Face Slab Research Articles

Utilizing Fiber Reinforced Concrete as the Concrete Face of Rockfill Dams

Construction of concrete faced rockfill dams (CFRD) includes pouring rock materials as the main body of the dam and compacting the rockfill to an acceptable compaction level. Then, a reinforced concrete slab is constructed on the upstream surface of dam to transfer water pressure to the rockfill materials. Considering the high permeability of rockfill materials, the concrete slab serves as a sealing element on the upstream side. Indeed, the required characteristics of concrete face are adequate flexibility in case of long-term settlement of dam, durability and its sufficiency in waterproofing the dam. In this study, to improve the quality of concrete face, increase its flexibility, optimize the construction cost and enhance the bearing capacity of concrete in tension, developed by bending moment in the face slab, three types of prevalent fibers (polypropylene, MEXO200 and steel) are utilized. The performance of each fiber is compared with the conventional reinforced concrete specimens by conducting some bending tests. Furthermore, carrying out numerical simulation, the efficacy of each fiber on performance of face slab is investigated. Based on the results, the fibers, especially the steel fibers, are suitable to substitute rebars of the conventional reinforced concrete employed as the concrete face of CFRDs. The steel fiber reinforced concrete has adequate capacity and ductility to mitigate the expected deformations from reservoir loading on the upstream face of CFRDs.

Quasidistributed optical fiber sensing system to monitor the displacement of joints on the face slab of concrete-faced rockfill dams

We propose a quasidistributed optical fiber-sensing system based on optical time-domain reflectometry (OTDR). The quasidistributed optical fiber-sensing system can be used to monitor the joint opening displacement or crack opening displacement of the face slab of concrete-faced rockfill dams (CFRDs). The crack sensors, which are based on the sensing principle of linear macrobending loss of optical fiber, are connected in series in each optical fiber link. In addition, multiple links can be connected with a single OTDR to construct an integrated monitoring system. Hence, the quasidistributed system overcomes some drawbacks of the distributed monitoring system, such as limited measurement range, fragility of fibers attached to the structural surface, and the nonlinear relationship between displacement and macrobending loss of optical fiber. To develop an optimal design plan for the monitoring system, some experiments are conducted to verify the independence of crack sensors and the minimum length of the connecting fiber between sensors. The analysis of the experiment results positively supports the application of optical loss in real structural monitoring.

Stiffness and Cavity Test of Concrete Face Based on Non-Destructive Elastic Investigation

A non-destructive testing (NDT) method was used in a concrete face rockfill dam (CFRD) to identify the condition of the concrete face slab and detect any existing cavities between the concrete face slab and the underlying support layer. The NDT for the concrete face slab was conducted using the impulse response (IR) method and the electrical resistivity tomography (ERT) method with the application of non-destructive electrodes. Information regarding the dynamic stiffness and average mobility of the concrete was obtained based on the mobility-frequency of the IR method, and cavity detection under the plate structures was analyzed using the two-dimensional (2D) electrical resistivity section of the ERT method. The results of the IR method showed that zones with low dynamic stiffness and high average mobility were expected to be found in concrete of poor quality and in cavities beneath the concrete face slab. The results of the ERT method showed that zones with high resistivity were expected to be cavities between the concrete face slab and the underlying support layer. As a result, the tendency toward low dynamic stiffness, high average mobility, and high resistivity in both methods implies unstable concrete conditions and the possible occurrence of a cavity. The results of the two methods also showed a good correlation, and it was confirmed that the NDT method was reliable in terms of cavity estimation.

