Dam Monolith Research Articles

Physical Model Tests of Concrete Buttress Dams with Failure Imposed by Hydrostatic Water Pressure

Although the failure of a concrete dam is a complex and highly dynamic process, the current safety assessments of concrete gravity and buttress dams rely on a simplified 2D stability analysis, which neglects the load redistribution due to 3D monolith interactions and the valley shape. In addition, the estimation of breach parameters in concrete dams is based on assumptions rather than analyses, and better prediction methods are needed. Model tests have been conducted to increase the understanding of the failure behavior of concrete dams. A scale model buttress dam, with a scale of 1:15, consisting of 5 monoliths that were 1.2 m in height and 4 m in width, was constructed and loaded to failure using water pressure. The model dam had detachable abutment supports and shear keys to permit variations in the 3D behavior. The results showed that the shear transfer was large between the monoliths and that the failure of a single dam monolith is unlikely. A greater lateral restraint gives not only a higher failure load but also a better indication of impending failure. These findings suggest that the entire dam, including its boundary conditions, should be considered during a stability assessment. The results also suggest that the common assumption in dam safety codes that a single monolith fails during flooding analysis is not conservative. The dataset obtained provides a foundation for the future development of dam-monitoring alarm limits and for predictive models of dam-breaching processes.

Underwater Explosion Resistance Performance of an Old Gravity Dam without Good Longitudinal-Joint Connection Integrity: Coupled Acoustic-Structural Analysis

A gravity dam monolith bounded by the transverse contraction joints on both sides is usually recognized as an independent element in various dynamic analyses. However, in most cases a gravity dam monolith is actually subdivided by longitudinal joints for mass concrete construction requirements. Previous studies of gravity dams subjected to underwater explosion (UNDEX) shock loadings have not dealt with the longitudinal-joint issue in much detail. In this work, a numerical simulation of the dynamic response of an old gravity dam without good longitudinal-joint connection integrity in an UNDEX event is performed. The coupled acoustic-structural analysis procedure with ABAQUS/Explicit is employed. The reliability of the procedure is validated by numerical simulations of two UNDEX impact tests in the literature. Nine calculation cases are considered to compare different responses of the dam with different connection states between monolith sections and underwater detonation depths. The results of this numerical investigation show that the longitudinal joints play an important role in the UNDEX response of the dam. The longitudinal joints greatly weaken the flexural rigidity of the dam and make the dam damaged in a piecemeal manner. This study adds to the rapidly expanding field of the UNDEX resistance performance evaluation of gravity dams by accounting more for the longitudinal-joint issue.

Method and Application of Spatial Positioning for Valid Temperature-measuring Optical Fibers in Concrete Dams

Spatial positioning of Valid Temperature-measuring Optical Fiber (VTOF) is of great significance for accurately grasping the real temperature distribution of a concrete dam. However, there is barely an effective way to accurately locate the measuring points of optical fiber in concrete dams. A comprehensive data symmetry analysis technique is presented for determining the optimal spatial positioning of valid temperature-measuring optical fibers (VTOFs) in concrete dams. This approach includes a mathematical model to confirm the center of symmetry in monitoring data, which is required to reconstruct the real spatial temperature distribution. This process can identify VTOF locations and verify the reliability of temperature predictions. Experimental validation was conducted using findings from research on a new mathematical model, site measurements from a concrete dam, and corresponding data from distributed optical fibers. These results were successfully applied to the construction of the Baihetan dam; the concrete temperature from 405 pouring blocks in 5 typical dam monoliths was accurately monitored, and the temperature-measuring points of VTOFs were more than 24711. Meanwhile, the optical fiber temperature measurement and temperature field analysis system of Baihetan dam located in southwest China was developed based on accurately acquired temperature measurement data, which provides data basis and engineering application platform support for temperature evolution and temperature field analysis and precise temperature control management of pouring blocks.

