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The aim of this study is to present an efficient analytical approach, based on a combination of transfer matrix formulations and modal superposition, to perform seismic response analysis of onshore wind turbines (WT) incorporating the effects of soil-structure interaction (SSI). The WT tower and rotor-nacelle assembly (RNA) are idealised as a segmented Timoshenko beam-column and lumped mass, respectively. SSI is taken into account using elastic springs at the bottom of the tower to model soil flexibility. The mass of the foundation is also considered in the mathematical model as a lumped mass at the base of the tower. As the serviceability limit state tends to be the governing design condition for WTs, the soil behaviour is idealised as linear-elastic with equivalent linear stiffness. Firstly, natural frequencies and mode shapes of the WT are obtained using the transfer matrix method (TMM), which is based on analytically-calculated state vectors of beam-column members, and state vectors of joints at the top and bottom of the tower. The entire vibrating structure is simplified into an equivalent single-degree-of-freedom (SDOF) system using mode shapes calculated from TMM. The seismic responses are derived using the Newmark-Beta method and modal superposition. The application of TMM for free vibration analysis is validated with reference to results from existing literature. Additionally, the accuracy of the method for calculation of seismic responses is verified using finite-element software. Three different ground models are considered, namely stratum over half-space, stratum over rigid base, and elastic half-space; and the effect of each on the free vibration and seismic responses are examined in terms of displacement and base shear time-histories of the WT with varying foundation geometries. The effects of varying soil properties on the natural frequencies and seismic responses of the WT are also investigated. Finally, the dynamic responses of the model subjected to different seismic time-histories are presented comparatively, to highlight the significant effects of the frequency content of ground motion records. The aim of this study is to present an efficient analytical approach, based on a combination of transfer matrix formulations and modal superposition, to perform seismic response analysis of onshore wind turbines (WT) incorporating the effects of soil-structure interaction (SSI). The WT tower and rotor-nacelle assembly (RNA) are idealised as a segmented Timoshenko beam-column and lumped mass, respectively. SSI is taken into account using elastic springs at the bottom of the tower to model soil flexibility. The mass of the foundation is also considered in the mathematical model as a lumped mass at the base of the tower. As the serviceability limit state tends to be the governing design condition for WTs, the soil behaviour is idealised as linear-elastic with equivalent linear stiffness. Firstly, natural frequencies and mode shapes of the WT are obtained using the transfer matrix method (TMM), which is based on analytically-calculated state vectors of beam-column members, and state vectors of joints at the top and bottom of the tower. The entire vibrating structure is simplified into an equivalent single-degree-of-freedom (SDOF) system using mode shapes calculated from TMM. The seismic responses are derived using the Newmark-Beta method and modal superposition. The application of TMM for free vibration analysis is validated with reference to results from existing literature. Additionally, the accuracy of the method for calculation of seismic responses is verified using finite-element software. Three different ground models are considered, namely stratum over half-space, stratum over rigid base, and elastic half-space; and the effect of each on the free vibration and seismic responses are examined in terms of displacement and base shear time-histories of the WT with varying foundation geometries. The effects of varying soil properties on the natural frequencies and seismic responses of the WT are also investigated. Finally, the dynamic responses of the model subjected to different seismic time-histories are presented comparatively, to highlight the significant effects of the frequency content of ground motion records.

This paper compares the application of two recently published guidance documents for risk-based assessment of hydraulic actions on bridges, namely the UK Design Manual for Roads and Bridges and the Italian Ministry of Infrastructure and Transport’s Guidelines, to two case study bridges (Staverton Bridge, UK; Borgoforte Bridge, Italy). This work is one of the first to illustrate how to apply these guidelines. Both documents present risk-based methods for the assessment of hydraulic actions, while exhibiting fundamental differences. For example, the UK method prescribes calculations for local and constriction scour, water depth, and velocity at several cross-sections; by comparison, the Italian method does not prescribe calculations to assess the risk level. For the case studies in this paper, the hydraulic risk obtained for Staverton Bridge resulted as ‘High’ using both methods. The scour score for the Borgoforte Bridge resulted higher using the Italian method (Medium-High), as compared to the UK approach (Medium). This difference is due to how the guidelines assess the vulnerability associated with the minimum clearance. The comparison of these two risk-based approaches and the resulting discussion may serve as a useful resource for those wishing to develop new risk-based methods for assessing hydraulic actions on bridges.

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.

