Large-diameter Piles Research Articles

Dynamic response analysis of monopile-supported offshore wind turbine on sandy ground under seismic and environmental loads

Monopiles are the most widely adopted foundation type for Offshore Wind Turbines (OWTs) in shallow waters. With the expansion of the construction of OWT, the number of OWT farms in seismic regions increases globally including the coastal areas of Japan and China. It is necessary to evaluate the impact of earthquakes including the vibration and soil liquefaction on the OWTs supported by the monopile foundation, while the effects of liquefaction on offshore structures, especially for OWTs with monopiles, have not been sufficiently studied. This study investigates the seismic response of the monopile-supported OWTs with the use of an advanced soil model. A three-dimensional numerical model is built, and dynamic analyses are carried out using the OpenSees framework. The pressure-dependent multi-yield (PDMY03) constitutive model is used to simulate the dynamic soil behavior. The applicability of the large-diameter pile modeling method for proper soil-pile interaction modeling in this numerical analysis is first validated through centrifuge tests on monopiles subjected to lateral loading. The dynamic analyses are then carried out to demonstrate the seismic response of the entire OWT system. The numerical results indicate that the contribution of higher modes of vibration is becoming of increased importance for large wind turbines and soil-structure interaction plays a significant role in the dynamic response. Moreover, the monopile-supported OWT in dense sand deposits experiences substantial lateral displacement and rotation under the combined action of wind and earthquake loads when liquefaction occurs.

Lateral Response Analysis of a Large‐Diameter Pile Under Combined Horizontal Dynamic and Axial Static Loads in Nonhomogeneous Soil

ABSTRACTThe large diameter piles are widely used in structures such as offshore wind turbines due to their superior lateral load‐bearing capacity. To explore the lateral response of a large‐diameter pile under combined horizontal dynamic and axial static loads in nonhomogeneous soil, a simplified analytical model of the pile–soil interaction is developed. This model represents the pile as a Timoshenko beam resting on the Pasternak foundation, incorporating the double‐shear effect by considering both pile and soil shear. The governing matrix equations for the pile elements are derived from the principle of virtual work. Further, the pile's lateral deformations and internal forces are obtained using the modified finite beam element method (FBEM) and then validated through existing analytical solutions. Finally, the contribution of various properties of pile, soil, and applied load to the pile's lateral vibration response are performed. It is found that both pile and soil shear effects significantly impact the lateral dynamic response of a large‐diameter pile. Additionally, in nonhomogeneous soil, decreasing surface soil strength and dimensionless frequency lead to increased lateral displacements and bending moments of the pile, which are significantly affected by the P‐Δ effect under increasing axial load.

Extended Winkler Model for design of offshore wind turbine large diameter monopiles

We propose a new model for the design of large diameter monopiles in this paper. These foundations are increasingly used for offshore wind turbines in water depths of up to 35 m. Our model extends the Winkler Model by incorporating rigid rotation effects relevant to large diameter piles, using conventional p-y and t-z curves to account for additional soil reactions. The current model primarily focuses on lateral loading; however, it is designed to accommodate axial loading as well, allowing for flexibility in future analyses should axial effects be incorporated. We demonstrate that our model adequately accounts for soil reaction due to rigid rotation by comparing its results with those of the PISA model. Furthermore, we validate our numerical results against available experimental data, showing that the proposed model can accurately predict pile behavior under lateral loading.

Optimization of Load-transfer Functions for a Large-diameter Pile in Multi-layered Geological Conditions via a Stochastic Search Algorithm

The paper presents the combination of the load transfer method (computational model) and the genetic algorithm (optimization model) for the automated inverse analysis of instrumented pile load tests. The output of this process are the input parameters governing the shape of the load-transfer functions and thus the prediction accuracy when the load-transfer method is used as a design tool for deep foundations. The optimization problem is converted into an unconstrained task by applying the static penalty approach. Two types of measurements are considered in a newly proposed objective function: the load-displacement curve monitored in a pile head and the axial force profiles along a pile derived from strain gauges. Firstly, the local sensitivity analysis via the Design of Experiments is carried out to identify the parameters of the genetic algorithm which significantly influences the rate of convergence during the optimization process. Subsequently, a fully automated inverse analysis of the loading test of the large-diameter bored pile in multilayered geological conditions is presented. The simultaneous combination of two instrumentation sources leads to a stable unique solution with a sufficient match between the prediction and measurements despite a larger number of unknown – optimized variables.

