Reliability analysis of earth dams using direct coupling

Numerical methods are helpful for understanding the behaviors of geotechnical installations.However,the computational cost sometimes may become prohibitive when structural reliability analysis is performed,due to repetitive calls to the deterministic solver.In this paper,we show how accurate and efficient reliability analyses of geotechnical installations can be performed by directly coupling geotechnical software with a reliability solver.An earth dam is used as the study object under different operating conditions.The limit equilibrium method of Morgenstern-Price is used to calculate factors of safety and find the critical slip surface.The conmercial software packages Seep/W and Slope/W are coupled with StRAnD structural reliability software.Reliability indices of critical probabilistic surfaces are evaluated by the first-and second-order structural reliability methods(FORM and SORM),as well as by importance sampling Monte Carlo(ISMC)simulation.By means of sensitivity analysis,the effective friction angle(φ′)is found to be the most relevant uncertain geotechnical parameter for dam equilibrium.The correlations between different geotechnical properties are shown to be relevant in terms of equilibrium reliability indices.Finally,it is shown herein that a critical slip surface,identified in terms of the minimum factor of safety(FS),is not the critical surface in terms of the reliability index.

Control of Seepage through Earth Dams Based on Pervious Foundation Using Toe Drainage Systems

Many dangerous effects arise from seepage through earth dams based on pervious layer. Therefore, the dam embankment must be provided with seepage control measures to avoid such effects. In the present work, different control methods were used such as flat slopes, toe drainage systems, and a catch drain in the tail water. The hydraulic performance of each control measure was evaluated using the analytical solutions, previously developed, to estimate the seepage quantity (q), the height of seepage surface (h3), and the coordinates of the free surface (hx). Study was conducted on a physical model for a dam embankment having a top width (b) = 10.0 meter, height (Hd) = 30.0 meter, and slope factor (m) = 1.5. The obtained results were analyzed and presented in dimensionless charts. Results showed that, the used control measures possess a great effect on the characteristics of seepage through earth dams based on pervious foundations. A comparative study was conducted between the studied toe drainage systems to enable the designers the better choice for design purposes.

Evaluation of behaviors of earth and rockfill dams during construction and initial impounding using instrumentation data and numerical modeling

In this study,the behavior of Gavoshan dam was evaluated during construction and the first impounding.A two-dimensional（2D） numerical analysis was conducted based on a finite difference method on the largest cross-section of the dam using the results of instrument measurements and back analysis.These evaluations will be completed in the case that back analysis is carried out in order to control the degree of the accuracy and the level of confidence of the measured behavior since each of the measurements could be controlled by comparing it to the result obtained from the numerical model.Following that,by comparing the results of the numerical analysis with the measured values,it is indicated that there is a proper consistency between these two values.Moreover,it was observed that the dam performance was suitable regarding the induced pore water pressure,the pore water pressure ratio r;,settlement,induced stresses,arching degree,and hydraulic fracturing probability during the construction and initial impounding periods.The results demonstrated that the maximum settlement of the core was 238 cm at the end of construction.In the following 6 years after construction（initial impounding and exploitation period）,the accumulative settlement of the dam was 270 cm.It is clear that 88% of the total settlement of the dam took place during dam construction.The reason is that the clay core was smashed in the wet side,i.e.the optimum moisture content.Whereas the average curving ratio was 0.64 during dam construction; at the end of the initial impounding,the maximum amount of curving ratio in the upstream was 0.81,and the minimum（critical） amount in the downstream was 0.52.It was also concluded that this dam is safe in comparison with the behaviors of other similar dams in the world.

