International Journal of Geomechanics

Offshore structures for oil and gas exploitation are designed to accommodate severe environments with large cyclic loads. These structures are either founded directly on the seabed, or they are moored to anchors installed in the seabed soil. The permanent and cyclic loading, the foundation or anchor geometry, and the nonlinear soil behavior may be very complex, and many interrelated aspects must be considered in the geotechnical design of the foundations. Finite-element analyses (FEAs) are used increasingly to deal with these complexities and offer the potential to increase accuracy, efficiency, and reliability and reduce the uncertainty of the design process. This paper presents the major geotechnical aspects in the design of foundations for offshore structures and examples from finite-element analyses carried out at the Norwegian Geotechnical Institute (NGI) to deal with these aspects. A brief review of the procedure used at NGI to obtain soil stress-strain-strength relationships from cyclic laboratory tests is included. It is demonstrated that FEA offers several benefits over classical methods, such as limiting equilibrium calculations.

The closure behavior of rock joints is intimately related to joint roughness. Here we utilize a micromechanical approach that explicitly considers asperity interactions on joint surfaces to study the rock-joint closure and wave propagation behavior. Elastic deformations and inelastic frictional sliding are considered at inclined asperity contacts. Rock-joint roughness is modeled through distributions of asperity heights and asperity contact orientations. The micromechanical approach developed in this paper establishes the link between the rock-joint closure behavior, the initial overlap of the joints, the asperity height distribution parameters, and the average asperity slope. The model is verified by comparison with experimental measurements. Subsequently a parametric study is performed. The results show that rock joints with the same roughness can exhibit a range of closure behavior depending upon initial overlap and rock intrinsic friction. Therefore, unique descriptions of rock-joint closure behavior are elusive. Finally, the model-predicted nonlinear normal and shear stiffness are used to investigate the reflection and transmission of plane waves at rock joints.

To analyze seismic wave propagation in geological structures, it is possible to consider various numerical approaches: the finite difference method, the spectral element method, the boundary element method, the finite element method, the finite volume method, and so on. All these methods have various advantages and drawbacks to analyze the propagation of seismic waves. In surficial soil layers, seismic waves may be strongly amplified because of the velocity contrast between these layers and, possibly, to topographic effects around crests and hills. The influence of the geometry of alluvial basins on the amplification process is also know to be large. Nevertheless, strong heterogeneities and complex geometries are not easy to take into account with all numerical methods. Two-dimensional/three dimensional (2D/3D) models are needed in many situations, and the efficiency/accuracy of the numerical methods in such cases is in question. Furthermore, the radiation conditions at infinity are not easy to handle with finite differences or finite/spectral elements, whereas it is explicitly accounted for in the boundary element method. Various absorbing layer methods (e.g., F-PML, M-PML, CALM) were recently proposed to attenuate the spurious wave reflections, especially in some difficult cases, such as shallow numerical models or grazing incidences. Finally, strong earthquakes may involve nonlinear effects in surficial soil layers. To model strong ground motion, it is thus necessary to consider the nonlinear dynamic behavior of soils and simultaneously investigate seismic wave propagation in complex 2D/3D geological structures. Recent advances in numerical formulations and constitutive models in such complex situations are presented and discussed in this paper. A crucial issue is the availability of the field/laboratory data to feed and validate such models.

This paper is a review of the use of particulate discrete element modeling (DEM) in geomechanics. The overall objective of the paper is to serve as an introduction to researchers and practitioners in geomechanics who are considering adopting DEM in their work or using the results of DEM simulations to guide other studies, for example, the development of constitutive models for continuum-based numerical analysis. It is hoped that prior converts to the use of DEM will also benefit from a relatively objective overview of current DEM use in geomechanics. The introductory sections present the background to the method and give an overview of the evolution of the use of particulate DEM in recent geotechnical research. The general principals of the algorithm are then presented, considering the types of particles typically used, the calculation of contact forces, and formulation of simulation boundary conditions. Some techniques available to interpret and postprocess of DEM results and provide the information to link the particle scale and overall response are outlined. Approaches used to validate and calibrate DEM models to verify that DEM simulation results representative of physical reality are discussed. An overview of the application of DEM modeling to field-scale problems is then presented. Finally the conclusions consider future developments in the area and emphasize the need to maintain quality in DEM simulations.

