Investigations of Blocks in Foundations and Abutments of Concrete Dams

This paper discusses investigations of potential block hazards in ten existing American concrete arch and buttress dams. Block theory was used to identify and describe all removable blocks, based on the study of construction photos and logs, and new mapping, often done on steep valley sides. Water forces on the block faces were calculated for stability analysis using a map of the phreatic surface or an estimated flow regime along the subsurface boundary of the block. In some cases, block hazards were shown to be nonexistent because there were no real intersections of significant discontinuities of sufficient extent to cut out a block, or there was no kinematically possible mode of failure with the prevailing force system. Several dams had blocks that were removable but judged sufficiently safe with the applicable friction angles. Three structures demanded attention: A spillway structure was armored to prevent further erosion that diminished the resistance of a key block in its foundation; a dome and buttress structure received a reinforced concrete buttress to add passive support against a block critically located in the abutment; and a drainage adit and deep drains drilled from the surface and from the adit were constructed to raise the safety factor of a large block beneath the abutment of an arch dam.     

Analysis of Gravity Dam on a Complicated Rock Foundation Using an Adaptive Block Element Method

In this Technical Note, the adaptive block element method of rock masses is formulated, in which the elastoplastic characteristics of both rock blocks and discontinuities are taken into account. The concept of an overlay element is illustrated first; then the displacement fields of rock blocks are expressed as functions of so-called general degree of freedoms using the shape functions of the hierarchical finite element method; the governing equations of the rock block system are deduced on the basis of the virtual work principle; and the p-version adaptive algorithm based on the energy norm error estimation of each block element is proposed. The method is applied to the deformation and stability study of a gravity dam, and the parallel laboratory physical test is used to check the validity and ability of the method.     

Effects of Environmental Action on Thermal Stress Analysis of Karaj Concrete Arch Dam

A three-dimensional finite element analysis is carried out to determine the thermal stresses of a concrete arch dam. Appropriate heat transfer boundary conditions in the dam body are used for air and reservoir temperature as well as solar radiation variations. A finite element model is used to determine annual variation of temperature and thermal stress in the body of Karaj arch dam in Iran as a case study. The rate of convergence of the numerical solution is examined. The temperatures predicted by the model are satisfactorily compared with the instrumentation records at Karaj Dam. Results of the finite element analysis show that probable cracks occur in a very narrow region of the downstream face. Thermal loads have the most significant effects for causing downstream cracks in comparison with self-weight and hydrostatic loads. The cracked areas of the downstream face conform to the regions that have the highest temperature in the downstream face. This can be associated with the solar radiation, which shows that two-dimensional analysis of an arch dam cannot yield accurate results and three-dimensional analysis is necessary.     

Analysis and Stabilization of Failing Earth Dam Founded on Claystone

A minor slide occurred in the downstream face of an earth dam. Initial observations and limited optical survey data suggested the failure was shallow, but continued measurements from inclinometers in the embankment and survey monuments, as well as visual observations of deformation patterns, revealed a more extensive and severe failure located in the underlying weathered claystone foundation. Deep inclinometers were installed, and inclinometer readings indicated that approximately 1 cm per day of movement was occurring along a very discrete zone in the foundation. Horizontal survey measurements confirmed this displacement, and an emergency soil buttress was constructed at the toe of the dam to reduce movement. The residual strength along the slip surface was estimated by back analysis of the sliding block, assuming a safety factor of 1.0 and incorporating end restraint to assess the lower-bound residual strength. The remediation scheme included installation of an internal drainage system and a soil buttress. Postconstruction monitoring shows that the dam is continuing to move at a decreasing rate and is apparently approaching static equilibrium.     

Effects of Space Distribution of Excitation on Seismic Response of Arch Dams

The effects of the ground motion spatial variation and of the canyon geometry on the dynamic response of arch dams during the event of an earthquake is studied in this paper. The seismic response of a dam subject to time harmonic longitudinal, shear, and Rayleigh waves impinging the dam site from different directions is analyzed. Several canyon and reservoir geometries are considered. A three-dimensional boundary element model which allows for the rigorous representation of the dynamic interaction between the dam, the foundation rock, and the water is used. The foundation rock is modeled as a uniform viscoelastic boundless domain where the incident traveling wave field is defined by its analytical expression, which may include any spatial variation. The obtained results show the importance of three-dimensional effects which are many times neglected.     

