Numerical parametric study on the influence of location and inclination of large-scale asperities on the shear strength of concreterock interfaces of small buttress dams

When assessing the sliding stability of a concrete dam,the influence of large-scale asperities in the sliding plane is often ignored due to limitations of the analytical rigid body assessment methods provided by current dam assessment guidelines.However,these asperities can potentially improve the load capacity of a concrete dam in terms of sliding stability.Although their influence in a sliding plane has been thoroughly studied for direct shear,their influence under eccentric loading,as in the case of dams,is unknown.This paper presents the results of a parametric study that used finite element analysis(FEA)to investigate the influence of large-scale asperities on the load capacity of small buttress dams.By varying the inclination and location of an asperity located in the concrete-rock interface along with the strength of the rock foundation material,transitions between different failure modes and correlations between the load capacity and the varied parameters were observed.The results indicated that the inclination of the asperity had a significant impact on the failure mode.When the inclinationwas 30and greater,interlocking occurred between the dam and foundation and the governing failure modes were either rupture of the dam body or asperity.When the asperity inclination was significant enough to provide interlocking,the load capacity of the dam was impacted by the strength of the rock in the foundation through influencing the load capacity of the asperity.The location of the asperity along the concrete-rock interface did not affect the failure mode,except for when the asperity was located at the toe of the dam,but had an influence on the load capacity when the failure occurred by rupture of the buttress or by sliding.By accounting for a single large-scale asperity in the concrete-rock interface of the analysed dam,a horizontal load capacity increase of 30%e160%was obtained,depending on the inclination and location of the asperity and the strength of the foundation material.

Influence of construction interfaces on dynamic characteristics of roller compacted concrete dams

To study the influence of construction interfaces on dynamic characteristics of roller compacted concrete dams(RCCDs),mechanical properties of construction interfaces are firstly analyzed. Then, the viscous-spring artificial boundary(VSAB) is adopted to simulate the radiation damping of their infinite foundations, and based on the Marc software, a simplified seismic motion input method is presented by the equivalent nodal loads. Finally, based on the practical engineering of a RCC gravity dam, effects of radiation damping and construction interfaces on the dynamic characteristics of dams are investigated in detail. Analysis results show that dynamic response of the RCC gravity dam significantly reduces about 25% when the radiation damping of infinite foundation is considered. Hot interfaces and the normal cold interfaces have little influence on the dynamic response of the RCC gravity dam.However, nonlinear fracture along the cold interfaces at the dam heel will occur under the designed earthquake if the cold interfaces are combined poorly. Therefore, to avoid the fractures along the construction interfaces under the potential super earthquakes,combination quality of the RCC layers should be significantly ensured.

Influence of location of large-scale asperity on shear strength of concrete-rock interface under eccentric load

The location and geometry of large-scale asperity present at the foundation of concrete gravity dams and buttress dams affect the shear resistance of the concrete-rock interface.However,the parameters describing the frictional resistance of the interface usually do not account for these asperities.This could result in an underestimate of the peak shear stre ngth,which leads to significantly conservative design for new dams or unnecessary stability enhancing measures for existing ones.The aim of this work was to investigate the effect of the location of first-order asperity on the peak shear strength of a concrete-rock interface under eccentric load and the model discrepancy associated with the commonly used rigid body methods for calculating the factor of safety(FS)against sliding.For this,a series of direct and eccentric shear tests under constant normal load(CNL)was carried out on concrete-rock samples.The peak shear strengths measured in the tests were compared in terms of asperity location and with the predicted values from analytical rigid body methods.The results showed that the large-scale asperity under eccentric load significantly affected the peak shear strength.Furthermore,unlike the conventional assumption of sliding or shear failure of an asperity in direct shear,under the effect of eccentric shear load,a tensile failure in the rock or in the concrete could occur,resulting in a lower shear strength compared with that of direct shear tests.These results could have important implications for assessment of the FS against sliding failure in the concrete-rock interface.