Post-earthquake effects on the seismic performance of concrete gravity dams

Seismic damage assessment of concrete gravity dams is a challenging task due to the difficulty in establishing the damage states in relation to target performance levels. Post-seismic loading and uplift in cracking is generally not considered in this complex assessment and the effects are not clear. A quantitative damage estimation approach considering post-earthquake effects is proposed in this work to investigate this issue. A novel procedure to include post-earthquake effects on the monolith cracking was proposed in which demands from the aftershocks and uplift are assessed on the cracked state after the main event. Damage state was quantified in terms of the extent of crack propagation on the dam monolith. Categorising performance as such, the effects of post-earthquake factors were investigated on three representative systems in two steps. First, over 2000 nonlinear time history analyses were conducted to obtain damage states for the main shaking event. Then, the change in these states due to the effect of post-earthquake factors, including water pressure penetrating the cracks and aftershocks, were quantified. The results, as well as the methodology proposed, can serve organisations managing dam stock risk in re-assessment of seismic analysis results considering post-earthquake effects.

Seismic analysis of a concrete gravity dam considering dam–water–sediment–foundation interaction

The dynamic behaviour of a concrete gravity dam–reservoir water–sediment–foundation system is very complex under seismic loading conditions. This study focuses on the investigation of the effects of variations in the material properties of a dam monolith and foundation bed, the consideration of with- and without-slip conditions at the dam–foundation interface and the variation in slope of settled bed-load sediment on the seismic behaviour of a concrete gravity dam. A finite-element model of the Koyna dam in India has been developed in the Abaqus software program to investigate the behaviour of the dam–reservoir–sediment–foundation interaction. The Koyna earthquake excitations in both horizontal and vertical directions of the dam have been used as the input data for the dynamic analysis. The main parameters investigated are contact pressure, friction shear stress at dam heel, heel stresses, crest displacement and accelerations, and tension damage. The findings of this numerical study are also compared with the results predicted from the analytical models proposed by Westergaard and Darbre using the added-mass concept for the reservoir effect.

Three-dimensional failure envelope of concrete dam shear keys

Shear keys provide interlocking mechanisms along the vertical contraction joints of many concrete gravity dams. The shear key ultimate shear capacity is typically estimated as a function of the friction and cohesion that could be mobilised across a 2D shear plane located at the base of the key when subjected to a normal confinement pressure, P. A two-dimensional response of dam monoliths and shear keys shearing-off at their bases, is assumed. However, evidence of 3D interlocking behaviour between dam monoliths and the possibility of different key failure mechanisms involving interacting axial, P, shear, V, moment, M, and torsion, T, indicates that typical empirical formulations could significantly overestimate the actual shear key capacity under floods and seismic loadings. This paper presents an evaluation of the load–displacement response of shear keys subjected to multiaxial loading via nonlinear finite element analyses. A concrete “Continuous Surface Cap Model” (CSCM), available in the computer program LS-Dyna, is first shown to best capture the experimental shear keys' ultimate and residual shear capacity responses among five constitutive models. The effects of the tensile strength, fracture energy, confinement pressure, friction coefficient, initial opening, and dilation conditions on the load–displacement response of keys are investigated. Failure envelopes considering the key multiaxial shear capacity with confinement pressure, P, moment, M, and torsion, T, are developed. The ratios between moment and shear, M/V, and torsion and shear, T/V, control the failure mechanism. Increasing these ratios significantly reduce the ultimate shear capacity.

Long-Term Thermal Stress Analysis and Optimization of Contraction Joint Distance of Concrete Gravity Dams