Bearing capacity of large-diameter pile foundation under combined VHM loadings in soft clay

ABSTRACT The large-diameter pile foundation is with an intermediate depth, widely used in engineering fields such as ports, breakwaters and rapid construction of artificial islands as a supporting structure to resist lateral loads. However, there is limited research on the bearing performance of such large-diameter, intermediate-depth pile foundations. In this paper, the finite element method is used to investigate the bearing capacity of the large-diameter pile foundation with different embedment ratios in homogeneous soil and normally consolidated soil and the prediction formulas for uniaxial and combined bearing capacity are developed. This study also sets up three different skirt wall–soil interface properties to discuss their influence on the bearing performance of the large-diameter pile foundation. The research is systematic, and relevant analysis can provide practical suggestions for engineering design.

New Design Criteria for Long, Large-Diameter Bored Piles in Near-Shore Interbedded Geomaterials: Insights from Static and Dynamic Test Analysis

This paper presents an analysis of long, large-diameter bored piles’ behavior under static and dynamic load tests for a megaproject located in El Alamein, on the northern shoreline of Egypt. Site investigations depict an abundance of limestone fragments and weak argillaceous limestone interlaid with gravelly, silty sands and silty, gravelly clay layers. These layers are classified as intermediate geomaterials, IGMs, and soil layers. The project consists of high-rise buildings founded on long bored piles of 1200 mm and 800 mm in diameter. Forty-four (44) static and dynamic compression load tests were performed in this study. During the pile testing, it was recognized that the pile load–settlement behavior is very conservative. Settlement did not exceed 1.6% of the pile diameter at twice the design load. This indicates that the available design manual does not provide reasonable parameters for IGM layers. The study was performed to investigate the efficiency of different approaches for determining the design load of bored piles in IGMs. These approaches are statistical, predictions from static pile load tests, numerical, and dynamic wave analysis via a case pile wave analysis program, CAPWAP, a method that calculates friction stresses along the pile shaft. The predicted ultimate capacities range from 5.5 to 10.0 times the pile design capacity. Settlement analysis indicates that the large-diameter pile behaves as a friction pile. The dynamic pile load test results were calibrated relative to the static pile load test. The dynamic load test could be used to validate the pile capacity. Settlement from the dynamic load test has been shown to be about 25% higher than that from the static load test. This can be attributed to the possible development of high pore water pressure in cohesive IGMs. The case study analysis and the parametric study indicate that AASHTO LRFD is conservative in estimating skin friction, tip, and load test resistance factors in IGMs. A new load–settlement response equation for 600 mm to 2000 mm diameter piles and new recommendations for resistance factors φqp, φqs, and φload were proposed to be 0.65, 0.70, and 0.80, respectively.

Influence of soil and pile properties on the critical length of laterally loaded piles

Piles could undergo various deformation and failure modes which were depended on their classification. Previous research primarily utilized the equation proposed by Poulos and Hull to classify the pile. While the Poulos and Hull method offers advantages in terms of its simplicity, it is not entirely suitable for contemporary applications involving large-diameter piles and complex soil profiles. In this study, a three-dimensional pile-soil coupling finite element model is proposed using the hypoplastic constitutive model. The main goal of this paper was to present extensive three-dimensional finite element analyses to examine the lateral response of a monopile embedded in sand. This research reevaluates five existing methods for determining the critical length of laterally loaded piles. The evaluation considers both directly assigned and indirectly calculated methods for determining the soil's elastic modulus. The findings reveal that Dobry's equation offers superior prediction for the critical length across various soil conditions. However, for low relative density soils, the Poulos and Hull method demonstrates better suitability. The numerical results presented in this study are expected to provide design references for further practical applications of the monopiles.