SHEAR BEAM MODEL FOR INTERFACE FAILURE UNDER ANTIPLANE SHEAR(Ⅱ)—INSTABILITY

Based on the (Ⅰ) of the present work, the behavior of shear beam model at crack initiation stage and at instable propagation stage was studied. The prime results include: 1) discriminant equation which clarifies the mode of instability, snap_back or snap_through, was established; 2) analytical solution was given out for the double shear beam and the load_displacement diagram for monotonic loading was presented for a full process; and 3) the problem of the energy release induced by instability was discussed.

混凝土面板与堆石料接触面模型及其应用

混凝土面板是面板坝防渗安全的生命线,其应力变形特性主要取决于坝体堆石料的应力变形特性及面板与堆石料之间的接触特性。在面板坝的应力变形计算中,通常采用接触面模型模拟面板与堆石料之间的荷载传递机制,因此接触面模型是否合理,直接影响到面板应力变形计算结果的可靠性。分析了传统双曲线接触面模型存在的不足,提出了一个考虑双向剪切耦合效应,且符合强度准则和标架无差异原则的接触面本构模型,该模型参数少,数值实现方便,可以定性地模拟出实际面板坝工程中出现的混凝土挤压破坏区域。

岩性特征对混凝土坝体-基岩原位直剪试验力学特性影响分析

以广西巴马县坡月水库工程为研究对象,采用平推法开展不同法向应力(0.4、0.8、1.2、1.6、2.0 MPa)下混凝土坝体-基岩原位直剪试验,探究岩性特征对混凝土坝体-基岩力学特性的影响。结果表明,不同法向应力下试样的剪切应力-位移曲线均表现出试样内部初始缺陷受力微裂纹扩展的稳定扩展阶段、微裂缝逐渐扩展形成宏观裂缝的不稳定扩展阶段及混凝土与岩体分离的开裂阶段3个阶段;法向应力、岩体岩性对试样的断裂能均有显著影响。不同法向应力下,试样断裂能的范围为16.65~177.28 N/m,断裂能随着法向应力或灰色砂岩含量增大而增大,随泥质粉砂岩含量或风化程度增大而减小;混凝土坝体-基岩的抗剪强度随基岩中灰色砂岩含量增大而增大,随泥质粉砂岩含量或基岩风化程度增大而减小。研究结果可为类似研究提供试验参考。

碾压混凝土分层碾压离散元模拟及施工成层细观机理研究

碾压混凝土(RCC)坝特有的大仓面分层碾压工艺带来的层间结合质量问题,是影响其安全运行的重要因素。从施工控制的角度分析RCC施工成层机理,进而合理调整碾压参数,可有效保证大坝的施工质量。首先,通过RCC分层碾压试验,分析了碾压过程中含层面RCC的沉降规律、压实度及抗剪强度,为后续离散元模拟提供了基础;然后,建立并验证了RCC双层碾压与抗剪试验离散元模型;最后,给出了表征层间结合质量的骨料嵌入值指标,进而从孔隙率、层间骨料嵌入、层面应力分布、层间抗剪强度等方面分析了RCC施工成层的细观机理。结果表明:碾压参数对RCC的嵌入值、压实特性、层面应力和层间结合质量均有一定影响;嵌入值是影响层间抗剪强度的重要因素,二者呈线性相关,嵌入值每增加1 mm,模拟抗剪强度约增加1.583 MPa;激振力的增加以及行进速度、碾压厚度的减小均有利于提高层间结合质量,且碾压厚度是影响层间结合质量的最敏感参数。研究结果可为施工中RCC层间结合质量控制标准的制定提供理论依据。

高心墙堆石坝接触黏土与岸坡接触特性的离心模型研究

为了提高砾石土心墙与岸坡之间的变形协调性,高心墙堆石坝通常在心墙与岸坡之间设置高塑性黏土,统称为接触黏土,接触黏土的变形特性对高心墙的应力变形性状和心墙的安全性有重大影响。开展了4组接触黏土与岸坡接触面的局部离心模型试验,通过调整施加的上覆荷载大小来探讨原型坝体不同高程处的接触面剪切位移发展规律。基于理论分析,提出了一个计算接触黏土与岸坡接触面离心模型试验中边界效应影响范围的解析表达式,以确定接触面离心试验中试验模型的相似比尺。试验结果表明:在原型坝体的不同高程处,接触黏土与岸坡之间均会产生一定的剪切位移,但不会出现分离现象;接触黏土和砾质心墙土变形主要以竖向下沉为主,并且接触黏土与砾质心墙土之间的沉降等值线变化较为连续,未出现陡变情况,这说明接触黏土起到了很好地协调变形的作用,因此在进行高心墙堆石坝的结构安全性分析过程中应考虑接触黏土与岸坡的接触效应。

