Exit Gradient Research Articles

EFFECT OF MUTUAL INTERFERENCE PILES ON SEEPAGE PROPERTIES UNDER HYDRAULIC STRUCTURES

The seepage phenomenon (uplift pressure - flow rate - exit gradient of hydraulic structures) is one of the main causes of failure or collapse of hydraulic structures, so it has been reduced using sheet piles under floor of hydraulic structures. In this study, effect of mutual interference piles were studied on seepage phenomenon by using finite elements program ANSYS. The results were verified with the practical results L-SAYED which given a good correlation. It was found that the use of the pile in the upstream reduced the uplift pressures by 8.36%, and the pile in the downstream increased it by 11.66%, the flow rate reduced by 66.8% and exit gradient of the hydraulic structures reduced by 28.28%.

Studying the seepage phenomena under a concrete dam using SEEP/W and Artificial Neural Network models.

Seepage under hydraulic structures is considered to be a dangerous phenomenon which may cause the collapse of the structure over time if neglected. In this research, a SEEP/W model was developed to find the seepage rate and exit gradient under a concrete dam provided with two sheet piles. The independent variables were head difference; coefficient of soil permeability; and the spacing, lengths, and inclined angles of the sheet piles. The model was run for three different values of each independent variable. The results obtained from SEEP/W model were then used to create two neural artificial network (ANN) models (A and B) in which the output variables were the seepage rate (model A) and exit gradient (model B). The most appropriate structure, which gave minimum relative errors, was (7 3 1) nodes for both models. The results of the ANN models indicated that the variable with the most effect on seepage rate was the coefficient of soil permeability, with an importance ratio of about 76%, followed by the difference in the head (8%), the distance between piles (5.5%), length of downstream pile (5%), length of upstream pile (4%), and downstream and upstream inclined angles of the sheet piles, with ratios of about 1% and 0.5%. In terms of exit gradient, the most influential factor was the distance between piles at 35%, followed by the downstream inclination angle, length of downstream pile, head difference, length of upstream pile, inclined angle of upstream pile, and soil permeability with importance of about 23%, 19%, 14%, 7.5%, 1% and 0.5%, respectively. These results are in agreement with an analysis of the SEEP/W model.

Stability Analysis of Flood Bunds: A Study on Geotechnical Health Evaluation of Embankments

Flood constitutes one of the world’s most serious environmental hazards. Flood bunds are the earthen hydraulic structures which are constructed along the River to control the Flood water to avoid damages to the infrastructures, crops, livestock and loss of human lives. Pakistan which lies in the Indus Basin has been facing severe threats and losses from the floods since histories. About 6807km length of flood embankment has been constructed to safeguard against the floods in the country. Punjab has been worst hit province by heavy floods and rains causing heavy loss. Geotechnical Evaluation is vital for proper functioning of such structures. In this study four flood bunds susceptible to potential embankment breaching during flood have been selected along river Chenab in district Muzaffar Garh Irrigation zone. Suite of analysis using GeoStudio software (SLOPE/W and SEEP/W for stability and seepage analysis respectively) has been performed by considering four different critical scenarios, (1) steady state at highest flood level (2) rapid drawdown from highest flood level (3) steady state at extreme condition with 3 feet free board (4) rapid drawdown from extreme condition with 3 feet free board. The safety of the flood bunds is evaluated in terms of River Embankment Breaching Vulnerability Index (REBVI), safe exit gradient and factor of safety against slope failure. Based on these calculated factors from the numerical analysis it is found that embankment at RD:25,29 are marginally safe, whereas at RD: 147, 157 are susceptible to pipping failure. Therefore, it is recommended to make embankment impervious using cohesive material and to install cut-off walls or berms to lengthen the seepage path.

