Effects Of Scour Research Articles

Dynamic analysis on monopile supported offshore wind turbine under wave and wind load

Monopile offshore wind turbine (OWT) accounts for more than 65% of all OWT types. Monopile OWT is a soil-pile-tower system and can be divided into three stages: the stage embedded in the soil, the stage submerged in the seawater and the stage exposed to the air. Based on the improved Tajimi formulation and dynamic principle, this paper establishes a three-stage monopile OWT model to predict the dynamic response of the structure. The displacement of the tower under different wind velocities is obtained. The solution is compared with the Finite Element Method (FEM) result to verify its correctness. The influences of different parameters on the structure are discussed. The results indicate that the structure natural frequency is sensitive to the tower length and nacelle-rotor mass. The increasing seawater depth due to scour effect can significantly decrease the foundation stiffness. The influence of axial force and wave load on the foundation stiffness is also investigated. The tower diameter and tower wall thickness are found of vital importance to the tower displacement.

Lateral Bearing Capacity of a Hybrid Monopile: Combined Effects of Wing Configuration and Local Scour

Pile foundations for offshore wind turbines are subjected to large lateral loads. By mounting wings on the perimeter of regular monopiles, winged monopiles have shown better performance in resisting deformation under horizontal loading. However, the hazardous effect of local scour on the lateral bearing capacity of winged monopiles installed in the sandy seabed has not been systematically evaluated. In this study, a modified Mohr–Coulomb model considering the pre-peak hardening and post-peak softening behavior of dense sand is adopted to simulate laterally loaded winged monopiles in the locally scoured sandy seabed, using three-dimensional finite element analyses. The effect of local scour depth on the lateral capacity of winged monopiles is examined and explained by soil failure mechanisms. The enhancement of lateral capacity with wings attached to the monopile is demonstrated to be more effective than extending pile embedment length. The effects of the relative density of sand and the wing load orientation are also discussed. Finally, the wing efficiency is evaluated to determine the optimal configuration of winged monopiles.

The Influence of Structural and Materials on Bridge Stability and Stabilization Measures

Bridges are now not only an auxiliary tool for travel, but it is also an important way to measure a country's comprehensive strength. Because the stability of a bridge structure is affected by a variety of professional knowledge and various aids which made by advanced technology. So whether the bridge is stable and how to be more stable are the focus of discussion. This paper mainly analyzes and summarizes the stability of the bridge from the perspective of influencing factors and structural principles, and puts forward some measures which could improve stability accordingly. It includes the most common natural factors: earthquake and flood, which need to be considered from different perspectives due to their different failure principles and structures. As for the bridge structure itself, its upper and lower structures have cracks or lack bearing capacity, resulting in uneven settlement of the foundation. The sliding and tilting of the foundation and the local voids in the foundation are all factors that endanger the stability of the bridge. As for the effects of cracks, freeze-thaw denudation, scouring and grinding, cavitation erosion, water erosion, weathering and denudation of materials, they should be considered as important factors. Moreover, the improvement of the existing technology, such as strengthening the anti-corrosion layer, waterproof layer and protective layer, are also important.

Numerical investigation on horizontal deformation of large diameter monopile under cyclic wave loading with local scour holes

Seabed liquefaction and local scour around the offshore wind turbine (OWT) monopile have significant effects on the horizontal bearing capacity of the foundation. The horizontal displacement of the monopile at the seabed mudline is one of important factors considered in the foundation design. In this study, the effects of local scour and soil liquefaction on the monopile horizontal displacement were investigated. The dynamic response of sandy seabed with various densities under wave loading was simulated by adopting the Cyclic Mobility (CM) model. The numerical model was compared with previous experimental results. Then, the effects of local scour and soil liquefaction on the monopile horizontal bearing capacity were discussed. The differences in the effects of local scour and liquefaction on the horizontal displacement of monopile in the case of sandy seabed with different densities were compared. Numerical results shown that the effect of soil liquefaction caused by the vertical cyclic wave loading on the monopile horizontal bearing capacity is more significant than that of local scouring and horizontal cyclic loading, especially in loose sandy seabed. For dense sandy seabed, the local scour of the seabed has a more significant impact on the horizontal bearing capacity of the foundation.

The Impact of Scour on Laterally Loaded Piles Bored and Socketed in Marine Clay

The majority of long-span bridges and offshore construction in deep water are supported by bored pile foundations. Pile foundations under water will be susceptible to scour, which will pose a significant danger to their safe operation. A three-dimensional finite element (FE) numerical analysis was conducted to examine the influence of scour on the responses of laterally loaded drilled piles in marine clays. First, the FE numerical model was calibrated using field test data of a drilled pile that did not include scour in soft marine clay. The findings of numerical modeling reveal that scour decreases the ultimate lateral capacity of socketed drilled piles, with the percentage of reduction being strongly related to scour types and depth, but moderately related to scour hole width and slope angle. Finally, a semi-empirical formula for calculating the effects of scour on the lateral capacity of socketed drilled piles following scour is presented. This method may be utilized fast and effectively to calculate scour’s effects.

