Effects Of Pile Diameter Research Articles

Investigation of torsion in asymmetric mid-rise structures on the pile-raft system considering soil-structure interaction

The adverse conditions of the soil such as soft soil can have seismic consequences that may either amplify seismic response or render the structure vulnerable. On one hand, the design and construction of asymmetric structures have become prevalent due to aesthetic considerations and structural space limitations. Studies indicate that structures with geometric irregularities are susceptible to seismic forces, leading to non-uniform distribution of floor drifts, excessive torsional behavior, and more, depending on the degree of irregularity. In this research, the main goal was to investigate the effect of soil and structure interaction on the torsion of asymmetric Mid-Rise Structures. To achieve this goal two asymmetric Steel moment frame system 15-story structures aim to address the effect of interaction between soil and structure on the seismic performance, one with a rigid base and the other with a flexible base, one with a rigid base and the other with a flexible base. To mitigate the settlement of soil-associated structures, a pile-raft system is employed, considering the softness of the soil. Additionally, to study the torsional behavior of these structures, their planes are asymmetrically designed in L-shape and C-shape configurations, subjected to four ground motion excitations in both far and near-field scenarios. Three-dimensional finite element numerical simulations are performed using Abaqus software. Various configurations of the pile-raft system for the asymmetric C-shape structure are examined to assess the effects of pile diameter and length on the torsional irregularity coefficient. The results show that the soil-structure interaction increases the maximum floor rotation angle, placing these structures in critical conditions concerning torsional irregularity under near-field and far-field seismic excitations.

Experimental study on pull-out behaviour of piles in cohesive sedimentary beds subject to water jet and vibration loading

With the development of offshore oil and gas platforms and the continuous promotion of offshore wind farms, offshore pile decommissioning will become an important issue. However, the complete removal of offshore piles is complex and significant. The difficulty in removing offshore piles embedded in a cohesive sedimentary bed is usually related to the properties of the cohesive sediments. In this study, we investigated the problem of pile pullout from cohesive sediments under the combined action of a water jet and vibration loading. Through a series of pile pull-out tests, we studied the effects of pile diameter, soil strength, water jet flow rate, vibration intensity, and vertical pull-out speed on the maximum pull-out force of the pile. By combining force and dimensional analyses with test data, we proposed a formula for calculating the maximum pull-out force of a pile embedded in a cohesive sedimentary bed and verified by other researchers.

Stochastic characterization of the dynamic response of a pile

The design of pile foundations subjected to large harmonic loads produced by machine vibrations is usually controlled by their service limit state. Verification of this condition under current design philosophy requires the estimation of the probability of failure. However, the deterministic nature of current design procedures does not facilitate such calculations. This paper presents a stochastic model for the dynamic pile response under harmonic loading considering the uncertainty in the mechanical parameters of the soil and the pile itself. The purpose of the model is to quantify the uncertainty in the pile response in terms of its impedance. A sensitivity analysis of the design parameters and stochastic variables is performed to quantify their impact on the uncertainty of the pile response. The effect of pile diameter and uncertainty of its properties, mean soil stiffness, and the selection of distribution models for the random variables are evaluated. Useful insights are gained into the stochastic behavior of the model and the characterization of the pile stiffness and damping components of the pile. This approach may provide a valuable first step towards the development of a comprehensive reliability analysis of this foundation type.

Cumulative cyclic response of offshore monopile in sands

Satisfactory performance of offshore wind turbines is governed by the cyclic response of its foundation under wind and wave. Most methods of evaluating the lateral cumulative deformation of monopile due to cyclic loading are based on small-diameter pile tests installed in dry sand. This paper investigates the cumulative deformation of large-diameter monopiles installed in saturated sand. A numerical model was developed employing FLAC3D and was validated by comparing its predictions with the results of triaxial tests and centrifuge tests. The validated numerical model was used to evaluate and compare the cyclic responses of monopiles between saturated case and no pore pressure case. To evaluate the cumulative deformation for offshore monopile due to cyclic loading, the numerical model was refined to account for the effects of number and amplitude and frequency of cyclic loading, soil permeability coefficient, soil relative density and pile diameter. It was found that the larger subsidence and the less capacity of soil around the pile is caused by accumulation and dissipation of transient excess pore pressure, thus the monopile in saturated sand would experience the larger cumulative deformation. Correspondingly, based on this parametric study, an analytical model was developed to calculate the pile-top cumulative displacement of offshore large-diameter monopile under lateral cyclic loading considering the effects of pile diameter. This analytical model was validated against the results of centrifuge tests and model pile tests in saturated sands.

