Seepage Gradient Research Articles

Experimental study of seawall piping under water level fluctuation

The hydraulic action, such as waves, tides and floods, is the main cause of piping failure in seawall engineering, while the suffusion failure mechanism in this case has not been specially studied. Indoor experimental studies of piping failure of seawall under the waves and tidal water level fluctuations are still in a blank state. Based on the designed experimental devices in the research work, six groups of model tests were carried out with crushed and round gravel. The seepage velocity, seepage gradient, permeability coefficient and the fine particle movement trends were analysed. The changing rules of these variables under water level cycle were studied. It has been found that the critical hydraulic gradient of the fine piping particles inside the seawall became smaller with the increase in water level fluctuation cycles. At the same time, the critical flow velocity tended to increase gradually. The piping occurrence probability in the round gravel-filled seawall was larger than that in the crushed gravel-filled seawall. The internal fine-grained structure showed dynamic changes in migration under the fluctuation of the water level cycle.

Seepage Stability Analysis for Surrounding Rock Mass of Diversion Tunnel by FEM

Based on the actual conditions of the project and related experience, the seepage model of diversion tunnel and its surrounding rock mass was built to analyze its seepage stability. The seepage field was calculated by using finite element method (FEM) and cut-off negative pressure method. Two typical cross-sections and working conditions were chosen to evaluate the seepage stability of the surrounding rock mass. According to the results, the seepage gradient is larger when one tunnel is filled with water while the other is empty. The maximum seepage gradient is less than the allowable seepage gradient, so the surrounding rock mass of the diversion tunnel can meet the demand of seepage stability.

Uplift force, seepage, and exit gradient under diversion dams

Correct estimation of uplift force, seepage discharge and exit gradient is very important in stability analysis of hydraulic structures. In this research, by carrying out a set of experiments on a laboratory model, the application of various methods for estimating uplift pressure and seepage discharge under hydraulic structures, and the exit gradient have been investigated. The results show that for the soil type used in the experiments, the method of Khosla et al. gives a better estimation of seepage effects than creep theories. Moreover, compared with Lane's method, it is found that Bligh's theory has better agreement with the observed data of uplift pressure, seepage discharge and piping. By using the finite-element method, the magnitude of pressure head upon a dam foundation in different conditions is calculated, and a relationship between the anisotropic conductivity ratio and the ratio of coefficients in Lane's creep theory is introduced. It is found that while the accuracy of Lane's theory is reduced by increasing the anisotropic conductivity ratio, the new relation estimates the amount of uplift force very well. Based on the finite-element method, a set of graphs is presented to estimate the exit gradient in different conditions with the presence of a cutoff wall at the downstream end or without any cutoff wall.

Modeling seepage erosion and bank retreat in a composite river bank

In large alluvial river courses where bank failure is prominent, seepage erosion may be a significant contributor that is often overlooked. Seepage erosion typically occurs simultaneously with fluvial erosion and other mass failure processes during the recession period of hydrographs. In this study, a functional relationship between seepage erosion rate and its controlling variables was developed using lysimeter experiments. This experimental relationship was used to formulate a mathematical seepage model that predicts the daily seepage erosion rate for a composite river bank. Soils collected from the Brahmaputra River banks were repacked in lysimeters with horizontal stratification similar to the actual bank stratification. A total of 71 lysimeter experiments were conducted with various combinations of the controlling variables. The results indicate that the seepage gradient has a dominating effect on the time taken in developing undercuts that lead to the bank collapse. Increase in the number of silt layers in the bank profile increased the time to collapse significantly. In situ bank erosion measurements were taken to validate the performance of a bank erosion model for the middle reaches of the Brahmaputra River in India. The model predictions of seepage erosion rate showed a close agreement with the in situ bank retreat measurements.

An Analysis of Two-Dimensional Steady Seepage Flow of Earth Dam Based on FEM

Through project examples, this paper set up a two-dimensional finite element seepage flow model with the help of the Auto-BANK software, to solve the soakage line overflow points, seepage quantity and seepage gradient of the earth dam based on the basic equation of steady seepage flow of earth dam and definite conditions. It is found that this method can effectively calculate and analyse multi-media seepage flow field with complex boundaries.

