Rockfill Dam Research Articles

Effect of size and strain rate on particle strength and stress–strain properties of rockfill materials

The stress-strain properties of the rockfill materials under cyclic loading conditions are critical for the seismic safety analysis of dams. However, due to the effect of size and strain rate, the parameters obtained from the scaled laboratory tests may not accurately calculate the earthquake response. In the presented study, a novel approach has been developed to infer the prototype parameters based on the single particle strength. Firstly, a series of particle crushing tests with different sizes and loading rates were conducted. The results show that as the strain rate increases, the particle strength enhances and the size effect on particle strength gradually weakens. According to the established particle strength model, it is assumed that two specimens have homothetic particle size distributions and the same compaction, and is geometrically similar, then the size effect relations on macroscopic stress and strain tensors can be derived, a set of monotonic and cyclic triaxial tests are used to validate the approach. Furthermore, the effect of particle size and strain rate on the equivalent linear model is determined, the small-strain modulus parameter K obtained from the laboratory test should be modified. This study provides a reference to the engineering design and seismic safety evaluation of high rockfill dams.

Large-scale shaking table model tests and numerical simulation for composite reinforcement aseismic measure of high earth rockfill dam

<p indent="0mm">The aseismic safety of high earth rockfill dams is an important problem in engineering construction in the intensive earthquake area of West China. A composite reinforcement aseismic measure (CRAM) for high earth rockfill dams was developed. The CRAM’s effectiveness and reliability were validated using comparative model tests on a large-scale shaking table. Numerical simulation was used to examine the CRAM’s reinforcement mechanism. The results demonstrated that the CRAM had an inhibiting effect on the acceleration response; the CRAM can effectively inhibit the loosening and slipping of rockfill at the crest and reduce earthquake-induced permanent deformation. The numerical simulation also revealed that the shear strain amplitude and distribution were significantly altered. A slightly larger shear strain appeared at the front of the CRAM’s internal reinforcement materials, and the dam slope’s potential sliding surface was transformed from shallow sliding to deep sliding. As a result, the CRAM can improve the high earth rockfill dam’s seismic safety. The research results can provide a reference for the seismic design of the high rockfill dam in the intensive earthquake area.

Criteria for ultimate aseismic capacity of high rockfill dam

<p indent="0mm">The ultimate aseismic capacity is important to evaluate the seismic safety of a rockfill dam. However, no specification in China explicitly stipulates the criteria for the ultimate aseismic capacity of a rockfill dam. Existing evaluation factors and their indices to represent earthquake-induced damage to rockfill dams are not unified, and there is a problem with the safety degrees obtained by the indexes of different evaluation factors not being coordinated. In this study, a large number of systematic analyses of overall elastic-plastic deformation and slip deformation of high rockfill dams (height &gt; <sc>200 m)</sc> under earthquake are performed in addition to a comparative study of measured engineering analysis. Based on the above studies, four basic principles for determining the criteria for the ultimate aseismic capacity of a high rockfill dam are proposed: strictly following the mapping relationship between the relative settlement ratio of dam crest and the damage degree of dams under earthquake, ensuring that the dam can not be a collapse in extreme earthquakes, taking the relative settlement ratio of dam crest as the key control evaluation factor, and ensuring the coordination of different evaluation factors such as the relative settlement ratio of dam crest and the slip deformation of dam slope. Finally, the criteria for the ultimate aseismic capacity of a high concrete face rockfill dam and a core-wall rockfill dam under earthquake conditions are proposed. The findings of the study could provide an important standard basis for determining the ultimate aseismic capacity of a high rockfill dam.

Two-dimensional DEM-FEM coupling analysis of seismic failure and anti-seismic measures for concrete faced rockfill dam

The concrete faced rockfill dam is composed of granular materials and the failure pathways typically exhibits multi-scale mechanical behavior. In this paper, a 2D polygonal DEM-FEM interface coupling analysis program is developed and introduced to the failure analysis of a 100 m high CFRD under strong earthquake. The polygonal DEM is developed to describe the strong-nonlinearity, large deformation and discontinuous of dam. The FEM and seismic wave input method is combined to simulate the dam-foundation interaction. A boned model is proposed to describe the deformation characteristics of face slab and anti-seismic measure. The results indicate that the initial seismic failure of CFRD occurs at the crest zone, and is characterized by the loosening, sliding and even rolling away of the shallow rockfill on the slope. Other initial failure modes include settlement of the dam crest, separation between the face slab and rockfill, downstream sliding of the wave wall and separation between wave wall and face slab. The installation of a nailed protective slab at the upper part of the downstream slope reduces the seismic damage by 27–41 %. The analysis method can intuitively reproduce the seismic failure mode and failure evolution of the CFRD and quantify the effect of the anti-seismic measure.