考虑土层界面效应的尾矿坝渗流稳定性分析

为研究渗流作用下尾矿土土层界面处力学行为对尾矿坝稳定性的影响,以河北省秦皇岛市某尾矿坝为研究对象,通过开展渗流直剪试验及FLAC3D数值建模,得出考虑土层界面效应的尾矿坝安全系数。研究结果表明:渗流状态下,随着水头高度上升,尾矿土抗剪强度下降;土层界面黏聚力呈下降趋势,内摩擦角变化幅度平缓;相同水头高度下,土层界面抗剪强度介于2种土层之间,尾细砂和尾粉质黏土界面黏聚力在多因素耦合作用下进一步降低;与Bishop法相比,考虑土层界面效应的尾矿坝在正常水位和洪水位下得到的安全系数分别下降29.0%,12.1%。研究结果可为多层尾矿土堆积而成的尾矿坝稳定性分析提供技术指导。

基于分区刚体-界面元方法的重力坝深层抗滑稳定分析

将坝体及坝基系统分界为可变形滑动界面、刚性滑动块体和刚性基床三部分,建立了基于滑动界面应力变形协调、分区刚体静力平衡的分区刚体—界面元分析方法,其未知量仅包含分区刚体变形及滑动界面上的接触力。结合基于极限平衡方程的迭代解法,可快速准确求解重力坝深层抗滑稳定安全系数。对局部滑面未知情形,可通过搜索求解最危险滑动面。该方法可同时满足上限极限分析的运动许可条件和下限极限分析的应力许可条件,克服了现有规范分析方法通过假定来满足下限极限分析条件的缺点。最后通过某重力坝深层抗滑稳定分析算例验证了该方法的准确性。

基岩地基上心墙堆石坝基座体型对基座结构的影响

为研究合适的基座体型方案,以重庆巫山庙堂水库为例,采用数值模拟的方法分析了基座上游侧角、下游侧角和宽度对心墙与基座连接结构的影响,并提出了基座体型优化方案。结果表明,对于基座应力,影响程度由大到小依次为下游侧角>基座宽度>上游侧角;对于心墙竖向位移,影响程度由大到小依次为上游侧角>下游侧角>基座宽度;优化方案有利于连接结构的应力安全和变形控制,各项指标均有不同程度下降,尤其是拉应力最大值较初始方案降低了71.1%,此优化方案可供同类工程参考。

中性载体DAM推动Ca^(2+)离子在水/硝基苯界面的转移

本文用卷积(半微分和半积分)循环伏安法研究了中性载体DAM(二安替比林甲烷)络合推动Ca^(2+)离子在水/硝基苯界面的转移过程,探讨了有关转移过程的机理,测定了相应的扩散系数和配合物的络合稳定常数。

基覆型边坡在地震作用下的动力响应与破坏模式研究

为探究基覆型边坡的动力响应特性、破坏模式以及基覆界面倾角的影响,以西南山区某基覆型边坡为原型,利用FLAC 3D软件模拟基覆型边坡在地震作用下的响应过程,分析不同地震作用下边坡的动力响应及破坏模式,在此基础上进一步改变基覆界面的上、下界面倾角,分析基覆界面倾角对动力响应特性及破坏模式的影响。结果表明:基覆型边坡的PGA放大系数沿坡表方向随高程增大先增大后减小,坡体内部的动力响应呈现明显的高程效应和趋表效应规律,且随着输入PGA增大而减小;在地震作用下边坡沿上覆土体内部形成的潜在滑动面发生剪切破坏,加载PGA=0.6g的EL波和Kobe波时,坡表的最大位移分别为228.1 cm和446.2 cm,加载Kobe波时模型的加速度响应与位移响应均要比加载EL波时更激烈;基覆界面倾角对动力响应特性及破坏模式有着显著影响,当上界面倾角增大时,坡表处动力响应程度有所增强,而下界面倾角增大时,坡表处动力响应程度有一定减弱,边坡破坏模式随着上下界面倾角差值增大由整体滑移破坏逐渐转变为局部滑移破坏,上下界面倾角差值越大,边坡稳定性越高。

