David M. Potts

A new versatile expression for yield and plastic potential surfaces

Abstract In any elasto-plastic constitutive model there are three main ingredients, namely a yield surface, a plastic potential surface and a hardening/ softening rule. In this paper a versatile mathematical expression is presented which can be used to describe the yield and plastic potential surfaces. The expression is defined completely by a maximum of only four parameters. These parameters can easily be obtained from observable soil behaviour in simple triaxial tests. A major advantage of the expression is that by suitable adjustment of the parameters a wide range of surface shapes can be achieved. For example, it is possible to reproduce the so called “bullet shape” typical of the plastic potential used in the original Cam clay model and the “tear shape” yield surfaces employed in the more recent models. In fact the expression is capable of accurately reproducing the shapes of many of the yield and plastic potential surfaces currently in use. The expression is also shown to be in good agreement with experimental data.

MODELLING SHEET PILE RETAINING WALLS

Abstract In finite element analysis of retaining wall problems, the wall is often modelled with two-dimensional elements. When analysing walls constructed from thin steel sections such as sheet pile walls the stiffness and thickness of the wall elements are determined by equating the bending stiffness and axial stiffness of the prototype and the finite element model. The element thickness is not arbitrary and can influence the results obtained. One-dimensional beam elements provide an alternate method for sheet pile retaining wall analysis. The results of analyses using two-dimensional elements to model the wall are compared to analyses in which one-dimensional beam elements are used. Beam elements are more accurate for modelling thin section walls such as sheet pile walls. Two-dimensional elements are recommended for modelling thick concrete walls.

Field testing of large diameter piles under lateral loading for offshore wind applications

The nature-inspired concept of self-healing materials in construction is relatively new and has recently attracted significant attention as this could bring about substantial savings in maintenance costs as well as enhance the durability and serviceability and improve the safety of our structures and infrastructure. Much of the research and applications to date has focused on concrete, for structural applications, and on asphalt, with significant advances being made. However, to date no attention has been given to the incorporation of self-healing concepts in geotechnical and geo-environmental applications. This includes the use of concrete and other stabilising agents in foundations and other geotechnical structures, grouts, grouted soil systems, soil-cement systems and slurry walls for ground improvement and land remediation applications. The recently established Materials for Life (M4L) project funded by EPSRC has initiated research activities in the UK focussing on those applications. The project involves the development and integration of the use of microcapsules, biological agents, shape memory polymers and vascular networks as healing systems. The authors are exploring development of self-healing systems using mineral admixtures, microencapsulation and bio-cementation applications. The paper presents an overview of those initiatives to date and potential applications and presents some relevant preliminary results.By contrast to studies in petroleum geology and, despite their world-wide occurrence, geotechnical studies of ancient fluvial sediments are rare. This paper introduces the main characteristics of these sediments by reference to a classic UK example. Attention is then drawn to a number of major overseas examples where, although the principal features can be recognised, large differences arise as a result of factors such as the tectonic setting, the volume and mineralogy of the source material and the climate at the time the sediments were deposited. The first, over-riding problem for their engineering evaluation comes during the site investigation phase with the difficulty of deducing the geological structure and distribution of the widely varying lithologies.ABSTRACT: nEarthen construction and soil-based construction materials (SBCMs) are expanding areas of interest worldwide. They offer nthe potential for low carbon and embodied energy, sustainability through recycling and an alternative to high energy materials such as fired nmasonry. The materials that are generally used in earthen construction can be identified as manufactured unsaturated soils. Until recently, nhowever, these materials have rarely been studied using a geotechnical approach, and there is a general lack of recognition of the key nmechanisms at work mechanically and hydraulically. In this paper we review geotechnical aspects of soil-based construction materials examining nthe effects of suction and environmental conditions, and demonstrating behaviour in shear, compression and fracture. We cover nmaterials which are both unstabilised, where the primary source of strength is suction, and materials which are stabilised with cement, lime nor fibres. The review is backed up by experimental results from laboratory and field testing undertaken over a number of years at Durham nand UWA. n nRESUME: nConstruction en terre (en utilisant des materiaux de construction a base de sol - ») est une extension du domaine de linteret ndans le monde entier en raison de faibles emissions de carbone potentiel et lenergie intrinseque , et la durabilite a travers le recyclage , et il nest possible d utiliser beaucoup plus pour remplacer les materiaux de haute energie tels que la maconnerie tire . Les materiaux qui sont generalement nutilises dans la construction en terre peuvent etre identifies comme les sols non satures fabriques . Jusqua recemment, toutefois n, ces materiaux ont rarement ete etudie en utilisant une approche geotechnique , et il ya un manque general de reconnaissance des principaux nmecanismes a loeuvre mecaniquement et hydrauliquement . Dans cet article, nous examinons les aspects geotechniques des materiaux nde construction a base de sol - examinant les effets de la succion et des conditions environnementales , et le comportement en cisaillement ndemontrer , la compression et de fracture. Nous traitons des materiaux qui sont a la fois stabilisees , ou la premiere source de force daspiration nest , et les materiaux qui sont stabilises avec du ciment , de la chaux ou de fibres . La revue est soutenue par experimentales re- sultats nde tests en laboratoire et sur le terrain entrepris depuis un certain nombre dannees a Durham et lUWA .Strain accumulation in granular soils due to dynamic loading is investigated through long term cyclic triaxial tests and cyclic triaxial tests according to ASTM D 3999-91. Soil parameters, test equipment and loading conditions have a significant influence on strain accumulation, therefore a parameterization of the silica sand and a description of the cyclic triaxial test device are explained. Cyclic triaxial tests are performed and test results are presented illustrating the evolution of Young’s modulus during long term cyclic loading. The influence of the width of the stress-strain loop and the initial void ratio on strain accumulation is investigated and validated with existing accumulation models. The usefulness of Miner’s rule on sand subjected to cyclic loading is demonstrated by two tests with different packages of loading cycles.