Results of the conducted research aiming to demonstrate the methodology of optimization of dam monolith length (distance between contraction joints), through monitoring the thermal tensile stresses during construction and service life of a concrete gravity dam that is built using the block method, are presented in this paper. A 3D space–time numerical model for phased thermal stress analysis is employed in a large concrete gravity dam case study. For the adopted block dimensions, schedule, and dynamics of construction and material parameters, the thermal stress analysis is conducted, taking into account the following: thermal physical properties of the material, the cement hydration process, heat exchange with the external environment and the reservoir, and self-weight of the structure. The main advantage of the proposed methodology is the possibility of controlling the cracks resulting from thermal tensile stresses in the monolith of a concrete gravity dam, by optimizing the monolith’s length to minimize the zones in which the tensile capacity of concrete is exceeded. The results obtained from the temperature field analysis show that the maximum temperature increase in the dam’s body results from the cement hydration process in combination with summer air temperatures in the construction phase. The aforementioned factors account for the increase in temperature of up to 45.0 °C, while during winter cooling of the structure occurs due to lower temperatures, especially in the surface zones. The results of the stress field analysis show that the extreme values of thermal tensile stresses are present in the process of a sudden or gradual cooling of the concrete when shrinkage occurs. Finally, it is shown that the reduction of the monolith length by 5.0 m (from 20.0 m to 15.0 m) results in a decrease in the extreme thermal tensile stress values by an average of 0.70 MPa (up to 12.0%) in winter and an average of 1.10 MPa (up to 20.0%) in summer; while for the entirety of the analyzed time period, results in a decrease in the extreme thermal tensile stress values by an average of 16.0% (0.93 MPa).

A modified applied element model for the simulation of plain concrete behaviour

A modified applied element model to simulate the behaviour of plain concrete continuum structures including discrete cracking is proposed in this study. In the classical applied element model, Poisson effects are fully ignored. To remediate this issue, diagonal elements are introduced to include the Poisson effect, and the constitutive parameters are rigorously determined using the Cauchy–Born rule and the hyper-elastic theory. The formulation is validated for linear elastic problems and the consistency and convergence behaviour of the numerical approach is shown. Tensile softening formulation using the concept of fracture energy is utilised for the nonlinear range. In this range, the approach is validated using the classical benchmark tests with pure tensile, split-tensile, combined shear-tensile and bending dominated push-over loading. The load–displacement behaviour and crack response were captured successfully, showing the proposed methodology can be used to quantify discrete cracks on large systems, such as dam monoliths, from initiation to significant damage levels.

Estimating the Ice Loads on Concrete Dams Based on Their Structural Response

In the assessment of concrete dams in cold climate, it is common that the theoretical stability becomes insufficient for load cases that include ice loads. However, the magnitude and return period of these ice loads have a high degree of uncertainty. This study estimates the magnitude of ice loads on eight concrete dam monoliths using measurements of their displacement from 29 winters. In the displacement signals, events are identified and assumed to be caused solely by ice loads. The observed displacement during an event is interpreted as an ice load using a load–displacement relationship derived from FE simulations of each dam. These simulations show that ice loads of the magnitudes given in design guidelines and recorded in previous measurements would significantly affect the structural response of the studied dams. However, only small traces of ice loads can be found in the observed responses of the studied dams. The estimated ice loads are significantly lower than the ice loads recorded in traditional ice load measurements. These results indicate that the average magnitude of ice load on an entire monolith is significantly lower than the measured local pressures. This would imply that ice loads may be a smaller concern regarding dam safety than previously believed.

Red Rock Hydroelectric Project – New hydro development at an existing flood control dam

The Red Rock Hydroelectric Project converted the existing Red Rock Dam near Pella, Iowa, from a non-powered dam into a multi-purpose dam. The existing flood control dam was constructed by the U.S. Army Corps of Engineers (USACE) in the 1960s on the Des Moines River near Pella, Iowa. The hydroelectric project was developed by Western Minnesota Municipal Power Agency and generates up to 55 MW with an average annual energy output of 178 gigawatt-hours. The new intake, penstock and powerhouse are located immediately adjacent to the spillway. Construction required large excavations into the upstream and downstream sides of the existing embankment dam and two penetrations through the existing gravity dam monoliths. Extensive water and earth-retention systems, prescriptive construction staging, and a robust dam safety surveillance program were designed to maintain and monitor the integrity of the existing dam throughout construction and avoid impacts to active USACE flood control operations. Construction was completed in 2020. This paper provides an overview of the unique challenges involved with the design and construction of a new hydroelectric project at an existing and active flood control dam.

