Global Scour Research Articles

Local scour effects on seismic behavior of pile-group foundations in cohesionless soils: Insights from quasi-static tests

Local scour has been reported as a common phenomenon for pile-group foundations in marine, coastal, and riverine sites, while its effects on the seismic behavior of pile-group foundations are yet to be well documented. This paper reported a pair of quasi-static cyclic loading tests on 2 × 3 scoured pile-group foundation specimens, one in global scour and the other in local scour, to understand the influence of local scour on the seismic behavior, particularly in terms of soil-pile interaction features and failure mechanisms. Test results indicate a flexural failure mode for both specimens. Local scour does not change the order of limit states but postpones the occurrence of concrete cover cracking and rebar yielding due to the reduced lateral stiffness of the locally scoured piles. Moreover, local scour rarely changes the aboveground damage regions at the top of outer piles (one time the side length of squared pile section, D), but significantly aggravates and deepens the underground damage regions at outer piles (from 4 ∼ 5D for global scour to 3.7 ∼ 6D for local scour). Besides, local scour reduces the displacement ductility factor from 2.50 to 1.25 for the Easy-to-Repair limit state, resulting in adverse impacts on pile-group foundations in general.

Spatio-temporal analysis of scour around complex offshore foundations under clear water and live bed conditions

This study investigates scour around offshore wind foundations, focusing on two complex structures with varying degrees of flow blockage: a novel hybrid gravity-based jacket (structure “A") and a conventional four-legged jacket (structure “B"). As offshore structures like jackets become more prevalent, mainly due to their structural stability and growth of offshore wind energy in general, understanding scouring phenomena around complex structures is crucial. Laboratory tests under steady flow clear-water and live-bed conditions, with measurements of 3D laser scans for test durations of 15, 90, and 420 min were conducted. In addition the scour development over time was measured and analyzed with eight echo sounders. The findings confirm that scouring around complex structures displays significant variability in dependency of the structure type, making standardization a challenging task. However, some common trends can be derived. Under live-bed conditions, both types of structures exhibit global erosion, regardless of the complexity or flow obstruction of the structure. The spatial erosion depth, relative to the footprint, is markedly higher (2.5 times) for the gravity-based structure as opposed to the jacket structure. In clear-water conditions, no global scour was observed for both structures and a very similar spatial erosion depth was reached after 420 min.

Scour Effect on the Lateral Bearing Behaviour of Monopiles Considering Different Slenderness Ratios

Scour leads to the loss of soil around monopile foundations for offshore wind turbines, which affects their structural safety. In this paper, the effect of scour on the lateral behaviour of monopiles was extensively investigated using finite element analysis, and calibration and comparison were undertaken using centrifuge tests. Piles with three slenderness ratios, i.e., 3, 5 and 8, were studied by keeping the diameter constant and varying the embedment length. Three scour types (local narrow, local wide and global) and four scour depths (0.5D, 1D, 1.5D and 2D; D signifies the pile diameter) were considered in this investigation. The results indicate that the lateral resistance of the pile is the greatest in the case of local narrow scour, followed by that in the cases of local wide scour and global scour. When the scour depth is larger than 1D, the influence of the scour type on the pile lateral bearing behaviour is insignificant. The influence of the scour type and scour depth on the pile lateral bearing behaviour is broadly similar for piles with slenderness ratios of 3, 5 and 8. However, the piles featured with smaller embedment lengths show a larger decrease rate in their lateral capacity, which means the effect of scour should cause more concern on small slenderness ratio monopiles.

Experimental Study on Influence of Different Patterns of an Emergent Vegetation Patch on the Flow Field and Scour/Deposition Processes in the Wake Region

AbstractFlume experiments were conducted to comprehend the impact of different patterns of an emergent vegetation patch on the flow field and the scour process in natural rivers. Velocity measurements, flow visualization, and scour tests were undertaken around different vegetation patch patterns, which were simulated inspired by the expansion process of a typical instream vegetation. The patch expansion process was idealized with an initially circular patch of rigid emergent stems becoming elongated due to positive and negative feedbacks. The expansion of the vegetation patch was considered to occur in three stages, in which the density of the patch from the previous stage was increased while the patch was also elongated by connecting at its downstream side with another sparser vegetation patch. These stages were replicated succesively by increasing the density and elongating the patch. In this way, two processes (i.e., elongation and decrease in permeability), which usually have hydrodynamically opposite effects on flow fields, were simulated at the same obstruction. Despite generally elongated obstacles being streamlined bodies, the morphometric analysis obtained by laser scanner revealed that streamlined elongation of permeable patches amplifies global scour and enhances localization of the local scour hole. This situation implies that as the patch expands, in the wake region, the steady‐wake region becomes shorter, turbulence diminishes, lateral shear stress enhances, and deposition cannot occur far from the patch. Consequently, as the patch expands, the hydrodynamic consequences may restrict further patch expansion after a certain length/density.