Finite element analysis of construction stability of Thika Dam

Abstract High pore water pressures were measured in the fill material of the Thika Dam embankment early in its construction. These caused concern about the subsequent short term stability. Therefore construction was stopped. Stability reassessment and drainage design was performed using finite element analyses before construction was allowed to continue. Different methods of modelling the pore water pressure allowed efficient analysis of various drainage options. The analyses highlighted zones of high shear stress and shear strain, and formation of failure surfaces. They established the necessary level of drainage required to ensure stability, control movement and reduce the risk of residual shear surfaces. Large toe movement was predicted immediately after construction recommences. ©

PISA: New Design Methods for Offshore Wind Turbine Monopiles

Improved design of laterally loaded monopiles is central to the development of current and future generation offshore wind farms. Previously established design methods have demonstrable shortcomings requiring new ideas and approaches to be developed, specific for the offshore wind turbine sector. The Pile Soil Analysis (PISA) Project, established in 2013, addresses this problem through a range of theoretical studies, numerical analysis and medium scale field testing. The project completed in 2016; this paper summarises the principal findings, illustrated through examples incorporating the Cowden stiff clay profile, which represents one of the two soil profiles targeted in the study. The implications for design are discussed.

Vertical ground motion and its effects on liquefaction resistance of fully saturated sand deposits

Soil liquefaction has been extensively investigated over the years with the aim to understand its fundamental mechanism and successfully remediate it. Despite the multi-directional nature of earthquakes, the vertical seismic component is largely neglected, as it is traditionally considered to be of much lower amplitude than the components in the horizontal plane. The 2010–2011 Canterbury earthquake sequence in New Zealand is a prime example that vertical accelerations can be of significant magnitude, with peak amplitudes well exceeding their horizontal counterparts. As research on this topic is very limited, there is an emerging need for a more thorough investigation of the vertical motion and its effect on soil liquefaction. As such, throughout this study, uni- and bidirectional finite-element analyses are carried out focusing on the influence of the input vertical motion on sand liquefaction. The effects of the frequency content of the input motion, of the depth of the deposit and of the hydraulic regime, using variable permeability, are investigated and exhaustively discussed. The results indicate that the usual assumption of linear elastic response when compressional waves propagate in a fully saturated sand deposit does not always hold true. Most importantly post-liquefaction settlements appear to be increased when the vertical component is included in the analysis.

