Automated Optimisation of Suction Caisson Foundations Using a Computationally Efficient Elastoplastic Winkler Model

This paper describes an automated approach for determining the optimal dimensions (length and diameter) of a suction caisson foundation subject to lateral loads, to minimise the foundation weight, whilst satisfying installation requirements, serviceability and ultimate limit states. The design problem was cast as a constrained optimisation problem. Solutions were initially developed using a graphical approach; the solution process was then repeated with an automated approach using an optimisation solver. Both approaches were feasible because a computationally efficient elastoplastic Winkler model was used to model the suction caisson foundation behavior under applied loading. The automated approach was found to be fast and reasonably accurate (when compared to more computationally expensive design procedures using three-dimensional finite element analyses). The benefits of this approach, made possible by the efficiency of the models employed, include better design outcomes and reduced design time

A systematic framework for formulating convex failure envelopes in multiple loading dimensions

The failure envelope approach is widely used to assess the ultimate capacity of shallow foundations for combined loading, and to develop foundation macro-element models. Failure envelopes are typically determined by fitting appropriate functions to a set of discrete failure load data, determined either experimentally or numerically. However, current procedures to formulate failure envelopes tend to be ad hoc, and the resulting failure envelopes may not have the desirable features of being convex and well-behaved for the entire domain of interest. This paper describes a new systematic framework to determine failure envelopes – based on the use of sum of squares convex polynomials – that are guaranteed to be convex and well-behaved. The framework is demonstrated by applying it to three data sets for failure load combinations (vertical load, horizontal load and moment) for shallow foundations on clay. An example foundation macro-element model based on the proposed framework is also described

Simplified method for the lateral, rotational, and torsional static stiffness of circular footings on a nonhomogeneous elastic half-space based on a work-equivalent framework

Although there are many methods for assessing vertical stiffness of footings on the ground, simplified solutions to evaluate lateral, rotational, and torsional static stiffness are much more limited, particularly for nonhomogeneous profiles of shear modulus with depth. This paper addresses the topic by introducing a novel “work-equivalent” framework to develop new simplified design methods for estimating the stiffnesses of footings under multiple degrees-of-freedom loading for general nonhomogeneous soils. Furthermore, this framework provides a unified basis to analyze two existing design methods that have diverging results. 3D finite element analyses were carried out to investigate the soil–footing interaction for a range of continuously varying and multilayered nonhomogeneous soils, and to validate the new design approach

Assessment of Bayesian changepoint detection methods for soil layering identification using cone penetration test data

This paper assesses the effectiveness of different unsupervised Bayesian changepoint detection (BCPD) methods for identifying soil layers, using data from cone penetration tests (CPT). It compares four types of BCPD methods: a previously utilised offline univariate method for detecting clay layers through undrained shear strength data, a newly developed online univariate method, and an offline and an online multivariate method designed to simultaneously analyse multiple data series from CPT. The performance of these BCPD methods was tested using real CPT data from a study area with layers of sandy and clayey soil, and the results were verified against ground-truth data from adjacent borehole investigations. The findings suggest that some BCPD methods are more suitable than others in providing a robust, quick, and automated approach for the unsupervised detection of soil layering, which is critical for geotechnical engineering design

Assessment of numerical procedures for determining shallow foundation failure envelopes

The failure envelope approach is commonly used to assess the capacity of shallow foundations under combined loading, but there is limited published work that compares the performance of various numerical procedures for determining failure envelopes. This paper addresses this issue by carrying out a detailed numerical study to evaluate the accuracy, computational efficiency and resolution of these numerical procedures. The procedures evaluated are the displacement probe test, the load probe test, the swipe test (referred to in this paper as the single swipe test) and a less widely used procedure called the sequential swipe test. Each procedure is used to determine failure envelopes for a circular surface foundation and a circular suction caisson foundation under planar vertical, horizontal and moment (VHM) loading for a linear elastic, perfectly plastic von Mises soil. The calculations use conventional, incremental-iterative finite-element analysis (FEA) except for the load probe tests, which are performed using finite-element limit analysis (FELA). The results demonstrate that the procedures are similarly accurate, except for the single swipe test, which gives a load path that under-predicts the failure envelope in many of the examples considered. For determining a complete VHM failure envelope, the FEA-based sequential swipe test is shown to be more efficient and to provide better resolution than the displacement probe test, while the FELA-based load probe test is found to offer a good balance of efficiency and accuracy

Probabilistic soil strata delineation using DPT data and Bayesian changepoint detection

Soil strata delineation is a fundamental step for any geotechnical engineering design. The dynamic penetration test (DPT) is a fast, low cost in situ test that is commonly used to locate boundaries between strata of differing density and driving resistance. However, DPT data are often noisy and typically require time-consuming, manual interpretation. This paper investigates a probabilistic method that enables delineation of dissimilar soil strata (where each stratum is deemed to belong to different soil groups based on their particle size distribution) by processing DPT data with Bayesian changepoint detection methods. The accuracy of the proposed method is evaluated using DPT data from a real-world case study, which highlights the potential of the proposed method. This study provides a methodology for faster DPT-based soil strata delineation, which paves the way for more cost-effective geotechnical designs

Investigation of local soil resistance on suction caissons at capacity in undrained clay under combined loading