Numerical Study on Lateral Response of Offshore Monopile in Sand under Local Scouring Conditions

Currently, p-y models have been broadly adopted for estimating the lateral bearing capacity of large-diameter monopiles in offshore engineering. However, the existing p-y curves cannot reflect the effect of pile diameter D on the lateral response of monopiles under local scouring conditions. In order to extend the existing p-y model to a large-diameter pile, a well-calibrated three-dimensional pile-soil model performed by ABAQUS is used to study the effect of the pile diameter D on the ultimate soil resistance P and the initial stiffness k of the Lin’s p-y model. Based on the numerical simulation results, two diameter-related parameters A and B, which represent the relationships between the ultimate soil resistance and the initial stiffness obtained by the numerical model and the Lin’s p-y model, respectively, are proposed and introduced into the Lin’s p-y model to present a modified p-y model. The comparison results show that the proposed modified p-y model is capable of providing a better estimation of the lateral response of large-diameter monopiles than the existing p-y model under a scouring condition.

Numerical Study on Seismic Response of Offshore Wind Turbine Monopile in Multi-Layered Soil Profile of Arabian Sea

The seismic performance of monopiled offshore wind turbine (OWT) structures was evaluated numerically. The aim was to analyze offshore wind farm sites on complicated layered seabed with high seismicity. Following pile soil analysis (PISA) model, three-dimensional (3D) numerical evaluations were undertaken for two separate locations under two independent seismic events. The effects of pile diameter, depth, site impact owing to transverse soil layering, uni-directional and multi-directional seismic loading and seismic acceleration magnitude are presented. Dynamic impedance depth variation and site response analysis via lateral displacement, lateral soil response and Fourier response amplitude are explored in frequency and time domain. In a seismic zone with stratified soil, pile diameter has a greater influence. Variations in soil profile affect wind turbine performance and seismic sensitivity. This research will give a strong platform for later studies to recommend a safe wind farm site based on simulation results.

Study on combination of p-y models for laterally loaded pile in offshore wind farm

The p-y curve method recommended by American Petroleum Institute (API) code is a mainstream design method of the laterally loaded large-diameter piles for offshore wind turbines. However, it has been long recognized that the conventional p-y models proposed for piles with a relatively large length to diameter ratio may not be appropriate for the large-diameter piles. In this study, the field tests of laterally loaded pile are analyzed in detail, particularly the bending moment and lateral displacement under multistage cyclic loading. The bending moment and lateral displacement of test pile are calculated by several combinations of clay p-y model and sand p-y model. A modified p-y model for clay is proposed considering the influence of pile diameter, and a modified p-y model for sand is proposed introducing the effect of sand layer depth and pile diameter on initial stiffness. The results indicate that the effect of pile diameter on p-y model for clay is apparent, and the influence coefficient of pile diameter on the critical parameter yc is recommended as 0.2–0.5. For the silty sand close to seabed, the formula of initial foundation stiffness of silty sand recommended by Sorensen appears to be more reasonable, and the initial foundation stiffness of silty sand at depth is suggested to calculate with formula suggested by Kallehave et al.

A semi-analytical one-dimensional model for offshore pile foundations considering effects of pile diameter and aspect ratio

A semi-analytical 1D model with the eight types of soil reactions is derived from the fishbone frame model for pile foundations of wind turbines, in which the coupling between axial and lateral resistances that matters for large pile diameters and small pile aspect ratios is modeled as the rotational soil reactions. The only correction factor for the calculation of the ultimate bearing capacities of the lateral and rotational soil reactions on the pile shaft is then identified by a curve-fitting approach and validated using a series of experiments and 3D FE analyses for both sand and clay. The proposed model shows favorable agreements with the experiments and 3D FE analyses in terms of the load-displacement curves at the ground level and the rotation and moment responses for the pile foundations with various pile diameters and aspect ratios. Finally, the proposed model is further verified by monopiles in layered soils and applied to investigate the effect of the coupling between axial and lateral resistances on the modal damping of monopile supported wind turbines. Without considering the coupling between axial and lateral resistances, the modal damping ratios for the monopile supported wind turbine are underestimated significantly.