Optimum Design of Bridge Abutments under Seismic Conditions: Reliability-Based Approach

The paper focuses on the reliability-based design optimization of gravity wall bridge abutments when subjected to active condition during earthquakes. An analytical study considering the effect of uncertainties in the seismic analysis of bridge abutments is presented. Planar failure surface has been considered in conjunction with the pseudostatic limit equilibrium method for the calculation of the seismic active earth pressure. Analysis is conducted to evaluate the external stability of bridge abutments when subjected to earthquake loads. Reliability analysis is used to estimate the probability of failure in three modes of failure viz. sliding failure of the wall on its base, overturning failure about its toe (or eccentricity failure of the resultant force) and bearing failure of foundation soil below the base of wall. The properties of backfill and foundation soil below the base of abutment are treated as random variables. In addition, the uncertainties associated with characteristics of earthquake ground motions such as horizontal seismic acceleration and shear wave velocity propagating through backfill soil are considered. The optimum proportions of the abutment needed to maintain the stability are obtained against three modes of failure by targeting various component and system reliability indices. Studies have also been made to study the influence of various parameters on the seismic stability.     

Allowable Bearing Pressures of Bridge Sills on GRS Abutments with Flexible Facing

Compared to geosynthetic-reinforced soil (GRS) retaining walls, GRS abutment walls are generally subjected to much greater intensity surface loads that are fairly close to the wall face. A major issue with the design of GRS abutments is the allowable bearing pressure of the bridge sill on the abutments. The allowable bearing pressure of a bridge sill over reinforced soil retaining walls has been limited to 200?kPa in the current NHI and Demo 82 design guidelines. A study was undertaken to investigate the allowable bearing pressures of bridge sills over GRS abutments with flexible facing. The study was conducted by the finite element method of analysis. The capability of the finite element computer code for analyzing the performance of GRS bridge abutments with modular block facing has been evaluated extensively prior to this study. A series of finite element analyses were carried out to examine the effect of sill type, sill width, soil stiffness/strength, reinforcement spacing, and foundation stiffness on the load-carrying capacity of GRS abutment sills. Based on the results of the analytical study, allowable bearing pressures of GRS abutments were determined based on two performance criteria: A limiting displacement criterion and a limiting shear strain criterion, as well as the writers’ experiences with GRS walls and abutments. In addition, a recommended design procedure for determining the allowable bearing pressure is provided.     

Block Element Method for the Seismic Stability of Rock Slopes

The seismic stability analysis of rock slope is implemented using a block element method (BEM) in this paper. Based on the formulations of the matrices of stiffness, mass, and damping, the dynamic governing equation for the rock block system is established. The Wilson method is used to solve the dynamic governing equation, and the viscoelastic artificial boundary condition is introduced to treat the unbound domain problem. The proposed method is applied to the seismic stability analysis of the intake slope in a hydropower project, from which the dynamic safety factors of key block element combinations during earthquake and their dynamic amplification factors of acceleration are evaluated.     

Hydrodynamic Modeling and Mathematical Simulation of Flow Field and Temperature Profile for Molten Stainless Steel in an Asymmetrical T‐Type Single‐Strand Continuous Casting Tundish with Arch or Round Hole(s) at Dam Bottom

In order to obtain the optimal structural parameters of the dug arch or round hole(s) at dam bottom in an 18–20 tons asymmetrical T‐type single‐strand continuous casting tundish, the flow field profiles and temperature profiles of molten stainless steel in the tundish with arch or round hole(s) at dam bottom have been investigated using hydrodynamic modeling coupled with mathematical simulation. The optimal structural parameters of arch hole(s) at dam bottom can be obtained from hydrodynamic modeling as that two arch holes with 30 mm as height and 50 mm as radius are symmetrically dug at dam bottom with the distance between arch hole center and dam center as 205 mm; or the optimal structural parameters of round hole(s) can be recommended as that one round hole with 70 mm as diameter is dug at left of the dam bottom with the distance between hole center and dam center as 205 mm. The results of mathematical simulation suggest that digging arch or round hole(s) at dam bottom with above‐mentioned structural parameters cannot obviously induce negative effects on streamlines and velocity vector profiles of molten stainless steel in the tundish by short circuit flow via arch or round hole(s) at dam bottom. The calculated temperature drop of molten stainless steel between the submerged ladle shroud and submerged entry nozzle in the tundish with arch or round hole(s) at dam bottom is about 3.0 K, the maximum temperature drop of molten stainless steel in the tundish is about 6.0 K.     

