Finite Element Seepage Analysis Research Articles

Investigation of the stability of a fly ash pond facility using 2D and 3D slope stability analysis

A numerical investigation of the effect of pore pressure regime on the safety factor and the critical failure mechanism is presented for fly ash storage facility. Pore pressures’ measurements from standpipe piezometers and pore pressure estimated from seepage analysis are used to compare the factor of safety for a fly ash slope. This was applied for considering static and seismic scenarios. A probabilistic approach was applied to account for the uncertainties resulting from the limited data available and support a qualitative risk assessment evaluation. Slope stability analysis is conducted in two and three dimensions, adopting the limit equilibrium analysis approach, and also a finite element seepage analysis, to assess the stability of the slope. The two-dimensional cross-sections were extruded to three-dimensional models to estimate the factor of safety and associated shear failure. The results from the performed analysis suggest an increase in safety factor values of 5%.

Modelling a geosynthetic clay liner in a centrifuge and its application to the dynamic stability of small-sized earth-fill dams

This study proposes a centrifuge modelling method for geosynthetic clay liners (GCLs). The development of such a centrifuge model has been challenging due to the complex hydraulic and mechanical behaviour of GCLs and their interface with the soil. Additionally, the proposed method was applied to the seismic stability of an earth-fill dam. Impermeable sheets were selected as the GCL model based on validation using finite element seepage analysis and similarity laws. Dynamic centrifuge model experiments were conducted on a 10-m-high embankment with a high phreatic line to visually verify the failure mechanism, as previous full-scale tests did not provide clear visualisation. A transparent plate imaging confirmed upstream translational failure of the GCL, which was installed in a staircase shape. With regard to improving the seismic resistance of the embankment, it was found that cement-stabilisation of the lower half of the water level upstream side of the GCL significantly reduced deformation.

Upper-bound optimized solution of unsaturated soil slope stability under steady and unsteady flows

The increasing frequency of extreme weather events is leading to a rise in unsaturated soil slope instability near the earth's surface year by year. Establishing the method for analyzing slope stability under complicated hydrological conditions is imperative. This study proposes an approach that combines finite element (FE) seepage analysis with upper-bound limit analysis to evaluate the stability of unsaturated soil slopes. Two kinematically failure mechanisms are constructed based on the spatial discretization technique via point by point including the rotational and rotational-translational combinational failure modes, which can adapt to the complex pore water pressure distribution and varying soil unit weight. Additionally, a novel optimized solution scheme that integrates dichotomy and meta-heuristic optimization approaches is adopted to determine the factor of safety (FoS) and critical slip surface of slopes. The effectiveness and practicality of this method are validated by comparing with the published examples of slope stability evaluation under horizontal and inclined water table. Moreover, the variation of failure mode during the rainfall infiltration is illustrated by a case study of the slope under heavy rainfall. The methodological framework presented in this study can provide a basis for evaluating the stability of saturated–unsaturated soil slopes with 2D seepage conditions.

Stochastic seepage analysis in embankment dams using different types of random fields

Random field theory has become a very popular approach for probabilistic analysis of earth dams. This study performs a stochastic analysis of seepage through an embankment dam to account for the spatial variability of hydraulic parameters. The originality of this study consists in using and comparing three types of Random Fields RFs for the seepage analysis: classical stationary RFs, conditional RFs (to consider the position of available measurements), and non-stationary RFs (to consider the mean variation with depth). The same sample of hydraulic conductivity values is considered to generate the three types of random fields. The hydraulic conductivity RFs are integrated into a Finite Element seepage analysis model to evaluate the flow velocity, hydraulic gradient, pore water pressure, and flow rate. The process is repeated by Monte Carlo simulations to characterize the spatial variability of the seepage results in terms of mean and standard deviation at each point of the studied dam and its foundation. The results show a higher dispersion for the stationary RF case and a lower dispersion for the conditional RF case. For the case study, the results also show that the use of different RFs for modelling the spatial variability of hydraulic conductivity induces differences that can be considered significant for flow velocity, moderate for hydraulic gradient and minor for pore pressure.