Experimental Study of Wave-Induced Response of Piles in Seabed with Various Permeability

Subjected to continuous wave loading, the responses of pile foundations and seabed develop gradually, severely affecting the serviceability of piled structures. This paper presents the results of a series of flume experiments on pile foundations in fine sandy and silty seabed under regular wave loading. Pile-head displacement and pore water pressure were measured and the effects of pile diameter, cross-section, pile stiffness and wave height were investigated. The experimental results indicate that the pore pressure in fine sandy seabed varied only slightly even under 640 s of wave loading but showed an increase of 15.7–25.9% around a pile. In silty seabed with much lower permeability, pore pressure accumulated quickly due to piles and oscillated impressively at the depth of soil liquefaction. Based on the comparison between the calculated and measured pile-head displacement, we found that the response of smaller-diameter piles in lower-permeability seabed was much more easily magnified by the induced pore pressure. Increasing the pile diameter and attaching fins could lead to a smaller response of piles. Wave height was a major factor in the experiments that affected the development of response.

BEHAVIOUR OF PILES SUBJECTED TO LATERAL FORCES IN COHESIVE SOIL

Although piles are elements designed primarily to support vertical loads, They are subjected to horizontal loads by serval sources such as earthquake, wind, impact loads and lateral earth pressure. Large number of studies have been carried out to investigate the effect of different parameters on the behavior of piles. These parameters are summarized as i) effect of pile diameter, ii) angle of the pile with the vertical, iii) piles slenderness ratio and soil type. In the present study an experimental program was carried out to identify the deflection of vertical and inclined piles subjected to a lateral force in cohesive soil. The experimental program comprised pile lateral load tests performed on model of single pile driven in cohesive soil. Using the experimental model results, relations were utilized to estimate the lateral deflection of piles.

Behavior of Offshore Pile in Calcareous Sand—Case Study

The erosion of limestone and calcarenite ridges that existed parallel to the Mediterranean shoreline forms the calcareous sand (CS) formation at the surface layer of Egypt’s northern coast. The CS is often combined with broken shells which are considered geotechnically problematic due to their possible crushability and relatively high compressibility. In this research, CS samples collected from a site along the northern coast of Egypt are studied to better understand its behavior under normal and shear stresses. Reconstituted CS specimens with different ratios of broken shells (BS) are also investigated to study the effect of BS ratios on the soil mixture strength behavior. The strength is evaluated using laboratory direct-shear and one-dimensional compression tests (oedometer test). The CS specimens are not exposed to significant crushability even under relatively high-stress levels. In addition, a 3D finite element analysis (FEA) is presented in this paper to study the degradation offshore pile capacity in CS having different percentages of BS. The stress–strain results using oedometer tests are compared with a numerical model, and it gave identical matching for most cases. The effects of pile diameter and embedment depth parameters are then studied for the case study on the northern coast. Three different mixing ratios of CS and BS have been used, CS + 10% BS, CS + 30% BS, and CS + 50% BS, which resulted in a decrease of the ultimate vertical compression pile load capacity by 8.8%, 15%, and 16%, respectively.

Observed Performance and FEM-Based Parametric Analysis of a Top-Down Deep Excavation in Soil-Rock Composite Stratum

This paper investigates the performance of a top-down deep excavation in soil-rock composite stratum. The behavior of the excavation bracing system, consisting of ground anchors and end-suspended piles, has not been well understood due to the lack of relevant research. Based on the observed data of a typical deep excavation case history for the May Fourth Square Station in Tsingtao, China, the characteristics of the horizontal and vertical pile displacements, ground surface settlements, building settlements, axial forces in ground anchors, earth pressure, and pore water pressure during excavation were analysed. Two-dimensional finite element simulations were carried out to further explore the deformation and internal force responses of end-suspended piles and to capture the effects of pile diameter, embedded depth, and rock-socketed depth on the horizontal displacement and bending moment distributions along the pile shaft. It was found that the pattern of the vertical pile displacements could be categorized into three types: rapid settlement, slow settlement, and rapid heave. The magnitudes of the ground and building responses can be well controlled within allowable limits by combining the top-down method with the adopted bracing system. Among the investigated parameters, pile diameter is dominant in affecting the horizontal pile displacement. The primary influence zone for pile bending moment varies, depending on the parameters. It is recommended that a combination of top-down method, ground anchors, and end-suspended piles be adopted for restraining excavation deformation and lowering construction costs of similar deep excavations in soil-rock composite stratum.