STUDY ON COUPLING MODEL OF (SEEPAGE-FIELD) AND STRESS-FIELD FOR ROLLED CONTROL CONCRETE DAM

Based on the construction interfaces in rolled control concrete dam(RCCD), the methods were proposed to calculate the influence thickness of construction interfaces and the corresponding physical mechanics parameters. The principle on establishing the coupling model of seepage_field and stress_field for RCCD was presented. A 3_D Finite Element Method(FEM) program was developed. Study shows that such parameters as the thickness of construction interfaces,the elastic ratio and the (Poisson's) ratio obtained by tests and theoretical analysis are more reasonable, the coupling model of seepage_field and stress_field for RCCD may indicate the coupling effect between the two fields scientifically, and the developed 3_D FEM program can reflect the effect of the construction interfaces more adequately. According to the study, many scientific opinions are given both to analyze the influence of the construction interfaces to the (dam's) characteristic, and to reveal the interaction between the stress_field and the seepage_field.

Crucial technologies in the design of Xiluodu Super High Arch Dam

Some super high arch dams （ SHADs）, like Xiluodu Arch Dam, after their heights reaching the magnitude of 300 m, confront lots of technical challenges in design and construction. Several crucial technologies of 6 SHADs will be reviewed and discussed in this and consecutive papers, including Xiluodu, Jinping I in China, Baktiary in Iran, ete. , on the topics of the research method, criterion for evaluation and engineering application of dam safety analysis and evaluation, reasonable dam base interface, dam shape optimization, comprehensive treatments of complex foundation, anti-seismic engineering, dam construction material, concrete placement and temperature control, instrumentation and monitoring of dam operation, etc. This paper will mainly focus on the overall safety of SHADs, reasonable dam base interface analysis and evaluation and their engineering application.

Reservoir water effects on earthquake performance evaluation of Torul Concrete-Faced Rockfill Dam

This study presents earthquake performance analysis of the Torul Concrete-Faced Rockfill （CFR） Dam with two-dimensional dam-soil and dam-soil-reservoir finite element models. The Lagrangian approach was used with fluid elements to model impounded water. The interface elements were used to simulate the slippage between the concrete face slab and the rockfill. The horizontal component of the 1992 Erzincan earthquake, with a peak ground acceleration of 0.515g, was considered in time-history analysis. The Drucker-Prager model was preferred in nonlinear analysis of the concrete slab, rockfill and foundation soil. The maximum principal stresses and the maximum displacements in two opposite directions were compared by the height of the concrete slab according to linear time-history analysis to reveal the effect of reservoir water. The changes of critical displacements and principal stresses with time are also shown in this paper. According to linear and nonlinear time-history analysis, the effect of the reservoir water on the earthquake performance of the Torul CFR Dam was investigated and the possible damage situation was examined. The results show that the hydrodynamic pressure of reservoir water leads to an increase in the maximum displacements and principal stresses of the dam and reduces the earthquake performance of the dam. Although the linear time-history analysis demonstrates that the earthquake causes a momentous damage to the concrete slab of the Torul CFR Dam, the nonlinear time-history analysis shows that no evident damage occurs in either reservoir case.