Numerical analysis of concrete-faced rockfill dams considering effect of face slab – cushion layer interaction

In this study, the effect of concrete face slab – cushion layer interface behavior on the performance of face slabs in concrete-faced rockfill dams (CFRDs) is investigated using the finite element method. The body of the CFRD is simulated by the cap elastoplasticity model while the interface zone between the concrete face slab and the gravelly cushion layer is simulated explicitly by an advanced interface constitutive model, developed in the framework of critical state soil mechanics and state parameters, and capable of simulating volumetric behavior and stress path dependency. The effect of elastic and elastoplastic material behavior and water level on the stress and displacement responses of the concrete face slab is examined, and the influence of the roughness at the interface area between the concrete face slab at the contact face and the cushion layer on the performance of the concrete face slab is investigated. Comparison of the results with and without the explicit consideration of the interface behavior shows the importance of incorporating advanced constitutive interface modeling in the design and analysis of CFRDs.

Temperature field analysis of hydraulic engineering structure with cracks based on XFEM

For the long-term exposure to temperature load, certain parts of a concrete structure have more or less cracks during their construction or operation period. Obviously, the existence of cracks disrupts the heat transmission, having a significant impact on the existed cracks, and is not conducive to operate the structure safely. Thus, it is necessary to calculate and analyze the temperature field. XFEM was introduced into the analysis of the temperature field in hydraulic structures for its unique advantage of solving discontinuous problems, in this paper. Firstly, the approximation form of the temperature field of cracked structure was analyzed and provided; and then the discrete equations for solving discontinuous temperature field was deduced, where the crack was simulated by isothermal form, namely, the temperature field is continuous but its first derivative is not continuous; and then the temperature field by XFEM was established. A numerical example of a cross shaped plate with slit was used to verify whether the proposed method is reasonable and reliable. Finally, this method is used to analyze the expansion of the cracks in the face slab of a concrete face rock-fill dam in winter. The results are consistent with the actual monitoring results.

Influence of strong motion duration on the seismic performance of high CFRDs based on elastoplastic analysis

Amplitude, frequency content and duration are the three main features of earthquake ground motions. The first two features have been fully considered in the seismic design of high concrete-faced rockfill dams (CFRDs). However, the role of ground motion duration on the seismic performance assessment of CFRDs remains unclear. In this paper, 40 as-recorded ground motions with a broad distribution of durations modified to match a 5% damped target spectrum are selected to investigate the influence of strong motion duration on the seismic performance of high CFRDs. A generalized plasticity model for rockfill materials, a plastic damage model for face slabs and a generalized interface plasticity model to describe the relative sliding behavior between the face slabs and rockfill are adopted for the dynamic response analysis based on a 2D 200-m-high regular CFRD. The vertical displacement, horizontal displacement, plastic shear strain and a face-slab damage index are used as the dam damage measures (DMs). The correlation coefficient R2 is selected to reflect the degree of the relationship between strong motion duration and DMs. The results indicate that the vertical displacement, horizontal displacement, and plastic shear strain are positively correlated with strong motion duration, but the face-slab damage index has a low correlation. Therefore, the influence of strong motion duration should be considered in the seismic performance assessment of high CFRDs, and the use of short durations of strong ground motion in the design and seismic performance assessment of CFRDs may overestimate dam performance.

A three-dimensional elasto-plastic dynamic response procedure for concrete-faced rockfill dams

Equivalent linear dynamic analysis is the primary method of analysis for concrete-faced rockfill dams during earthquake. However, this method cannot be directly used to estimate permanent deformation during earthquake, which is important to evaluate the dynamic safety of concrete-faced rockfill dams. To bridge this gap, an elasto-plastic dynamic response procedure based on finite element method is presented to estimate the construction, impoundment, and dynamic characteristics of concrete-faced rockfill dams. This procedure involves a generalized plasticity model for rockfill materials and an ideal elasto-plastic model for interface between concrete face slab and rockfill material. The construction, impounding process, and seismic behavior of a 150-m-tall concrete-faced rockfill dam were simulated using seismic motion based on Chinese code for seismic design of hydraulic structures of hydropower project with the peak ground acceleration of 0.3 g as an excitation to validate the procedure. On the basis of the numerical simulation results of the step-by-step construction, reservoir impoundment and seismic response of a concrete-faced rockfill dam, dam settlement, joint deformation, stress, and deformation of slab were analyzed. The results indicate that the dam settlement and deformation of joint can be directly obtained by the procedure during earthquake. The construction, impoundment, and seismic analysis could be considered in one procedure.