Behavior of single pile subject to oblique uplift load in sandy soil under global scouring

Soil scouring is one of the most dangerous causes of geotechnical collapse, which can result in catastrophic disasters. An experimental scaled laboratory model was used to investigate the effects of lowering the embedded length of tension pile owing to surface scouring and varying relative densities of sand soil for vertical and angled uplift force. The designs for the uplift piles beneath the concentric oblique loads exposed to surface soil scouring have not been made known up until this point. The lateral and vertical responses of the uplift piles in sandy soil were examined in a series of 1-g experimental tests that included secondary characteristics such as soil relative density, the inclination angle of the uplift load, and various applied uplift loads. By applying statistical analysis to the experimental data, it was found that global scour caused reductions in overall pile capacity where the sum of the applied uplift load ratio and the scour depth ratio was equal to 0.70. Idealized P-y curves for piles with axially vertical, horizontal, or oblique uplift loads were developed, and they can be utilized to calculate the ultimate uplift capacity at the preliminary design stage.

Local scour around tandem square porous piles in steady current

Experimental study on the local scour around tandem square porous piles in steady current has been carried out in the live-bed flow condition. The influence of the porosity (P) of both front and rear piles in the tandem configuration (P = 0–38.5 %) and the gap ratios (G/D = 2–12) are quantified based on the measurement of the development of the local scour, time scale, three-dimensional scour profiles and the scour characteristics were analysed. Three regimes are identified regarding on the porosity distributions of the front and rear pile. The distinct features were found at the porosity of the front was small than its counterpart of the rear. The global scour can be recognized at G/D < 4. The dimensions of the scour hole for the rear pile decrease first as a trade-off is observed between the sediment deposit landed around the rear pile and its weak erosion. A critical gap ratio, at which the minimum scour dimensions were observed is strongly depend on the difference of the porosity between the front and rear piles.

Vulnerability of bridges to individual and multiple hazards- floods and earthquakes

Building resilient bridges, that are able to withstand multiple natural stressors with minimal damage and quickly restore their functionality is paramount to delivering climate-resilient transport infrastructure. Nevertheless, bridges are proven to be vulnerable to natural hazards, with floods and earthquakes being the main causes of failure. The available research and practice for assessing the vulnerability of river-crossing bridges is predominantly qualitative and therefore relies heavily on visual inspections, while ignoring important characteristics of the complex water-soil-bridge interaction. This is a knowledge gap that this paper aims to fill. This work provides novel fragility models for hydraulically induced stressors and/or combinations of hydraulic and seismic hazards. To achieve this, unique detailed two- and three- dimensional numerical models are employed, for a typical three-span prestressed box-girder river-crossing bridge. This paper is a primer on the vulnerability of flood-critical bridges as it models the entire water-soil-bridge system, taking into account critical hydraulic stressors (scour, debris accumulation, hydraulic forces), the uncertainty in scour hole formation, and all components of integral and isolated bridges: deck, bearings, piers and abutments, backfill, and the foundation soil. A detailed description of the damage modes for each component is given and sets of fragility curves for floods and combinations of hydraulic stressors and earthquakes are developed. The study concludes that integral bridges are in most cases more vulnerable to local scour than bridges with bearings, since the latter are more flexible and can therefore adapt to changes in their geometry. The opposite is true for global scour and/or seismic earthquake excitations. The generated fragility models are useful tools for quantitative risk assessment of transport systems and provide practical means in resilience-based asset management by owners and operators of transport infrastructure.

Centrifuge Modeling of the Impact of Local and Global Scour Erosion on the Monotonic Lateral Response of a Monopile in Sand

Abstract 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.