The Shear Effect of Large-Diameter Piles under Different Lateral Loading Levels: The Transfer Matrix Method

In various analytical models, modeling the behavior of large-diameter monopiles and piles can be challenging due to these foundations with huge body sizes carrying mechanisms of lateral loads to the surrounding soils. In this paper, the transfer matrix method with the Timoshenko beam theory was used to modify the shear rotation of pile sections under different loading stages, including serviceability limit stages and the ultimate loading stage. In this transfer matrix method, a large-diameter pile is considered according to the Timoshenko beam theory, and the recurring variables in the matrix equation are replaced with constants to simplify the calculation steps. Two model test cases were used to verify the accuracy of the method. Then, a series of comparisons between the Timoshenko beam and the Euler–Bernoulli beam theories, with the relative pile–soil stiffness being equal to 0.15, 0.45, and 0.75, was conducted to investigate the differences in pile response after considering the shear deformation. The results show that the effect of shear deformation of large-diameter piles changes with different loading levels. The values of the pile deformation based on the Timoshenko beam theory divided by those of that based on the Euler–Bernoulli beam theory were in the range of 1.0 to 1.10, and they increased slightly with increasing loads, reaching their maximum value, and then rapidly decreased to 1.0 when close to the ultimate lateral load; the maximum value was influenced by the relative pile–soil stiffness. Furthermore, the ratio of the shear rotation of the pile section to the slope of the deflection curve was in the range of 1.0 to 1.10; these also showed similar but more moderate trends compared with the values of pile deformation based on the Timoshenko beam theory divided by those of that based on the Euler–Bernoulli beam theory.

Dynamic response of a large-diameter end-bearing pile in permafrost

Vertically dynamic model of a large-diameter pile in frozen soil is established, in which the frozen soil is described to a saturated frozen porous media, and the large diameter end-bearing pile is simplified to a one-dimensional rod considering the influence of the transverse inertia effect. Analytical solutions of the longitudinal coupling vibration between the end-bearing pile and the frozen soil are obtained using Helmholtz decomposition and variable separation methods in the frequency domain. By comparing the dynamic responses of the longitudinal vibration of the large diameter end-bearing pile with the traditionally one-dimensional pile, as well as the impedance factor of the frozen soil layer induced by the pile vibration, these demonstrate the influence of the transverse inertia effect on the high frequency vibration of large diameter pile is significant, and the influence on the pile with a smaller slenderness ratio is larger. The temperature and the Poisson’s ratio also have significant effects on the vertical vibration of large diameter piles in frozen soil, which cannot be ignored in the analysis.

Predicting Load-Settlement Behaviour of Large Diameter Piles using Runge-Kutta numerical Method.

A numerical solution for predicting the axial load and corresponding settlement of large diameter piles is proposed based on the hyperbolic curve load transfer function in this paper. It is important to note that the most reliable approach to determine the axial load capacity of a pile is to use the P-S curve, which is usually obtained through an on-site load test. Performing a pile load test on a large diameter pile (pile diameters over 3m), requires a thousand tons of test load, which sometimes is unachievable. The load-settlement relationship or P-S curve of this type of pile can be determined by the Runge-Kutta numerical method for practical application especially for offshore piles. Comparison is made between the results obtained by this proposed numerical, the test data, analytical solution and pile load test data, and close agreement has been found thus verifying the accuracy of the proposed method. However, the rigid body displacement of the piles did not show any significant effect on the proposed numerical when compared to the analytical and onsite pile load test results. It is expected that the proposed numerical method can be used to predict the load and corresponding settlement of large diameter piles.