An Analysis of Two-Dimensional Steady Seepage Flow of Earth Dam Based on FEM

Through project examples, this paper set up a two-dimensional finite element seepage flow model with the help of the Auto-BANK software, to solve the soakage line overflow points, seepage quantity and seepage gradient of the earth dam based on the basic equation of steady seepage flow of earth dam and definite conditions. It is found that this method can effectively calculate and analyse multi-media seepage flow field with complex boundaries.

Seepage Field Study of Xiaowan Hydropower Project

The Xiaowan Hydropower Project is located in the middle and lower reaches of the Lancang River in Yunnan Province, China. The arch dam of this project, with a height of 294.5m, is by far the highest dam of its type that has been completed in the world. The great water potential makes it necessary to assess the seepage behavior in the dam-foundation system. Due to the complexity of geological conditions and seepage control system, the research is full of challenges. In this paper, the finite element method (FEM) is adopted to simulate the three-dimensional seepage field. The initial flow method and the boundary pre-adjustment method are combined to obtain the seepage free surface and overflow surface under complicated boundary conditions. The hydraulic potential contours, seepage gradient, drainage discharges, and so on, are all analysed and the results show that the seepage control system is reasonable and effective. Furthermore, the results of the seepage field can provide suitable seepage load in the stress-strain and stability analysis for the dam-foundation system of Xianwan. At present, the project is in good operation.

Numerical Calculation and Safety Evaluation of Impervious Effects of the Reinforced Dyke

Based on the finite element method, the seepage field of reinforced dykes of a reservoir is analyzed from hydro geological nature and characteristics of the impermeable wall. According to the typical cross-section of the reinforced dyke, performance-parameters of the dyke such as total water head, seepage velocity, seepage gradient are compared with the counterparts of the original dyke. Results in steady seepage are obtained. Generally seepage quantity decreases 45% from the original dyke. Permeability coefficients and defects of the impermeable wall are the most factors of affecting effectiveness of dyke reinforcement. Because piezometric tubes work well, measured dada are reliable, both measured and calculated data also coincide with each other well. The results show that the impervious reinforcing plan of the dyke is fit, the reinforced dyke can reach predeterminate impervious effects and meet the technical specifications. Safety of the reservoir is guaranteed.

Calculation and Control of Seepage of Hardfill Dam with Different Foundation

As a new dam type, Hardfill dam has been arisen recently. The suitable geologic condition of t Hardfill dam construction was analyzed based on considering its characteristics in this paper; it was also studied how to choose the reasonable impervious and drainage system to control the dam body and foundation’s seepage on a given geologic condition to make sure the safety of the project. On the basis of analyzing the foundation geologic conditions of many common dam types, two kinds of typical Hardfill dam foundation were put forward. The seepage field law of dam foundation was analyzed by dam foundation seepage discharge, uplift pressure and seepage gradient of some typical positions after setting reasonable impervious and drainage measures on the two kinds of dam foundations. It is indicated by the calculation results that the adaptability of Hardfill dam foundation is comparatively nice, and the safety of dam foundation can be ensured if appropriate impervious and drainage measures are adopted.

Technology Research of Large Underwater Ultra-deep Curtain Grouting in Zhong-guan Iron Ore

Abstract Problems in Zhong-guan Iron Ore are complicated hydrogeological conditions, larger water inflow in mine ore, all ore bodies buried under the water table, ordovician limestone aquifer in the system directly to the roof for the ore body. Paper used ring-type single-row curtain grouting closed ground plan. This has not only achieved the safety of mining, but also protected ground water resources and hydro-geological environment. Study has shown that: the elevation of purdah base is -96 m ∼ -568 m, the average drilling depth is 523.92 m, the minimum hole depth is 321 m, and the maximum is 810 m, holes depth greater than 600 m take up about 30.8 A single slurry material can allow seepage gradient and the curtain can withstand the maximum head difference design curtain thickness T > 10 m, grouting hole spacing is designed to 12 m; curtain grouting pressure is 2 times of the head pressure. Research improves reference for similar mines.