Quantity of Intraretinal Hyperreflective Foci in Patients With Intermediate Age-Related Macular Degeneration Correlates With 1-Year Progression.

The purpose of this study was to evaluate the correlation between quantity of intraretinal hyperreflective foci (HRF) in the eye with intermediate AMD and progression to late AMD. Volume optical coherence tomography (OCT) scans from 114 eyes of 114 patients were retrospectively reviewed. HRF were assessed both qualitatively and quantitatively. Five sequential en face slabs from midretina were thresholded to isolate the HRF. These five slabs were recombined, and HRF area was measured in the whole 6 × 6-mm image (HRFTOT) and within the central 3-mm (HRF3mm) and 5-mm (HRF5mm) regions. These measurements were correlated with the development of late AMD (defined as choroidal neovascularization [CNV] and/or complete RPE and photoreceptor atrophy [cRORA]) after 1 year of follow-up. HRF area in all three regions showed significant correlations with progression to late AMD: R = 0.610 for HRF3mm, R = 0.622 for HRF5mm, and R = 0.614 for HRFTOT (all P < 0.001). Correlations remained significant with progression to cRORA alone, though not for progression to CNV alone. While qualitative assessment of HRF (i.e., presence of HRF: yes or no) also showed a significant correlation with progression to late AMD (R = 0.454, P < 0.001) and atrophy alone (R = 0.445, P < 0.001), they were weaker than by HRF quantification. The area of HRF from en face OCT in eyes with intermediate AMD correlates with the 1-year risk of progression to late AMD, and in particular with the development of atrophy.

Influence of Abutment Slope Angle Variety on the Deformation and Stress of the Concrete-Faced Rockfill Dam During Initial Impoundment

Favorable valley terrain for building dams is gradually decreasing. The primary objective of this study is to investigate the influence of abutment slope angle variety on the deformation and stress of concrete-faced rockfill dam (CFRD). First, the effect of abutment slope angle on perimeter joint deformation is investigated using CFRD monitoring data in China. A normalized deformation index (CDS) is proposed to evaluate the relationship between perimeter joint deformation and the steepest abutment slope angle. Results indicate that perimeter joint deformation increases nonlinearly with the steepest abutment slope angle. Then, numerical analysis is conducted on Miaojiaba CFRD under three types of abutment slope variety from upstream to downstream. We focus on the effect of abutment slope angle variety from upstream to downstream on CFRD behavior. In the analysis, the terrain shape line is assumed to be constant at the downstream slope foot, and the abutment slope angle increases linearly from upstream to downstream. The Duncan–Chang E–B model is used in nonlinear analysis for rockfill, and the slab–rockfill interfaces are modeled using contact elements with Coulomb friction. An influence sphere is proposed for the different types of abutment slope angles. Results indicate that face slab deformation is more dependent on abutment slope angle than rockfill deformation.

A nonlinear analysis of dynamic interactions of CFRD–compressible reservoir system based on FEM–SBFEM

Reservoir water compressibility is a major factor affecting dynamic dam–reservoir interactions, and most studies focus on the analysis of concrete dams. However, few studies have been conducted with respect to the effects of reservoir water compressibility on the dynamic response of CFRDs (concrete-faced rockfill dams). Furthermore, new researches show the hydrodynamic pressure is significant for evaluating the seismic performance of high CFRD. In this paper, a method to calculate the nonlinear dynamic interactions of CFRD–compressible reservoir water system in the time-domain is established based on a FEM–SBFEM approach. The CFRD is discretized using the finite element method (FEM), and the hydrodynamic pressure of the semi-infinite water on the upstream face of the dam is calculated using the efficient scaled boundary finite element method (SBFEM), in which the degrees of freedom of the hydrodynamic pressure to be solved for are relatively few. The objective of this work is to investigate the effects of reservoir water compressibility and the sediment absorption layer at the reservoir bottom and bank slopes on the dynamic response of a high CFRD–reservoir-coupled system under seismic action in various directions. When the reservoir water is considered compressible, partial absorption of the hydrodynamic pressure wave by the sediments can be considered. The results show that water compressibility can significantly affect the stress in the face slabs.