Relevance of the Foundation Input Motion in the assessment of the seismic response of pile-supported bridge piers

The seismic response of pile-supported bridge piers is commonly studied avoiding the assessment of the Foundation Input Motion (FIM) (i.e., the motion at the foundation level, which is different compared to that in the free-field, due to the filtering effect exerted by the piles) and assuming fixed-base conditions. A better approach is represented by the substructure method which consists of 3 steps: 1) the assessment of the FIM; 2) the computation of the impedance functions of the soil-foundation system, 3) the analysis of the superstructure response resting on a compliant-base at which is applied the FIM. In many cases this method is not used in a rigorous way (as the assessment of the FIM is not commonly considered an easy task by the structural engineers) and the seismic motion at the base of the model is assumed to be the same as in the free-field. Thus, the effect of Soil-Structure Interaction (SSI) is neglected. Nevertheless, this effect might be beneficial, as piles can filter out some high-frequency components of the free-field motion, especially in the case of large-diameter piles and soft soil conditions (i.e., in those cases in which piles are generally necessary). On the other hand, special attention should be given to those foundations composed by very small pile groups, since the FIM will consist of both a horizontal and a rotational component (which would play a detrimental role). Moreover, it should be remembered that considering SSI would lengthening the fundamental period of the system compared to that under fixed-base hypothesis. This aspect would lead to both positive and negative effects based on the seismic motion properties. This contribution will attempt to show the relevance of a proper assessment of the FIM while studying the seismic response of pile-supported bridge piers. To this end a specific software has been developed to properly compute the FIM considering the filtering effect exerted by the piles.

Development of a high-energy centrifuge model impact hammer for pile-driving

The offshore wind energy industry continues to expand rapidly around the world in response to the demand for clean energy. Research to investigate monopile performance under cyclic lateral loading needs to replicate the installation process as well as the cyclic loading regimes. This has provided the impetus for the development of model pile driving hammers for use in geotechnical centrifuges. This paper presents a new model-scale centrifuge impact hammer that is capable of in-flight driving of large-diameter piles into dense sediments with the flexibility of varying the energy during a test for a more controlled installation. The new hammer is activated by a pair of rotating cams, improving on the pneumatically activated hammer developed in the 1980s, giving greater energy and much better reliability. This paper provides full details of the hammer, together with data obtained at 80g acceleration, driving prototype 4 m wide (50 mm in model scale) piles into dense, dry sand to depths of over 20 m (over 5D) in <1000 blows.

Behaviour of a laterally loaded short-finned pile located on sloping ground

Finned piles are considered a novel solution to replace large-diameter piles supporting transmission towers, bridge abutments and so on and are often implemented beneficially for mooring dolphins in offshore areas. This experimental investigation was attempted to examine the lateral response of short-finned piles installed in the proximity of a typical slope of 1V:2H. The 1g model testing of regular and finned piles at different load eccentricities comprised three lateral load tests on horizontal ground and 24 lateral load tests on the slope with the pile at varying distances from the crest. The fin efficiency was observed to decrease with an increase in load eccentricity due to the poorer mobilisation of soil resistance on fins at higher load eccentricities. The finned pile installed at a distance of two pile diameters away from the crest exhibited a net efficiency closer to unity, indicating its ability to improve the receptible subgrade reaction near a loose and steeper sandy slope. An in-depth study of soil resistance developed in the finned pile located at the crest reveals that the fins effectively reduced the soil resistance, specifically in the region above the pivot point.

Numerical and experimental study on the effect of the installation meth-od on the lateral response of offshore monopiles