Scour protection effects on the dynamic response of jacket structures under extreme load events

A well-validated Finite Element (FE) model of a jacket structure (OC4 jacket) with pile foundation together with a Computational Fluid Dynamic (CFD) model (OpenFOAM-based) set-up for extreme waves on the structure are applied to perform systematic comparative analyses of the dynamic response of the structure to extreme wave loads with and without scour effects. The pile foundation model is considered by implementing a non-linear soil model to the foundation using load-deflection (p-y) approach to capture the non-linear behaviour of the soil for large pile foundation displacements. The results show that, the scour protection improves the foundation bearing capacity and consequently, decreases the failure probability in terms of both serviceability and ultimate limit states. Moreover, it enhances the dynamic response of the structure under extreme load events caused by breaking waves. Accordingly, an innovative solution for scour protection of offshore-fixed bottom structures has been introduced by “Biodegradable Soft Rock” (sand filled containers produced from bio-degradable geotextile product) which is the newest development in this section.

ANALYZING THE EFFECT OF WATER SEEPAGE AND SCOUR ON SLOPE STABILITY USING FINITE ELEMENT METHOD

On every slope, there is always the possibility of landslides. One of the hypotheses tested was the effect of water flow and scour on slope stability at the investigated location. A brief inspection was carried out to determine the condition and influence of the river, in addition to soil investigations, for proper follow-up arrangement. At landslide sites, water flow has an effect in raising the groundwater level so as to reduce the soil shear strength. Its causes the slope safety factor to drop below the safe rate. The scouring disrupts the stability of the slope but does not reduce the slope safety factor significantly. The parametric studies are chosen for their strong relation between measured displacement within the slope, to verify the measurements and the numerical analysis (Finite Element Method) results. The parametric study simulation shows that time-dependent numerical model could be useful as forecasting tool, including various possible scour and non-scour condition.

Coupled effects of long-term cyclic loading and scour on the mechanical responses of monopile-supported offshore wind turbines

Monopile-supported offshore wind turbines (OWTs) inevitably undergo the coupled effects of long-term cyclic loads and scour, resulting in cumulative deformation and the migration of the natural frequency of the foundation-superstructure system and the simultaneous development of scour holes around monopiles, which brings notable challenges to the design of OWTs. Hence, a series of model tests on monopile-supported OWTs in sands were conducted in a flume equipped with a custom long-term cyclic loading device. The mechanical responses of the OWT system and the scour process under separate and coupled effects of long-term cyclic loading and scour were studied, with special attention given to the cumulative rotation, natural frequency, ultimate bearing capacity of the OWT system, and scour hole development. A preliminary investigation of the armoring effect of the overlying coarse-grained soil under combined action was also conducted. The experimental results revealed that the coupled effect of long-term cyclic loading and scour not only has a great impact on the mechanical response of the foundation-superstructure system, but it also resulted in an acceleration of the initial scour rate and reduced the equilibrium scour depth. Moreover, the coupled effect reduced the scour resistance of coarse-grained soils under the layered soil conditions.

Effects of scour on the first natural frequency of monopile in different layered foundations

Scour is ubiquitous in offshore wind farms and threatens the stable state of the natural frequency of offshore wind turbines. The foundation of offshore wind turbines is a layered seabed, which mainly consists of clay, silt, and fine sand layers. However, little published literature can be found on the natural frequencies of monopiles in layered foundations with scour. The objective of this study was to reveal the influence of scour in layered foundations on the natural frequencies of monopoles and study its mechanism based on the dynamic characteristics of layered foundations. Therefore, based on an actual layered seabed, three typical layered foundations were established, and the first natural frequency of the monopile in terms of sensitivity to scour as well as the ability to resist continuous scour in the layered foundations were revealed. Moreover, the experimental results were analysed based on the stiffness and damping variations of the layered foundations under scour conditions. Importantly, a competitive relationship between the stiffness and damping of the layered foundations on the variation of the first natural frequency with scour was found and elaborated for the first time. The competitive relationship was influenced by the proportion of clay in the layered foundation.

