A case study on the seismic performance of earth dams

The seismic non-linear behaviour of earth dams is investigated by using a well-documented case study and employing advanced static and dynamic coupled-consolidation finite-element analysis. The static part of the analysis considers the layered construction, reservoir impoundment and consolidation, whereas the dynamic part considers the response of the dam to two earthquakes of different magnitude, duration and frequency content. The results of the analysis are compared with the recorded response of the dam and exhibit a generally good agreement. The effects of the narrow canyon geometry, the reservoir impoundment and the elasto-plastic soil behaviour on the seismic dam behaviour are investigated. Finally the implications of the adopted constitutive modelling assumptions on the predicted response are discussed

Numerical and analytical investigation of compressional wave propagation in saturated soils

In geotechnical earthquake engineering, wave propagation plays a fundamental role in engineering applications related to the dynamic response of geotechnical structures and to site response analysis. However, current engineering practice is primarily concentrated on the investigation of shear wave propagation and the corresponding site response only to the horizontal components of the ground motion. Due to the repeated recent observations of strong vertical ground motions and compressional damage of engineering structures, there is an increasing need to carry out a comprehensive investigation of vertical site response and the associated compressional wave propagation, particularly when performing the seismic design for critical structures (e.g. nuclear power plants and high dams). Therefore, in this paper, the compressional wave propagation mechanism in saturated soils is investigated by employing hydro-mechanically (HM) coupled analytical and numerical methods. A HM analytical solution for compressional wave propagation is first studied based on Biot’s theory, which shows the existence of two types of compressional waves (fast and slow waves) and indicates that their characteristics (i.e. wave dispersion and attenuation) are highly dependent on some key geotechnical and seismic parameters (i.e. the permeability, soil stiffness and loading frequency). The subsequent HM Finite Element (FE) study reproduces the duality of compressional waves and identifies the dominant permeability ranges for the existence of the two waves. In particular the existence of the slow compression wave is observed for a range of permeability and loading frequency that is relevant for geotechnical earthquake engineering applications. In order to account for the effects of soil permeability on compressional dynamic soil behaviour and soil properties (i.e. P-wave velocities and damping ratios), the coupled consolidation analysis is therefore recommended as the only tool capable of accurately simulating the dynamic response of geotechnical structures to vertical ground motion at intermediate transient states between undrained and drained conditions

On the assessment of energy dissipated through hysteresis in finite element analysis

The accurate reproduction of the hysteretic behaviour exhibited by soils under cyclic loading is a crucial aspect of dynamic finite element analyses and is typically described using the concept of damping ratio. In this paper, a general algorithm is presented for assessing the damping ratio simulated by any constitutive model based on the registered behaviour in three-dimensional stress-strain space. A cyclic nonlinear elastic model capable of accurately reproducing a wide range of features of soil behaviour, including the variation of damping ratio with deformation level, is chosen to illustrate the capabilities of the proposed algorithm. The constitutive model is described and subsequently employed in two sets of finite element analyses, one involving the dynamic response of a sand deposit subjected to different types of motion and another focussing on the simulation of a footing subjected to cyclic vertical loading. The application of the presented algorithm provides insight into the processes through which energy is dissipated through hysteresis

Accounting for partial material factors in numerical analysis

The concept of a safety factor in the design of geotechnical structures has traditionally been developed within the framework of classical soil mechanics, where the analysis methods for its calculation involve simple limit equilibrium or limit analysis approaches. Therefore the inclusion of a safety factor within an advanced analysis method, such as finite elements or finite differences, is a more complex issue. In particular, the problem arises with design codes, such as Eurocode 7, in which partial factors on soil strength (or partial material factors) must be accounted for. Eurocode 7 implies that a numerical analysis should be performed accounting for a characteristic strength, which is reduced by partial factors. There are two ways in which such partial factors can be included in numerical analysis: one in which the strength is reduced at the beginning of the analysis, and the other in which this is done during the analysis. Eurocode 7 gives no guidance as to which one of these two approaches is more appropriate to apply. More importantly, there is no guidance on the appropriate numerical procedure that should be implemented in any software in order to perform the required strength reduction during the analysis in the latter approach. Therefore different software programs account for this in different ways, and mostly only for simple constitutive models. This paper presents, first, a consistent methodology for accounting for partial material factors in finite-element analysis, which can be applied to any constitutive model. It then demonstrates the implications of the two ways the partial material factors can be introduced in any analysis, using the example of a bearing capacity problem and employing constitutive models of increasing complexity. The paper shows that the two approaches for accounting for partial material factors may lead to different results, and that it is therefore necessary to develop a rational set of guidelines for their inclusion in advanced numerical analysis. </jats:p