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In this paper, an empirical relationship between the Unconfined Compressive Strength (UCS) of intact rock and the unit shaft resistance of piles penetrating rock is investigated. A growing number of civil engineering projects are utilizing steel piles driven into rock where a significant portion of the pile capacity is derived from the shaft resistance. Despite the growing number of projects utilizing the technology, little to no guidance is offered in the literature as to how the shaft resistance is to be calculated for such piles. A database has been created for driven piles that penetrate bedrock. The database consists of 42 pile load tests of which a majority are steel H-piles. The friction fatigue model is applied to seven of the pile load tests for which sufficient UCS data exists in order to develop an empirical relation. The focus of this paper is on case histories that include driven pipe piles with at least 2 m penetration into rock.

The growing demand for clean renewable energy sources and the lack of suitable nearshore sites are moving the offshore wind industry toward developing larger wind turbines in deeper water locations further offshore. This is adding significant uncertainty to the geotechnical design of monopiles used as foundations for these systems. Soil testing becomes more challenging, rigid monopile behavior is less certain, and design methods are being applied outside the bounds of the datasets from which they were originally derived. This paper examines the potential impact of certain elements of geotechnical uncertainty on monotonic load–displacement behavior and design system natural frequency of an example monopile-supported offshore wind turbine (OWT). Geotechnical uncertainty is considered in terms of spatial variability in soil properties derived from cone penetration tests (CPT), parameter transformation uncertainty using the rigidity index, and design choice for subgrade reaction modeling. Results suggest that spatial variability in CPT properties exhibits limited impact on design load–displacement characteristics of monopiles as vertical spatial variability tends to be averaged out in the process to develop discrete soil reaction-lateral displacement (p-y) models. This highlights a potential issue whereby localized variations in soil properties may not be captured in certain models. Spatial variability in CPT data has a noticeable effect on predicted system frequency responses of OWTs employing a subgrade reaction model approach, and the influence of subgrade reaction model choice is significant. The purpose of this paper is to investigate the effect of uncertainty in soil data, model transformation, and design model choice on resulting structural behavior for a subset of available design approaches. It should be noted that significant further uncertainty exists and a wide variety of alternative models can be used by designers, so the results should be interpreted qualitatively.

Scour is a significant issue for bridges worldwide that influences the global stiffness of bridge structures and hence alters the dynamic behaviour of these systems. For the first time, this article presents a new approach to detect bridge scour at shallow pad foundations, using a decentralized modal analysis approach through re-deployable accelerometers to extract modal information. A numerical model of a bridge with four simply supported spans on piers is created to test the approach. Scour is modelled as a reduction in foundation stiffness under a given pier. A passing half-car vehicle model is simulated to excite the bridge in phases of measurement to obtain segments of the mode shape using output-only modal analysis. Two points of the bridge are used to obtain modal amplitudes in each phase, which are combined to estimate the global mode shape. A damage indicator is postulated based on fitting curves to the mode shapes, using maximum likelihood, which can locate scour damage. The root mean square difference between the healthy and scoured mode shape curves exhibits an almost linear increase with increasing foundation stiffness loss under scour. Experimental tests have been carried out on a scaled model bridge to validate the approach presented in this article. Science Foundation Ireland National Science Foundation (NSF) Invest Northern Ireland US-Ireland R&D

The majority of installed offshore wind turbines are supported on large-diameter, open-ended steel pile foundations, known as monopiles. These piles are subjected to vertical and lateral loads while in service. In current design practice, interaction of vertical and lateral loads are not considered, rather piles are designed to resist vertical and lateral loads independently. Whilst interaction effects are widely studied for shallow foundations, the limited research on this topic for pile foundations often produces conflicting results. This paper reviews the research of the influence of vertical loading on the lateral response of pile foundations under combined loads, from the perspective of analytical research, numerical research, and experimental research from tests performed on 1-g (gravitational acceleration) model, centrifuge, and full-scale piles. The potential reasons for the differences among the results of previous research are discussed. Some guidance for future research on the effect of vertical loads on the lateral response of piles is provided.