A new method based on equivalent surfaces for simulation of the post-cooling in concrete arch dams during construction

In arch dams the thermal field has a crucial role on the structural behaviour. Throughout construction, the internal rise of temperature due to the heat released by cement hydration often results in significant volumetric deformations. When restrained either internally or externally, these deformations may lead to the development of considerable tensile stresses and, consequently, to the cracking of concrete. For this reason, establishment of temperature control measures is frequently deemed as necessary. One of the most relevant procedures is the circulation of water in embedded coils, which is also used to complete the cooling of concrete to the desired temperature for grouting the contraction joints.Despite its importance, detailed thermal analyses of arch dams during the construction phase are relatively scarce in literature, eventually because the existing methodologies based on the Finite Element Method (FEM), which represent the coils in a discrete way, require a huge computational effort. To overcome this drawback, the present paper proposes a new methodology to simulate the post-cooling effect, where the heat removal by the cooling coils is reproduced using fictitious convective boundaries. The framework to quantify the boundary coefficients is laid out, and comparisons are made in regard to analyses where explicit modelling of the cooling coils are performed. The proposed approach is also used to predict the thermal behaviour of a real arch dam monolith during construction, for which in-situ monitoring is available.

Feedback Design of Temperature Control Measures for Concrete Dams based on Real-Time Temperature Monitoring and Construction Process Simulation

Crack prevention is a significant issue in the construction process of concrete dams. The vast majority of concrete cracks are related to temperature variations, and hence, temperature control is a primary method used to prevent cracks. This paper presents a new integrated concept, named Feedback Design (FD) of temperature control measures, which incorporates real-time temperature monitoring, temperature field simulation and construction process simulation in a large system to optimize a temperature control scheme for concrete dams. We used a fiber-optic Distributed Temperature Sensing (DTS) system to monitor the temperature variation process of typical dam monoliths in real time. By incorporating a genetic algorithm, neural network algorithm, and finite element theory, we proposed an intelligent inversion method to obtain thermal parameters of concrete dams based on the temperature monitoring data. Meanwhile, we built a simulation model of a concrete dam’s construction process based on a cycling network technique to obtain accurate construction environmental parameters. Finally, we simulated the temperature field of a typical dam monolith based on initial temperature control measures, inversion thermal parameters, and accurate construction environmental parameters. Temperature variance lines of typical points in the model obtained from the finite element method (FEM) accord well with the measured values obtained from the DTS system. We applied the FD approach to the Xiluodu arch dam located in southwest China to forecast the temperature fields and optimize the temperature control scheme of the dam blocks planned for pouring. The FD approach has been proven reliable and efficacious.

Vibration-based damage detection of concrete gravity dam monolith via wavelet transform

This study has presented a method to estimate the damage location in the concrete gravity dam monolith. The first four vibration mode of the highest monolith of the Koyna dam was estimated and analyzed by the combined discrete and continuous wavelet transform. The position of the damage scenarios was identified by using bior 3.9 wavelet in the spatial variation of the data response. The sensitivity of wavelet to random signal noise was investigated by using noise to displacement response of cracked monolith. Three noise levels were introduced in the model and the capacity of this method to detect damage from noisy data was discussed.