Probability of failure estimation for highway bridges under combined effects of uncorrelated multiple hazards

Majority of the highly populated regions of the United States are susceptible to multiple natural hazards. In such regions, the design and construction of structures under multiple hazards are critical to achieve the appropriate structural performance and infrastructure resilience. Multi-hazard reliability analysis of structural systems evaluates the system response under multiple random loads, some of which may occur simultaneously, or the effect of one may weaken the structural system before the occurrence of the next event. This paper studies the combined effects of scouring and earthquakes, as two uncorrelated extreme events, on the performance of reinforced concrete highway bridges. In a continuous effort to support future improvement in understanding the impact of multi-hazard loading scenario on bridges and to develop mitigation actions, this paper assesses the seismic vulnerability of a reinforced concrete highway bridge experiencing the effect of erosion due to the increase in frequency of flood events. The analytical fragility approach uses a three-dimensional nonlinear finite element model of the bridge cases with various levels of scouring. Because a bridge is system of components, a component level fragility curve is used to track the response of the components for a given ground motion intensity. The system fragility curves are developed to consider the vulnerability of critical components to assess the probability of bridge damage. The results indicate that under multi-hazard scenarios, the component governing the fragility of the bridge system varies depending on the level of scour sustained by the structure.

Field tests and numerical analysis of the effects of scour on a full-scale soil–foundation–structural system

Scour is the prevailing cause of bridge failure worldwide, leading not only to traffic disruption, but also to social and economic losses and even to casualties. Many vibration-based monitoring techniques have been proposed for identifying the scour location and extent, based on the evaluation of the changes of the bridge modal properties due to scour. This study describes the experimental and numerical research carried out to investigate the effects of scour on the dynamic properties of structures with shallow foundations. Although these are the most vulnerable ones, they have received less attention compared to structures founded on pile foundations. To fill some existing knowledge gaps, field experiments were carried out on EuroProteas, a structural prototype with shallow foundation that was subjected to increasing levels of scour. The changes of the dynamic properties of the system are evaluated by postprocessing the ambient vibration recordings and by developing various models of the soil–foundation–structural system with different descriptions of the soil–structure interaction problem. The study results shed light on the effects of scour on systems with shallow foundations and on the accuracy of alternative modelling approaches. They are presented here to inform the development and validation of vibration-based techniques and modelling strategies for bridge scour identification.

A test method to study scour effects on the lateral response of rigid piles in sand under real scour conditions

Rigid piles are common foundations for offshore wind turbines. Scour around piles leads to the loss of surrounding soil, which seriously threatens the safe operation of offshore wind turbines. However, most of the existing experimental studies on the scour effects on marine foundations are carried out in the test box without a hydraulic environment, resulting in deviations between the test results and the actual ocean engineering. Therefore, a test method to study scour effects on the lateral response of rigid piles is proposed. The key to this test method lies in the design of four types of parameters: pile, soil, hydraulic (wave and current), and load. Considering the characteristics of scour tests and scour effects tests, the design method of the four types of parameters is proposed. Combined with an example, the test method is described in detail. Subsequently, the defects of this test method are discussed. The research shows that this test method is a good attempt to study the loss of lateral ultimate capacity and the increase of cyclic cumulative deformation of rigid piles under real scour conditions.

Dynamic response of pile group in two-layered soils under scour condition by FEM-ALEM approach

Pile groups for the bridge and jacket foundations in the marine or traffic geotechnical engineering are significantly influenced by the current scour in service. Under the effects of scour and dynamic load, the capability of the pile foundations to resist deformation is reduced and thus causes structural failure. This paper presents a FEM-ALEM coupling approach to study the dynamic interaction between a pile group and layered soils under scour condition. Behaviors of the pile group are analyzed by applying the finite element method. Fundamental solutions of the layered soils under scour condition are obtained by adopting the analytical layer element method. Comparisons with existing solutions are performed to confirm the correctness of the present approach, and numerical examples are provided to discuss the effects of the scour type, scour depth, scour width and layered property of soil on the dynamic behaviors of the pile group. The present coupling approach can be used to evaluate the responses of a pile group embedded in layered saturated soils under scour due to a vertical dynamic load.

Scour effect on a sea-crossing bridge under combined action of service and extreme seismic loads

Scour effect on a sea-crossing bridge under combined action of service and extreme seismic loads

A Large Bridge Pier in an Alluvial Channel: Local Scour versus Morphological Effects and the Role of Physical Models

A Large Bridge Pier in an Alluvial Channel: Local Scour versus Morphological Effects and the Role of Physical Models

Numerical evaluation of scour effects on lateral behavior of pile groups in clay

Scour can cause the water-induced failure of fluvial and marine bridges and structures. Previous studies have focused on the scour mechanisms and their effects on the load capacity of single piles, although deep foundations consist mostly in pile groups. In this paper, scouring on pile groups embedded in soft clay is studied when piles are laterally loaded and affected by the formation of scour holes. This boundary problem is simulated using the three-dimensional finite element method. The scour depth, slope angle, and pile spacing are analyzed as main influence factors. Summary charts quantify how the dimensions of scour holes affect the lateral load capacity of 3x3 pile groups for varying pile spacing and their corresponding p-multipliers. Importantly, results indicate that it is unreasonable to design pile foundations by ignoring the influence of scour holes or directly removing the soil layer above the scour depth, as frequently assumed in practice.

