Rockfill Modulus and Settlement of Concrete Face Rockfill Dams

A method is presented for estimating the modulus of compacted rockfill in dams based on the particle size, unconfined compressive strength of the rock, compaction layer thickness, compactive effort, and the applied vertical stress. Also presented are methods for predicting the crest settlement and face slab deformation of concrete face rockfill dams during first filling and in the long term. It is demonstrated that the modulus is stress dependent and guidance is provided on how to assess this, as well as effects of construction in narrow valleys where arching may affect the stresses in the dam. These methods are based on analysis of the 35 dams with good quality information on construction materials and placement methods, and good quality internal and surface settlement monitoring records.     

Hydrostatic, Temperature, Time-Displacement Model for Concrete Dams

This paper presents frequency domain solution algorithms of the one-dimensional transient heat transfer equation that describes temperature variations in arch dam cross sections. Algorithms are developed to compute the temperature T(x,t), spatial distribution, and time evolution for the “direct” problem, where the temperature variations are specified at the upstream and downstream faces, and for the “inverse” problem, where temperatures have been measured at thermometers located inside instrumented dam sections. The resulting nonlinear temperature field is decomposed in an effective average temperature, Tm(t), and a linear temperature difference, Tg(x,t), from which the dam thermal displacement response can be deducted. The proposed frequency domain solution procedures are able to reproduce an arbitrary transient heat response by appending trailing temperatures at the end of thermal signals, thus transforming a periodic heat transfer problem in a transient one. The frequency domain solution procedures are used to develop the HTT (hydrostatic, temperature, time) statistical model to interpret concrete dam-recorded pendulum displacements. In the HTT model, the thermal loads are arbitrary and can contain temperature drift or unusual temperature conditions. The explicit use of Tm(t) and Tg(x,t) in the HTT dam displacement model allows extrapolation for temperature conditions that have never been experienced by the dam before (within the assumption of elastic behavior). The HTT model is applied to the 131-m-high Schlegeis arch dam, and the results are compared with the HST (hydrostatic, seasonal, time) displacement model that is widely used in practice.     

Effect of Seismic Uplift Pressure on the Behavior of Concrete Gravity Dams with a Penetrated Crack

Proper consideration of the uplift pressure at the base of a concrete gravity dam is of great importance in practical engineering, since it is crucial to the safety of the dam, specifically for a cracked dam under seismic conditions. However, constant uplift pressure, which is suitable for the static case only, was adopted in almost all the seismic analyses of cracked concrete gravity dams. To adequately estimate the seismic behavior of cracked concrete gravity dams, a seismic uplift pressure model is proposed for a penetrated crack. In this model, the amount and the distribution of the uplift pressure along the assumed rigid crack walls are determined by the earthquake acceleration, the water heads, the aperture of the crack, and the opening/closing velocity. Application of the model to a typical concrete gravity dam with a penetrated crack at the base reveals that the seismic behavior of the dam is markedly affected by the seismic uplift pressure. In general, the residual downstream sliding is considerably enlarged compared to that of constant uplift pressure. Computations show that the seismic uplift pressure can be several times higher than the constant one, increasing the dynamic instability of the cracked dam. It is also revealed that the dynamic water flow plays the role of a wedge while the upper mouth of the crack is closing. When the dam rocks back to upstream, the uplift pressure increases until it is so high that the pivot at the toe is raised up and the whole dam loses its contact. Then the resultant uplift pressure remains constant until the dam is inclined to the upstream. During this period of time, the cracked dam is normally drifting towards the downstream due to the hydro pressure.     

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.     

Energy Release Rate due to Friction at Bimaterial Interface in Dams

The interface between concrete dam and rock foundation is one of the most important regions governing the strength and stability of gravity dams. Many researchers have attempted to extend the fracture mechanics approach to study this rock concrete interface assuming stress free crack surfaces. In a real-life situation, because of the combined compression and shear loading, the crack faces come in contact resulting in a sizeable contact zone near the crack tip. Thus, frictional contact of the crack surfaces cannot be neglected. The frictional contact alters the stress singularity to become either weaker or stronger than the inverse square root singularity observed in homogeneous crack problems. Consequently, the strain-energy release rate as conventionally defined, either vanishes or becomes unbounded and thus cannot be used as a fracture parameter. In this work, an attempt is made to include the effect of friction associated with the sliding of crack surfaces and compute the energy dissipated during crack propagation. It is shown that the total energy release rate decreases with crack length when friction is accounted for between the rock-concrete interface in gravity dams.     