Simplified model for dam rockfill creep and influence of constructive delay of concrete face

Advances in equipment and tools implemented for geotechnics have been allowing design to be supported by numerical simulations. However, even with sophisticated resources, the dam geotechnical engineering industry still lacks few information on laboratory parameters as in the case of Concrete Face Rockfill Dams (CFRD). It is necessary to simplify the use of back-analysis that represent real CFRD, in search of tools that can be applied in the industry. The objectives include the simplification of the behavior of the linear modules associated with a linear coefficient representing the vertical creep and evaluation of the influence of the constructive delay of the concrete face in a specific case. The text goes through examples of real CFRD simulations, as a theoretical background. The CFRD calibration model includes an elegant representation of the vertical orthotropic rockfill creep with only two calibration parameters. The displacements obtained from a CFRD are represented with a linear elastic constitutive model with linear vertical deformation in time. The result is well adjusted both in the construction phase and during filling. Further on, six different slab constructive delay sequencing arrangements were simulated. The study uses a numerical device that makes it possible to drive the concrete slab in its correct design thickness. Among the stress results, the step-by-step sequencing of the slab following the rockfill reached the worst horizontal stresses, reaching almost 45 MPa, which exceeds the typical compressive strengths of conventional concretes that can affect the structural integrity of the slab. The results suggest that step-by-step sequencing should be avoided. In the case studied, the balanced behavior could be achieved in a concreting scenario maintaining the constructive delay of the slab with at least half the height of the dam. It is important to point out that all this work considerations serve for a specific example. Any generalization should be avoided or associated with broader complementary studies, as each dam is a unique work and must be studied individually, case by case.

Packing, compressibility, and crushability of rockfill materials with polydisperse particle size distributions and implications for dam engineering

In rockfill dam engineering, particle breakage of rockfill materials is one of the major factors resulting in dam settlement. In this study, one-dimensional compression tests on a series of coarse granular materials with artificially-graded particle size distributions (PSDs) were carried out. The tests focused on understanding the role of initial PSDs in the dense packing density, compressibility and crushability of coarse granular materials. The effects of fractal dimension (D) and size polydispersity (θ) of PSDs were quantitatively analyzed. Two different loading stages were identified from the logarithms of the stress–strain relationships, with the turning point marked as the yield stress. A similar effect of initial PSDs was observed on the packing density and low-pressure modulus of coarse granular materials. The packing density and low-pressure modulus increased monotonically with θ, and their peak values were attained at a D value of approximately 2.2. However, there was no unique correspondence between the dense packing density and low-pressure modulus. The particle breakage was influenced differently by the initial PSDs, and it decreased with the values of D and θ. The emergence of the unique ultimate state was also identified from both the compression curves and PSDs of the samples after the tests. The potential implications of the test results in the design of both low and high rockfill dams were also demonstrated.

Prediction for the Settlement of Concrete Face Rockfill Dams Using Optimized LSTM Model via Correlated Monitoring Data

Settlement prediction is of great importance for safety control of concrete-face rockfill dams (CFRDs) during the operation stage. However, the prediction accuracy achieved by the commonly used hydrostatic–seasonal–time (HST) methods, without the consideration of the previous conditions of influencing factors, is not competitive. Moreover, in most methods, settlement data at each monitoring point are modeled individually; the correlation relationships between settlements are neglected. In this paper, a method based on an optimized long short-term memory (LSTM) model is proposed to predict the settlement of CFRDs, modeling multiple monitoring data series with strong correlation relationships simultaneously. In the method, settlement data series are classified into several categories, firstly according to a global relevance measure. Then, the cuckoo search (CS) algorithm is applied to optimize the hyper-parameters in the neural network structure of LSTM. Ultimately, the LSTM model is utilized to predict the multiple settlement data series classified in the same category. Results indicate that the proposed method has a better prediction performance compared with the LSTM model, the back propagation neural network (BPNN) model, and the HST with single monitoring point.