Seismic response of high concrete face rockfill dams subjected to non-uniform input motion

Analysis of the seismic response of high CFRDs under non-uniform ground motion input is conducted using a novel non-uniform input motion calculation method combined with nonlinear FEM. The non-uniform input motion calculation method and its basic assumption are validated. The response of CFRDs under uniform and non-uniform input is compared to discuss the necessity of conducting seismic analysis of high CFRDs under realistic non-uniform ground motion input. When the acceleration at the surface of the free field for dynamic simulations with uniform and non-uniform input is kept consistent, the seismic response of CFRDs under non-uniform input is in general significantly smaller, while the dynamic tensile stress around the edges of the concrete face slab is greater. The simulation results suggest that non-uniformity of the ground motion input has important effects on the seismic response of high CFRDs and should be considered in the seismic design of CFRDs. The influence of the incident angle of seismic waves is also investigated, with results indicating that the influence is waveform dependent, while being frequency independent.

Internal characterization of embankment dams using ground penetrating radar (GPR) and thermographic analysis: A case study of the Medau Zirimilis Dam (Sardinia, Italy)

The stability of embankment dams without an impermeable core depends on the characteristics of the face slab that prevents internal erosion, piping and eventual collapse of the structure. Under a Mediterranean climate, the impermeable asphaltic face slab is subjected to high solar radiation and consequent temperature changes, which can generate the creation of cracks and joints. The Medau Zirimilis Dam, located in the Casteddu River (Sardinia), is an embankment dam that has undergone seepage and continuous repairs in its asphalt face slab. These reparations have been conducted because of the occurrence of cracks and relative movement of different segments of the slab. To evaluate if seepage endangers the integrity of the dam, GPR was used, with different antennas (100, 250 and 500 MHz), along its crest and upstream and downstream faces, and the data were integrated with infrared thermographic images. Although geophysical data do not show structural changes affecting the main dam structure, deformation structures at shallow levels and in particular in the upstream face and along the crest of the dam have been identified. Such deformation affects the road atop the crest, the face slab and underlying levels, resulting in landslides that include material from several meters below the surface. The analysis permitted the identification of the origin of surficial cracks and their effects on the face slab. These sectors, independent of current movement, define the most unstable areas against water level changes that can affect the dam integrity. GPR analysis at the embankments usually has the handicap of high clay content that precludes electromagnetic wave penetration; however, in this case, the obtained resolution and extent of penetration using the different antennas was sufficient, due to the absence of an inner waterproof unit, and permitted the evaluation of the inner structure of the dam and the application of GPR for construction quality surveillance, internal structural characterization and dam monitoring.

Shear Stress Analysis and Crack Prevention Measures for a Concrete‐Face Rockfill Dam, Advanced Construction of a First‐Stage Face Slab, and a First‐Stage Face Slab in Advanced Reservoir Water Storage

Due to the demand in flood season for power generation, the first‐stage face slab of a high concrete‐face rockfill dam often must be constructed ahead of schedule, and advanced water storage is needed for the reservoir. Since the dam‐body filling has not yet been completed at this point, the internal stress of the first‐stage face slab is more complicated than that of normal construction. Taking Buxi Power Station as an example, the first‐stage face slab temporary construction seam showed large areas of shear stress damage during the rise in reservoir water levels during the second segment of the second construction stage. The concrete‐face slab showed large‐piece brittle bulging, and the steel rebar was exposed and developed contortional deformation. Based on the monitoring data for Buxi Power Station along with the first‐stage fracture characteristics of Shuibuya concrete face, this paper applied a numerical analysis to conduct research on the causes of fracture mechanics. The results indicate that the cracks occurred on the face slab during the second segment of second‐stage water storage primarily due to the advanced concrete pouring of the first‐stage face slab; during the first stage of reservoir water storage, the internal stress of the first‐stage face slab was not reduced or eliminated prior to second‐stage face slab pouring. Thus, with the rise in the reservoir water level, the shear stress increased continuously, eventually leading to partial large‐scale shear stress failure of the first‐stage face slab. The research results provide important references for the design and construction of concrete‐face rockfill dams.