Structural Assessment of a Concrete Dam Based on Uplift Pressure Monitoring

The context of structural monitoring of concrete dams as part of hydropower assets management is described. A tool that fits well into this context is the control of the uplift pressure of concrete dams. A monitoring technique suitable for this purpose was developed based on an automated water-level measurement technique using time-domain reflectometry and standard air-dielectric coaxial cable sensors. The signal is interpreted automatically by applying a threshold method to determine the apparent water level, which is then used to calculate the uplift pressure. A field test at a concrete dam column displayed consistent results, which were used as input to a reliability-based stability safety analysis. The results show that this technique can be very useful for uplift pressure monitoring. The information determined can serve as input to the maintenance work as well as assist engineering decisions.     

Assessment and Rehabilitation of Embankment Dams

A series of observations, studies, and analyses to be made in the field and in the office are presented to gain a proper understanding of how an embankment dam fits into its geologic setting and how it interacts with the presence of the reservoir it impounds. It is intended to provide an introduction to the engineering challenges of assessment and rehabilitation of embankments, with particular reference to a Croton Dam embankment.     

Behavior of Concrete-Faced Rockfill Dams during Initial Impoundment

Centrifuge tests to investigate the behavior during initial reservoir filling of a concrete faced rockfill dam (CFRD) with face slab stiffnesses that vary by a factor of about two are described. The two centrifuge models exhibited similar deformations at the crest and along the face slab, with crest settlements averaging 0.19H (%) and maximum face slab deformations averaging 0.88H (%). The centrifuge test results suggest that the face slab stiffness had little effect on deformations, at least for the range of stiffnesses examined here. A parametric study of transition (supporting) zone stiffness was performed using a numerical model calibrated using the centrifuge results. The numerical results indicated that face slab deformation is more influenced by transition zone stiffness than face slab stiffness, supporting the centrifuge results. Deformation measurements for 25 in-service CFRDs (including six Korean CFRDs—one of which was used as the basis for the centrifuge model dam) are presented and compared with the experimental and numerical results. The centrifuge experiments exhibited crest settlements similar to the Korean CFRDs; however, the centrifuge models exhibited considerably larger maximum face slab deflections. The larger values measured in the centrifuge tests likely resulted from some experimental limitations. These limitations, as well as suggestions for improving future centrifuge studies of CFRDs, are discussed.     

Seasonal Thermal Displacements of Gravity Dams Located in Northern Regions

Seasonal temperature displacements are an important component of the total displacements recorded by pendulum measurements at gravity dams located in northern regions. A hybrid dam displacement model is presented in this paper to interpret these displacements and extrapolate the response for an extreme thermal event not yet experienced by the dam. The hybrid model uses a simplified deterministic structural dam representation with beam elements in complement to a hydrostatic seasonal time (HST) statistical displacement model. Comparisons are first established between 1D heat transfer analyses of typical gravity dam sections, and 2D finite-element (FE) analyses. Thermomechanical displacements are compared to show the validity of the proposed simplified deterministic beam model for typical dams. A case study of an actual 40?m gravity dam located in Quebec, Canada is then presented. It is shown that the deterministic model can be calibrated using the pendulum displacements and the HST model. The calibrated deterministic model is then used to extrapolate the displacement response for extreme thermal events not yet experienced by the dam. The proposed methodology represents a simple extension of the gravity method, widely used to verify gravity dam stability, as a first step to interpret recorded pendulum displacements and set appropriate warning and alarm levels on a rational basis before developing 2D and 3D thermomechanical FE deterministic dam models that require a lot of resources and expertise to be used effectively.     

Influence of Sediment Layers on Dynamic Behavior of Aged Concrete Dams

The assessment of response of an aged concrete dam is important for the prediction of its behavior during earthquakes, so that remedial measures can be taken at the right time to withstand future earthquakes. The assessment is necessary, since the analyses procedures may become obsolete and the state of the art may change as the time of construction and the structural material may deteriorate due to harsh environmental conditions. At the same time, the sediment will be accumulated at the reservoir bottom on the upstream side of the dam. The process of siltation depends on the reservoir topography and climatic variability, i.e., increased flood frequency and river inflow. The decision of retrofitting or strengthening the aged dam necessitates accurate analysis of the same in the presence of accumulated sediment. In this paper, an approach to include the time-dependent degradation of concrete owing to environmental factors and mechanical loading in terms of isotropic degradation index is presented. The absorption of pressure waves at the bottom of the reservoir due to the presence of sediments has been incorporated in the hydrodynamic pressure equation. The response of an aged concrete dam in the presence of sediment layers under seismic excitation is studied. The outcomes show the efficiency of the proposed algorithm.     