Effect of CNTs and MEA on the creep of face-slab concrete at an early age

Abstract The creep of face-slab concrete in a rockfill dam is critical for determining the restrained stress and cracking resistance of the concrete at an early age. In this article, the mix proportion of the face-slab concrete for a rockfill dam under construction without any cracking resistance additive was taken as the reference concrete (called JC) mix proportion. The carbon nanotubes (CNTs) and magnesium oxide expansion agent (MEA) were incorporated into JC to prepare face-slab concretes called NC and PC, respectively. The temperature–stress tests under temperature matching curing (TMC) and constant temperature curing (CTC) modes were conducted on these three kinds of concretes to investigate the effects of CNTs and MEA on the early-age creep properties of the face-slab concrete under variable stress conditions. The results showed that the creep performance of NC concrete under CTC mode was lower than that under TMC mode. Combined with mercury intrusion porosimetry test results, the mechanism of the effect of CNTs and MEA on creep was analyzed. The results showed that the temperature change may lead to the CNTs debonding from the cementitious matrix or matrix cracking for the NC concrete. The incorporation of CNTs can increase the early-age creep and improve the cracking resistance of concrete.

Post-construction safety assessment of a high concrete face rockfill dam

The most important issue regarding the safety and performance of concrete face rockfill dams (CFRDs) is to determine stresses and deformations. However, predictions of crest settlements of CFRDs are usually performed using empirical relationships. In recent years, CFRDs have been constructed much higher and at more complicated topographical and geological dam sites, so numerical analysis is required to evaluate the stress and deformations in the entire dam body. Also, assessing long-term rockfill behaviour and the concrete face membrane crack potential are important for post-construction dam safety. This study evaluates post-construction deformations and crest settlements of a high CFRD under two different loading cases, considering the non-linear elasto-plastic behaviour of the rockfill structure. As a constitutive model, the Duncan–Chang hyperbolic stress–strain model was applied in finite-element simulations considering the failure criterion is isotropic and hardening. Calibration of the rockfill's hyperbolic parameters was performed by comparing the calculated displacements with actual settlement measurements. Computed crest settlements were also compared with those of some CFRDs given in the literature. Calculated crest settlement and other deformations are in good agreement with the rockfill settlement measurements and in acceptable ranges, and do not threaten the dam's safety.

Study on the Tensile Properties and Application of Gravelly Soil Reinforced by Polypropylene Fiber

Incorporating fibers into gravelly soil is an effective method to prevent the core wall of high earth core rockfill dams (ECRDS) from cracking. In this study, a new type of soil tensile device was used to carry out tensile tests on gravelly soil with different gravel contents and fiber contents. The test results show that as the gravel content increases from 0% to 50%, the improvement in tensile strength decreases from 48.9% to 6.4%, which means the increase in gravel content reduces the improvement in tensile strength significantly. The ultimate tensile strain, tensile strength, and post-peak tensile strength of fiber-reinforced gravelly soil are positively correlated with the fiber content. Combined with the Scanning electron microscopy (SEM) results, the reinforcement effect of the three types of fiber interfaces on the gravel soil is qualitatively analyzed, and the microscopic mechanism of the improvement of the tensile strength of fiber-reinforced gravelly soil is revealed. The energy absorption capacity (EAC) results showed that the lower the gravel content in the soil, the higher the degree of improvement in the EAC value. In practical application, it is recommended to use gravel soil with low gravel content and high fiber content. Finally, a regression model considering the gravel content and fiber content was proposed for fast predicting the tensile strength of the soil. The related results can provide references for the anti-cracking design of the core wall of high ECRDS.

Effects of Fly Ash Dosage on Shrinkage, Crack Resistance and Fractal Characteristics of Face Slab Concrete

The crack resistance of face slab concretes to various shrinkages is crucial for the structural integrity and the normal operation of concrete-faced rockfill dams (CFRDs). In this work, the effects of fly ash with four dosages (i.e., 10%, 20%, 30% and 40%) on the drying shrinkage, autogenous shrinkage and the cracking resistance of face slab concrete were studied. Besides, the difference in shrinkage behavior due to fly ash addition was revealed from the viewpoint of the pore structure and fractal dimension of the pore surface (Ds). The findings demonstrate that (1) the incorporation of 10–40% fly ash could slightly reduce the drying shrinkage by about 2.2–13.5% before 14 days of hydration, and it could reduce the drying shrinkage at 180 days by about 5.1–23.2%. By contrast, the fly ash addition could markedly reduce the autogenous shrinkage at early, middle and long-term ages. (2) Increasing fly ash dosage from 0 to 40% considerably improves the crack resistance of concrete to plastic shrinkage. Nevertheless, the increase in fly ash dosage increases the drying-induced cracking risk under restrained conditions. (3) The pore structures of face slab concrete at 3 and 28 days become coarser with the increase in fly ash dosage up to 40%. At 180 days, the pore structures become more refined as the fly ash dosage increases to 30%; however, this refinement effect is not as appreciable as the fly ash dosage increases from 30% to 40%. (4) The Ds of face slab concrete is closely related with the concrete pore structures. The Ds of face slab concrete at a. late age increases from 2.902 to 2.946 with increasing of the fly ash dosage. The pore structure and Ds are closely correlated with the shrinkage of face slab concrete. (5) The fly ash dosage around 30% is optimal for face slab concretes in terms of lowering shrinkage and refining the pore structures, without compromising much mechanical property. However, the face slab concretes with a large fly ash dosage should be well cured under restrained and evaporation conditions at an initial hydration age.