Winkler modelling offers a flexible and computationally efficient framework for estimating suction caisson capacity. However, there is a limited understanding of the local soil resistance acting on caissons at capacity under combined six degrees-of-freedom (6DoF) loading, which is essential for accurately estimating caisson failure envelopes. Furthermore, existing simplified design models for caissons cannot assess capacity under non-planar lateral and moment loading, which is common in offshore wind applications. To address these limitations, this paper presents a comprehensive three-dimensional (3D) finite element analysis (FEA) study, which investigates the local soil resistance acting on the caisson at capacity in undrained clay under combined 6DoF loading. The paper introduces the concept of ‘soil reaction failure envelopes’ to characterise the interactions between soil reactions at capacity. Closed-form formulations are derived to approximate these soil reaction failure envelopes. An elastoplastic Winkler model is then developed, incorporating linear elastic perfectly plastic soil reactions based on these formulations. The results demonstrate that the Winkler model can provide efficient and reasonably accurate estimations of caisson capacity under combined 6DoF loading, even for irregular soil profiles that pose much uncertainty and challenges to existing macro-element models

Bayesian optimization for CPT-based prediction of impact pile drivability

Pile drivability predictions require information on the pile geometry, impact hammer, and the soil resistance to driving (SRD). Current SRD prediction methods are based on databases of long slender piles from the oil and gas industry and new, robust, and adaptable methods are required to predict SRD for current offshore pile geometries. This paper describes an optimization framework to update uncertain model parameters in existing axial static design methods to calibrate SRD. The approach is demonstrated using a case study from a German offshore wind site. The optimization process is undertaken using a robust Bayesian approach to dynamically update uncertain variables during driving to improve simulations. The existing method is shown to perform well for piles with geometries that reflect the underlying database such that only minimal optimization is required. For larger diameter piles, relative to the prior best estimate, optimized results are shown to provide significant improvements in the mean calculations and associated variance of pile drivability as more data is acquired. The optimized parameters can be used to predict SRD for similar piles in analogous ground conditions. The demonstrated framework is adaptable and can be used to develop site-specific calibrations and advance new SRD methods where large pile driving data sets are available

Time-critical design methods for suction caisson foundations

This thesis is centred around the development of a family of computationally efficient design methods called ‘oxCaisson’, that can predict the behaviour of suction caisson foundations under six degrees-of-freedom loading. These design methods were developed for applications where timeliness or speed is a crucial factor; hence the term ‘time-critical design methods’. oxCaisson is based on the Winkler framework, in which the soil response is represented by Winkler-type soil reactions that are either distributed along the caisson skirt length or concentrated at the caisson base. These soil reactions were calibrated against the results of rigorous, three-dimensional finite element (3DFE) analyses, making ox- Caisson effectively a surrogate model for the 3DFE method. The design methods developed are hierarchical in nature, where each subsequent design method builds upon previous ones. First, a design method called ‘oxCaisson-LE’ was developed for a ‘toy problem’, in which the soil was idealised as a homogeneous, linear elastic material. Then, the assumption of soil stiffness homogeneity is relaxed as an energy-based design method was developed for non-homogeneous stiffness profiles. Following that, the assumption of a fully rigid caisson is relaxed as the caisson skirt is modelled using deformable frame elements, which allows the prediction of flexible caisson behaviour (i.e. caissons with deformable skirts). Next, a design method called ‘oxCaisson-NLE’ was developed to predict the caisson behaviour in small-strain, non-linear elastic soil. Then, a new elasto-plastic design method called ‘oxCaisson-LEPP’ was developed to predict the caisson behaviour in linear elastic, perfectly plastic soil. oxCaisson-LEPP combines the previously developed linear elastic soil reactions with plastic yield surfaces. To model cohesive soil, the plastic yield surfaces were calibrated against 3DFE analyses using linear elastic, perfectly plastic soil with a von Mises yield criterion. Altogether, these design methods allow a rapid turnover of ‘3DFE-approximate’ design evaluations, which is crucial for time-critical applications such as large scale foundation design optimisation.</p

Time-critical design methods for suction caisson foundations

This thesis is centred around the development of a family of computationally efficient design methods called âoxCaissonâ, that can predict the behaviour of suction caisson foundations under six degrees-of-freedom loading. These design methods were developed for applications where timeliness or speed is a crucial factor; hence the term âtime-critical design methodsâ. oxCaisson is based on the Winkler framework, in which the soil response is represented by Winkler-type soil reactions that are either distributed along the caisson skirt length or concentrated at the caisson base. These soil reactions were calibrated against the results of rigorous, three-dimensional finite element (3DFE) analyses, making ox- Caisson effectively a surrogate model for the 3DFE method. The design methods developed are hierarchical in nature, where each subsequent design method builds upon previous ones. First, a design method called âoxCaisson-LEâ was developed for a âtoy problemâ, in which the soil was idealised as a homogeneous, linear elastic material. Then, the assumption of soil stiffness homogeneity is relaxed as an energy-based design method was developed for non-homogeneous stiffness profiles. Following that, the assumption of a fully rigid caisson is relaxed as the caisson skirt is modelled using deformable frame elements, which allows the prediction of flexible caisson behaviour (i.e. caissons with deformable skirts). Next, a design method called âoxCaisson-NLEâ was developed to predict the caisson behaviour in small-strain, non-linear elastic soil. Then, a new elasto-plastic design method called âoxCaisson-LEPPâ was developed to predict the caisson behaviour in linear elastic, perfectly plastic soil. oxCaisson-LEPP combines the previously developed linear elastic soil reactions with plastic yield surfaces. To model cohesive soil, the plastic yield surfaces were calibrated against 3DFE analyses using linear elastic, perfectly plastic soil with a von Mises yield criterion. Altogether, these design methods allow a rapid turnover of â3DFE-approximateâ design evaluations, which is crucial for time-critical applications such as large scale foundation design optimisation.</p