This paper experimentally investigates a vibration-based scour monitoring approach applicable to bridges with multiple simply supported spans on shallow foundations. A monitoring strategy based on the relative changes in pier-mode shape amplitudes due to scour is postulated. The first global mode shape of a bridge structure with multiple spans is extracted from acceleration measurements using an output-only approach, frequency domain decomposition (FDD). The relative changes of the pier-mode shape amplitudes under scour are then tracked. Here, each pier-mode shape value is compared with the mean values of the remaining piers in a process that creates a mean-normalized mode shape (MNMS). The approach is demonstrated on a scaled model of a bridge with four spans, supported on sprung foundations, where scour is simulated by the replacement of springs, with springs of lower stiffness corresponding to a reduction in foundation stiffness. It is shown that at a given “scoured” pier, significant increases in the MNMS value occur, suggesting that the location of the scour can be identified. The magnitude of the MNMS at a given pier also increases with an increase in stiffness loss due to scour. In practice, the approach would work best by carrying out a visual inspection of the bridge to establish the initial health condition at the time of sensor installation. After this initial process, the bridge can be monitored remotely for scour on an ongoing basis. Science Foundation Ireland

This paper investigates the applicability of Cone Penetration Test (CPT)-based axial capacity approaches, used for estimating pile static capacity, to the prediction of pile driveability. An investigation of the influence of various operational parameters in a driveability study is conducted. A variety of axial capacity approaches (IC-05, UWA-05 and Fugro-05) are assessed in unmodified and modified form to appraise their ability to be used in estimating the driveability of open-ended steel piles used to support, for example, offshore jackets or bridge piers. Modifications to the CPT-based design approaches include alterations to the proposed base resistance to account for the resistance mobilized under discrete hammer impacts and the presence of residual stresses, as well as accounting for the effects of static capacity increases over time, namely ageing. Furthermore, a study on the influence of various operational parameters within a wave equation solver is conducted to ascertain the relative impact of uncertain data in this respect. The purpose of the paper is not to suggest a new design procedure for estimating pile driveability, rather to investigate the influence of the various operating parameters in a driveability analysis and how they affect the magnitude of the resulting predictions. The study will be of interest to geotechnical design of piles using CPT data.

The majority of offshore wind turbines are founded on large-diameter, open-ended steel monopiles. Monopiles must resist lateral loads and overturning moments because of environmental (wind and wave) actions, whereas vertical loads tend to be comparatively small. Recent developments in turbine sizes and increases in hub heights have resulted in pile diameters increasing rapidly, whereas the embedment length to diameter ratio (L/D) is reducing. Soil erosion around piles, termed scour, changes the soil strength and stiffness properties and affects the system's load resistance characteristics. In practice, design scour depths of up to 1.3D are routinely assumed during the turbine lifetime; however, the impact on monopiles with low L/D is not yet fully understood. In this article, centrifuge tests are performed to assess the effect of scour on the performance of piles with low L/D. In particular, the effect of combined loads, scour type (global, local), and depth are considered. A loading system is developed that enables application of realistic load eccentricity and combined vertical, horizontal, and moment loading at the seabed level. An instrumented 1.8-m-diameter pile with L/D = 5 is used. A friction-reducing ball-type connection is designed to transfer lateral loads to the pile without inducing any rotational pile-head constraint, which is associated with loading rigs in tests of this nature. Results suggest that vertical and lateral load interaction is minimal. Scour has a significant impact on the lateral load-bearing capacity and stiffness of the pile, leads to increases in bending moment magnitude along the pile shaft, and lowers the location of peak pile bending moment. The response varies with scour type, with global scour resulting in larger moments than local scour. The size of the local scour hole is found to have a significant impact on the pile response, suggesting that scour hole width should be explicitly considered in design.