Assessment of bridge natural frequency as an indicator of scour using centrifuge modelling

One of the most prevalent causes of bridge failure around the world is “scour”—the gradual erosion of soil around a bridge foundation due to fast-flowing water. A reliable technique for monitoring scour would help bridge engineers take timely countermeasures to safeguard against failure. Although vibration-based techniques for monitoring structural damage have had limited success, primarily due to insufficient sensitivity, these have tended to focus on the detection of local damage. High natural frequency sensitivity has recently been reported for scour damage. Previous experiments to investigate this have been limited as a result of the cost of full-scale testing and the fact that scaled-down soil-structure models tested outside a centrifuge do not adequately simulate full-scale behaviour. This paper describes the development of what is believed to be the first-ever centrifuge-testing programme to establish the sensitivity of bridge natural frequency to scour. A 1/60 scale model of a two-span integral bridge with 15 m spans was tested at varying levels of scour. For the fundamental mode of vibration, these tests found up to a 40% variation in natural frequency for 30% loss of embedment. Models of three other types of foundation, which represent a shallow pad foundation, a deep pile bent and a deep monopile, were also tested in the centrifuge at different scour levels. The shallow foundation model showed lower frequency sensitivity to scour than the deep foundation models. Another important finding is that the frequency sensitivity to “global scour” is slightly higher than the sensitivity to “local scour”, for all foundation types. The level of frequency sensitivity (3.1–44% per scour depth equivalent to 30% of embedment of scour) detected in this experiment demonstrates the potential for using natural frequency as an indicator of both local and global scour of bridges, particularly those with deep foundations.

Influence of scour depth and type on p–y curves for monopiles in sand under monotonic lateral loading in a geotechnical centrifuge

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.

Experimental study of scour around pile groups in steady flows

This paper presents comprehensive experimental results of scour around pile groups with different pile arrangements under different steady flow conditions, with the main aim of analyzing the influence of pile spacing (G/D) and Froude number (Fr). The temporal developments of scour depth measured at different positions around different piles are provided and yield exponential functions. The temporal variations of scour depth at the front pile and the rear pile are significantly different for small values of Fr. The equilibrium scour depth of pile groups decreases with G/D, and the scour depth at the rear pile is apparently smaller than that at the front pile. The critical value of G/D representing the pile group effect tends to increase with Fr. The equilibrium scour topography is divided into local scour at individual pile and global scour over entire area of pile group, and the local scour is less obvious with the increase of Fr. Additionally, the results of scour hole dimensions show that the length of the scour hole upstream of the pile group decreases with G/D while the width of the scour hole increases slightly with G/D. Finally, empirical relationships are obtained to predict the equilibrium scour hole area and volume.

Volume-Based Assessment of Erosion Patterns around a Hydrodynamic Transparent Offshore Structure

The present article presents results of a laboratory study on the assessment of erosion patterns around a hydrodynamic transparent offshore foundation exposed to combined waves and currents. The model tests were conducted under irregular, long-crested waves in a scale of 1:30 in a wave-current basin. A terrestrial 3D laser scanner was used to acquire data of the sediment surface around the foundation structure. Tests have been conducted systematically varying from wave- to current-dominated conditions. Different volume analyzing methods are introduced, which can be related for any offshore or coastal structure to disclose physical processes in complex erosion patterns. Empirical formulations are proposed for the quantification of spatially eroded sediment volumes and scour depths in the near-field and vicinity of the structure. Findings from the present study agree well with in-situ data stemming from the field. Contrasting spatial erosion development between experimental and in-situ data determines a stable maximum of erosion intensity at a distance of 1.25 A, 1.25 times the structure’s footprint A, as well as a global scour extent of 2.1–2.7 A within the present study and about 2.7–2.8 A from the field. By this means, a structure-induced environmental footprint as a measure for erosion of sediment affecting marine habitat is quantified.

