the head drop along the rock joint; is the dynamic viscosity of the water(1.005Pas ); Is the j
31、oint hydraulic aperture; and G is the shape factor, which depends on the geometry of flow. For straight flow, G=W/L (where W and L are the width and length, respectively, of the joint); and for divergent radial flow, G=2/ln (re/), where and re are the borehole and external cylindrical surface radius
32、es, respectively.Jointed rock mass permeability change with depthAlternatively, the rock mass equivalent permeability, km, can be expressed in the same form as the modified cubic law, or in terms of hydraulic aperture, to account for spacing of the joints, S:Changes in jointed rock mass permeability
33、 due to overburden and confining stresses were calculated using eqs. 1 3.The results of a theoretical relationship of rock mass permeability, k, for depths up to 1000 m, using eq. 5 are presented in Fig.2.The reduction of hydraulic apertures with increasing overburden stresses results in a rock mass
34、 permeability that decreases with an increase in depth from cm/s near the surface to cm/s at depths of 600 1000 m.The rock mass permeability estimates were obtained assuming f=1.0, = and =10, which are representative of the values obtained in laboratory tests carried out in granitic formations(Alvar
35、ez et al.1995)similar to those of the Brazilian test location described in this section. Overburden stresses were estimated using a unit weight of 26.0 kN/m3.In this case it was assumed that horizontal and vertical stresses are about the same (coefficient of earth pressure at rest Ko=1.0), which are
36、 also considered to be representative of the igneous formations at the Brazil test location, but other values of in situ stresses could be estimated, e.g., for Ko1.0, vertical joints would have larger permeabilities.Field permeability measurements obtained in Packer tests at a deep open-pit mining p
37、roject in granitic rock in Brazil are also plotted in Fig.2 for comparison with the theoretical relationship. Values of joint spacing observed from borehole cores are in the range of a few meters, and thus a constant spacing of 5m was assumed in the computations. Values of aho in the range of 300100
38、0m were used to determine the theoretical relationships of km=f (z), where z is the depth, and compare with field measurements.Measured permeability values in the two boreholes are relatively high at depths between 100 and 200m, probably denoting the presence of a sheared zone or a zone of more join
39、ted rock. The measured rock permeabilities decrease steadily with an increase in depth, however, and their values correspond well with the theoretical trend of rock mass permeability estimated with the model. Typical hydraulic apertures of 400500m and joint stiffness following a hyperbolic relations
40、hip with =10V mc and = seem to agree well with observed field behavior for these crystalline rock masses.Fig.2.Theoretical jointed rock mass permeability relationship with depth.Although real Hydromechanical behavior of jointed rock masses is site specific and depends on geologic factors, which need to be taken into account, the proposed approach provides a framework to estimate rock mass permeability during design stages where information is not yet available.