Application of a Genetic Algorithm for the Optimization of a Location and Inclination Angle of a Cut-Off Wall for Anisotropic Foundations Under Hydraulic Structures

A genetic algorithm technique, integrated with a numerical model (finite element method), was used in this study to compute the optimal cut-off location and angle of inclination for barrages constructed on homogenous anisotropic soil foundations. The exit gradient function is minimized as an objective function in the problem formulation. Constraints included uplift pressure functions and safety factors taking account of a minimum concrete floor thickness. Various degrees of anisotropy and different values of a range of heads have been studied. Around 1400 different situations (cases) were simulated using SEEP2D GMS software. Statistical nonlinear regression models were used to predict the pressure head and exit gradient for anisotropic soil foundations under various degrees of inclination (θ), relative location (b1/b) and relative depth (d/b). A genetic algorithm optimization model was also performed using Matlab. The study results indicated that the computed optimum cut-off locations and relative inclination angles were affected by changing the anisotropic ratio and relative cut-off depth. For isotropic soil foundations, for cases with a cut-off ratio (d/b) of up to 0.4, the optimum inclination angle ranged between 59° and 68° for upstream locations. In other relative cut-off depth cases, the cut-off can work efficiently when situated in the last third of the concrete floor foundation, this considered the optimum location. For anisotropic soil, optimum locations tended to be upstream for most d/b values.

A Multi-objective Simulation–Optimization Approach for Design of Cutoff Walls and Apron of Diversion Dams

Various seepage problems such as uplift pressure and high exit gradient lead to less efficient hydraulic structures since aims of the hydraulic design are substantial to be achieved. Cutoff walls and aprons are effective implements to control the problematic consequences of seepage flow. This paper presents a novel methodology for optimal design of cutoff walls and apron beneath Yusufkand diversion dam in Iran based on Non-dominated Sorting Genetic Algorithms-ΙΙ (NSGA-ΙΙ) multi-objective optimization model, Multi-Layer Perceptron (MLP) neural network model and Young Conflict Resolution Theory (YCRT). First, input–output data of a simulation model of seepage flow through the foundation of an existing diversion dam using Geostudio software are utilized to train and validate the MLP neural network, which can model the hydraulic performance of cutoff walls and the upstream apron. Then, the validated MLP neural network models are linked to NSGA-ΙΙ multi-objective optimization model and a trade-off curve is acquired among the hydraulic and cost criteria. Finally, YCRT is used to find the compromise optimal solution on the Pareto-front. Results show that by increasing length of the apron, when cutoff walls are at their minimum depth, the reduction of 18, 14 and 17% have been resulted in vertical exit gradient, uplift force and seepage flow discharge, respectively. Conversely, the same increment in length of the apron caused 2, 0 and 2% reduction in the mentioned parameters when other two cutoff walls are at their maximum depth. Moreover, the construction cost pertained to 1 meter of cutoff wall is nearly twice as much as for 1 meter of apron.

Effect of hydraulic conductivity of unsaturated soil on the earth dam performance

In this paper, the finite element method is uzed to solve the governing equations of flow through earth dams. The computer program Geo-Slope is used in the analysis through its sub-program named SEEP/W. A case study is considered to be Al-Adhaim dam which consists of zoned embankment with a total length of 3.1 km. The dam in its actual design is analyzed. Then, an attempt is made to study the seepage in unsaturated zone of the dam through studying the effect of several parameters including the effect of changing the unsaturated hydraulic conductivity with the degree of saturation of the core soil and changing of water level in the reservoir. A procedure is proposed to define the hydraulic conductivity function from the soil water characteristic curve which is measured by the filter paper method. Fitting methods are applied through the program SoilVision. A parametric study was carried out and different parameters were changed to study their effects on the behavior of partially saturated soil. The study reveals that the rate of flow is decreased by about 20 - 27% when the degree of saturation of the core material is decreased from 100% to 50% at water level 115.75 m, while the exit gradient of flow is decreased by about 13 -15%. This decrease in flow rate becomes 13-15% and 8-9.5% when the reservoir water level is 131.5 m and 143.5 m, respectively, while the exit gradient of flow is increased by about 23-29.5% and 29-29.5% when the reservoir water level is 131.5 m and 143.5 m, respectively. When the state of soil changes from fully saturated S= 100% to partially saturated S= 90%, a rapid increase in head gradient and pore water takes place at the embankment base for different water levels in the reservoir. This decrease plateaus out on further decrease in the degree of saturation.