Three-Degree-of-Freedom Rigid Model for Seismic Analysis of Cracked Concrete Gravity Dams

A rigid model with three-degrees-of-freedom is proposed for the purpose of seismic analysis of cracked concrete gravity dams. The model considers the geometry of the dam and all its possible modes of motion: sliding, rocking, rock-sliding, and drifting. The governing equations for all the modes are derived with the Mohr-Coulomb friction assumption at the crack, and corresponding conditions to initiate and maintain the modes are also given. For impact that follows rocking and drifting modes, postimpact velocities of the model are explicitly determined according to the momentum principle and the concept of restitution from classical point collision. Studies with the proposed model on rectangular blocks demonstrate two different types of rocking according to the slenderness. Applications to dams indicate that a large coefficient of friction does not necessarily prevent sliding, and rocking and drifting modes should not be neglected in estimating the stability of concrete gravity dams cracked at the base or at a height.     

Thermal-Structural Modeling and Temperature Control of Roller Compacted Concrete Gravity Dam

A coupled thermal-structural analysis is carried out using both a two- and a three-dimensional finite-element method. The computer program ANSYS is used and simulates the construction process of a roller compacted concrete (RCC) dam. Thermally induced stresses are computed for the 60 m high RCC Tannur Dam in Jordan. The actual temperature distribution in the body of the dam measured by thermocouples is compared with that obtained by ANSYS; generally, a good agreement is obtained. The study demonstrates that detailed thermal stress analysis should be performed for large RCC dams to provide a basis to minimize and control the occurrence of thermal cracking.     

Analysis of Arch Dams Using Coupled Trial Load and Block Element Methods

This paper presents the use of the trial load method and the block element method with elastoviscoplastic discontinuities for analysis of arch dams. The arch dam is considered as an arch-cantilever system and the foundation as a block element system. With the displacement compatibility condition at the contact surface of the dam and the foundation (including abutment), the governing equations of the arch dam and foundation are established. These methods are used for the analysis of the double curvature arch dam with complex geology conditions of the Xiaowan Hydroelectric Project in China. The deformation and stress states in both the dam body and the foundation are determined. Furthermore, the stability safety factors of the foundation and the abutment are calculated at the same time, which allows for an optimal design of the arch dam considering the strength, the deformation and the stability of the dam and foundation.     

Cyclic Behavior of Asphalt Concrete Used as Impervious Core in Embankment Dams

Asphalt concrete is used as a water barrier (interior core or upstream facing) in embankment dams. This paper investigates the behavior of hydraulic asphalt specimens subjected to cyclic loading in a triaxial cell. The specimens were tested at various sustained static stress states and temperatures and at maximum cyclic shear stress levels corresponding to severe earthquake shaking of the dam. The cyclic modulus versus mean sustained static stress showed an approximately linear relationship in a logarithmic diagram, and an empirical expression was developed to determine the cyclic modulus. At a mean sustained stress of 1.0?MPa, the cyclic modulus at 20°C was about 900?MPa; at 9°C, it was 1900?MPa and at 3.5°C, about 2500?MPa. The damping ratio was found to be between 0.07–0.30, depending on stress state and temperature level. The number of load cycles (up to 6000) had no significant effect on the magnitude of cyclic strain, and the cyclic loading was documented to have little effect on the postcyclic monotonic stress-strain-strength behavior and permeability (watertightness) of the asphalt concrete.     

