Effects Of Local Scour Research Articles

Probabilistic Failure Envelopes of Monopiles in Scoured Seabed Based on a New Nonstationary Random Field Model

Monopiles, which have been most extensively used in the offshore wind industries, are usually subject to threats from local scour. The effects of local scour on the bearing capacity of monopiles have been studied based on deterministic soil properties and local scour dimensions. However, the properties of the seabed soil and the dimensions of the local scour both have uncertainty. In this study, a new nonstationary random field model, which can better simulate the nonstationary characteristics of the seabed clayey soil and result in a safer probabilistic design of foundations, is proposed to characterize the spatial variability of the seabed clayey soil. Then, the effects of the spatial variability of the seabed clayey soil and the uncertainty of the local scour depth on the bearing capacity of monopiles are investigated under both uniaxial loading and vertical-horizontal-moment (VHM) combined loading using the random finite-element method combined with kriging metamodeling technique and Monte Carlo simulation. The undrained shear strength of the seabed clayey soil was simulated with a lognormally distributed nonstationary random field, and the local scour depth was treated as a uniform random variable. The negative correlation between them was also considered. The results show that the factor of safety required for the local scoured seabed decreased from 3.3 to 2.6 as the negative correlation was enhanced, greater than or equal to that [factor of safety (FS)=2.6] for the intact seabed. The procedure of analyzing the design failure envelop of a monopile in spatially variable clayey soil with a local scour proposed in this study can provide a reference for geotechnical engineers.

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.

Effects of local scour on failure envelopes of offshore monopiles and caissons

Local scour may induce bearing capacity loss of the foundation significantly by removing soil around the foundation and it has become one of the pivotal factors causing the failure of structures in offshore engineering. Therefore, it is indispensable to investigate the effect of local scour on bearing capacity behaviour of foundations. This paper focuses on the effects of dimensions of local scour hole on both ultimate bearing capacities and failure envelopes of the monopile and the caisson in undrained soil with different soil strength heterogeneity ratios using three-dimensional finite element method. The results show that the scour depth has a more significant effect on the ultimate bearing capacity than the scour hole angle. Meanwhile, the scour depth has a more significant effect on the lateral bearing capacity (i.e., horizontal bearing capacity and moment) than on the vertical bearing capacity. For given dimensions of local scour hole, the effect of local scour on the bearing capacity decreases as soil strength heterogeneity ratio increases. Moreover, local scour affects not only the size of the failure envelopes of the monopile and the caisson, but also their shape. Based on the above findings, a procedure of designing the monopile and the caisson to consider local scour effect is proposed, which could be helpful to the design of the foundations in practical engineering.

Experimental assessment of the performance of a bridge pier subjected to flood-induced foundation scour

Scouring around piers is recognised as one of the main causes of bridge failure. Local scour refers to the localised erosion of soil at piers and abutments caused by flow-induced vortices around their base. This paper studies experimentally the response of a bridge pier, supported on a cylindrical embedded foundation, subjected to flood-induced scour. A hybrid two-step methodology is developed to study the hydraulic and the mechanical part of the problem. In the first step, the hydraulic problem of local scour around a pier is experimentally modelled in 1g using a recently developed miniaturised tidal generator. The experimentally generated scour hole is then three-dimensionally scanned to produce a three-dimensional (3D)-printed mould. The latter is used in the second step to reproduce the scour hole in an Ng model, subsequently tested in a drum centrifuge to study the mechanical part of the problem under proper stress scaling. Foundation performance before and after local scour is studied through vertical, lateral monotonic and slow cyclic pushover tests. The effects of general (uniform) scour are also investigated by removing a soil layer of constant thickness. Local scour is shown to have a minor effect on vertical bearing capacity. In stark contrast, the lateral performance is significantly affected, with the foundation moment capacity Mult being reduced by up to 38%. The effect of general scour is even more pronounced, leading to 48% reduction of Mult. The rate of cyclic settlement accumulation is also much more severely affected by general scour as compared to local scour. Overall, the effects of local scour on foundation performance differ substantially to those of general scour, bringing into question the common simplification of ignoring the geometry of the scour hole, making no distinction between local and general scour.

Effects of scour on horizontal bearing behavior of monopile foundations for offshore wind turbines

Monopile is widely used as a typical foundation for offshore wind turbines (OWT). Local scour usually occurs around offshore pile foundations, which may bring the reduction of both the effective stresses in the surrounding soil and the effective pile embedment. Until now, the effects of local scour on the lateral pile-soil interaction have not been well understood. In this study, a three-dimensional finite element model for the lateral pile-soil interaction is proposed and verified with experimental results. The p-y curves describing the pile-soil interaction are extracted from a series of numerical results with various sandy soil conditions and pile parameters. A parameterized expression of the p-y curves for large-diameter monopile in sand is proposed. The results indicate that scour could induce a significant transition of pile behavior for a monopile typically employed for OWT. If the pile exhibits rigid structural behaviors after scour, the pile’s tip and shaft resistance may become a significant component of the whole soil resistance, which would render the traditional p-y approach inapplicable. For a given depth below the scour base, the p-y curves get significantly stiffer with increasing scour depth, especially at relatively shallow depths. The scour effects on the variation of p-y curves from the present numerical results are generally consistent with the existing centrifuge results. As the slope angle of scour holes decreases, the effect of the remaining sloping overburden soil above the level of the scour base reduces and the stiffening of the p-y curves at a given depth below the scour base is correspondingly alleviated.