Numerically predicting seepage gradient forces and erosion: Sensitivity to soil hydraulic properties

summary Research has suggested that streambank seepage can be an important mechanism of bank instability; however, limited information is available on the level of soil characterization necessary to accurately predict seepage gradient forces and erosion. The objective of this research was to quantify the expected range of predicted seepage gradients for various degrees of site characterization. Uncertainty analysis on seepage gradient predictions was performed relative to variability in soil hydraulic properties. A two-dimensional unsaturated/saturated groundwater flow model was used to simulate a homogeneous soil layer for sand and loamy sand soils packed at various bulk densities, qb. A pedotransfer function (ROSETTA), designed to estimate unsaturated hydraulic properties from surrogate soil data (i.e., texture and bulk density), was used to derive the saturated hydraulic conductivity, Ks, and water retention parameters for various levels of site information (i.e., only textural class; percent sand, silt, and clay (%SSC); %SSC and qb; and %SSC, qb, and Ks). Statistical distributions were derived for each soil hydraulic parameter and Monte Carlo simulations were performed to generate distributions of maximum seepage gradient. The deviation in predicted seepage gradient was calculated using assumed baseline conditions. Ranges in predicted soil hydraulic parameters and maximum seepage gradients were considerably reduced when using %SSC as compared to soil texture. Therefore, at a minimum, soil samples should be taken for particle size analysis. For qb between 1450 and 1500 kg m � 3

Discussion of “Overview of New Orleans Levee Failures: Lessons Learned and Their Impact on National Levee Design and Assessment” by G. L. Sills, N. D. Vroman, R. E. Wahl, and N. T. Schwanz

The article concludes that the primary IHNC north breach failure was a result of “differences between actual conditions and assumptions used as design.” Specifically, the breach was attributed to elevation differences and rotational failure as a result of wall deflections and the formation of a “gap” on the flood side of the I-wall. The article also concludes that overtopping was the only possible failure mechanism for earthen levees. While these failure modes are certainly possible, they are neither exclusive, nor exhaustive. Additional failure modes exist and should be accounted for and incorporated into the failure analyses. There were two major breaches along the IHNC east bank: a short 50-m breach at the north end and a much longer breach of 250 m near the southern end. These two major failures of the floodwall were strongly influenced by deep excavations that had been completed adjacent to the outboard side of the hurricane flood protection system just prior to hurricane Katrina Bea 2006a, 2008a. These excavations were part of a site demolition and clearing operation removal of structures, utilities, underground storage tanks, and contaminated soils for a new navigation lock to be constructed in the East Bank Industrial Area EBIA located immediately outside of the floodwall levee Fig. 1. Multiple excavations were dug to depths in excess of 7.6 m 25 ft that were backfilled with loose sand as a result of sand placement below the water table and “native” soils from this area Fig. 2. These excavations penetrated the buried marsh layers that underlie this entire area down to depths in excess of 9 m Figs. 3 and 4. Since the sheet piling supporting the floodwall were driven to tip elevations above the bottom of the marsh layers of approximately 3 m, seepage and water pressures generated by the hurricane Katrina surge waters were able to rapidly communicate under the levee floodwall and destabilize the flood protection structures. The impacts of these excavations on the performance of the flood protection system at the North Breach were evaluated by the authors. The primary failure mode impacted by the excavation work was floodwall underseepage piping. Fig. 5 summarizes results from a two-dimensional finite-element analysis of the seepage gradients and pressures at the north breach Bea 2008a. The overlying less-permeable fill and levee soils act as a blanket on Fig. 1. Color Excavations for soil borrow pit on waterside of floodwall at Lower 9th Ward Bea 2008a

Borax Lake, a thermal lake confined within a biomorphic mound, Pueblo Valley, Oregon, USA