In this paper, a novel pier scour indicator is introduced, which uses the ratio between mode shape amplitudes identified at two points on an integral bridge structure to monitor the progression of scour erosion. The mode shape ratio (MSR) is investigated as an additional parameter to complement the use of changes in natural frequency as a scour indicator. The approach is demonstrated using numerical modelling and the MSR is extracted from acceleration signals arising in the structure due to modelled ambient and vehicle-induced vibrations. The MSR shows higher sensitivity to scour erosion than the more commonly researched natural frequency. Furthermore, the variation in MSR under temperature fluctuations is inversely related to that of frequency, in that it increases with increasing temperature whereas frequency decreases with increasing temperature. This inverse relationship potentially enables the separation of the scour effect from the temperature influence on the dynamics of the system.

The influence of combined loading on the response of monopiles used to support offshore wind turbines (OWTs) is investigated in this paper. In current practice, resistance of monopiles to vertical and lateral loading is considered separately. As OWT size has increased, the slenderness ratio (pile length, L, normalised by diameter, D) has decreased and foundations are tending towards intermediate footings with geometries between those of piles and shallow foundations. Whilst load interaction effects are not significant for slender piles, they are critical for shallow footings. Previous research on pile load interaction has resulted in conflicting findings, potentially arising from variations in boundary conditions and pile slenderness. In this study, monotonic lateral load tests were conducted in a geotechnical centrifuge on vertically loaded monopiles in dense sand. Results indicate that for piles with L/D = 5, increasing vertical loading improved pile initial stiffness and lateral capacity. A similar trend was observed for piles with L/D = 3, when vertical loading was below ≈45% of the pile’s ultimate vertical capacity. For higher vertical loads considered, results tended towards the behaviour observed for shallow footings. Numerical analyses conducted show that changes in mean effective stress are potentially responsible for the observed behaviour.

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

Scour erosion poses a significant risk to bridge safety worldwide and remains among the top causes of failure. Scour at bridge foundations changes the stiffness of the soil-foundation system, resulting in global changes in the dynamic behavior of the bridge. In this paper, a new approach to detect the loss in foundation stiffness resulting from scour at multiple foundation locations is proposed, using wavelet-based Operating Deflection Shape (ODS) amplitudes. A numerical model of a bridge with four simply supported spans resting on piers is used to introduce and test the approach. Scour erosion is modelled as a reduction in vertical foundation stiffness under one or multiple bridge piers. A fleet of passing trucks, modelled as half-car vehicles, are used to excite the bridge to enable structural accelerations be calculated at an ‘accelerometer’ (sensor node) located at each support. The proposed method is shown to be effective with only one accelerometer at each support location in a multi-span bridge. Using a statistical population of passing vehicles, the temporal accelerations measured at each support are averaged and transformed into the frequency–spatial domain, in order to estimate the wavelet-based ODS for a given scour case. A damage indicator is postulated based on differences between the ODS of healthy and scoured bridge cases. The damage indicator enables visual identification of the location of scoured piers considering a range of natural frequencies of the system.

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

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 influence of scour on the lateral response of monopile foundations for offshore wind turbines is investigated in this paper. Application of lateral load-displacement (p-y) curves to predict the lateral pile behaviour is subject to uncertainty as many of the presently used design approaches have been derived for long, slender piles. These piles, with typical length/diameter ratios (L/D) of greater than 10 behave differently compared to rigid monopiles, with L/D typically less than 6. In this paper, centrifuge tests are conducted on a monopile model under various scour scenarios and p-y curves are derived from strain gauges embedded along the model pile wall. Global scour and two different shapes for local scour holes are studied. Using the piecewise polynomial method for extraction of p-y curves from sparsely distributed strain measurements, it is recommended to use a 4th order polynomial for the moment profile to extract soil reaction and a 7th order polynomial for the moment profile to calculate pile deflection. Results indicate that the pile behaviour is significantly influenced by the nature (size, shape) of the scour holes affecting the pile–soil system and suggest that the p-y curves should be appropriately modified to account for this behaviour.

It is generally accepted that climate change is leading to increased frequency of extreme weather events worldwide, and this is placing heavier demands on an already aging infrastructure-network. Bridges are particularly vulnerable infrastructure assets that are prone to damage or failure from climate-related actions. In particular, bridges over waterways can be adversely affected by flooding, specifically the washing away of foundation soils, a mechanism known as scour erosion. Scour is the leading cause of failure for bridges with foundations in water as it can rapidly compromise foundation stiffness often resulting in unacceptable movements or even collapse. There is growing interest among asset managers in applying health monitoring approaches to assess the real-time performance of bridges under damaging actions, including scour. Sensor-based approaches involve the acquisition of data such as dynamic measurements, which can be used to infer the existence of scour or other damage without the laborious requirements of undertaking visual inspections. In this paper, a framework is proposed to assess the benefit obtained from health monitoring systems as compared to the scenario where no monitoring system is employed on a bridge, to ascertain how useful these systems are at assisting decision-making. Decisions typically relate to the implementation of traffic restrictions or even partial or complete bridge closure in the event of damage being detected, which has associated consequences for a network. A case study is presented to demonstrate the approach postulated in this paper.