The successful deployment of offshore wind turbines requires the utilization of foundation installation methods that are fast, low-cost, and reliable. Offshore wind turbines foundations are commonly installed by impact driving. The noise generated during the driving process is a major constraint as it impacts marine life [1] and induces high stresses which reduce the fatigue lifetime of the foundation. Vibratory pile driving offers a promising alternative to traditional impact driving for the installation of monopiles [2], the limited data on its effects on the mechanical properties of the soil during installation in offshore conditions and consequently the mechanical response of the structure throughout its lifetime has prevented its adoption. During vibratory driving, the repeated and high intensity vibration can cause an increase in pore pressures and a decrease in both the inter-particle soil friction and the effective stress, which can eventually lead to liquefaction [3, 4].
 This research aims at investigating the influence of the installation method on the soil properties around monopiles and how it affects the lateral response of such structures. In order to comprehensively understand the different phenomena involved, this problem is investigated through both numerical modeling and laboratory testing.
 A numerical axisymmetric model based on the finite elements method that reflects on an offshore monopile was developed to describe the installation process using fixed boundaries or Perfectly Matched Layers (PML) [5] to realistically simulate the dynamic soil-pile interaction and ensure radiation of waves towards infinity. This model takes into account the reduction in soil strength around the pile shaft during the installation by impact or vibratory driving, along with the dynamic force applied by the vibro-hammer and the pore water evolution. Cyclic triaxial tests have been conducted on a sand extracted from the North Sea. They are used to calibrate the constitutive equations of the model. The results obtained provide insight into the onset and progression of liquefaction, the distribution of pore water pressures, and the resulting deformation of the soil-pile system and how the boundary conditons and the frequency of the imposed load affect those mechanisms.
 These numerical results are also compared with 1g reduced scale experiments composed of a large cylindrical box filled with dry and saturated sand, in which a fully instrumented model pile (tube 60mm in diameter) is vertically driven. The impact or vibrations are imposed at the pile head by a scaled vibratory hammer or scaled impact hammer. In addition, the relative density and the homogeneity of the sand is carefully controlled using the pluviation method [6] allowing to test the sand at different controlled densities. After the installation phase, the lateral behavior of the pile is investigated by imposing a horizontal force with a piston. The test results will also be compared to similar tests on large diameter piles (2 meters) than are planned for fall 2023.

Study on p-y curve of large diameter pile under long-term cyclic loading

The pile foundation used in offshore wind turbine system is located in a complex marine environment and is subjected to various long-term cyclic loads such as wind, wave and current. However, the attenuation effect of long-term cyclic load on seabed soil has not been considered by most of the existing p-y curve calculation models. In order to solve this problem, the finite element numerical model of pile-soil interaction of large diameter single pile foundation of offshore wind turbine is established in this paper. Based on the Stokes second-order wave theory, the time history curve of wave and current loads on piles with different diameters are derived and calculated by programming. Then the soil stiffness attenuati on model subjected to marine cyclic loading is embedded into the finite element model by subroutine. Considering the influence of the number of ocean load cycles on the ultimate soil resistance of the soil around the pile and the initial foundation reaction modulus, the calculation results of different pile diameters and soil depths are analyzed and regressed. Finally, the p-y curve calculation model of large diameter single pile foundation of offshore wind turbine under long-term ocean cyclic load is obtained.

A new non-axisymmetric 3D model of the pile-soil system for dynamic analysis of large-diameter piles under eccentric excitation load

With the widely application of large-diameter piles for strengthening soft soil and supporting the offshore structures, the three-dimensional (3D) effect of large-diameter piles have been realized. However, most of the existing 3D theoretical models are axisymmetric, which consider the vertical and radical propagation of stress waves in the pile body during the low-strain pile integrity test (LSPIT). But it is difficult to place the excitation hammer accurately at the center point of the large-diameter pile top in the LSPIT, thus, the hammer-pile-soil system is usually non-axisymmetric. In this paper, a new non-axisymmetric 3D theoretical model is proposed to evaluate the dynamic characteristics of the large-diameter pile-soil system under eccentric excitation load, in which the pile and soil are simulated by the 3D continuum model. The rationality of the new non-axisymmetric solution is verified by comparing it with the results calculated by the finite element model (FEM) and the results derived from the existing 3D theoretical models. Finally, the optimal relative positions of the impact point and the test point in the LSPIT are acquired to weaken the high-frequency interference of the large-diameter pile top. The conclusions drawn from this study provide some useful guidance for evaluating the results of LSPIT.

Centrifuge Model Tests Investigating Initiation and Propagation of Pile Tip Damage during Driving