Numerical analysis of nonlinear dynamic behavior of earth dams

A numerical study is conducted to investigate the dynamic behavior of earth dams.The numerical investigation employs a fully nonlinear dynamic finite difference analysis incorporating a simple elastic perfectly plastic constitutive model to describe the stress-strain response of the soil and the Rayleigh damping to increase the level of hysteretic damping.The extended Masing rules are implemented into the constitutive model to explain more accurately the soil response under general cyclic loading.The soil stiffness and hysteretic damping change with loading history.The procedures for calibrating the constructed numerical model with centrifuge test data and also a real case history are explained.For the latter,the Long Valley(LV)earth dam subjected to the 1980 Mammoth Lake earthquake as a real case-history is analyzed and the obtained numerical results are compared with the real measurements at the site in both the time and frequency domains.Relatively good agreement is observed between computed and measured quantities.It seems that the Masing rules combined with a simple elasto-plastic model gives reasonable numerical predictions.Afterwards,a comprehensive parametric study is carried out to identify the effects of dam height,input motion characteristics,soil behavior,strength of the shell materials and dam reservoir condition on the dynamic response of earth dams.Three real earthquake records with different levels and peak acceleration values(PGAs)are used as input motions.The results show that the crest acceleration decreases when the dam height increases and no amplification is observed.Further,more inelastic behavior and more earthquake energy absorption are observed in higher dams.

Seismic response of an earth dam:finite element coupling analysis and validation from centrifuge tests

Variations in acceleration and excess pore water pressure during a seismic event are critical early-warning indicators of an impending dam collapse.To assess these variations,the seismic responses for three simplified model dams,based on cross-sections through a real earthen dam,were assessed with numerical simulations and centrifuge tests.A normalized root-mean-square error was utilized as a comparison index to assess the closeness between simulated and the recorded values.Assuming that the experimental records are reliable,the reliability of the numerical program was evaluated using this root-mean-square error estimation approach.Explanations for inconsistency between the two approaches are presented.The conclusions are drawn from the results of the three model dams.

Seepage safety monitoring model for an earth rock dam under influence of high-impact typhoons based on particle swarm optimization algorithm

Extreme hydrological events induced by typhoons in reservoir areas have presented severe challenges to the safe operation of hydraulic structures. Based on analysis of the seepage characteristics of an earth rock dam, a novel seepage safety monitoring model was constructed in this study. The nonlinear influence processes of the antecedent reservoir water level and rainfall were assumed to follow normal distributions. The particle swarm optimization （PSO） algorithm was used to optimize the model parameters so as to raise the fitting accuracy. In addition, a mutation factor was introduced to simulate the sudden increase in the piezometric level induced by short-duration heavy rainfall and the possible historical extreme reservoir water level during a typhoon. In order to verify the efficacy of this model, the earth rock dam of the Siminghu Reservoir was used as an example. The piezometric level at the SW1-2 measuring point during Typhoon Fitow in 2013 was fitted with the present model, and a corresponding theoretical expression was established. Comparison of fitting results of the piezometric level obtained from the present statistical model and traditional statistical model with monitored values during the typhoon shows that the present model has a higher fitting accuracy and can simulate the uprush feature of the seepage pressure during the typhoon perfectly.

Top coal caving mining technique in thick coal seam beneath the earth dam

It is important to study the mining technology under structures for raising the coal resources recovery ratio. Based on the geological and mining conditions, the top coal caving harmonic mining technique in thick coal seam beneath the earth dam was put forward and studied. The 5 factors such as the panel mining direction, panel size, panel location, panel mining sequence and panel advance velocity were taken into account in this technique. The dam movement and deformation were predicted after the thick coal seam mining and the effects of mining on the dam were studied. By setting up the surveying stations on the dam, the movement and deformation of the dam were observed during mining. By taking some protective measures on the dam, the top coal caving mining technique in thick coal seam beneath the earth dam was carried out successfully. The study demonstrates that harmonic mining in thick coal seam is feasible under the dam. The safety of the earth dam after mining was ensured and the coal resources recovery ratio was improved.