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.

Bridges founded in water can be prone to scour erosion, an effect which alters the dynamic properties of the bridge. In seismic regions, bridges are prone to excitation from earthquakes, which can be damaging to the structures. These two hazards, while independent from one another, can alter how each affects a bridge in the sense that an earthquake may pose a more serious effect on a scoured bridge than on the unscoured structure. The magnitude and location of a scour hole affecting a structure can alter the seismic bridge response to varying degrees. In this paper, a finite-element model of a multi- span bridge is created using OpenSees, and some preliminary results of a numerical investigation into scour at multiple locations and the resulting impact on the seismic behavior is reported. The bridge is subjected to ground motion considering several defined scour conditions, where scour is modelled as the incremental increase in length of a given pier. The influence of scour depth and location on the internal shear forces, displacement and accelerations of the structure are studied with a view to ascertaining critical damage combinations.

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.

Several regions in Europe are prone to both seismic and scour hazards thus bridges can suffer from the joint actions of these two phenomena. The two types of hazards are actually independent as to the generation process but the loss of surrounding soil due to scour may significantly modify the dynamic parameters of the structure thus changing its response to a subsequent earthquake. Permanent monitoring through vibration-based methods can be an effective support in bridge assessment procedures providing updated information about the structural state and performance. Information from the monitoring system allow both the prompt detection of a possible damage state after an extreme event and also support for long term assessment of the structural conditions. For bridges under multiple hazard the possibility of using the same network of sensors for different hazards can increase the benefit/cost ratio related to the use of a monitoring system. In this paper, some preliminary results of a numerical investigation carried out on the finite element model of a bridge under flood and seismic actions are reported. The numerical model of a multispan bridge, developed using OpenSees, is subjected to a seismic base input considering several defined scour conditions. Scour has been modelled as the incremental increase in length of one of the piers and the effect of scour on the seismic response is investigated. The evolution of modal periods and of deck peak accelerations under seismic excitation have been studied in order to investigate the feasibility of these parameters as performance indicators able to detect the presence of scour in one piers for a bridge under seismic excitation. The first part of the paper reports a brief description of the main damage scenarios connected to scour and to seismic actions. The second part of the paper reports the results of a non-linear dynamic analyses of a scoured bridge under seismic input. Several scour depths have been considered in order to study the sensitivity of several vibration-based parameters to the intensity of the scouring.

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.

The 8th Conference on Computational Stochastic Mechanics (CSM8), Paros, Greece, 10-13 June 2018

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.

Detecting scour by analysing bridge vibrations is receiving an increasing amount of attention in the literature. Others have considered changes in natural frequency to indicate the presence of scour damage; however, little work has been reported on identifying the location of a scour hole based on vibration measurements. In this paper, a numerical study is carried out using a bespoke vehicle–bridge–soil dynamic interaction model to examine how the first six vibration modes (Eigen frequencies) of a typical integral bridge are affected by scour at different locations. It is found that depending on the location of the scour hole, some modes are much more affected than others in terms of frequency changes. In fact, a clear pattern emerges as to which modes are affected by which scour location. Using this knowledge, the location of a scour hole can potentially be detected on a real bridge. However, recognising that it is not possible to undertake an eigenvalue analysis on an actual bridge, an analysis is performed by collecting acceleration signals from various points on the structure. The bridge is loaded by a realistic vehicle model, incorporating vehicle–bridge interaction effects, which leads to the generation of discrete acceleration signals at various ‘sensor’ locations on the bridge. In this paper, it is found that it is possible to detect the location of a scour hole using a relatively small number of ‘sensors’. However, to achieve this, careful signal processing is necessary and advice on a number of pertinent issues is provided.

The high profile failure of the Malahide Viaduct, in Dublin, Ireland which is a part of the EU TEN-T network of critical transport links was caused by foundation scour. In a study of five hundred bridge failures that occurred in the United States between 1989 and 2000, flooding and scour were the cause of 53% of the recorded failures (Wardhana and Hadipriono 2003). Scour is a common soil-structure interaction problem. In light of current changes in climate, increased frequency of flooding, coupled with the increased magnitude of these flood events, will lead to a higher risk of bridge failure occurring. Monitoring scour is of paramount importance to ensure the continued safe operation of the aging bridge asset network. Most monitoring regimes are based on using expensive underwater instrumentation that can often be subject to damage during times of flooding, when scour risk is at its highest. This paper presents a critical review of existing scour monitoring equipment and methodologies with a particular focus on those that use the dynamic response of the structure to indicate the existence and severity of the scour phenomenon affecting the structure. A sensitivity study on a recently developed monitoring method is also undertaken. European Commission - Seventh Framework Programme (FP7) Higher Education Authority European Regional Development Fund (ERDF)