Plausible failure mechanisms of wildlife-damaged earth levees: insights from centrifuge modeling and numerical analysis

Earth levees are subject to a wide range of wildlife intrusion patterns that cause mass removal and subsequent serious deformations. Such invasive activities leave the body of an earth embankment with burrow systems too complex to map and model using conventional techniques. This study investigates the impact of different idealized configurations of animal burrows on the geotechnical performance of levees. For this purpose, centrifuge testing was conducted on homogenous scaled-down 1 horizontal : 1 vertical (1H:1V) levee models built from silty sand material. Modeling involved introducing horizontal cylinder-shaped waterside and landside burrows at different elevations within the levee section. The reference (intact) and deteriorated levee models were subject to a centrifugal acceleration of 35g, which was kept constant as the water level behind the levee model was gradually increased. The deformation profile of the model was tracked, and the crest displacements were concurrently measured. Miniature pore pressure transducers (PPTs) embedded within the levee body provided pore pressure measurements. A three-dimensional finite element model was developed to investigate the hydraulic performance and verify the failure patterns of the deteriorated levees. Compared with an intact levee, the presence of animal intrusions was found to increase the exit hydraulic gradient for both waterside and landside intrusions. Lower animal burrows appeared to cause larger exit gradients than higher ones. Similarly, waterside burrows exhibited a notably higher pore pressure and larger hydraulic gradient. Waterside damage resulted in a quicker and more violent failure than landside burrows. The failure mechanisms for both the waterside and landside burrows are dissimilar despite their similarly abrupt nature.

Optimum Dimensions of Hydraulic Structures and Foundation Using Genetic Algorithm coupled with Artificial Neural Network

A model using the artificial neural networks and genetic algorithm technique is developed for obtaining optimum dimensions of the foundation length and protections of small hydraulic structures. The procedure involves optimizing an objective function comprising a weighted summation of the state variables. The decision variables considered in the optimization are the upstream and downstream cutoffs lengths and their angles of inclination, the foundation length, and the length of the downstream soil protection. These were obtained for a given maximum difference in head, depth of impervious layer and degree of anisotropy. The optimization carried out is subjected to constraints that ensure a safe structure against the uplift pressure force and sufficient protection length at the downstream side of the structure to overcome an excessive exit gradient. The Geo-studio software was used to analyze 1200 different cases. For each case thelength of protection (L) and volume of structure (V) required to satisfy the safety factors mentioned previously were estimated for the input values, namely, the upstream cutoff depth (S1), the downstream cutoff depth (S2), the foundation width (B), the angle of inclination of the upstream cutoff (Ɵ1) and the angle of inclination of the downstream cutoff (Ɵ2), H (differencehead), kr (degree of anisotropy) and D (depth of impervious layer). An ANN model was developed and verified using these cases input-output sets as its data base. A MatLAB code was written to perform a genetic algorithm optimization modeling coupled with this ANN model using a formulated optimization model. A sensitivity analysis was done for selecting the crossover probability, the mutation probability and level,the number of population, the position of the crossover and the weights distribution for all the terms of the objective function. Results indicate that the most factors that affects. the optimum solution is the $ number of population required. The minimum value that gives stable global optimum solution of this parameter is (30000) while other variables have little effect on the optimum solution.

Optimizing Location of Cutoffs to Improve Safety and Economy of Hydropower Projects

Large numbers of perennial rivers flowing through steep gradient has made Nepal an ideal place for hydropower development. Development of hydropower project in most economical way possible is the first prerequisite to get maximum benefit from harnessing water resources. The maximum benefit can be achieved only through optimal design of the project. Each project component must be designed considering the required level of factor of safety, performance standards of each component and economy. On this backdrop, this paper intends to optimize the location of cutoffs in headworks to get maximum benefits from use of it in terms of safety and economy.Improper placement of cutoff not only reduces the factor of safety against piping but also curtails the project benefits due to excessive seepage flow. Piping (internal erosion of soil particles under structure) is associated with high exit gradient. It threatens the structural stability and ultimately leads to failure of structure while as excessive seepage flow limits the availability of flow for power generation and reduces the project benefits. So, the optimal design of headworks is an attempt to identify the best location of cutoff to control seepage flow and reduce exit gradient. Five different cases representing different location of cutoff were analyzed and their roles in controlling seepage flow and reducing exit gradient was evaluated to optimize the cutoff location. 2D Finite Element Method (FEM) was used for the seepage analysis. The analysis showed that cutoffs are essential to control seepage flow when dam/weir is founded in pervious soil. However, the best location to place cutoff must be adroitly identified to reap maximum benefit from use of cutoff. The analysis reveals that the role of central cutoff in controlling seepage flow and reducing exit gradient is very limited. Likewise, the u/s cutoff has minimum effect in reducing exit gradient but the d/s cutoff seemed very effective in reducing exit gradient resulting increased factor of safety against piping. Hence, this paper concludes that the d/s end is the optimal location to place cutoff to improve safety and economy of the project. In addition, the use of both u/s and d/s cutoffs extend positive roles both in controlling seepage flow and reducing exit gradient. However, in author’s opinion the construction cost of two cutoffs must be compared with benefit added by use of two cutoffs over use of single d/s cutoff. HYDRO Nepal JournalJournal of Water Energy and EnvironmentIssue: 21, July, 2017Page: 42-44Upload Date: July 18, 2017