A number of incidents of pile tip damage and extrusion buckling have been reported within the offshore industry, generally arising during driving of relatively thin-walled tubular pile foundations through hard layers or potentially heterogeneous sediments. The issue has become of particular concern with modern trends toward larger diameter piles, with higher ratios of diameter to wall thickness, for monopile or jacket foundations of offshore wind turbines. The paucity of good-quality data available in the public domain about this kind of failure has impaired the development of simple design guidelines for industry to address the issue. The paper presents results from a series of centrifuge model tests where pile tip damage and extrusion buckling were observed during driving of either (1) undamaged cylindrical piles into a sand bed containing a layer of different sizes of boulders, or (2) predented piles into medium dense homogeneous sand. Following a previously reported pilot study, a new and higher energy model pile-driving hammer was designed to permit driving through the hard layers and allow pile embedment by up to five diameters. Two test beds were prepared, one of which contained boulder layers of different sizes either near the surface or at a depth of about one pile diameter, into which piles of two different diameter to wall thickness ratios were driven. The test series led to observations of pile tip damage that ranged from abrupt tip crumpling to gradual extrusion buckling. For 2 of the 22 piles installed, dent growth during installation was detected by optical fiber sensors attached to the piles. For the uninstrumented piles, damage was deduced from the trends in blow counts during installation, with later confirmation from detailed visual inspections and laser scanning, and also from careful layer by layer excavation of each sample. The high-quality database generated from the model tests forms a valuable resource for further study and validation of advanced numerical models to simulate pile tip damage and for calibration of simple guidelines for practical design.

Holding Capacity of Suction Anchor in Cohesive Soils based on Soil-Spring Model

In this paper, a soil-spring model was applied to evaluate the holding capacity according to the loading position of the suction anchor. The API(2010) method, which is mainly applied to small-diameter piles, the PISA(2015) method, which recently introduced research results in offshore wind power design, and the Jeanjean(2009) method, which was presented as a model suitable for large-diameter piles in cohesive soil, were analyzed. A comparative review of the finite element analysis(ALE) was applied was performed for the conditions. A procedure for calculating the holding capacities for each loading position of the suction anchor was established using the soil-spring model, and it was confirmed that it was a method that could be used practically. When the soil-spring model of Jeanjean(2009) was applied, the results (5~10%) most similar to the FEM(ALE) analysis results in tendency and holding capacity could be obtained.

Cyclic overlay model of p–y curves for laterally loaded monopiles in cohesionless soil

Abstract. The bearing behaviour of large-diameter monopile foundations for offshore wind turbines under lateral cyclic loads in cohesionless soil is an issue of ongoing research. In practice, mostly the p–y approach is applied in the design of monopiles. Recently, modifications of the original p–y approach for monotonic loading stated in the API regulations have been proposed to account for the special bearing behaviour of large-diameter piles with small length-to-diameter ratios. However, cyclic loading for horizontally loaded piles predominates the serviceability of the offshore wind converters, and the actual number of load cycles cannot be considered by the cyclic p–y approach of the API regulations. This research therefore focuses on the effects of cyclic loading on the p–y curves along the pile shaft and aims to develop a cyclic overlay model to determine the cyclic p–y curves valid for a lateral load with a given number of load cycles. A stiffness degradation method (SDM) is applied in a three-dimensional finite element model to determine the effect of the cyclic loading by degrading the secant soil stiffness according to the magnitude of cyclic loading and number of load cycles based on the results of cyclic triaxial tests. Thereby, the numerical simulation results are used to develop a cyclic overlay model, i.e. an analytical approach to adapt the monotonic (or static) p–y curve to the number of load cycles. The new model is applied to a reference system and compared to the API approach for cyclic loads.

A corrected p-y curve model for large-diameter pile foundation of offshore wind turbine

With the development of pile-soil interaction theory, at present the sand p-y model of API specification is adopted to the analysis and calculation of large diameter piles under horizontal static load, but the results of the ultimate soil resistance at different depths is underestimated and the initial foundation reaction modulus is overestimated, thus the calculation results are obtained with large errors. For this problem, a finite element numerical model of pile-soil interaction is established in this study, and the calculation models under horizontal static load with different pile diameters and soil depth are analyzed and the data regression is carried out. A correction coefficient is introduced to consider the size effect of large diameter pile, and the variation law of influence index of different factors is discussed. Finally, a corrected sand p-y curve model for single pile foundation with large diameter more than 2 m is proposed. Compared with the results of model test and field test, the calculation results of ultimate soil resistance and initial foundation reaction modulus from the corrected model is more in accord with the actual situation.