砌石拱坝坝后加固结合面力学特性研究

针对某砌石拱坝上游坝面中下部存在大面积拉应力超标现象,基于该砌石坝加固工程中的两种坝后局部加固方案,采用接触单元模拟新老坝体结合面的连接特性,分析了规范中五种工况下结合面的受力特性及变形特性。研究表明,加固后,两种方案结合面的张开特性和受力特性在整体上相似,其中肋墩方案中边墩存在不对称张开及受力现象;高水位工况下加固结构主要在上部承力,下部应力较小并可能张开;低水位工况下结合面应力水平较低,加固结构基本不承受荷载;加固后温降工况下,砌石坝体以加固结构上部为支点进行转动变形,在低水位条件下结合面会出现大面积张开;温升工况下,低水位时坝体向上游变位,导致结合面间隙分布在水平方向上呈对称光滑的抛物线形分布。

新疆大石峡水利枢纽混凝土碱-硅酸反应抑制研究

新疆大石峡水利枢纽混凝土选用的骨料存在碱活性,可能导致工程产生碱骨料破坏问题,本文研究了当地粉煤灰和矿粉对碱-硅酸反应(ASR)的影响,并采用XRD和SEM-EDS测试分析了水化产物和界面过渡区的形态。结果表明:粉煤灰掺量≥20%(质量分数)或矿粉掺量≥40%(质量分数)都能显著抑制碱-硅酸反应;在纯水泥样品界面过渡区可以观察到无定形相,在含有粉煤灰或矿粉的样品中未观察到无定形相,这意味着碱-硅酸反应发生在纯水泥样品中;添加粉煤灰或矿粉降低了界面过渡区物相的Ca/Si摩尔比,抑制了碱-硅酸反应。

适用二维切割体网格的THINC/QQ算法扩展研究

为了扩展基于二次曲面和高斯积分的双曲正切函数界面捕捉算法THINC/QQ的单元适用类型,本文将适用切割表面式非结构单元的THINC/QQ-SF算法原理应用于二维切割体网格。在进行界面重构时,先将带悬挂点的复杂单元虚拟简化为常规的四边形或三角形单元;而在计算流体体积流量时再考虑各单元的真实拓扑结构。针对典型流体体积对流和溃坝算例,本文设计了一系列二维切割体网格,并验证了所用扩展方法的可行性。研究表明:与高分辨率界面捕捉HRIC算法相比,二维THINC/QQ-SF算法的计算精度更高,能有效抑制多相流界面发生大变形时的流体体积数值耗散。

黄土高原坝地层状土壤剖面孔隙分布特征

以明确自然界中广泛存在的层状结构土壤孔隙分布特征为目的。选黄土高原典型坝地土壤剖面为研究对象,利用CT扫描技术分析耕作层、犁底层、心土层土壤三维结构,探讨层状土孔隙参数随土层结构变化及界面层孔隙特征。结果表明:(1)瘦长型孔隙(孔隙形状系数≤0.2)是各土层土壤孔隙的主要形态。各土层的孔隙参数具有明显差异。耕作层的总孔隙度最高,大孔隙(≥1 mm)分布较多;犁底层孔隙总数量最高,含有较多孤立的小孔隙(≥0.1~0.5 mm)和微小孔隙(≥0.06~0.1 mm),但连通性最差;心土层孔隙参数多数介于耕作层与犁底层之间。(2)坝地剖面层状土壤具有一定厚度的界面层,各孔隙参数均在土层界面位置存在突变点,具有明显分界现象。孔隙特征参数均呈过渡位置渐变、界面位置突变的规律。在界面层,犁底层土壤总孔隙度和连通性与耕作层相比,分别显著下降41.6%,69.8%;心土层在界面层的土壤总孔隙度和连通性与犁底层相比,分别显著增高30.4%,52.3%。(3)孔隙孔径大小与孔隙形态密切相关,大孔隙形态多呈瘦长型,而微小孔隙形状则多呈规则型。(4)土层越深,连通性与总孔隙度、孔隙总数量、孔径分布及孔隙形状的相关性越强。界面层出现改变相邻土层的土壤孔隙质量。人为作用改变耕作层和犁底层的土壤孔隙特征,主要表现在对耕作层翻耕扰动,使得犁底层被动承压,心土层由于受到干扰较小,土壤整体呈现自然状态。研究结果可丰富非均质土壤结构的认识,为从微观层面理解层状土壤孔隙特征及其对土壤水气热传输的可能影响提供科学依据。