The dynamic response of structures in contact with soil is receiving increasing interest and there is a growing need for more accurate models capable of simulating the behaviour of these systems. This is particularly important in the field of offshore wind turbines, where accurate estimates of system frequency are needed to avoid resonance, and in the structural health monitoring fields, where accurate reference damage models are used. Previous work has shown that there is significant uncertainty in how to specify mobilised soil stiffness for dynamic soil-pile interaction modelling. Moreover, the contribution of soil mass in dynamic motion is often ignored. This paper applies a finite-element iterative model updating approach previously developed by the authors to two experimental piles to ascertain the mobilised soil stiffness and mass profiles from impact test data. The method works by obtaining a frequency response function (FRF) from an impact test performed on a test pile, developing a numerical model of this system, applying initial estimates of soil mass and stiffness, and updating these properties to match the experimental FRF with that generated in the numerical model. A range of elements are investigated including multiple runs of the approach to test repeatability, the influence of different starting estimates for stiffness, the effect of variability in experimental test data, and the influence of the pile length over which masses are distributed. Moreover, potential sources of error are discussed. The method provides reasonably consistent estimates of the soil stiffness and mass acting in the lateral dynamic motion of a given pile tested in this paper. The approach may be useful in the continued improvement of Soil-Structure Interaction (SSI) modelling for dynamic applications.

This paper numerically investigates the feasibility of using bogie acceleration measurements from a passing train to detect the presence of bridge scour. The Continuous Wavelet Transform is used to process the simulated acceleration measurements for a number of train passages over a scoured bridge, with scour represented as a local reduction in stiffness at a given pier. Average Wavelet coefficients are calculated for a batch of train runs passing over the same bridge. A scour indicator is developed as the difference in average coefficients between batches from the healthy bridge and when the bridge is damaged by scour. The method is assessed using a blind test, whereby one author simulated trains passing over a bridge in various states of health. The remaining authors were provided only with the train accelerations and had to predict the state of scour without any prior knowledge. This scour indicator performed quite well in the blind test for normal vehicle operating conditions.

Offshore pile driving is a high-risk activity as delays can be financially punitive. Experience of pile driving for offshore jacket structures where pile diameters are typically < 2 m has led to the development of empirical pile driveability models with proven predictive capability. The application of these methods to larger diameter piles is uncertain. A major component of driveability models involves estimating the static resistance to driving, SRD, a parameter analogous to pile axial capacity. Recent research on axial capacity design has led to improved models that use Cone Penetration Test, CPT data to estimate pile capacity and include for the effects of friction fatigue and soil plugging. The applicability of these methods to estimating pile driveability for larger diameter piles is of interest. In this paper, recent CPT based axial capacity approaches, modified for mobilised base resistance and ageing, are applied to estimating driveability of 4.2 m diameter piles. A database of pile installation records from North sea installations are used to benchmark the methods. Accounting for factors such as pile ageing and the relatively low displacement mobilised during individual hammer blows improves the quality of prediction of pile driveability for the conditions evaluated in this study.

Accurate characterisation of soil behaviour in Dynamic-Soil-Structure Interaction (DSSI) applications remains a significant challenge. Knowledge of the operational soil-structure interaction stiffness is important for applications ranging from earthquake engineering to offshore structures subjected to wind and wave loading. A number of methods have been derived to couple soil and structural properties using beam-Winkler models. One of the key drawbacks of these approaches is the disparity in predicted stiffness depending on the formulation chosen. Moreover, the contribution of soil mass in the dynamic motion of foundations is often neglected. In this paper, a method is presented that uses a Frequency Response Function (FRF) measured from a laterally-impacted pile to estimate operational stiffness and mass profiles acting along the pile. The method involves creating a beam-Winkler numerical model of the soil-pile system, applying a starting estimate of the soil stiffness and mass profiles and calculating weighting factors to be applied to these starting estimates to obtain a match between the measured FRF from the test pile and the calculated FRF from the numerical model. This paper presents the formulation of the iterative updating approach, and demonstrates its functionality using simulated experimental data of typical piles. Simulated data is used as it enables testing a wide range of circumstances including possible issues relating to the influence of the shape of the operational soil stiffness profile, soil density, effects of sensor noise and errors in damping estimation. The method may be useful in finite-element (FE) model updating applications where reference numerical models for soil-structure interaction are required.