Evaluation of the Ground Motion Scaling Procedures for Concrete Gravity Dams

The seismic safety of dam structures is often evaluated using time history analyses conducted with a limited number of ground motions. The selection and scaling of the ground motions is usually the most effective factor determining the results of the safety assessment. The inherent variability in the ground motion as well as the difficulty of conducting the analyses for a large number of ground motions renders the selection as the most important factor in the analysis results. The guidelines for the nonlinear transient analyses of buildings, such as the one presented in ASCE/SEI-7-10, are well studied. For dams, however, it is not clear how the selection and scaling of the accelerograms should be conducted with the goals of a) consistency b) reliability and the c) practicality of the analyses. In this context, consistency implies obtaining consistent results for the same problem, reliability implies a reduction in the variability in the results while practicality implies the completion of the process with lesser effort. The selection and scaling of the ground motions for use in the nonlinear seismic analysis of the concrete gravity dams was investigated in this study with the aforementioned goals focused on the efficient prediction of the seismic demands on these structures. Three different concrete gravity dam monoliths were selected for this purpose, using 15 selected ground motions for the appropriate local site conditions. The material nonlinearity, dam-reservoir interaction and vertical component of ground motions were considered in the analyses. The engineering demand parameters were selected as the crest displacement, the maximum crest acceleration and the crack extent, a direct indicator of the damage on the monoliths. Nine different scaling methods were investigated. The effectiveness in the prediction of the mean demand and the corresponding dispersion levels were compared. The required number of motions to conduct effective analyses was determined.

OPTIMIZATION OF THE TEMPERATURE CONTROL SCHEME FOR ROLLER COMPACTED CONCRETE DAMS BASED ON FINITE ELEMENT AND SENSITIVITY ANALYSIS METHODS

Achieving an effective combination of various temperature control measures is critical for temperature control and crack prevention of concrete dams. This paper presents a procedure for optimizing the temperature control scheme of roller compacted concrete (RCC) dams that couples the finite element method (FEM) with a sensitivity analysis method. In this study, seven temperature control schemes are defined according to variations in three temperature control measures: concrete placement temperature, water-pipe cooling time, and thermal insulation layer thickness. FEM is employed to simulate the equivalent temperature field and temperature stress field obtained under each of the seven designed temperature control schemes for a typical overflow dam monolith based on the actual characteristics of a RCC dam located in southwestern China. A sensitivity analysis is subsequently conducted to investigate the degree of influence each of the three temperature control measures has on the temperature field and temperature tensile stress field of the dam. Results show that the placement temperature has a substantial influence on the maximum temperature and tensile stress of the dam, and that the placement temperature cannot exceed 15 °C. The water-pipe cooling time and thermal insulation layer thickness have little influence on the maximum temperature, but both demonstrate a substantial influence on the maximum tensile stress of the dam. The thermal insulation thickness is significant for reducing the probability of cracking as a result of high thermal stress, and the maximum tensile stress can be controlled under the specification limit with a thermal insulation layer thickness of 10 cm. Finally, an optimized temperature control scheme for crack prevention is obtained based on the analysis results.

Effect of the impounding process on the overall stability of a high arch dam: a case study of the Xiluodu dam, China

Effects of the first filling process of the Xiluodu super-high arch dam are studied by both field monitoring and numerical modelling methods. The numerical modelling is conducted by the three-dimensional nonlinear finite element method, which takes into account the nonlinear contact of the transverse joints between dam monoliths and adopts both the design and feedback parameters of the dam. The results obtained from the various numerical analysis cases for different impounded water levels are comprehensively compared with those obtained from the field monitoring. Throughout this study, it is concluded that: (1) changes of the hydrostatic load resulting from the filling process have a significant effect on the deformation and stress states of the dam and foundation. The foundation settles as a consequence of consolidation of the reservoir bed under the effect of the water mass. During the filling process, the compressive stress of the dam in the sealed region increases in the arch direction while that in the cantilever direction decreases, which reveals an enhancement of dam thrust into the two banks. (2) The transverse joints between the dam monoliths are compressed almost over the entire height of the poured dam, and their aperture gradually decreases as the impounded water level increases. (3) The overall deformation of the Xiluodu dam is obviously affected by the uneven foundation settlement due to the complex reservoir basin. The tensile stress concentration appears at the crossing zone of the top of the dam’s sealed region and the dam–foundation interface. Thus, reasonable layout of steel bars and suitable jointing design are essential in these tensile stress concentration regions. (4) The dam behaviour predicted using the three-dimensional numerical modelling is in good agreements with that from the field measuring in terms of the stress and strain distribution characteristics in the dam and foundation, especially when the feedback parameters are adopted for the numerical analysis. The results from both the field monitoring and numerical modelling reveal that overall stability of the dam during the first filling process meets the dam operational requirements.