Torsional Capacity of CFRP Strengthened Reinforced Concrete Beams

Many buildings and bridge elements are subjected to significant torsional moments that affect the design, and may require strengthening. Fiber-reinforced polymer (FRP) has shown great promise as a state-of-the-art material in flexural and shear strengthening as external reinforcement, but information on its applicability in torsional strengthening is limited. Furthermore, available design tools are sparse and unproven. This paper briefly recounts the experimental work in an overall investigation of torsional strengthening of solid and box-section reinforced concrete beams with externally bonded carbon fiber-reinforced polymer (CFRP). A database of previous experimental research available in literature was compiled and compared against fib Bulletin 14. Modifications consistent with the space truss model were proposed to correct the poor accuracy in predictions of CFRP contribution to strength. Subsequently, a design tool to analyze the full torsional capacity of strengthened reinforced concrete beams was validated against the experimental database.     

Performance of Mechanically Fastened FRP Strengthened Concrete Beams in Flexure

The use of adhesively bonded fiber-reinforced polymer (FRP) materials has become widely accepted for use in flexural strengthening applications; however, the method of attachment presents drawbacks in application. These include extensive time and labor investments, as well as a tendency of the system to fail in a brittle manner. This paper presents a study of a series of reinforced concrete beams each strengthened in flexure with an FRP strip attached with large diameter concrete screws. The concrete screws were arranged in a variety of patterns. The effect of fastener number and spacing, as well as the effect of fastener pattern on the behavior of the beam, was investigated through the use of two groups of specimens. The beams in each group were tested to failure to verify the behavior of the strengthening system. Measured behavior was then used to determine an analytical approach for prediction of load response behavior of mechanically fastened systems. It was found that the strengthening method investigated improved the flexural capacity of the specimens 12 to 39% with little or no loss in ductility.     

Flexural Reliability of Reinforced Concrete Bridge Girders Strengthened with Carbon Fiber-Reinforced Polymer Laminates

This paper presents the development of a resistance model for reinforced concrete bridge girders flexurally strengthened with externally bonded carbon fiber-reinforced polymer (CFRP) laminates. The resistance model is limited to pure flexural failure and does not address shear failure, laminate debonding, or delamination. The resistance model is used to calculate the probability of failure and reliability index of CFRP-strengthened cross sections. The first-order reliability method is employed to calibrate the flexural resistance factor for a broad range of design variables. The study shows that the addition of CFRP improves reliability somewhat because the strength of CFRP laminates has a lower coefficient of variation than steel or concrete. However, the brittle nature of CFRP laminates necessitates a reliability index that is greater than that generally implied in the AASHTO LRFD for 1998. This leads to a lower resistance factor than is currently accepted for reinforced concrete sections in flexure.     

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.     

Fatigue Performance of CFRP Strengthened RC Beams under Environmental Conditioning and Sustained Load

Carbon fiber-reinforced polymers (CFRPs) have become increasingly important in recent years in bridge rehabilitation. Significant research has been done on the static behavior of CFRP-strengthened reinforced concrete (RC) structures; however, the fatigue behavior of such structures with interface defects subjected to harsh environmental conditions still needs to be investigated. Hence, an experimental program has been carried out to investigate the fatigue behavior, under a load range, which generates service load stress levels, of RC beams strengthened with CFRP fabrics. The effect of aggressive environments was studied by subjecting the test members to freeze–thaw, extreme temperature, ultraviolet light exposure, and relative humidity cycles. All beams survived 2 million fatigue cycles without showing significant bond degradation between composite and substrate. However, significant flexural stiffness degradation was observed in the conditioned specimens. The presence of defects also affected specimen stiffness; however, limited growth in defect size was observed due to fatigue cycling.     

Behavior of CFRP Strengthened Reinforced Concrete Beams in Corrosive Environment

This paper reports the test results of 11 reinforced concrete beams strengthened with carbon fiber-reinforced polymer (CFRP) sheets and subjected to an aggressive environment. In this study, eight beams were cracked and repaired with CFRP sheets, while the remaining three beams were kept uncracked as a control. The beams were 150?mm wide by 250?mm deep by 2,400?mm long and lightly reinforced with a reinforcement ratio of 0.6%. Two types of carbon FRP products were considered: Sheets and strips. In terms of environmental exposure, three beams were kept at room temperature and eight beams were subjected up to 300 wetting and drying cycles with deicing chemicals (3% NaCl). Following the exposure, the beams were tested to failure in four-point bending. In addition, nondestructive tests were performed to determine the corrosion rate, as well as destructive tests to determine chloride diffusion and reinforcing bar mass loss. Based on the findings of the study, the long-term effectiveness of the CFRP strengthened reinforced concrete in aggressive corrosive environments was established.