Geotechnical characterization of the Miocene formations at the location of Ivens shaft, Lisbon

The design of complex underground structures in an urban environment requires in the first instance an appropriate characterization and interpretation of the ground conditions and of the mechanical behaviour of soil formations in the ground profile. With such information it is then possible to select and calibrate appropriate soil constitutive models for application in advanced numerical analysis, with the objective of predicting the induced ground movements and the potential damage to existing structures and services. This paper provides an interpretation of the site investigation data collected for the numerical analysis and design of the Ivens shaft excavation in Lisbon, Portugal. For the first time a comprehensive set of interpreted data is obtained for two of the main formations in the Lisbon area, Argilas e Calcários dos Prazeres (AP) and Areolas da Estefânia (AE), improving the understanding of their mechanical behaviour and making the data available for application in most soil constitutive frameworks. It is evident from the results that even with careful testing procedures the data may appear to be inconsistent, requiring further assumptions when deriving soil parameters. Such assumptions are discussed and emphasis is placed on the need to combine data from laboratory and field investigations.

PISA: Recent Developments in Offshore Wind Turbine Monopile Design

This paper provides a brief overview of the Pile Soil Analysis (PISA) project, recently completed in the UK. The research was aimed at developing new design methods for laterally loaded monopile foundations, such as those supporting offshore wind turbine structures. The paper first describes the background to the project and briefly outlines the key research elements completed. The paper concludes with a brief description of the anticipated impact of the work and describes initiatives that have followed since.

Numerical Modelling of Thermo-Active Shafts

Geotechnical structures, such as foundation piles, retaining walls and tunnel linings, are increasingly employed to produce geothermal energy for space heating and cooling. However, the exchange of heat between the structure and the ground induces additional structural forces and contributes to further structural and ground movements, which may affect the serviceability and stability of such structures. While numerous field and numerical studies exist regarding the response of geothermal piles, no investigations have been carried out to characterise the response of thermo-active shafts. This paper presents a numerical study of the short and long term behaviour of hypothetical thermo-active shafts through fully coupled thermo-hydro-mechanical (THM) finite element (FE) analyses using the Imperial College Finite Element Program (ICFEP), where the effect of changing the structure’s geometric characteristics is investigated.

The Implications of Advanced Monopile Design Methodologies in Offshore Wind Turbines

The design of Offshore Wind Turbines (OWT) is a complex process involving several stages: wind turbine selection, tower and sub-structure design, as well as foundation design and installation. A successful design requires close interaction between these components in order to satisfy the main design requirements, namely the capacity and accumulated rotation for the foundation and dynamic response and fatigue for the whole system. Recent research has revealed that the current design methods for laterally loaded piles, when applied to short and stubby OWT monopiles, underestimate their initial stiffness and capacity. Advanced Finite Element (FE) analysis, with realistic modelling of the ground conditions can accurately reproduce soil response around a monopile, and hence improve the design, ultimately leading to cost reduction of monopile foundations. In the present paper, the impact of economies in foundation design on the overall design of a realistic OWT is explored. The NREL 5 MW baseline wind turbine is modelled through FE analysis under several characteristic design load cases. The advantages of using FE analysis when compared to traditional methods, in particular with respect to capacity and dynamic response, are demonstrated and discussed. pitfalls of traditional methods when applied to the design of offshore wind turbine monopiles. To ensure the chosen example is realistic, the main design constraints and realistic sources of information are used, including site conditions, soil characterisation, wind turbine characteristics and industry basis for design. The final objective is to describe a methodology for setting-up integrated models capable of representing the whole wind turbine system. The derived model can then be used for future fatigue assessment and quantification of the response of the wind turbine under varying environmental and operational conditions. Resulting models are able to reproduce accurately the following aspects of the system: the servocontrol of the wind turbine, the dynamic response of the super-structure, hydrodynamic loading and soilstructure interaction.