Relations among geothermal system dynamics, evolution of an 8-m thick biomorphic mound, and requirements to protect critical habitat of Gila boraxobius, a fish protected under the Endangered Species Act of 1973, were explored at Borax Lake, a shallow thermal lake in the tectonically active northwestern Basin-and-Range Province. Neutral pH, thermal water at ~ 105 °C discharges within a carrot-shaped vent at a depth of 29 m and rapidly mixes within the vent and lake which is mostly < 1 m deep. The 14 hectare thermal lake lies at the crest of a biomorphic mound composed of diatoms, silt-size opal-A silica particles, silica encrusted and permineralized vascular plant fragments, sinter fragments, and discontinuous carbon-rich plant and charcoal layers. Siliceous sinter lines the vent, part of the lake bed, and forms a discharge apron that caps the mound and gently dips (< 3°) away from the shore. The hydrothermal deposit unconformably overlies lacustrine deposits of Pleistocene Lake Alvord and began to accumulate prior to BC 3775 to 3655, the age of a charcoal layer near the mound crest. Slope degradation occurs primarily from rilling, gullying, and solifluction as the result of seasonal seepage, overland flow and freeze/thaw cycles. Winter seepage on the flanks of the mound ceases as the water table drops (~ 1.5 m) during the summer dry season. Piping from an upward seepage gradient also degrades slopes. Stability analysis indicates that the mound is stable under current static stress conditions, however, high water table and cohesionless diatomaceous silt with shear wave velocities less than 164 m s − 1 and proximity of seismic sources indicates the mound may be susceptible to liquefaction and slope failure during dynamic earthquake loading. Borax Lake is the unique product of the geothermal system and the biological communities it supports. Portions of the mound may have failed in the past during seismic loading, eroded, and been rebuilt by processes similar to those active today.

Incipient Motion of Riverbank Sediments with Outflow Seepage

Based on a force analysis, an expression is derived to describe the critical Shields number for incipient motion of uniform cohesionless sediment particles on a riverbank slope in terms of flow parameters, outflow seepage, and physical and mechanical properties of sediments. Parametric studies were conducted to investigate quantitatively the effects of hydraulic gradient of seepage, slope angle, and flow direction on the critical Shields number. The results show that the critical Shields number decreases with an increase in the hydraulic gradient. Where bank collapse is concerned, the most dangerous direction of hydraulic gradient of outflow seepage is at an angle equal to the effective internal angle of friction of the sediment mass with respect to slope surface. At a certain value of hydraulic gradient, the critical Shields number decreases with increasing slope angle. Open flow becomes more erosive when the current direction changes from horizontally parallel with the riverbank line to turning downward.

Seepage caused tension failures and erosion undercutting of hillslopes

Summary Seepage has been suggested as an important factor in gully and river bank erosion. This study investigated the underlying mechanisms of instability by seepage in laboratory studies. A 25-cm tall, 50-cm wide, and 20-cm long soil block with a focused inflow reservoir was constructed to investigate seepage gradient forces and the three-dimensional nature of seepage particle mobilization (i.e., seepage erosion) and undercutting. Experiments included sand and loamy sand soil blocks packed at prescribed bulk densities (1.30–1.70 Mg m−3) and with an outflow face at various angles (90°, 75°, and 60°). Constant heads of 15, 25, and 35 cm were imposed on the soil to induce flow. A laser scanner was utilized to obtain the three-dimensional coordinates of the bank and undercut surfaces at approximately 15–30 s intervals. The bulk density of the two different soil types controlled which seepage failure mechanism occurred: (1) tension or “pop-out” failures due to the seepage force exceeding the soil shear strength which was being concurrently reduced by increased soil pore-water pressure, or (2) particle entrainment in the seepage flow, particle mobilization, bank undercutting, and bank collapse when the initial seepage force gradient was less than the resistance of the soil block. For cases experiencing particle mobilization and undercutting, seepage erosion initiated as unimodal (i.e., concentrated at one point) or as multimodal (i.e., initiating at several locations across the bank face), and this result was largely controlled by the bank angle. A five parameter Gaussian function was fitted to the measured three-dimensional undercut shapes to derive parameters for the maximum depth of undercutting, position of the center of the peak, and the vertical and lateral spreads of the undercut. The parameters of this distribution can be useful in the development of improved sediment transport functions and the incorporation of this failure mechanism into hillslope stability models.