A quantitative model for danger degree evaluation of staged operation of earth dam reservoir in flood season and its application

Based on the natural disaster risk evaluation mode, a quantitative danger degree evaluation model was developed to evaluate the danger degree of earth dam reservoir staged operation in the flood season. A formula for the overtopping risk rate of the earth dam reservoir staged operation was established, with consideration of the joint effect of flood and wind waves in the flood sub-seasons with the Monte Carlo method, and the integrated overtopping risk rate for the whole flood season was obtained via the total probability approach. A composite normalized function was used to transform the dam overtopping risk rate into the danger degree, on a scale of 0-1. Danger degree gradating criteria were divided by four significant characteristic values of the dam overtopping rate, and corresponding guidelines for danger evaluation are explained in detail in this paper. Examples indicated that the dam overtopping danger degree of the Chengbihe Reservoir in China was 0.33-0.57, within the range of moderate danger level, and the flood-limiting water level （FLWL） can be adjusted to 185.00 m for the early and main flood seasons, and 185.00-187.50 m for the late flood season. The proposed quantitative model offers a theoretical basis for determination of the value of the danger degree of an earth dam reservoir under normal operation as well as the optimal scheduling scheme for the reservoir in each stage of the flood season.

Seismic rehabilitation and analysis of Chaohe earth dam

Stability of earth dams during earthquakes has been a major concern for gcotechnical engineers in seismic active regions. Liquefaction induced slope failure occurred at the upstream slope of a major earth dam in the suburb of Beijing, China, during the 1976 Tangshan Earthquake. The gravelly soil with loose initial condition liquefied under relatively small ground vibration. In recent years, a major seismic rehabilitation project was carried out on a similar earth dam nearby using dumped quarry stone. Seismic stability analysis was carried out using model test, finite element simulation, and pseudostatic slope stability program after taking into account the influence of excess pore pressure.

Application of 3D Electrical Resistivity Tomography for Diagnosing Leakage in Earth Rock-Fill Dam

The leakage occurs during operation of the dam in Liuhuanggou reservoir. It’s a threat to the safety of the people’s lives and property in downstream. In order to eliminate the hidden danger of reservoir, ensure the safety of the dam, play better the function of flood control and water storage of the reservoir etc., we apply the 3D electrical resistivity tomography detecting technology and volume rendering image processing technology, make the measurement in field, process the data and combine the field survey to find out the leakage channels inside the dam. The results show that the 3D resistivity images appear the low resistivity zone corresponding with the leakage channels. There are two main leakage channels that come from different location inside the dam. It is feasible to diagnose the leakage in earth rock-fill dam by applying 3D electrical resistivity tomography.

A general framework for modeling long-term behavior of earth and concrete dams

Many problems are linked with the long-term behavior of both earthdams and concrete dams.They range from hydraulic fracturing to alkali-silica reaction(ASR)and to repair work in concrete dams,from seismic behavior to secondary consolidation in earthdams.A common framework for the simulation of such systems is shown,based on the mechanics of multiphase porous media.The general model is particularized to specific situations and several examples are shown.

Effect of freeze-thaw cycles on mechanical behavior of clay-gravel mixtures

The use of clay-gravel mixtures(i.e.,adding excavated or natural gravel particles into clay soil matrix)as the main filling materials is increasing in the anti-seepage system of high Earth Core Rockfill Dams(ECRDs).With the continuous construction of high ECRDs in the Chinese plateaus and cold regions,it is of great urgency and importance to understand the physical and mechanical characteristics of compacted clay-gravel mixtures under freeze-thaw action.To this end,laboratory freezing-thawing tests,computed tomography(CT),and triaxial compression tests were conducted to evaluate the effects of freezethaw cycles on moisture loss,pore structure characteristics,stress-strain behavior,failure strength,elastic modulus,cohesion,and internal friction angle of compacted clay-gravel mixtures.The results demonstrate that,1)the freeze-thaw cycle significantly changed the mechanical characteristics of the clay-gravel mixture samples,but the shape of the stress-strain curve is less sensitive to it.2)The failure strength of samples exhibits a significant decrease after the first freeze-thaw cycle,but shows a certain increase as the number of freeze-thaw cycles increases from 1 to 2.3)The elastic modulus of samples first decreases and then increases with increasing freeze-thaw cycle,and the most severe deterioration was observed after the first freeze-thaw cycle.4)Regardless of the number of freeze-thaw cycles,there is a linear relationship between failure strength and elastic modulus for a sample that has suffered freeze-thaw weathering.5)The cohesion of samples decreases firstly and then slightly increases with increasing freeze-thaw cycles,while the internal friction angle is hardly affected.