Effect of Pile Diameter on Plugging Phenomenon of Open-ended Piles

Effect of Pile Diameter on Plugging Phenomenon of Open-ended Piles

Failure mode of monopile foundation for offshore wind turbine in soft clay under complex loads

Offshore wind turbines (OWT) subjected to long-term complex marine loads are prone to foundation damages, affecting their regular operation. However, the failure mode of the large-diameter monopile foundation under complex marine loads is not well understood. Hence, failure modes of monopile foundation for OWT in soft-soil seabed under unidirectional and multidirectional loads are investigated. Firstly, the mechanical characteristics of soft soil are tested indoors. Subsequently, the monopile foundation failure modes under unidirectional loads are examined, which indicates capacity failures of monopile foundation are progressive, and two distinct soil failure mechanisms are identified. Then, the failure modes and failure envelopes under multidirectional loads (horizontal load, vertical load, and bending moment）are explored. Results show that the failure envelopes in H–V and M–V load space are oval-shaped, and failure modes in H–V and M–V load space are characterized by wedge failure. More critically, the service performance of the monopile foundation in soft soil in a full lifecycle can be estimated by the location of the actual loads in the failure envelope. Finally, the effect of pile diameter on the bearing capacity of the monopile foundation for OWT under unidirectional and multidirectional loads is discussed.

Nonlinear lateral response of offshore large-diameter monopile in sand

This paper presents a nonlinear analysis method for laterally loaded large-diameter monopiles. The method accounts for the effects of pile diameter and soil deformation on the modulus of horizontal subgrade reaction by considering a modified modulus of horizontal subgrade reaction. The solution considers the force and moment equilibrium of soil-pile system to calculate the response of the monopile to lateral loads. The influence of the soil side friction reaction on the ultimate horizontal resistance is evaluated considering the degradation of modulus of horizontal subgrade reaction due to large pile displacement. The predictions of the developed method compared favorably with the results obtained from full-scale field tests, centrifuge model tests and laboratory model tests as well as existing analytical methods. The developed analytical method is then applied to study the rigid rotation behaviour of offshore monopiles. The results demonstrated that under lateral loading at the pile head, the depth for rigid rotation point lies within 0.62L–0.73L (L is pile length), and the depth of the rotation point increases as the pile diameter increases. It was also found that the load eccentricity has a significant effect on the depth of the rotation point.

Calculation of monopile foundation in sand to long-term lateral cyclic load

Monopiles are widely used to support the upper structures of wind turbines in offshore engineering. Due to harsh environmental conditions, they are subjected to long-term lateral cyclic loading from waves and winds during the design life. A robust beam on the Winkler foundation model is presented to analyze the responses of monopile foundation under long-term lateral cyclic load, incorporating cyclic stiffness evolution of sand based on the results of drained cyclic triaxial tests. The reasonability of the model is proved by comparison with a published centrifuge model test. Furthermore, a parametric analysis is performed to study the effects of pile diameter, pile eccentricity, embedment depth and cyclic load amplitude to the accumulation of deformation at the pile head during the long-term lateral cyclic load, based on which an empirical expression for estimating the accumulated displacements of a long-term lateral loaded monopile under cyclic loads is given.