Scour development around a jacket structure in combined waves and current conditions compared to monopile foundations

This paper presents the results of an experimental study on the scour development of a hydraulic-transparent offshore foundation exposed to combined waves and current. Irregular waves propagating perpendicular to a current were simulated in a wave-current basin. The physical model tests were conducted in a length scale of 1:30 while measurements of the scour development over time were achieved by echo sounding devices placed at several locations at the upstream and downstream side of the jacket structure. Insights were gained on the scour development and time scale of the scouring process around a complex jacket structure for different wave-current conditions. The results were presented with respect to the Keulegan-Carpenter KC number and the relative wave-current velocity. Wave conditions were adjusted so that KC numbers between 6.7 and 23.4 could be tested in a systematic wave-current test program with tests reaching from wave dominated conditions up to current dominated conditions. Measured scour depths were critically assessed by an extrapolation to expected equilibrium scour depths. With respect to the current flow direction, the experiments showed generally larger scour depths at the upstream side and lower scour depths on the downstream side for each pile of the jacket structure. The development of global scour around the structure intensified with increasing relative wave-current velocity. As a result, a practical formulation is proposed for the reliable prediction of local scour depths around a jacket foundation in combined wave-current conditions. Finally, dimensionless time scales and observed as well as predicted scour depths are compared to values for the scour development around monopiles.

Effect of scour on the structural response of an offshore wind turbine supported on tripod foundation

A simplified scour model for tripod foundation taking into account both local scour and global scour is proposed in this paper. The model is incorporated into a three-dimensional (3D) finite element model for analysis of a full-scale offshore wind turbine founded on a tripod structure using realistic structural properties. Applicability of the 3D finite element model is validated using full-scale load test data. Four different scour conditions under two wave situations are examined for the ultimate limit state (ULS), serviceability limit state (SLS) and fatigue limit state (FLS). The results show that scour has a minor effect on the natural frequency of the tripod-supported wind turbine but can significantly increase the maximum cross-sectional von Mises stress of piles under the ULS and increase the deflection of piles within nearly 20 m below the original seabed under the SLS. As for the fatigue life of the tripod structure, it can also be reduced by the effect of scour. These findings provide insights which are useful for development of safe and economic design of offshore wind turbines supported by tripod foundations.

Impacts of Scour on Lateral Pile Behavior in the Marine Environment

Piles supporting marine structures are subjected to lateral loads due to berthing, mooring forces, waves and current forces. Scour around piles can have significant impact on the lateral loading capacity and hence affect the stability of marine structures. This paper presents the impact of scour on lateral loading of single piles and pile groups installed in coheshionless soils. Finite element modeling and Winkler model were used in the analyses. Different parameters were considered such as scour depth, magnitude of lateral load, pile group arrangement, pile spacing, pile slenderness ratio and pile batter angle. The results showed that global scour has greater impact on lateral pile capacity compared to local scour. Scour has greater impact on lateral loading of pile groups compared to its impact on single piles. Pile groups with side-by-side arrangement exposed to scour are more critical than single piles and piles groups with tandem arrangement due to the combined effect of scour and pile-soil-pile interaction. Some recommendations for scour protection are provided based on pile group arrangement, pile spacing and slenderness ratio

Effect of Local and Global Scour on Lateral Response of Single Piles in Different Soil Conditions

Marine structures, offshore platforms and bridge piers are usually supported on foundation piles. These piles are subjected to lateral loading due to wind, waves and currents. Piles installed in marine or river environments are susceptible to scour depending on wave and current characteristics and soil types. In this paper, the effect of local and global scour on behavior of laterally loaded piles installed in different soil conditions has been investigated. Finite element model (FEM) using the software program PLAXIS and Winkler model using the software program LPILE were used in the analyses. Different parameters were investigated such as soil types, scour depth, scour hole dimension, pile material, submerged condition, magnitude of lateral load and load eccentricity. The results showed that scour has a significant impact on piles installed in sand and a less significant impact on piles installed in clay. Global scour has a significant impact on pile lateral displacement and bending stresses. The effect of scour is more significant if piles are subjected to large lateral loads due to the nonlinear response of pile-soil system. Effect of scour of stiff clayey soils on piles is more pronounced than that of soft clayey soils.

Surface-based bedload transport relation for gravel rivers

Bedload transport in gravel-bed rivers is accomplished by means of the mobilization of grains exposed on the bed surface. This mobilization is due to the action of fluid forces on the exposed grains. Substrate particles can participate in the bedload only to the extent that local or global scour results in their exposure on the surface. It follows that a calculation of the bedload transport rate of mixtures should be based on the availability of each size range in the surface layer. Herein an existing empirical substrate-based bedload relation for gravel mixtures, developed solely with reference to field data, is transformed into a surface-based relation.