Bridges can be subjected to damaging environmental actions due to flooding and seismic hazards. Flood actions that result in scour are a leading cause of bridge failure, while seismic actions that induce lateral forces may lead to high ductility demand that exceeds pier capacity. When combined, seismic actions and scour can lead to effects that depend on the governing scour condition affecting a bridge. Loss of stiffness under scour can reduce the ductility capacity of a bridge but can also lead to an increase in flexibility that may reduce seismic inertial forces. Conversely, increased flexibility can lead to deck collapse due to support loss, so there exists some uncertainty about the combined effect of both phenomena. A necessary step towards the performance assessment of bridges under flooding and seismic actions is to calibrate numerical models that can reproduce structural responses under different actions. A further step is verifying the achievement of performance goals defined by codes. Structural health monitoring (SHM) techniques allow the computation of performance parameters that are useful for calibrating numerical models and performing direct checks of performance goal compliance. In this paper, various strategies employed to monitor bridge health against scour and seismic actions are discussed, with a particular focus on vibration-based damage identification methods.

The high profile failure of the Malahide viaduct in Dublin in late 2009 was attributed to erosion of the supporting soils around the bridge piers, commonly referred to as foundation scour. This is a widespread geotechnical-structural problem, where foundation scour has been identified as the number one cause of bridge failure in the United States. Monitoring scour is of paramount importance to ensure the continued safe operation of the ageing bridge asset network. Most monitoring regimes rely on expensive underwater instrumentation that is often subject to damage during times of flooding, when scour risk is at its highest. Scour causes a rapid reduction in foundation stiffness and can lead to complete failure of one or more sub-structural components of a bridge. In this paper, a novel scour monitoring approach based on dynamic measurement techniques is described. The investigation is based on using accelerometers mounted on the structure of interest to detect losses in foundation stiffness due to scour, which manifest itself as a change in vibration characteristics. Experimental and numerical analyses were performed to validate the potential of this new monitoring framework. A significant advantage of this monitoring method over traditional approaches is that the structure itself is used to monitor the damage. Therefore, if failure is likely, it is assumed that the dynamic characteristics will indicate such and remediation works may be implemented.

The trend for development in the offshore wind sector is towards larger turbines in deeper water. This results in higher wind and wave loads on these dynamically sensitive structures. Monopiles are the preferred foundation solution for offshore wind structures and have a typical expected design life of 20 years. These foundations have strict serviceability tolerances (e.g. mudline rotation of less than 0.25° during operation). Accurate determination of the system frequency is critical in order to ensure satisfactory performance over the design life, yet determination of the system stiffness and in particular the operational soil stiffness remains a significant challenge. Offshore site investigations typically focus on the determination of the soil conditions using Cone Penetration Test (CPT) data. This test gives large volumes of high quality data on the soil conditions at the test location, which can be correlated to soil strength and stiffness parameters and used directly in pile capacity models. However, a combination of factors including; parameter transformation, natural variability, the relatively small volume of the overall sea bed tested and operational effects such as the potential for scour development during turbine operation lead to large uncertainties in the soil stiffness values used in design. In this paper, the effects of scour erosion around unprotected foundations on the design system frequencies of an offshore wind turbine is investigated numerically. To account for the uncertainty in soil-structure interaction stiffness for a given offshore site, a stochastic ground model is developed using the data resulting from CPTs as inputs. Results indicate that the greater the depth of scour, the less certain a frequency-based SHM technique would be in accurately assessing scour magnitude based solely on first natural frequency measurements. However, using Receiver Operating Characteristic (ROC) curve analysis, the chance of detecting the presence of scour from the output frequencies is improved significantly and even modest scour depths of 0.25 pile diameters can be detected.

There have been significant developments in the area of vibrationbased bridge scour monitoring in recent years. Traditional scour monitoring using either visual assessment or diving inspections are now recognised to be very unreliable and highly subjective. There has been a concerted effort to move towards reliable systems capable of either direct measurement of scour or indirect measurement, based on monitoring the response of the structure to damage. The developments have unearthed new challenges and problems. This paper describes some recent developments in the field. In addition, remaining challenges that act as a barrier to the successful wide-scale deployment of the methodologies are discussed. In particular, it addresses issues related to how to measure key performance indicators (such as the vibration response of the structure) and the potential of these approaches in real-world applications.

