Large Earth Dams Research Articles

Analysis of Slope Stability and Soil Liquefaction of Zoned Earth Dams Using Numerical Modeling

The design of dams requires comprehensive studies to ensure the safety and feasibility of these important engineering projects, as any possible failure case may lead to considerable losses in human life and properties. Specifically, analyses should be performed to evaluate seepage, slope stability, and soil liquefaction of large earth dams. In this study, numerical modeling, based on finite element methods, was used to analyze seepage, slope stability, and liquefaction of Makhoul Dam which is a large zoned dam, currently under construction on Tigris River in the north of Iraq. Earthquake shakings impose additional hysteric and short-term loads that may lead to dam failure due to high pore water pressure, piping, and soil liquefaction. Therefore, the dynamic stability of the dam and soil liquefaction were also evaluated, as a result of applying an earthquake shaking to the dam. For the static condition, the dam was safe against internal erosion and slope failure, as the calculated value of the safety factor was greater than the allowable value. However, the results obtained from the dynamic analysis indicated that a possible earthquake, with an acceleration of 0.38g and 10 seconds period, led to upstream slope failure, a relative displacement as high as 2 meters at the dam crest, and soil liquefaction at the upstream slope. As discussed herein, dam redesign or geotextile reinforcement may be considered to reduce the effects of earthquakes on the dam.

Seismic response of large earth dams in near-source areas

The paper focuses on the seismic response of earth dams close to active faults. The engineering implications of such an issue may be crucial since many existing dams, which have been in operation for several years, are located nearby or even on seismogenic faults. Owing to their size, large earth dams could dramatically be compromised by near-source ground motion. The spatial variability of the incoming seismic wave could promote differential settlements in the dam body and consequently fractures in the watertightness elements with an overall decrease of dam safety. The paper refers to the iconic case of Conza Dam in Italy. During its construction, it was seriously damaged by the catastrophic 1980 Irpinia earthquake (M = 6.9), whose epicenter was very close to the dam site. To model the seismic wave propagation from the causative fault to the dam site, a physics-based approach has been adopted by implementing the domain reduction method to simulate the whole boundary value problem. Two models were hence considered: a regional model, containing the source and the propagation medium of the Campania region basin, and a site model containing the upper foundation soils and the dam body.The paper compares the different response of the dam in the hypothesis of contemplating or disregarding near-source effects. Finally, the dam performance to seismic scenarios resembling the source mechanism of the 1980 Irpinia earthquake was analysed to ascertain the resilience of the dam to future earthquake scenarios having source characteristics similar to those of the 1980 event.

A Short Review on Characteristics of Kolkata Soil on The Basis of Liquefaction Susceptibility

The present study describes liquefaction of soil due to loss of strength of cohesionless soils in saturated condition due to sudden increase of pore water pressure during earthquake in undrained condition. Many failures of various structures such as seismic dams, retaining structures, and harbours have been attributed to liquefaction of saturated soils. There have been very limited studies regarding the behaviour of soil liquefaction at large depths, yet this situation commonly occurs at foundations of some structures such as large earth dams. This paper describes effects, mechanism, susceptibility, evaluation and prevention of liquefaction and it concludes by proposing that dynamic consolidation is the best suitable method of soil improvement which can be adopted before constructing any structures in Kolkata in those places are highly prone to liquefaction.