Assessing rainfall-induced wetting band depth for stability analysis of unsaturated soil slopes

Because of factors such as plant root action, soil capillary action, and evapotranspiration, the soil near the surface of a slope tends to be drier and in an unsaturated state. However, when rainfall infiltrates the soil, the soil near the surface can become saturated, creating a wetting band that may trigger shallow landslides on the slope. This study performed transient finite element seepage and slope stability analyses on unsaturated soil slopes during rainfall. The analysis considered a representative rainfall pattern observed in Taiwan from 2010 to 2022, accounting for potential impacts of climate change. The objective of this study was to explore the impact of wetting band development on the stability of unsaturated soil slopes under rainfall conditions. Additionally, the investigation aimed to analyze the variations in pore water pressure and volumetric water content throughout the rainfall process. These analyses were conducted to determine the slope safety factor and investigate the relationship between the wetting band and the potential sliding surface. The results showed that the soil pore water pressure and water content near the surface of the slope were highly influenced by rainfall. The wetting band gradually extended downward from the surface with increasing rainfall duration, resulting in an increased wetting band depth (WBD), while the potential sliding surface approached the slope surface. When the safety factor was at its lowest, the potential sliding surface intersected the edge of the wetting band and was included within the range of the wetting band. This observation indicates a significant relationship between the potential sliding surface and the evolution of the wetting band during slope failure at the moment of minimum safety factor. The study also observed that water infiltration near the toe of the slope was more rapid, leading to a deeper wetting band at this particular location.

Effect of fines content on soil moisture responses to wetting & drying cycles

Studies of soil moisture responses to climatic conditions are essential for understanding the mechanism of rainfall-induced landslide. This study aims to investigate the effects of fines content on soil moisture responses under repetitive wetting and drying cycles. Remoulded residual soil samples were prepared at 5 different fines contents, i.e. 15%, 30%, 45%, 60% and 75%. The soils were instrumented with soil moisture sensors and they were subjected to 2 cycles of wetting and drying. Results showed that all the soil specimens experienced faster reductions in soil moisture during the second cycle of drying than the first cycle. During wetting, the recovery of soil moisture, however, was less affected by the repetitive wetting and drying cycles. The fines content has a significant influence on the soil moisture reduction during drying. The degree of saturation of the specimen with 15% fines was found to be 3.5–6 times lower than that of 75% fines. The response of soil moisture to wetting was only marginally affected by the fines content. Results from numerical seepage analyses showed that the wetting soil moisture responses can be simulated to a reasonable accuracy. However, the simulated drying soil moisture responses showed relatively large discrepancies with the actual measurements. The results implied that the soil may have experienced changes in soil structure, void ratio or formation of desiccated crack during the repetitive wetting and drying cycles which cannot be captured by a typical finite element seepage analysis.

Seepage characteristics of three-layered landfill cover system constituting fly-ash under extreme ponding condition

A multi-layered final cover system is constructed over the landfill after it reaches its full capacity to minimize water ingress into the underlying hazardous waste. Three layered landfill cover are designed for areas experiencing very humid climatic conditions. Under the effects of climate change, the occurrences of extreme rainfall events become more frequent and this has resulted in catastrophic floods and hence extreme ponding. This study investigates the seepage characteristics of three-layered capillary barrier cover systems under an extreme ponding condition of 1.5 m water head, through detailed laboratory column tests and finite-element seepage analysis. Four 1.2 m-tall columns having different configurations (C1–C4) were studied. Fly ash (FA) was used to amend the surface and barrier layers in columns C2 and C4, in line with the novel concept of “waste protect waste”. Spatiotemporal variations of volumetric water content of the four columns were monitored for three years continuously. With FA amendment in the surface layer and an inclusion of a 0.01 m thick geosynthetic clay liner between the drainage and barrier layers, the onset of basal percolation was significantly delayed until 700 days of ponding, compared to 115 days without FA amendment. Capillary flow dominated the gravitational flow and perched water table was formed as waterfront advanced from the drainage to barrier layers. Further seepage analysis considering a realistic humid climate boundary condition showed that all four configurations were successful in preventing basal percolation for 800 days.

Design of grout curtain for temporary embankments on rock foundation

ABSTRACT The design of curtain grouting for dams on rock foundations and the importance of finite element seepage analysis is reported by highlighting a case study of the upstream cofferdam of Gulpur Hydropower Project, Pakistan. Results from seepage analysis decreased the scope of curtain grouting by almost 3 times as compared to initial scope based on results from empirical correlations and water pressure tests. The total grouting holes were decreased from 185 to only 66 and this meant a decrease of construction time by 72% and saved a huge amount of capital and resources as well. The performance of the grout curtain is concluded to be satisfactory, for an operation phase of 3 years which includes two major floods as well and during this period, no visible erosion, quick condition or saturation was found on the downstream end. The paper concludes that if the safety criteria are satisfied, even with high inflows of the river, the scope of curtain grouting can be partially reduced or completely eliminated for temporary dams having an operational life of 1–5 years.