Design Gradient on the Interfacial Heaving of Clay Soils of Seepage-Control Structures of Dams

The interface between the core and the filter of a dam looks like a “fir tree”—core teeth sinking into the filter and shoulder of the dam. The exit gradient in the cases of straight core face (as per the SNiP) and toothed core face is considered, the gradient being higher in the latter case.

Effects of spatial autocorrelation structure of permeability on seepage through an embankment on a soil foundation

Theoretical autocorrelation functions (ACFs) are generally used to characterize the spatial variation of permeability due to the limited number of site investigation data. However, many theoretical ACFs are available in the literature, and there are difficulties in selecting a suitable ACF for general cases. This paper proposes using the random finite element method to investigate the effects of ACF on the seepage through an embankment. Five commonly used ACFs—the squared exponential (SQX), single exponential (SNX), second-order Markov (SMK), cosine exponential (CSX) and binary noise (BIN) ACFs in the literature—are compared systematically by a series of parametric studies to investigate their influences on the seepage flow problem. Both stationary and non-stationary random fields are considered in this study. The results show that the commonly used SQX and SNX ACFs may overestimate and underestimate the seepage flow rate, respectively. It is also known that the maximum exit gradient associated with the SNX ACF is larger than those obtained using the other four ACFs. Additionally, it is proved that the deterministic approach-based design is on the conservative side and tends to be too conservative when dealing with soils with greater variation in the properties. It is also found that the SQX ACF has a higher probability of providing a more conservative design in practice. Overall, the differences between different ACFs are not significant and are within acceptable ranges.

Seepage Analysis through an Earth Dam (KHASA-CHAI Dam) as a Case Study

In this research KHASA-CHAI Dam that consists of zoned embankment was investigated by using finite element method. The finite element computer software SEEP/W was used. Experimental works were done to the soils that enters in the construction of the dam to obtain the different parameters that SEEP/W software need in order to complete the analysis. The dam at its actual design was investigated by considering the water in the reservoir to be at maximum, minimum and half filled with water. Then the control of seepage and exit gradient through the dam were investigated by studying the effect of changes in the construction of the dam. It was concluded that the core in the dam has an important effect on decreasing the seepage quantity through the dam body. The presence of filters in the dam has small effect on increasing seepage quantity, but they have great effect on decreasing exit gradient.

Darcian flow under/through a leaky cutoff wall: Terzaghi–Anderson's seepage problem revisited

SummaryAn analytical solution is obtained for 2‐D steady Darcian flow under and through a cutoff wall partially obstructing a homogeneous isotropic foundation of a dam. The wall is leaky; that is, flow across it depends on the ratio of hydraulic conductivity of the wall and the wall thickness that results in the third‐type (Robin) boundary condition along the wall, as compared with the Terzaghi problem for an impermeable wall. The Laplace equation for the hydraulic head is meshlessly solved in a non‐standard flow tube. A Fredholm equation of the second kind is obtained for the intensity of leakage across the wall. The equation is tackled numerically, by adjusted successive iterations. Flow characteristics (total Darcian discharge and its components through the wall and the window between the wall top and horizontal bedrock, stream function, head distribution, and Darcian velocity along the wall and tailwater bed) are obtained for various conductivity ratios, head drops across the structure, thicknesses of the foundation, and the degree of its blockage by the wall. Comparisons with the Terzaghi limit of an impermeable wall show that for common wall materials and thicknesses, the leakage may constitute tens of percent of the discharge under the dam. The through‐flow hydraulic gradients on a vertical wall face (Robin's boundary condition) as well as the exit gradients along a horizontal tailwater boundary (Dirichlet's boundary condition) acting for decades have deleterious impacts on dam stability because of potential heaving, piping, and mechanical–chemical suffusion. Copyright © 2017 John Wiley & Sons, Ltd.