Numerical Modelling of Passively Loaded Pile Groups

Piled foundations could be affected negatively as a result of passive loadings caused by nearby soil movement-induced activities, and failure of piles could happen in some sever cases. This paper deals with the numerical analysis of passively loaded pile groups and piled raft in sand. The complexity involved in such problems due to pile–soil, pile–pile, pile–cap, soil–cap, and moving soil-stable soil interaction needs a powerful tool to make three dimensional analysis possible. In the current study, PLAXIS 3D software was used to back analyse laboratory tests carried out by the authors. “Embedded pile” feature in which the pile is represented by beam elements, while soil-pile interaction along the pile shaft and at the pile tip is described by special interface elements was employed. The Mohr–Coulomb elastic–plastic constitutive model was used to describe the sand behaviour. Although an overestimation of the predicted deflection was obtained, the general trend of bending moment profiles of piles was in a reasonable agreement with those obtained experimentally. A number of limitations were identified as possible reasons behind the overestimation of the predicted deflections. Furthermore, parametric studies are adopted to consider the effects of pile diameter, pile–soil stiffness and pile group configuration on the response of passively loaded pile groups.

Dynamic response of monopiles in sand using centrifuge modelling

Monopiles are one of the most commonly used offshore foundation for wind turbines. Their static capacity, p-y curve and cyclic loading behaviour have been studied using 1 g tests and centrifuge tests, but there is little experimental data regarding their natural frequency, especially using centrifuge testing. The design of offshore wind turbine foundations is largely governed by natural frequency as resonance due to cyclic loading can cause damage and even failure. Understanding the dynamic response of the monopile under free vibration is thus critical to design. This paper presents the results of novel monopile (large diameter) and single pile (small diameter) tests in a centrifuge to for the first time directly determine the natural frequency (fn) of the pile-soil system. An experimental methodology was used to define the natural frequency via measured acceleration and force time histories and their fast Fourier transforms (FFT) under a force applied at a controlled frequency. The effects of pile diameter, embedded length, free length of the tower and soil density on fn were investigated in the centrifuge tests. The same models used in the centrifuge test at 50 g were also tested at 1 g in order to assess the relevance of earlier 1 g investigations into system behaviour. The measured natural frequency of wind turbine monopiles in centrifuge models during harmonic loading from a piezo-actuator, confirmed that soil structure interaction at an appropriate stress level must be taken into account to obtain the correct natural frequency. The experimental data was compared to theoretical solutions, giving important insights into the behaviour of these systems.

An Investigation into the Effect of Scour on the Loading and Deformation Responses of Monopile Foundations

Severe foundation scour may occur around monopile foundations of offshore wind turbines due to currents and waves. The so-called p-y curves method is suggested in the existing design recommendations to determine the behavior of monopiles unprotected against scour and the reduction of effective soil stress is accounted for by the extreme scour depth. This conservative design approach does not consider the geometry of the scour hole and the effect of pile diameter on the soil resistance. An underestimated foundation stiffness would be obtained, thereby influencing the predicted overall response of the support structure of an offshore wind turbine. In this study, we calculated the load-deformation response and foundation stiffness of a monopile when scour occurred. The influence of pile diameter on the initial modulus of subgrade reaction, and the modification of the ultimate soil resistance of a monopile subject to scour are evaluated. The commercial software BLADED was used to simulate the dynamic response of the reference offshore wind turbine with monopile unprotected against scour at Chang-Bin offshore wind farm in Taiwan Strait. The results showed that when the p-y curve suggested by existing design regulation was used to calculate the load-deformation response, the foundation stiffness was underestimated where the scour depth was greater than the pile diameter, but the foundation stiffness was overestimated when the scour depth was less than the pile diameter.

Modelling effects of pile diameter

The most commonly used program for the analysis of piles under static lateral loading is LPILE. The program uses the nonlinear Winkler springs recommended by the American Petroleum Institute (API) to model soil–pile interaction. The p–y (load–displacement) curves were developed from field tests, with pile diameters in the range 0.324–0.67 m. When these p–y curves are used to analyze load tests on piles with larger diameters, the computed load–deflection curves underestimate the stiffnesses of the test piles. This effect is referred to as the pile diameter effect. In this technical note, a very different approach is presented to evaluate the pile diameter effect. Both LPILE and a continuum-based finite element program VERSAT-P3D were calibrated to closely simulate the results of two lateral load tests on small-diameter piles at two different sites. VERSAT-P3D modelled the volume of the pile and LPILE did not. Each program was used to develop p–y curves for increasingly larger pile diameters up to 2.0 m. An important finding for practice is that there was no pile diameter effect for displacements up to 60 mm. LPILE can be used with confidence in practice in this displacement range. Thereafter, the load–deflection curves from LPILE became softer and the pile diameter effect became evident.