Monopiles are the most common foundation system for supporting Offshore Wind Turbines (OWT’s), accounting for more than 80% of all OWT substructures installed in Europe to date [1]. Significant reductions in the cost of developing OWTs have been realized over the past few years, to the point where offshore wind can now be developed subsidy free in favorable locations. Optimizing the engineering design of these structures has played a key role in ensuring these cost reductions are possible. The largest uncertainty with respect to modelling the dynamic response of an OWT often relates to the geotechnical design. This paper examines the influence of soil-structure interaction on the dynamic response of an OWT structure. The below ground pile-soil behavior was modelled using (i) a conventional DNV (De Norske Veritas) ‘p-y’ approach and (ii) an advanced in-situ calibrated 3D FE geotechnical design approach. The results for the soil-structure interaction were inputted into a separate dynamic wind turbine model and the dynamic response using the two separate SSI approaches were compared.

Dynamic Soil-Structure Interaction (DSSI) is an area of much ongoing research and has wide and varied applications from seismic response analysis to offshore wind foundation response. DSSI covers a wide range of load regimes from small-strain vibrations to large-strain cyclic loading. One of the most common ways to model DSSI uses the Winkler model, which considers the soil as a series of mutually independent springs. The difficulty with modelling DSSI arises with the inelastic and nonlinear load–displacement response of soil with increasing strain, therefore modelling of large-strain DSSI relies on the specification of many interrelated parameters. The relative magnitude of these parameters can have a significant effect on the modelled response. In this paper, the specification of an initial stiffness coefficient to model the elastic (small-strain) response of a soil–pile system is investigated. The coefficient of subgrade reaction method can be used to generate spring stiffness moduli for Winkler type models. A number of subgrade reaction theories have been proposed and their application to the problem of static loading has been widely studied. However, relatively little research concerning the application of these models for small-strain dynamic loading has been undertaken. This paper describes a sensitivity study in which a number of subgrade reaction models were used to estimate the frequency response at small-strain levels for a range of pile geometries and ground conditions. A field investigation was undertaken on two piles with different slenderness ratios to estimate the frequency response and damping ratios. The experimental results were compared to predictions of damped natural frequency obtained from numerical models using the force input and measured damping ratio from each experiment. The ability of each subgrade reaction formulation to model the response at small-strain levels is evaluated. European Commission - European Regional Development Fund European Commission - Seventh Framework Programme (FP7) Higher Education Authority Earth and Natural Sciences (ENS) Doctoral Studies Programme EU FP7 SMART RAIL

Scour around bridge foundations is one of the leading causes of bridge failure. Up until recently, the monitoring of this phenomenon was primarily based around using underwater instrumentation to monitor the progression of scour holes as they develop around foundation systems. Vibration¿based damage detection techniques have been used to detect damage in bridge beams. The application of these vibration based methods to the detection of scour has come to the fore in research in recent years. This paper examines the effect that scour has on the frequency response of a driven pile foundation system, similar to those used to support road and rail bridges. The effect of scour on the vibration characteristics of the pile is examined using laboratory and field testing. It is clear that there is a very clear reduction in the natural frequency of the pile as the severity of scour increases. It is shown that by combining state-of-the-art geotechnical techniques with relatively simple finite element modelling approaches, it is possible to accurately predict the natural frequency of the pile for a given scour depth. Therefore, the paper proposes a method that would allow the estimation of scour depth for a given observed pile frequency. European Commission - Seventh Framework Programme (FP7) Higher Education Authority European Regional Development Fund (ERDF)

Probabilistic slope stability analysis typically requires an optimisation technique to locate the most probable slip surface. However, for many slopes particularly those containing many different soil layers or benches several distinct critical slip surfaces may exist. Furthermore, in large slopes these critical slip surfaces may be located at significant distances from each other. In such circumstances, finding and rehabilitating the most probable failure surface is of little merit, as rehabilitating that surface does not improve the safety of the slope as a whole. Unfortunately, existing slip surface search techniques were developed to converge on one global minimum. Therefore, to implement such methods to evaluate the stability of a slope with multiple failure mechanisms requires the user to define probable slip locations prior to calculation. This requires extensive engineering experience and places undue responsibility on the engineer in question. This paper proposes the use of a locally informed particle swarm optimisation method which is able to simultaneously converge to multiple critical slip surfaces. This optimisation model when combined with a reliability analysis is able to define all areas of concern within a slope. A case study of a railway slope is presented which highlights the benefits of the model over single objective optimisation models. The approach is of particular benefit when evaluating the stability of large existing slopes with complicated stratigraphy as these slopes are likely to contain multiple viable slip surfaces.