谈片石混凝土桥基、台身和翼墙施工监理质量控制

对石拱桥桥基、台身、翼墙浇捣及其支模、拆模等施工监理工作内容进行了详细论述，力求提高监理工作实效和所监理工程质量。     

尾矿坝抗震设计研究进展

国内外震害表明,尾矿坝浸润线较高时,大部分坝体处于饱和状态,地震作用下易发生液化破坏。因此,研究实用的尾矿坝简化抗震设计方法是非常有意义的工作。围绕尾矿坝地震液化评价和稳定分析简化方法两方面内容,较全面地概述了国内外尾矿坝抗震设计的研究成果和最新进展,并对尾矿坝抗震设计存在的问题进行了探讨,为尾矿坝抗震设计的发展指明方向。     

Evaluating Safety of Concrete Gravity Dam on Weak Rock: Scott Dam

Scott Dam is owned and operated by Pacific Gas & Electric Co. (PG&E) as part of the Potter Valley Project. Although it is an unimpressive concrete gravity dam [233 m (765 ft) long with maximum water surface 33.4 m (110 ft) above tailwater], the dam has unusually complex and weak foundation rocks; this condition caused design changes during construction, numerous subsequent special investigations, and several corrections and additions. A main stumbling block to clarification of the dam safety issue for Scott Dam has always been difficulty in characterizing the foundation material. This paper discusses an approach to this problem as well as how the safety of the dam was subsequently confirmed, following a comprehensive program of research, investigations, and analysis from 1991 to 1997.     

Response of a Buckled Beam Constrained by a Tensionless Elastic Foundation

In this paper, we study the deformation and stability of a pinned buckled beam under a point force. The buckled beam is constrained by a tensionless elastic foundation, which is flat before deformation. From static analysis, we found a total of five different deformation patterns: (1)?noncontact, (2)?full contact, (3)?one-sided contact, (4)?isolated contact in the middle, and (5)?two-sided contact. For a specified set of parameters, there may coexist multiple equilibria. To predict the response of the buckled beam foundation system as the point force moves from one end to the other, we have to determine the stability of these equilibrium configurations. To achieve this, a vibration method is adopted to calculate the natural frequencies of the system, taking into account the slight variation of the contact range between the buckled beam and the tensionless foundation during vibration. It is concluded that among all five deformation patterns, Deformations 1, 2, 3, and 4 may become stable for certain loading parameters. In the extreme case in which the foundation is rigid, on the other hand, only Deformations 1 and 3 are stable.     

强度折减法在某尾矿坝边坡稳定性分析中的应用

利用有限元强度折减法对某尾矿坝的边坡稳定性进行了计算,同时利用简化毕肖普法和瑞典圆弧法对该坝体的边坡稳定性进行了计算。通过对比分析,有限元强度折减法的计算值与考虑条块间作用力的简化毕肖普法的计算值基本一致,证明基于有限元理论的强度折减法的计算结果是合理的,其安全系数值可以作为该尾矿工程定量判断的依据。     

Fully Coupled Analysis of Failure and Remediation of Lower San Fernando Dam

The extent of flow deformation in an embankment dam is determined by the driving forces and the residual strength of the soil, as well as by the kinematic constraints. The soil conditions of berm and buttress, as well as of foundation, are also critical factors affecting seismic performance of an embankment dam. A careful examination of these factors is necessary when proposing remedial measures to a seismically deficient dam. This paper presents a set of fully coupled finite element analyses of the response of the well-known lower San Fernando Dam during the 1971 earthquake. A critical state model incorporating the concept of state-dependent dilatancy was employed to describe soil behavior over the full range of loading conditions encountered. The results show clearly that a flow slide occurred on the upstream side, and indicate that a downstream flow slide would occur, too, if the downstream berm had not been constructed before the event. The analyses show also that the addition of an upstream berm could effectively prevent the upstream flow slide.     