Stability of sandy slopes under seepage conditions

Stability against shallow mass sliding in saturated sandy slopes under seepage depends on the flow direction and hydraulic gradient, particularly near the ground surface. Two modes of instability i.e., Coulomb sliding and liquefaction have been studied and the critical flow directions discussed. The utility of the numerical approach in solving complex flow problems with irregular boundaries and surface topography is demonstrated by means of two slope examples with different internal drainage conditions. The numerical results for the seepage gradients at different points are compared with those predicted by the simple expression derived in this study, and the corresponding effects on the stability are evaluated.

Installation of Suction Caissons in Sand with Silt Layers

Suction caissons are increasingly being used for offshore anchors because of their ease of installation. However, for soil profiles that have sand overlain by layers of silt, there are reasons to believe that it may not be possible to install suction caissons. This may happen because the low permeability of the silt will create a hydraulic blockage, and thus diminish the upward hydraulic gradient required to reduce the penetration resistance in the sand to enable caisson penetration. The tendency of the silt to be sucked up, and its effect on the underlying sand, are not clearly understood. Furthermore, the blockage by the silt layer may be less than if a clay layer is present. This is because scouring of the silt may occur, allowing an upward seepage gradient to develop, and hence causing a reduction in penetration resistance. A series of suction caisson installation tests have been conducted in a geotechnical centrifuge to provide data on the penetration resistance and mechanisms for soil profiles where silt layers are present, either on top, or interbedded between layers of sand. In these tests, the thickness of the silt layer and its depth below the sand surface have also been varied. It has been found that if silt overlies the sand, a larger suction force is required for installation than when installation occurs in homogeneous sand, but that this force is still much smaller than the soil resistance when the caisson is pushed into the ground. When a silt layer is present, the suction pressures are observed to follow a similar trend that is independent of the position and thickness of the silt layer. Scouring of silt was evident in the tests, and significant upward movement of the soil plug inside the caisson was observed in all soil profiles when silt layers were present. The plug height was seen to reduce after the suction was turned off, suggesting that the resulting heave was unstable.

Seepage Effects on Dune Dimensions

This note presents an experimental study of seepage effects on the dimensions of a dune: its height, length and lee-side slope. The critical slope of a cohesionless material was found to be related to the ratio of the hydraulic gradient of seepage to its critical value under the quick condition. The theoretical relationship is successfully used to predict seepage effects on the lee-side slope of dunes. Measured laboratory data support the assumption that the effect of seepage on the lee-side slope of dunes is the same as that on the critical slope of the sediment. The data reveal that the slope is reduced and increased with injection and suction, respectively. Additionally, the study explores seepage effects on the development of dune height and length. Experimental results show that suction and injection increase and decrease the equilibrium height of dunes by 25 and 40%, respectively. On the other hand, the dune length is found to increase by approximately 30% and decrease by 20% under the influence of suction and injection, respectively.

General expression for the critical seepage gradient in an infinite slope

With reference to the ideal case of an infinite slope of granular (c' =0) homogeneous and isotropic soil subject to uniform seepage with a vertical component directed upwards, the possibility, in principle, of a slope instability due to static liquefaction (quick condition) is examined. Such instability takes place when the seepage gradient is the critical one (ic). In this paper, a general expression for ic, is obtained.

Incipient sediment motion with upward seepage

This study investigates the effect of upward bed seepage on the critical condition of incipient sediment motion in open channel flow both analytically and experimentally. The critical condition was derived by analyzing the forces acting on a sediment particle lying on a permeable horizontal bed subject to seepage. The ratio of the critical shear velocity with seepage to that without seepage depends on the ratio of theJiydraulic gradient of seepage to its critical value under the quick condition. Experimental results concerning incipient motion of cohesionless uniform sediments in open channel flow show that for a particular size of sediment, the critical shear velocity decreases with increasing seepage velocity. All measurements generally support the theoretically derived expression of the critical shear velocity in the presence of an upward seepage.