Earth Dams in Near-fault Areas: From the Regional to the Site Model

The seismic response of large embankments in near-source conditions may be investigated by a multi-scale and multidisciplinary approach, where the seismological and geological aspects of the problem are firstly studied on a regional scale while the local and geotechnical aspects are studied on a detailed scale, including the structure of interest. A detailed knowledge of the basic seismological aspects of the area on a regional scale is necessary to properly simulate the seismic source and the wave propagation pattern within the rock basement to get the input motion at the bedrock formation for the reference site. The proper characterization of the motion at the bedrock level is crucial for large earth-dams or embankments for which a strong variability of the input motion may be dangerous, since differential settlements and fractures of the embankment may be promoted, with the consequent reduction of structure safety. Worth mentioning is the case-history of Conza Dam (AV) during the 1980 Irpinia earthquake: the embankment was partially jeopardized by the seismic event, being the dam site very close to the Irpinia source (≈10km). To merge the different scales of analysis (macro and micro) the mathematical formulation of the Domain Reduction Method (DRM), initially proposed by [1], has been recently implemented [14] in a FDM commercial code. One of the main advantage of the DRM procedure is the possibility of sub-structuring the real problem, dealing the generation and propagation of the seismic waves (model on a regional scale) separately from the seismic response of the specific site and structure, carried out on a reduced and detailed model (site model). According to the DRM approach, the dam is inserted in the refined internal domain and will receive the seismic motion from the external domain (which contains the source and propagation medium) through an interface boundary. To quantitatively evaluate the variability of the ground motion at the bedrock of the reference site, synthetic parameters were selected and suitable represented.

Hydraulic head field estimation using kriging with an external drift: A way to consider conceptual model information

This study presents an approach to hydraulic head mapping based on kriging with an external drift in which the auxiliary variables are obtained by finite-element modeling. The approach relies on the idea that numerical solutions stemming from simple conceptual models are suitable candidates for the hydraulic head field drift. Indeed, these numerical head fields obey the groundwater flow equations and incorporate information gathered during site investigations such as parameter estimates, geology and boundary conditions. The approach is tested on 2D and 3D synthetic case studies, and is then applied to a real case involving the mapping of hydraulic heads within two large earth dams. In each case, the hydraulic head maps are compared quantitatively and qualitatively to those obtained by ordinary kriging and universal kriging with first order polynomials. It is shown that kriging with an external drift can improve both the precision and realism of the mapped head fields.

Analysis of deformations of large earth dams

Safety of earth dams depends on the proper design, construction, and monitoring of actual behaviour during the construction and during the operation of the structure. The most critical factor in the assessment of the safety threshold value of any structure is the acceleration of its deformation. Therefore, the designed accuracy of monitoring surveys must fulfill requirements of detecting accelerations at critical locations of the investigated object. As an example, time dependant behavior of a large embankment dam during filling up the reservoir has been analyzed and verified by comparing monitoring results with the deterministic (prediction) model of the deformation. The geotechnical and geodetic monitoring besides providing a warning system in case of an abnormal behaviour of the dam, may be used as a tool for a verification of design parameters where geotechnical parameters are of the highest importance. Modeling of deformation of earth dams is a complex process in which one should consider the nonlinear behaviour of the construction material, interaction between the structure and the underlying foundation strata, influence of water load on the structure and on the foundation bedrock, and the effects of water saturation. Due to the uncertainty of the model parameters, careful monitoring of the dam and its surroundings are required in order to verify and enhance the model. In addition, with properly designed monitoring surveys, one may also determine the actual deformation mechanism. The finite element method may be useful tool in the proper design of the monitoring scheme by providing information on the locations and magnitude of the expected maximum displacements and velocites of movements. The discussed problems are illustrated by three types of earth dams located in California, U.S.A. and in Quebec, Canada.

Modelling of seismically induced liquefaction under high confining stress

Evaluation of seismically-induced liquefaction under high confining stress, such as might relate to the foundation soil under large earth dams, is based on the extrapolation of observed behavior and correlations at shallow depths. In practice, the behavior of saturated sand and other cohesionless soils under these conditions is not well understood. Experiments were conducted, at the 100 g-ton RPI centrifuge facility, to investigate confining stress effects on liquefaction potential, under the boundary and loading conditions of a deep horizontal saturated sand deposit. Three level ground models, with either a homogeneous profile or a dense layer over a loose layer, were tested in a laminar box. In two of the models, a steel plate surcharge was placed on the soil to increase confinement. Recorded accelerations and pore pressures, vertical surface settlement, and shear strains and stresses derived from the accelerations using System Identification, were analyzed to study the effect of high confining stress on the development of soil liquefaction.