Deformation analysis of high CFRD considering the scaling effects

In this paper, a predictive method accounting for the scaling effects of rockfill materials in the numerical deformation analysis of rockfill dams is developed. It aims to take into consideration the differences of engineering properties of rockfill materials between in situ and laboratory conditions in the deformation analysis. The developed method is based on the modification of model parameters used in the chosen material model, which is, in this study, an elasto-plastic model with double yield surfaces, i.e., the modified Hardening Soil model. Datasets of experimental tests are collected from previous studies, and a new dataset of the Nam Ngum 2 dam project for investigating the scaling effects of rockfill materials, including particle size, particle gradation and density, is obtained. To quantitatively consider the influence of particle gradation, the coarse-to-fine content (C/F) concept is proposed in this study. The simple relations between the model parameters and particle size, C/F and density are formulated, which enable us to predict the mechanical properties of prototype materials from laboratory tests. Subsequently, a 3D finite element analysis of the Nam Ngum 2 concrete face slab rockfill dam at the end of the construction stage is carried out using two sets of model parameters (1) based on the laboratory tests and (2) in accordance with the proposed method. Comparisons of the computed results with dam monitoring data indicate that the proposed method can provide a simple but effective framework to take account of the scaling effect in dam deformation analysis.

Effect of Low Temperature and Freeze-thaw on Stress and Deformation of Concrete Faced Rockfill Dams

The freezing and freeze-thaw cycles may reduce the mechanical strength of the material for a concrete faced rockfill dam (CFRD), resulting in the risk of structural damage to the dam under the design load. In order to study the influence of freezing and freeze-thaw on the stress and deformation of the CFRDs, some experimental results and theoretical analysis for the influence of freezing and freeze-thaw cycles on the mechanical parameters of concrete face slab and rockfill were introduced at first. The finite element method (FEM) was adopted for the structural analysis of a CFRD in the high altitude area of Qinghai-Tibet Plateau of China. Using the FEM analysis results, the distribution of stress and deformation of concrete face slab and dam body were obtained and a comparison was made with the analysis results when the freezing and freeze-thaw cycles were not taken into account. Some conclusions were drawn from these analysis results and the research results provide a reliable basis for the design and construction of the CFRD in the future.

Bond-breaker impact on slab behaviour in concrete face dams

The actual technique of construction of concrete face slabs of concrete face rockfill dams and concrete face gravel dams is oriented to the protection of the upstream slope by a series of concrete extruded curbs. The face slab leans on these curbs and in the current designs a bond-breaker element is placed between the face slab and the curbs in order to break the stress transference between them. The main aim of this research is to highlight the importance of this treatment while at the same time a technical analysis of the transference of stresses is done by generating a model of the face slab that studies the cases of: not placing a bond-breaker element, placing a partially bond breaker and a total bond breaker, between the face slab and the concrete curbs. In addition, the relevance of the angle of the upstream slope onto which the face slab leans has been also analysed.

A statistical review of the behaviour of concrete-face rockfill dams based on case histories