Scour effects on the bearing capacity of multi-bucket jacket foundation for offshore wind turbines

The Multi-Bucket Jacket Foundation (MBJF) is presently being installed in Chinese wind farms with increasing numbers. Compared to monopiles, MBJF is a shallow foundation and the effect of scouring on the bearing capacity is more significant. This research proposes a universal three-dimensional model of the MBJF with local scour to efficiently estimate the effect of scouring on the MBJF bearing capacity. Firstly, using the proposed model, the ultimate bearing capacity is calculated under a single load, and the discounting effect of different scour ranges and scour depths on the ultimate bearing capacity is evaluated. Furthermore, the fixed load-displacement approach is used to calculate the failure envelope of the foundation under composite loading mode, and the influence of different scour patterns on the failure envelope is compared. The findings reveal that when the scour extent and depth increase, the foundation's bearing capacity decreases non-linearly, and the foundation bearing envelope line gradually shrinks. Under the condition that the ultimate vertical load is not exceeded, a rise in vertical load can enhance the ultimate load capacity after scouring to a certain extent. Finally, the calculated envelopes are integrated with engineering applications to quantify the safety redundancy of the MBJF bearing capacity for different seabed morphologies. The method and results can provide methodological support for the accurate prediction and efficient assessment of the MBJF's post-scour bearing capacity.

Turbulence approach for predicting scour at abutments

This study proposes a scour equation for abutments that is based on a combination of Newton’s second law and a turbulence model comprised of a pair of isotropic vortices. A control volume of the scour hole is considered on which the sum of forces equals nil. The effect of abutments on local scour is usually modelled by an empirical K-factor for which there is no physical description available. Here, we deduce a relation between abutment shape and the maximum scour depth by using engineering tools from turbulence science. We pay attention to the scour effects of several types of abutments, such as submerged, non-submerged, permeable and impermeable and the impact of bed protection. We consider the highly nonlinear combination of constriction and local scour. The unknowns in the proposed scour equation are calibrated and validated with several scour tests.

Numerical modelling of the effects of foundation scour on the response of a bridge pier

Foundation scour can have a detrimental effect on the performance of bridge piers, inducing a significant reduction of the lateral capacity of the footing and accumulation of permanent settlement and rotation. Although the hydraulic processes responsible for foundation scour are nowadays well known, predicting their mechanical consequences is still challenging. Indeed, its impact on the failure mechanisms developing around the foundation has not been fully investigated. In this paper, numerical simulations are performed to study the vertical and lateral response of a scoured bridge pier founded on a cylindrical caisson foundation embedded in a layer of dense sand. The sand stress–strain behaviour is reproduced by employing the Severn-Trent model. The constitutive model is firstly calibrated on a set of soil element tests, including drained and undrained monotonic triaxial tests and resonant column tests. The calibration procedure is implemented considering the stress and strain nonuniformities within the samples, by simulating the laboratory tests as boundary value problems. The numerical model is then validated against the results of centrifuge tests. The results of the simulations are in good agreement with the experimental results in terms of foundation capacity and settlement accumulation. Moreover, the model can predict the effects of local and general scour. The numerical analyses also highlight the impact of scouring on the failure mechanisms, revealing that the soil resistance depends on the hydraulic scenario.

Scour effects on lateral responses of pile groups in clays

In contrast to single piles, pile groups experience more capacity loss by scour due to not only geotechnical group effect but also hydraulic group effect. However, existing standards only include design considerations for scoured single piles, and scour effects on lateral responses of pile groups are not fully appreciated. This study proposes a practical method to analyze lateral responses of pile groups in soft clays affected by local scour. The proposed method was verified against the results of 3D continuum finite element (FE) models, in which the baseline no-scour 3D FE model was established based on a reported full-scale load test. Parametric analyses were conducted to investigate the effects of scour-hole dimensions and pile group configurations on the lateral responses of pile groups in soft clays. The suitability of applying the methods specified in the various standards for a scoured single isolated pile to a scoured pile group was assessed by comparing them with the proposed method and 3D FE analyses. The results also highlighted the importance of the hydraulic group effect in the evaluation of lateral behavior of scoured pile groups. This research recommends a rectangular pile group shape with greater pile spacing in the foundation design against scour.