Rockfill material segmentation and gradation calculation based on deep learning

The gradation of rockfill materials affected the rolling quality and body strength of earth-rockfill dam. To address the limitations of manual screening and traditional computer vision methods, a gradation detection method suitable for rockfill dam construction sites was proposed based on deep learning, which can output the detection results end-to-end. The method utilized the modified MASK R-CNN model to locate and segment rockfill materials. Specifically, the model used ResNeXt101 as the backbone network and adds the squeeze-and-excitation block to enhance the feature extraction capability of the model. The detection results of 500 test images showed that proposed method had high accuracy, the average precision and intersection over union value of the model reach 0.934 and 0.879. The morphological characteristics of rockfill particles were extracted by rotating calipers algorithm and used for grading calculation. Comparing this calculation method with manual screening, the results indicated that the overall fitting effect of the gradation detected by proposed method was in accord with the actual gradation parameters. The gradation of rockfill materials can be obtained within 60 s. This paper provided a new method for rapid analysis of particle size distribution, which can be used for auxiliary construction.

Predicting the Crest Settlement of Concrete Face Rockfill Dams by Combining Threshold Regression and Support Vector Machine

Predicting the Crest Settlement of Concrete Face Rockfill Dams by Combining Threshold Regression and Support Vector Machine

Analysis on damage causes of built-in corridor in core rock-fill dam on thick overburden: A case study

Analysis on damage causes of built-in corridor in core rock-fill dam on thick overburden: A case study

Seismic response analysis of concrete-faced rockfill dams with dual mortar contact method

Many high concrete-faced rockfill dams (CFRD) are constructed or planned with high seismic intensity. It is challenging to evaluate the resultant seismic response involving multi-body interactions. In this paper, a dual mortar contact method for multi-process sequential analysis accounting for staged construction, water storage, creep effects and seismic loading is presented. A complete derivation of the strong and weak forms, spatial and temporal discretizations, material and contact non-linearity treatments is provided as a solid basis to incorporate the rockfill constitutive behavior and stable algorithm for thin-walled inter-slab interfaces. The present method based on using nonconforming meshes is validated by the comparisons with conforming meshes and then applied to an ideal CFRD. The issues of post-earthquake extrusion damage, slab–slab separation and slab–cushion separation, which are major concerns in CFRDs, are discussed in detail. The additional extrusion stress of the concrete face during an earthquake can be up to 20 MPa and therefore calls for considerations in the design and safety assessment.

Evaluation of the Effect of Varying the Angle of Asphaltic Concrete Core on the Behavior of the Meijaran Rockfill Dam

The use of asphaltic concrete cores for sealing embankments and rockfill dams is very important. The self-healing properties of bitumen, simple construction in cold and rainy conditions compared to clay cores, good flexibility and connection with embankment materials are the essential characteristics of asphaltic concrete. The main concern regarding the use of asphaltic concrete cores in Iran is mainly the performance of these dams under seismic loads. The evaluations of the performance of these types of dams in other countries show that asphaltic concrete cores perform satisfactorily in the static state, but in earthquake conditions, the situation may be different. In this paper, the static and seismic behavior of the Meijaran dam in Iran, Mazandaran, is evaluated for three core angles of 90°, 60° and 45°. This evaluation was conducted at the end of the impounding stage and after applying seismic loads using FLAC 2D software and Mohr–Coulomb consitutive models. The results were matched with the ICOLD recommendation to use angled cores in dams with asphaltic cores and showed that the dam performs better with angled cores. Finally, for the Meijaran dam, the results from the dynamic analysis are compared with the results from the centrifuge test.

Failure of the Downstream Shoulder of Rockfill Dams Due to Overtopping or Throughflow

This paper presents the results of an extensive laboratory set of tests aimed to study the failure of the downstream shoulder of highly permeable rockfill subjected to overflow. The experimental research comprised testing 114 physical models by varying the following elements: (i) the median size of the uniform gravels (7 to 45 mm); (ii) the configuration of the dam, i.e., upstream and downstream shoulders and crest or just the downstream shoulder; (iii) the dam height (from 0.2 to 1 m), (iv) the crest length (from 0.4 to 2.5 m), (v) the downstream slope (from 1 to 3.5 H:V), (vi) the type of impervious element (i.e., central core, upstream face, and no impervious element). The tests allowed us to identify two failure mechanisms, slumping and particle dragging. In addition, the downstream slope was observed to be one of the most important variables in this parametric study, as it influenced the pore water pressures inside the dam, the failure discharge, and the occurrence of one or the other mechanism of failure.