A labor consumption measurement system based on real-time tracking technology for dam construction site

Abstract The aim of the study presented in this paper is the development of a labor consumption measurement system based on real-time tracking technology for use on a dam construction site. Such technology includes Global Positioning System (GPS), and Geographic Information System (GIS). The system is three-layered and aims to solve several labor computational and usability challenges. The software for processing the collected data, presenting real-time site state visualization, and the accurate analysis of on-site labor consumption is run by smart phones with GPS devices, on-site private wireless base stations, and servers. The benefits of this quantitative labor consumption measurement approach for hydropower projects include the provision, for each dam monolith, of precise information regarding both the number and category of laborers together with their respective working hours. Such knowledge provides adequate quality management information for the owner and the supervisor and importantly aids payment negotiations with contractors. The proposed system was firstly applied to the world's third largest hydropower project. On-site tests successfully demonstrated the feasibility and effectiveness of the whole system. The results achieved show that the system is able to log entire on-site trajectories of laborers, and calculate the accurate labor consumption of each dam monolith.

Seismic Fortification Analysis of the Guoduo Gravity Dam in Tibet, China

The primary aim of this research was to analyze the seismic performance of the Guoduo gravity dam. A nonlinear FEM method was implemented to study the deformation, stress, and overall stability of dam under both static and dynamic loading conditions, including both normal and overloading conditions. A dam seismic failure risk control method is proposed based on the cracking mechanism induced by the dynamic load to ensure dam safety and stability. Numerical simulation revealed that (1) under normal static and dynamic loading the symmetry of the displacement distributions is good, showing that the dam abutments and riverbed foundation have good overall stiffness. The stress distribution is a safe one for operation under both normal water loading and seismic loading. (2) Attention should be paid to the reinforcement design of outlets of the diversion dam monoliths, and enhance the capability of sustaining that tensile stress of dam monoliths. (3) The shape of the dam profile has a significant effect on the dynamic response of the dam. (4) By employing the “overload safety factor method,” the overall seismic fortification is as follows:K1=1.5,K2=2~3, andK3=3~4.

Analysis on the Impact of the Status of the Fresh-Old Concrete Bonding Interface on Dam Safety

The bonding problem of interface between the fresh and old concrete is one of the main technological difficulties in the Heightening Project of Danjiangkou Dam in the South-to-North Water Diversion Project. At present, no mature experiences or engineering examples are found in China for concrete dam heightening. Because of the external environmental impacts of temperature change, aging and hydraulic fracturing on the interface between the fresh and old concrete, crack is likely to occur, which brings risks to the safe operation of the dam. In this article, the finite element direct force method was used to calculate the internal force. The safety of No. 1 dam monolith was calculated in accordance with the standard method for gravity dam in three conditions of fresh-old concrete bonding interface, i.e., perfect condition, cracked interface and cracked interface with water seepage. Suggestions for their solutions were also provided.

Simulation and feedback analysis of the temperature field in massive concrete structures containing cooling pipes

Simulation and feedback analysis of the temperature field are key difficult problems in massive concrete structures. A method combining composite element method (CEM) with genetic algorithm (GA) is studied in this paper. The CEM allows for the cooling pipes embedded in composite elements for discrete simulation. The GA is used for feedback analysis to obtain key thermal parameters of the concrete in site. To verify the method, a multi-lifts concrete model is simulated. Subsequently, the method is applied in the simulation and feedback analysis of the temperature field of the Xiaowan 22# dam monolith during the construction period. The method does not need to refine or change the analysis mesh, and the calculation results tallies with the monitoring results well, which is proven to be feasible and effective. The research shows that the method proposed in this paper is suitable for the simulation and feedback analysis of the temperature field.