The concrete-face rockfill dam (CFRD) has become a preferred dam type, but its design remains largely based on past experiences. Understanding the behaviour of CFRDs is critical to the design and safety assessments of these dams. The main objective of this study is to review the behaviour of CFRDs from the perspective of statistical analysis. A set of monitoring records and construction details of 87 case histories of in-service CFRDs constructed in the past 50 years is collected. The analysed data that are presented are based on a review of the literature. Detailed statistical analyses of post-construction crest settlement and face slab behaviour, settlement of the dam body during construction and leakage after reservoir filling are conducted on the basis of the above-mentioned case histories. Several suggested empirical relationships are obtained by regression analysis to estimate the deformation and seepage behaviour of CFRDs. Findings offer reliable insights into dam behaviour and provide an engineering analogy and reference for the planning, design and construction of CFRDs in the future. The statistical analyses also attempt to identify quantitatively the major influencing factors of the deformation of CFRDs, such as intact rockfill strength, foundation characteristics, valley shape and seepage flow. Intact rockfill strength and foundation characteristics are found to be the main influencing factors of dam deformation behaviour. The results from the statistical analysis of the case histories and the analyses performed are also presented herein.

Tensile Stress Responses of CFRD Face Slabs during Earthquake Excitation and Mitigation Measures

Tensile Stress Responses of CFRD Face Slabs during Earthquake Excitation and Mitigation Measures

A Methodology for Optimal Layout Design of Pressure Cells for Concrete Faced Rockfill Dams

In this study, a methodology for optimal layout design of pressure cells for concrete faced rockfill dams is developed. A representative dimensionless stress distribution model was formed for obtaining the magnitudes and location of different stress zones as a function of dam height. This information enabled development of a procedure for proper location and the number of pressure cells throughout the dam body. A vertical placement algorithm based on error minimization was first developed, which is followed by an approach to find the number and location of pressure cells on a particular elevation of the dam body. The effects of face slab cracking and earthquake are interpreted. Furthermore, the performance of the proposed model was also tested when instruments are installed to different elevations in the dam body than those recommended by the model developed. The proposed optimization scheme provides a basis for economical layout design of pressure cells with sufficient information allowing realistic assessment of the structural behavior. The application of the proposed model is illustrated for some existing dams. It is observed that this algorithm gives satisfactory results.

Natural History of Subclinical Neovascularization in Nonexudative Age-Related Macular Degeneration Using Swept-Source OCT Angiography

Swept-source (SS) OCT angiography (OCTA) was used to determine the prevalence, incidence, and natural history of subclinical macular neovascularization (MNV) in eyes with nonexudative age-related macular degeneration (AMD). Prospective, observational, consecutive case series. Patients with intermediate AMD (iAMD) or geographic atrophy (GA) secondary to nonexudative AMD in 1 eye and exudative AMD in the fellow eye. All patients were imaged using both the 3×3 mm and 6×6 mm SS OCTA fields of view (PLEX Elite 9000; Carl Zeiss Meditec, Inc, Dublin, CA). The en face slab used to detect the MNV extended from the outer retina to the choriocapillaris, and projection artifacts were removed using a proprietary algorithm. Prevalence of subclinical MNV and time to exudation with Kaplan-Meier cumulative estimates of exudation at 1 year. From August 2014 through March 2017, 160 patients underwent SS OCTA (110 eyes with iAMD and 50 eyes with GA). Swept-source OCTA identified subclinical MNV at the time of first imaging in 23 of 160 eyes, for a prevalence of 14.4%. Six eyes demonstrated subclinical MNV during the follow-up. Of 134 eyes with follow-up visits, a total of 13 eyes demonstrated exudation, and of these 13 eyes, 10 eyes were found to have pre-existing subclinical MNV. By 12 months, the Kaplan-Meier cumulative incidence of exudation for all 134 eyes was 6.8%. For eyes with subclinical MNV at the time of first SS OCTA imaging, the incidence was 21.1%, and for eyes without subclinical MNV, the incidence was 3.6%. There was no difference in the cumulative incidence of exudation from pre-existing MNV in eyes with iAMD or GA (P= 0.847, log-rank test). After the detection of subclinical MNV, the risk of exudation was 15.2 times (95% confidence interval, 4.2-55.4) greater compared with eyes without subclinical MNV. By 12 months, the risk of exudation was greater for eyes with documented subclinical MNV compared with eyes without detectable MNV. For eyes with subclinical MNV, recommendations include more frequent follow-up and home monitoring. Intravitreal therapy is not recommended until prospective studies are performed.