Influence of Rainfall Infiltration on Transient Volumetric Water Content of Coal Measure Soil Slope

With coal measure soil side slope in Jiangxi’s Pingxiang-Lianhua Expressway selected as the research object, GeoStudio, a finite element seepage analysis application, was adopted for numerical simulation on the variation pattern of volumetric moisture content in the side slope in rainfall as well as its spatial distribution characteristics. The results show that rainfall infiltration mainly influences the surface layer of the side slope and that its influence decreases with the increase of depth. The volumetric moisture content of undisturbed coal measure soil remains basically unchanged under the weathered layer. When soil mass permeability coefficient is less than rainfall intensity, the rainwater permeates quickly in the outermost layer of a weathering zone, but exerts a minor influence. When it is larger than the rainfall intensity, rainfall permeates slowly but exerts a major influence. When rainfall intensity reaches 8.33 ×10−7 m/s, soil moisture contents in the superficial layers of the slope toe and halfway up the slope surface become saturated. The saturation depth is 80% of the weathered layer. When the rainfall intensity reaches 1.1 ×10−6 m/s, it exerts a more extensive influence on the saturation area in the side slope, but the corresponding affected depth is only 50% of that in the weathered layer.

A method to develop the input parameter database for site-specific debris flow hazard prediction under extreme rainfall

The predictive hazard analysis at a detailed scale for debris flow runout analysis can be improved significantly through reliable estimation of the input parameters. In this study, a method for database establishment of input parameters at a site-specific scale was laid out for the predictive-based debris flow hazard assessment under extreme rainfall. The adoption of the DAN-3D code necessitated the estimation of three main input parameters: initial volume, bulk basal frictional angle, and growth rate. The initial volume was assessed using a 3D coupled finite element seepage and limit equilibrium-based slope stability analysis. An artificial neural network-based model was developed using 27 debris flow events for predicting the basal bulk frictional angle and consisted of eight factors: plan curvature, profile curvature, percentage of fine content, D50, initial unit weight, initial volume, relative relief ratio, and channel length. Finally, the growth rate was estimated using the previously assessed initial volume, soil depth, and the approximate runout length. The proposed method was validated by application to the Raemian slope in the Woomyeon mountain region, Seoul, for the extreme rainfall event of 27 July 2011. The analysis yielded a final volume of 53,067.9 m3, a velocity upon arrival on the road of 26.81 m/s, and an approximately 0.5-m debris thickness concentrated near the Raemian apartments. The comparison of the predicted debris flow path and debris flow velocity with the actual event demonstrates good similarity and provides a conservative estimate of the volume. This study therefore illustrates the importance of an input parameter database in providing a reliable debris flow runout hazard assessment.

Design and implementation of site-specific rainfall-induced landslide early warning and monitoring system: a case study at Nam Dan landslide (Vietnam)

ABSTRACTThis paper proposes and implements an early warning and monitoring system for rainfall-induced landslide (named as EWMRIL) with a case study at the Nam Dan landslide (northern Vietnam). The proposed system consists of six sensor nodes and one rainfall station that are used to sense large amounts of data in real-time such as soil moisture, pore-water pressure (PWP), movement status, and rainfall. In addition, a new flexible configuration for the wireless communication system is proposed that is capable not only to save the energy consuming but also to ensure the reliability of the system. Using wireless communication system, the sensed data were sent to the computer station for analyzing and predicting the instability of the landslide in terms of factor of safety (FoS) using the finite element seepage analysis and the limit equilibrium slope stability analysis methods. These methods are available in the SEEP/W and SLOPE/W modules of the GeoStudio software. Based on the analyzing results, the system proposed three warning levels for the landslide Early, Intermediate, and Imminent. Experiment result in the rainy season from August to September 2016 has proven the validity of the EWMRIL system. The result of this study is useful for landslide risk prevention and management in landslide prone-areas.

Anti-Seepage Evaluation of Reinforcement Effect for Fengchan Earth Dam

Currently, there is a large amount of earth dams should be reinforced because of the running incessantly in a long period in our country. And the anti-seepage evaluation is an extremely critical and indispensable index to scrutinize the reinforcement effect of earth dam comprehensively. According to the real engineering conditions of the dam, the finite element seepage analysis method is used to compute the seepage characteristics in the Fengchan reservoir before and after the reinforcement in this paper. The results show that the effect of the anti-seepage system is well for adopting the concrete slope protection and concrete anti-seepage wall. The seepage state of foundation and dam body is improved evidently. The seepage gradient and seepage quantity of each material zone reduce significantly and meet the standard requirement of specification.

