Gudmund Reidar Eiksund

A mechanistic–empirical model for base-reinforced flexible pavements

A mechanistic–empirical model for geosynthetic base-reinforced flexible pavements is proposed. The model uses traditional components of an existing unreinforced mechanistic–empirical model developed in the USA through NCHRP Project 1-37A. These components include a finite element response model, material models for the asphalt concrete, unbound aggregate base and subgrade and damage models for asphalt concrete fatigue cracking and permanent deformation in the pavement cross-section layers. New components for the reinforcement are introduced and include structural elements for the reinforcement, a material model for the reinforcement, a model for reinforcement–aggregate shear interaction, additional response modelling steps that account for the influence of the reinforcement on lateral confinement of the base aggregate during construction and subsequent traffic loading, and a modified permanent deformation damage model used for aggregate within the influence zone of the reinforcement. This paper describes the basic components of the model with a focus on the ability of the model to predict permanent deformation, which is compared to results from test sections. This comparison shows favourable agreement that is on the level seen with existing unreinforced mechanistic–empirical models and a large improvement over previously proposed models for reinforced pavements.

Cone penetration data classification with Bayesian Mixture Analysis

ABSTRACT This paper presents an application of the Bayesian Mixture Analysis (BMA) to deal with the classification of spatially variable soil data from the cone penetration test. The cone penetration data classification postulates a problem where a set of cone penetration measurements is used to identify “hidden or unobserved” soil classes. The problem is formulated as an incomplete-data Gaussian mixture where the observed data are defined by the natural logarithm-transformed values of the normalized friction and the normalized cone resistance, while the soil classes to be identified are considered as hidden data. The solution for the incomplete-data problem which consists of class-dependent mixture probabilities and Gaussian distribution parameters is defined in a Bayesian framework. The implementation of conjugate priors for the Gaussian mixtures enables an efficient sampling of the posterior parameters by the Gibbs algorithm of the Markov Chain Monte Carlo method. When compared to the well-established Robertson classification charts, the BMA formulation has an advantage due to the Bayesian framework which enables the definition of soil classes through mixture priors, class-dependent posterior parameter estimates, and a probabilistic soil classification. The presented approach is applied to the cone penetration data from the Sheringham Shoal Offshore Wind Farm site.

Dynamic Model Test of Monopile for Offshore Wind Turbines

In current practice, the most common foundation type for Offshore Wind Turbine generators (OWT’s) at moderate water depths is the monopile. A model pile in scale 1:20 of a typical monopile foundation for offshore wind turbine has undergone lateral vibration testing in dry laboratory sand. Eigen-frequencies are determined based on acceleration measurements. The aim of the tests is to provide benchmark results for validation of different calculation methods for offshore wind monopile foundations. The stiffness contribution from the sand is evaluated on behalf of measuring the first natural frequency of the pile-soil interaction system. Preliminary results from back-calculations of the model tests using both 3D -FEM and a simple beam on elastic foundation model indicate that strain-dependent soil stiffness plays an important role for determining the system stiffness. In this paper, the model tests and their results are presented, along with the preliminary results from the back-calculation.Copyright

Characterisation of Residual Shear Strength at the Sheringham Shoal Offshore Wind Farm

For offshore foundations, the residual shear strength is an important soil parameter for the evaluation of installation resistance and axial pile capacity (for jacket foundation). Estimation of residual shear strength can be conducted in a shear box test in the conventional way, or with the introduction of an interface to evaluate the change in residual shear strength under influence of friction between soil and the interface. In addition, the residual effective friction angle can be measured in the ring shear test using the Bromhead apparatus. In this study, the three above-mentioned methods are employed to estimate the values of residual shear strength of two soil units: the Swarte Bank Formation and the Chalk Unit sampled from the Sheringham Shoal offshore wind farms. The Swarte Bank Formation is dominated by heavily over-consolidated stiff clay, while the Chalk Unit is characterized by putty white chalk which behaves in a similar manner to stiff clay if weathered, or to soft rock if unweathered. These soil units are located at the bottom of the soil profile at the Sheringham Shoal wind farm and hence are important in providing axial capacity to the foundation.Samples from the two soil units are tested and compared at different rates of shearing to evaluate the change in axial capacity and installation resistance of the offshore wind turbine foundations under various possible loading and drainage conditions. Comparison is also made between residual shear strength with and without a reconsolidation period to assess the potential for soil set-up and its influence on the soil capacity. The results show that, for both the clay and the chalk, the estimated residual shear strengths are quite similar between the conventional and interface shear tests and tend to increase with increasing shearing rate. This can be attributed to the increasing dominance of the turbulent shearing mode. Relative to the peak shear strength, the values of residual shear strength are approximately 5 to 35% lower in most cases. Reconsolidation for a period of 24 hours appears to have, if any, marginal positive effect on residual shear strength of the two soils in both shear box and interface shear box tests. The residual friction angles derived from the shear box and ring shear tests are comparable and fall in the immediate range of shear strength. The various test results imply that the pile foundations at the Sheringham Shoal would have considerably large axial capacity, assuming that the horizontal stress is similar to the normal stress used in testing. The test data however should be used with caution and combined with piling experience in comparable soils where possible. The study aims to provide a source of reference for design of pile foundations for sites with similar soil conditions.Copyright