The disturbed state concept (DSC) is used to characterize the deformation behavior of glacial tills, which may have a significant role in the motion of overlying glaciers with attendant implications for global climate dynamics. The model includes elastic, plastic, and creep deformation; microstructural modifications leading to softening and failure; and effect of rate of loading. This paper also presents a procedure to construct the static yield surface, which is essential to model the full creep response of the material. Laboratory triaxial tests are performed to calibrate the model. Predictions are compared with the test data, which include independent validations. A finite-element (FE) program with the implementation of the DSC model is used to predict the motion of an idealized ice sheet as affected by the deformation in the till. A novel concept is proposed to predict the failure and resulting motion of ice sheets. The concept involves the definition of state in the deforming till when stress, strain, and the disturbance reach critical values beyond the peak stress. The proposed concept is considered to be more realistic than the commonly used Mohr-Coulomb (M-C) criterion, in which failure and motion are assumed to occur at the peak stress with very small (elastic) strains.

Numerical models fitting long-term monitoring results represent a useful tool for understanding behavior of dams, especially if unusual performance jeopardizing safety is observed and rehabilitation measures should be suggested. Path-dependent and time-dependent constitutive models suitable for simulating observed behavior of concrete and rockfill dams are described in this paper. For the mechanical part of the solutions, a combined model using path-dependent incrementally linear elasticity in the prepeak regime and theory of plasticity in the peak regime is applied. For the time-dependent part of the solutions, the theory of multiface viscoplasticity is used. Three-surface-viscoplastic flow combined with Feda’s experimental relationships was successfully applied for modeling long-term creep of rockfill undergoing permanent structural changes attributable to grain breakage. By using a synthesis of these models, the reason of unexpectedly large deformations of Sance Rockfill Dam was pinpointed, future performance of the dam predicted, and cracking and hydraulic-fracturing hazard to the clay core estimated. The modeling results provided input for the rehabilitation concept of the dam.

Calibration, on the basis of data from centrifuge and shake table experiments, continues to promote the development of more accurate computational models. Capabilities such as coupled solid–fluid formulations and nonlinear incremental-plasticity approaches allow for more realistic representations of the involved static and dynamic/seismic responses. In addition, contemporary high-performance parallel computing environments are permitting new insights, gained from analyses of entire ground-foundation-structural systems. On this basis, the horizon is expanding for large-scale numerical simulations to further contribute toward the evolution of more accurate analysis and design strategies. The studies presented in this paper address this issue through recently conducted three-dimensional (3D) representative research efforts that simulate the seismic response of (1) a shallow-foundation liquefaction countermeasure, (2) a pile-supported wharf, and (3) a full bridge-ground system. A discussion of enabling tools for routine usage of such 3D simulation environments is also presented, as an important element in support of wider adoption and practical applications. In this regard, graphical user interfaces and visualization approaches can play a critical role.

The paper deals with the seismic stability assessment of two existing earth dams in Italy using a fully coupled effective stress nonlinear approach implemented in a finite-element (FE) code. The mechanical behavior of the involved clayey and granular soils is described through advanced elastoplastic constitutive models, calibrated on laboratory and in situ test results. Before the application of the seismic motions, appropriate FE static analyses are performed in both cases to define the initial stress state and the internal variables of the material models. The stability of both dams during dynamic loading is proved by inspection of the cumulated horizontal and vertical displacement time histories of the monitored solid nodes, which become constant immediately after the end of the seismic actions. Moreover, the computed crest settlements induced by the earthquakes are considerably smaller than the service freeboard of the dams. Because the applied seismic actions are characterized by high return periods, the presented results are indicative of a satisfactory dynamic performance of the two embankments during extreme dynamic loading conditions.

Mechanized tunneling is characterized by a staged procedure of excavation and lining erection and continuous support of the soil by means of supporting fluids (or compressed air) at the tunnel face and pressurized grouting of the tail gap. The interactions between the tunnel boring machine (TBM), the support measures, and the soil, including the groundwater, determine the efficiency, safety, and effects on the existing infrastructure. In this paper, a process-oriented numerical simulation model for mechanized tunneling and its integration in the context of an integrated optimization platform for tunneling (IOPT) is addressed. The simulation model is based upon the finite-element method and considers the transient excavation process and all relevant components, support measures, and processes, along with their interactions during tunnel advance. In particular, the model allows the investigation of the effects of drilling and stand-still periods upon the generation of a filter cake at the tunnel face. This is demonstrated by the numerical analysis of a straight tunnel advance by means of a hydroshield machine in water-saturated soft soil.

This paper deals with the usefulness and feasibility of three-dimensional (3D) finite-element analysis in geotechnical engineering practice. The usefulness of 3D analysis is illustrated through two examples. The first example deals with a collapse problem and highlights the presence of significant three-dimensional effects even in what may appear to be a two-dimensional problem. The second example deals with the effect of soil flow on piles preinstalled within an excavation area. This problem is increasingly encountered in urban high-rise construction wherein piles are often preinstalled before basement excavation. The lessons learned from these two examples are then discussed, and the viability of conducting three-dimensional analysis with relatively modest computing means is explored.