A new laboratory apparatus for measuring leakage through geomembrane holes beneath mine tailings

The design and performance of a new laboratory apparatus for measuring leakage through geomembrane holes beneath mine tailings is presented. Finite element seepage analysis shows a negligible effect of the lateral boundary on leakage through the geomembrane hole and that the laboratory apparatus provides an excellent idealization of the deep burial conditions expected in the field. Results from four experiments are then reported to demonstrate the effectiveness of the new apparatus and gain insight on the effect of having a low permeable layer on top of the geomembrane on leakage, as may be expected for containment applications involving mine tailings. Two of the experiments simulated having fine grained tailings above the geomembrane (containing a 10 mm diameter hole) in a deep tailings storage facility with applied vertical and pore pressures of 3000 and 1500 kPa. Leakage through the hole with low permeable layer on top of the geomembrane was found to be two to four times smaller than the leakage from two other experiments with a very high permeable layer above the geomembrane (i.e., much more like a solid waste landfill configuration).

Parametric study of horizontal drains for slope stability measure: A case study in Putrajaya, Malaysia

Horizontal drains are commonly used in stabilizing slopes. This paper presents a case study of slope failure in Precinct 9, Putrajaya, Malaysia which buried 23 cars and caused the evacuation of 1000 people from their apartments. Recorded data showed that it had been raining heavily with a total cumulative rainfall of 210 mm two days prior to the slope failure occurrence. Post failure investigations suggested that the slope failure is caused by the rise in groundwater level that increased the pore water pressure thus reducing the shear strength. Therefore, horizontal drains were proposed as a remedial measure to enable the lowering of groundwater level to a safe level. Parametric study was carried out to determine the suitable length and ideal location of horizontal drains installation. Geophysical electrical resistivity survey was carried out to determine the ideal location for installing the horizontal drains. The Finite Element seepage analysis and Limit Equilibrium method were used to examine the effect of horizontal drains on slope stability during rainfall. The minimum length required for the horizontal drains is 22.5 m and the best location for installation is at the toe below the lowest berm of the slope. Field monitoring results from piezometers and measurements of discharge rate verified that the installation of horizontal drains are able to lower the groundwater level effectively.

Rehabilitation of the Massingir Embankment Dam: A Concept Based on Finite Element Seepage Analysis

In this paper, several rehabilitation alternatives are discussed and evaluated leading to the analysis of the preferred concept. The examination was mainly performed by means of the finite element program, 2D PlaxFlow, which is a computer%aided program created for groundwater flow analysis in the domain of geotechnical, environmental, agricultural and hydrological engineering. It may also be used as an add%on to th e Plaxis 2D program, which serves to analyse deformation, stability and dynamics in the geotechnical domain. After the presentation of the examined concept, an outlook is given concerning the next necessary steps.

Sensitivity Study of Input Parameters for Seepage Flow Simulations using Parallel Computers

This paper describes a comprehensive sensitivity study that was performed using high performance parallel computers to understand the importance of input parameters to a transient partially saturated finite element seepage analysis for a levee with separate soil layers of sand, silty sand, and clay. Seepage flow in this paper refers to the type of flow of water that occurred through the failed levees in New Orleans, Louisiana, USA, as a result of Hurricane Katrina. The input parameters tested were saturated hydraulic conductivity, volumetric compressibility, residual moisture content, saturated moisture content, and two van Genuchten unsaturated flow parameters. The output data compiled to show the sensitivity of the input parameters were the simulation times (days) to achieve 25%, 50%, and 75% of the steady-state values of pore pressure at the toe of the levee beneath its blanket, flow rate through the landside flux section, and the level of saturation in the levee. The use of high performance parallel computers enabled the running of thousands of scenarios using different values for the input variables. A sensitivity investigation of this magnitude has not been previously performed. The results of this investigation indicated that the more sensitive soil parameters were the saturated hydraulic conductivity and the volumetric compressibility. The unsaturated van Genuchten parameters of the landside blanket had a larger than anticipated impact on the duration of time to achieve steady state. This practical example is an excellent success story for high performance computing in that running a given simulation for a couple of hours on thousands of processors in parallel replaced over a year work using a PC.