Placed Riprap Deformation Related to Axial Load at Toe Support: Physical Modelling

Rockfill dams are hydraulic structures of major importance. They can be exposed to extreme flood events, in turn leading to overtopping. These phenomena erode and affect structural and geotechnical integrity, which in turn can cause dam breach. Ripraps are broadly used for rockfill dam protection against such erosion processes. For steep slopes, as the one considered in this study (S = 1:1.5, vertical: horizontal), understanding the riprap behavior during overtopping is an important issue to improve dam design and reinforcement techniques. In this work, datasets are obtained from five experimental models of placed riprap built on a rock filter layer in a flume, at the hydraulic laboratory of the Norwegian University of Science and Technology, Trondheim. The riprap stones were placed in an interlocking pattern with a metallic support at the toe. The models were subjected to successive and incremental overtopping discharges until their complete failure. A laser traverse system was used to measure the coordinates (3D) of individual marked riprap stones between each discharge increase. Six load cells located at the toe measured the imposed loads during the entire procedure. From the total load values, two different types of load contributions could be distinguished: the self-weight of the stones and the hydraulic load depending on the discharge level of the overflow. This article highlights the strong relation in each of the five tests between riprap stone displacements, axial reaction load values measured at the toe and overtopping discharges. Moreover, as demonstrated in previous works, a buckling deformation of the riprap layer was observed and described. The results demonstrate that as the hydraulic load induces 2D deformations of the riprap, a larger part of the riprap weight is supported at the toe. Thus, the measured axial load during overtopping arises both from the hydraulic load and from the load imparted due to the compaction of the riprap layer. This compaction effect induces an even greater load than the one imposed due to the hydraulic contribution. The results from this study are finally put into perspective with the Norwegian Water Resources and Energy Directorate recommendations for full scale dams and suggest the great resistance of supported riprap at the toe.

Embankment Dam Design with Dispersive Soil: Solutions and Challenges

The configuration and safety factor of rockfill dams design depend on materials used in construction of the dam. In some regions, the available materials for using as impermeable element of the dam body are dispersive and it is inevitable to consider the dispersivity of the material in the design and construction. Internal erosion through cracks or other openings in the embankment is the most concern of using the dispersive soils. The authors have designed an embankment dam by using dispersive soil in East Africa after finalizing the dispersivity of materials based on the required tests. The foundation of dam and borrow areas consisted of dispersive soil and the effective measures has been considered in the design to safely deal with dispersivity according to recommendation of the design codes. The selected solution was the combination of chimney filter, filter drain, and selective placement of materials in the dam body as the effective solution considering the specification of the project. More precautions in addition to general recommendation have been incorporated into the design criteria due to placing the project in high hazard category. The local instability in the contact zones was the concerns of designers and bilinear parameters has been considered in the stability analysis as the base case. A drainage layer was added into half of the length of filter blanket and the feature of the design was determined according to nonlinear static and dynamic analysis of the dam body. This paper presents the measures and precautions that were taken into account in dealing with dispersive soil in order to ensure the safety of design.

The Inversion Analysis and Material Parameter Optimization of a High Earth-Rockfill Dam during Construction Periods

Inversion analysis is usually an efficient solution to process the monitoring data of earth-rockfill dams. However, it is still difficult to obtain calculation results that are consistent with monitoring data due to different construction statuses. To deal with this situation and to introduce a new solution to improve calculation accuracy, the general method of inversion analysis based on back-propagation neural networks and the original step-by-step inversion method assuming that the parameters of the constitutive model vary with construction periods are introduced and verified in this work. Then, both methods are applied in the inversion analysis of a high gravelly soil core rock-fill dam during construction periods. Moreover, the relationship between the inversed material parameters and the stress values of the core wall is discussed. The material parameters are further optimized to obtain more accurate displacement values. The results show that the step-by-step inversion method has a higher accuracy in vertical compression values compared with the conventional inversion method, the trend of material parameter K is more significant than other parameters, and the proposed variable parameter constitutive model has an accuracy between the step-by-step and conventional inversion methods. Conclusions can be drawn that the original step-by-step inversion method has more advantages than the conventional method and the variable parameter constitutive model proposed in this paper might be more suitable for the analysis of a high earth-rockfill dam during construction periods.