Bridge scour is the number one cause of failure in bridges located over waterways. Scour leads to rapid losses in foundation stiffness and can cause sudden collapse. Previous research on bridge health monitoring has used changes in natural frequency to identify damage in bridge beams. The possibility of using a similar approach to identifying scour is investigated in this paper. To assess if this approach is feasible, it is necessary to establish how scour affects the natural frequency of a bridge, and if it is possible to measure changes in frequency using the bridge dynamic response to a passing vehicle. To address these questions, a novel vehicle–bridge–soil interaction (VBSI) model was developed. By carrying out a modal study in this model, it is shown that for a wide range of possible soil states, there is a clear reduction in the natural frequency of the first mode of the bridge with scour. Moreover, it is shown that the response signals on the bridge from vehicular loading are sufficient to allow these changes in frequency to be detected.

Damage detection in bridges using vibration-based methods is an area of growing research interest. Improved assessment methodologies combined with state-of-the-art sensor technology are rapidly making these approaches applicable for real-world structures. Applying these techniques to the detection and monitoring of scour around bridge foundations has remained challenging; however this area has gained attraction in recent years. Several authors have investigated a range of methods but there is still significant work required to achieve a rounded and widely applicable methodology to detect and monitor scour. This paper presents a novel Vehicle-Bridge-Soil Dynamic Interaction (VBSDI) model which can be used to simulate the effect of scour on an integral bridge. The model outputs dynamic signals which can be analysed to determine modal parameters and the variation of these parameters with respect to scour can be examined. The key novelty of this model is that it is the first numerical model for simulating scour that combines a realistic vehicle loading model with a robust foundation soil response model.This paper provides a description of the model development and explains the mathematical theory underlying the model. Finally a case study application of the model using typical bridge, soil, and vehicle properties is provided. University College Dublin Earth and Natural Sciences (ENS) Doctoral Studies Programme Programme for Research at Third Level Institutions, Cycle 5 (PRTLI-5) European Regional Development Fund (ERDF)

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

Rapid expansion of the offshore wind industry has stimulated a renewed interest in the behaviour of offshore piles. There is widespread acceptance in practice that pile design methods developed for the offshore oil and gas industry may not be appropriate for designing wind turbine foundations. To date, the majority of offshore wind turbines are supported by large diameter monopiles. These foundations are sensitive to scour which can reduce their ultimate capacity and alter their dynamic response. In this paper, the use of a vibration-based method to monitor scour is investigated. The effect of scour on the natural frequency of a model monopile was measured in a scale model test. A spring-beam finite element numerical model was developed to examine the foundation response. The model, which used springs tuned to the small-strain stiffness of the sand, was shown to be capable of capturing the change in frequency observed in the scale test. This numerical procedure was extended to investigate the response of a full-scale wind turbine over a range of soil densities, which might be experienced at offshore development sites. Results suggest that wind turbines founded in loose sand would exhibit the largest relative reductions in natural frequency resulting from scour. Higher Education Authority European Regional Development Fund

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.

Soil supporting foundations for infrastructural assets such as bridges are repeatedly subjected to dynamic loading from passing traffic. The load-displacement characteristics are nonlinear at even small to medium strains, however this effect is mostly modelled using secant stiffness models with effective linear stiffness. This simplification enables a pseudo linear model to encapsulate the effects of a nonlinear system with some accuracy. Linear models have a frequency related to the amplitude of the load used to specify the secant stiffness. Nonlinear models, however, have a range of frequencies due to the changing stiffness of the system resulting from the amplitude of the input load. There is growing uncertainty surrounding the response of foundation systems to damage effects. In particular scour erosion, which is the term used to describe removal of soil from around foundation elements by hydraulic action, presents a significant hazard to infrastructure and is becoming more prevalent with increased flooding risk due to climate change effects. This paper describes a numerical framework to incorporate a basis for nonlinearities in the load-displacement response modelling of pile foundations. The Newmark-Beta nonlinear integration solver is implemented in the study. A case study is presented of a pile foundation subjected to a range of load types and progressive scour damage, and the calculated response from the various conditions is evaluated. Periodic loads of varying amplitude are implemented. Effective linear models are developed based on the secant stiffness approach and compared to the nonlinear systems. The paper presents the numerical basis for the analysis, results of the case study and advice on modelling issues relevant to analyses of this nature.