Use of Dual Capillary Barrier as Cover System for a Sanitary Landfill in Singapore

Construction and demolition of buildings together with the reconstruction of roads produce a lot of solid wastes every year. These wastes create problems related to cost and space for disposal, especially for countries with limited land area like Singapore. Therefore, it is important to choose suitable landfill management system (i.e. cover system) for optimization of landfill area in Singapore. Landfill cover offers many geo-environmental benefits, including reducing water infiltration, isolating waste and controlling landfill gases. In Singapore the cover system is located within the unsaturated zone above the groundwater table. Therefore, it is necessary to incorporate unsaturated soil mechanics principles in designing the cover system for the landfill. Soil properties in the unsaturated soil zone affect the rate of wetting front movement from the ground surface. As a result, the rates of changes in pore-water pressures during and after rainfall will vary in accordance with the characteristics of unsaturated soil properties. In this study, the performance of dual capillary barrier (DCB) in minimizing rainwater infiltration into a sanitary landfill in Singapore was investigated. The DCB consists of fine recycled asphalt pavement (RAP) and coarse RAP as the materials for the fine- and coarse-grained layers, respectively. The recycled materials were used in the DCB to reduce the cost associated with the construction of a cover system in the field and to maintain environmental sustainability. Laboratory tests were conducted to characterize the index properties and hydraulic properties (i.e. soil–water characteristic curve (SWCC) and permeability function) of the RAP for the cover system under saturated and unsaturated conditions. The SWCC and permeability function were used in the finite element seepage analyses to study the effect of climate change on the soil cover system. The results from the seepage analyses show that the DCB was effective in minimizing rainwater infiltration into the sanitary landfill.

Laboratory Modeling of the Mechanisms of Piping Erosion Initiation

A laboratory modeling program has been conducted to assess the mechanics of initiating the piping erosion process in sandy soils. The models were performed on several soils, differing in gradation, grain size, grain shape, and specific gravity. Observations and monitoring of pore pressures within the samples during the modeling identified four stages in the development of piping initiation: initial movement, progressive heave, boil formation, and total heave. By linking the observed behavior with the measured pore-pressure regime in the sample, a model for the mechanics of piping development has been developed. Finite-element seepage analyses were performed to model the progression of piping development in the laboratory models and corroborate the developed model of mechanics. The findings of the study identified a new model for the initiation of piping development that can be applied to the assessment of piping in the field.

Comprehensive Instrumentation for Real Time Monitoring of Flux Boundary Conditions in Slope

Rainfall-induced slope failures occur in response to climatic changes in many parts of the world. Climatic changes result in dynamic flux boundary conditions across slope surface, such as infiltration and evaporation. In addition, trees, shrubs and grasses generate another dynamic process of transpiration across slope surface. As a result, stability of a slope as quantified by factor of safety is not constant over time, but varies in accordance with variation in flux boundary conditions.Effects of flux boundary conditions on slopes can be assessed using real-time monitoring. In this paper, different types of instruments for real time monitoring of flux boundary conditions are illustrated to study the effect of rainfall and evaporation on pore-water pressure and water content in slope. Several geotechnical instruments such as piezometer, tensiometer, time-domain reflectometry and devices for measuring changes in local climate such as rainfall gauge and weather station can be installed within the slope area. The data from these instruments were captured automatically using a data acquisition system and the data were transported to a secured website in real time. Finite element seepage analyses were carried out to investigate the effect of rainwater infiltration and evaporation on pore-water pressure distribution of a residual soil slope at Telok Blangah, Singapore. The numerical analysis results showed a good agreement with those obtained from field measurements if the evaporation was applied in the analyses. This indicated that rainfall and evaporation play important roles in pore-water pressure changes within the residual soil. Seepage analyses were also conducted using different coefficients of saturated permeability and soil-water characteristic curves of the soil. The results showed that good quality laboratory test results are required to closely simulate pore- water pressure distributions within the soil layers.

Study the Infiltration and Stability of the Earthfill dams (Proposed Kashkan dam as a Case study)

This paper performed two steps of analysis. The first step used Soil Conservation Service (SCS) method to estimate surface water infiltration rates from the earth fill dams (proposed Kashkan dam north of Duhuk city as a case study). In the second step, a two dimensional finite element seepage analysis using (Seep/w) software was employed to determine seepage rates. The main purpose of constructing Kashkan dam is to store the rainfall water during rainy season and use it during summer season to provide the human requirements of water as well as to performed the agricultural requirements. Mean monthly rainfall data recorded for selected watersheds during the period (1986-2008) was used. The maximum infiltration rate during the depended period asymptotically tends to the value of the saturated permeability. The computed infiltration rates were used as input data for the seepage analysis by assigning the ground surface boundary condition. Finally, positive and negative pore water pressure distributions obtained