Stochastic analysis of seepage under water-retaining structures

This paper investigated the problem of confined flow under dams and water-retaining structures using stochastic modelling. The approach advocated in the study combined a finite-element method based on the equation governing the dynamics of incompressible fluid flow through a porous medium with a random field generator that generates random hydraulic conductivity based on log normal probability distribution. The resulting model was then used to analyse the confined flow under a hydraulic structure. Cases for a structure provided with cut-off wall and when the wall did not exist were both tested. Various statistical parameters that reflected different degrees of heterogeneity were examined and the changes in the mean seepage flow, the mean uplift force and the mean exit gradient observed under the structure were analysed. Results reveal that under heterogeneous conditions, the reduction made by the sheet pile in the uplift force and exit hydraulic gradient may be underestimated when deterministic solutions are used.

Effect of Intermediate Sheet Piles in Non-Homogenous Soil on Seepage Properties Under Hydraulic Structure Using SEEP/W Program

The seepage through a permeable soil under hydraulic structure exerts uplift pressure and may carry soil particles there by leads to piping. This paper concerns to study the effect of using intermediate sheet pile under the apron of hydraulic structure besides the upstream and downstream piles rest on non-homogeneous soil layer. This configuration aim to show how it affect the uplift pressure, exit gradient and seepage discharge at toe of hydraulic structure by using computer program SEEP/W Package.From the software test carried out two cases, first case using two sheet pile one at the upstream and the other at the downstream, then compare its results with the second case when the sheet pile at upstream, downstream and intermediate pile introduced Also for each run the quantity of uplift pressure, exit gradient and discharge at toe of hydraulic structure were determined to develop an empirical equations. Also, the results have been verify with artificial neural network (ANN), this verification shown good agreement between them.

Quantifying groundwater dependency of riparian surface hydrologic features using the exit gradient

AbstractNumerical groundwater flow models necessarily are limited to subsurface flow evaluation. It is of interest, however, to examine the possibility that, for unconfined aquifer systems, they could be used to proportionately measure the magnitude of seepage they estimate when these aquifers intersect the landscape surface. Our goal in this study was to determine the degree to which an unconfined groundwater model can estimate run‐off or seepage at the land surface during winter time wet season conditions, as well as in the dry season, when evapotranspiration is a major part of the water balance, using a lowland basin‐fill example study area in the Pacific Northwest. The exit gradient is a metric describing the potential for vertical seepage at the landscape surface. We investigated the spatial relationship of mapped surface features, such as wetlands, streams and ponds, to the model‐predicted mapped exit gradient. We found that areas mapped as wetlands had positive exit gradients. During the wet season, modelled exit gradients predicted seepage throughout extensive areas of the groundwater shed, extending far beyond mapped wetland areas (355% increase), associated with previously observed increases in nitrate‐nitrogen in streams in wet season. During the dry season, exit gradients spatially corresponded with wetland areas. The increase in in‐stream nitrogen corresponds with shorter residence times in carbon‐rich wetland zones because of the onset of saturation overland flow. We present results that suggest that the exit gradient could be a useful concept in examining the groundwater–surface water linkage that is often under represented physically in watershed flow models. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.

Effects of blanket, drains, and cutoff wall on reducing uplift pressure, seepage, and exit gradient under hydraulic structures

Upstream blankets, drains and cutoff walls are considered as effective measures to reduce seepage, uplift pressure and exit gradient under the foundation of hydraulic structures. To investigate the effectiveness of these measures, individually or in accordance with others, a large number of experiments were carried out on a laboratory model. To extend the investigation for unlimited arrangements, the physical conditions of all experiments were simulated with a mathematical model. Having compared the data obtained from experiments with those provided from the mathematical model, a good correlation was found between the two sets of data indicating that the mathematical model could be used as a useful tool for calculating the effects of various measures on designing hydraulic structures. Based on this correlation a large number of different inclined angles of cutoff walls, lengths of upstream blankets, and various positions of drains within the mathematical model were simulated. It was found that regardless of their length, the blankets reduce seepage, uplift pressure and exit gradient. However, vertical cutoff walls are the most effective. Moreover, it was found that the best positions of a cutoff wall to reduce seepage flow and uplift force are at the downstream and upstream end, respectively. Also, having simulated the effects of drains, it was found that the maximum reduction in uplift force takes place when the drain is positioned at a distance of 1/3 times the dam width at the downstream of the upstream end. Finally, it was indicated that the maximum reduction in exit gradient occurs when a drain is placed at a distance of 2/3 times of the dam width from upstream end or at the downstream end.