Simulating the Behavior of GRS Bridge Abutments

Geosynthetic-reinforced soil (GRS) bridge-supporting abutments are similar in principle to GRS retaining walls, except that GRS abutments are typically subjected to a much higher area load, and that the loads are close to the wall face. The GRS abutment technology is relatively new, but it has great potential, and it has been gaining some popularity in recent years. This paper describes the finite element analyses of two full-scale loading tests of GRS bridge abutments referred to as the “National Cooperative Highway Research Program (NCHRP) experiment.” The analysis was carried out using the computer program Dyna3d, developed at the Lawrence Livermore National Laboratory. The finite element analysis of the NCHRP experiment will help with the understanding of the complex behavior of GRS structures in general, and the behavior of GRS bridge abutments with modular block facing in particular. The analysis of the two full-scale loading tests allows the loading conditions that are of greatest concern in the design of the bridge abutments to be examined rationally. The analysis shows that the performance of a GRS abutment, resulting from the complex interaction among the various components, while subject to a service load or a limiting failure load can be simulated in a reasonably accurate manner. In addition, a parametric study was conducted to investigate the performance of the modular block facing GRS bridge abutments subjected to live and dead loads from a bridge superstructure. This study investigated the performance of the GRS bridge abutments as they are affected by backfill properties, reinforcement stiffness properties, and reinforcement vertical spacing.     

Composite Element Analysis of Gravity Dam on a Complicated Rock Foundation

This paper presents the formulation and application of a composite element method, which is intended for numerical modeling of discontinuous rock masses. This method allows analysis of fractured rock masses using regular meshes that do not need to rigorously respect the orientations and positions of discontinuities. It can be incorporated in conventional finite-element programs. The performance of this method are illustrated through its use for the analysis of the mechanical behavior of the Baozhusi gravity dam which is constructed on a complex rock foundation.     

Load Rating of Masonry Arch Bridges: Refinements

In a paper previously published by the first writer, a procedure for load-rating masonry arch bridges was introduced. The procedure uses the Load Factor Method of the 1994 American Association of State Highway and Transportation Officials Manual for the Condition Evaluation of Bridges, applied to a frame analysis model of a masonry arch spanning from abutment to abutment. The procedure is based on the assumption of the arch barrel having no tensile strength. The objective of this technical note is to complement the initial procedure by enabling the assessing engineer to exercise discretion in deciding whether or not a small value of tensile strength should be allowable in determining a suitable rating for masonry arch bridges. In addition the initially proposed strength values, which are considered overly conservative, are increased. The introduction of these refinements will allow a more accurate assessment of the nation’s stock of stone masonry bridges.     

Asymptotic Prediction of Energetic-Statistical Size Effect from Deterministic Finite-Element Solutions

An improved form of a recently derived energetic-statistical formula for size effect on the strength of quasibrittle structures failing at crack initiation is presented and exploited to perform stochastic structural analysis without the burden of stochastic nonlinear finite-element simulations. The characteristic length for the statistical term in this formula is deduced by considering the limiting case of the energetic part of size effect for a vanishing thickness of the boundary layer of cracking. A simple elastic analysis of stress field provides the large-size asymptotic deterministic strength, and also allows evaluating the Weibull probability integral which yields the mean strength according to the purely statistical Weibull theory. A deterministic plastic limit analysis of an elastic body with a through-crack imagined to be filled by a perfectly plastic “glue” is used to obtain the small-size asymptote of size effect. Deterministic nonlinear fracture simulations of several scaled structures with commercial code ATENA (based on the crack band model) suffice to calibrate the deterministic part of size effect. On this basis, one can calibrate the energetic-statistical size effect formula, giving the mean strength for any size of geometrically scaled structures. Stochastic two-dimensional nonlinear simulations of the failure of Malpasset Dam demonstrate good agreement with the calibrated formula and the need to consider in dam design both the deterministic and statistical aspects of size effect. The mean tolerable displacement of the abutment of this arch dam is shown to have been approximately one half of the value indicated by the classical deterministic local analysis based on material strength.     

安庆铜矿深部开采-400m隔离矿柱稳定性分析

采用拉格朗日非线性程序,数值模拟计算与分析了安庆铜矿1“矿体-400m水平隔离矿柱在深部矿体开采过程中的力学效应显现规律．研究结果表明：隔离矿柱在上部矿体开采后,因覆盖岩体卸载产生了应力释放,呈现“凸起”垂直位移变形;在深部厚大矿体开采扰动下,隔离矿柱局部区域应力集中,部分区域发生塑性变形,呈现应力拉力拱现象,产生下向变形位移,与矿山常规位移变形监测结果近似．综合分析,隔离矿柱在深部矿体开采过程中整体稳定性良好．