Application of a genetic algorithm for the optimization of a cutoff wall under hydraulic structures

A genetic algorithm (GA) model coupled with finite element modeling was developed here to find the optimal values of inclination angle and cutoff location for a hydraulic structure of a given cutoff depth to floor length ratio. The objective function to be minimized is the exit gradient function. The main constraint models are those that satisfy a factor of safety against uplift pressure which correspond with minimum floor thickness. More than 3500 different cases were modeled and analyzed using SEEP2D modeling. Statistical Procedures for Social Sciences software was used to obtain a nonlinear regression model for estimating pressure head at nodal points and the exit gradients behind inclined cutoff walls for a given relative depth (d/b), relative location (b1/b) and angle of inclination (θ), using the data obtained from SEEP2D modeling. A Matlab code was used to perform a GA optimization modeling. The results indicated that the optimum location of cutoff is at upstream with an inclination angle from 59° to 68° for a (d/b) equal to or more than 0.4. For other relative depth values studied, the optimum location is at the last third of the concrete floor. The obtained optimum solutions were compared with the SEEP2D modeling to find the uplift pressure and exit gradient for the optimum solutions. Values estimated were found to be comparable to the obtained values from the GA model.

Worth of the three dimensional simulations of seepage in the vicinity of heading-up structures

This paper evaluates the three dimensional effects in studying seepage in the vicinity of heading up structures. An artificial heading-up structure installed on pervious soil is studied using both a 3D model (Seep3D) and a vertical 2D model (SEEP2D). The uplifts under the apron and the exit gradients resulting from both models are compared for different structure configurations including the presence and lack of sheetpiles for both laterally homogeneous and heterogeneous conditions. In addition, the effects of downstream wing walls (guide walls) on seepage are also assessed. The results showed that the exit gradient in the 3D simulations can reach almost double its value in the 2D simulations in different configurations. The existence of downstream guide walls can also lead to significant increase in the exit gradient. The paper proves that studying seepage in 3D can significantly become critical over the traditional 2D approach especially in cases of significant lateral heterogeneity.

Electrospray droplet exposure to polar vapors: delayed desolvation of protein complexes.

Fragile non-covalent complexes are susceptible to dissociation upon introduction into and transmission through the mass spectrometer. The exposure of nanoelectrospray droplets to various polar vapors, which are introduced into the curtain gas, is shown to stabilize non-covalent protein complexes even under relatively energetic ion transfer conditions. This study probes the mechanism by which polar vapor exposure appears to stabilize non-covalent protein complex ions in the gas phase. Holomyoglobin and hemoglobin were dissolved in either aqueous 1 mM ammonium acetate or ammonium bicarbonate solutions and ionized via nanoelectrospray ionization in the positive polarity. Polar vapors were entrained within the counter-current drying gas and exposed to nanoelectrospray droplets for circa 1 ms within the interface of a quadrupole/time-of-flight mass spectrometer. Mass spectra were acquired using various voltage gradients within the mass spectrometer. In the absence of added reagent vapors, significant fragmentation of holomyoglobin ions is noted with high voltage gradients for ions either entering or departing q0, a transmission quadrupole closely coupled to the skimmer exit. However, upon the introduction of reagent vapors, essentially 100% of the holomyoglobin complex can be preserved. Significant stabilization is noted at both relatively high q0 entrance and exit gradients when ions are transmitted through q0. These results indicate that upon vapor exposure the holomyoglobin ions are not completely desolvated as they enter or exit q0 under normal ion transmission conditions. The apparent stabilization of protein complexes and other non-covalent complexes noted here and elsewhere is attributed to the delayed desolvation of the ions. This allows the solvated ions to be transmitted through relatively high voltage gradients without disrupting the non-covalent interactions holding the complexes together.