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Use of helical piles has become increasingly popular since last few decades for numerous advantages and installation convenience. They may consist of isolated helical plate welded to a central shaft or continuous screw type piles. The load transfer mechanism of helical piles is complex compared to the conventional vertical driven piles. Although limited contributions are available on the performance of helical piles including theoretical and experimental studies, still there are major scopes for further research. The authors have conducted a series of laboratory experimentations on axial load-settlement characteristics of model helical piles made of high tension steel in soft clayey soil, followed by 3D finite element analysis using ABAQUS. The numerical results were found to be close to the experimental observations. Using the numerical model, extensive parametric studies were conducted. The load-settlement response was found to be mostly logarithmic. The pile capacity was found to be influenced significantly by the pitch and embedded pile length. The FEM analysis revealed maximum plastic strain and displacement to develop adjacent to the pile-tip and the interface, while negligible at the boundaries. The interface shear stress distributions are also studied. From the entire investigation, a set of important conclusions are drawn. [ABSTRACT FROM AUTHOR]

Machine learning is frequently used in various geotechnical applications nowadays. This study presents a statistics and machine learning model for settlement prediction of helical piles that relates compressive service load and soil parameters as a group with the pile parameters. Machine learning algorithms such as Decision Trees, Random Forests, AdaBoost, and Artificial Neural Networks (ANN) were used to develop the predictive models. The models were validated using cross-validation techniques and tested on an independent dataset to assess their accuracy and generalizability. Numerical investigation is used here to supplement the field data by simulating various soil conditions and pile geometries that have not been tested in the field. This study compiled numerical results of 3600 models. As the models are well-calibrated and validated, the data from these models can be reasonably assumed to simulate the ground situation. At the end of this study, a comparative analysis of statistic learning and machine learning (ML) was done using the field axial load tests database and numerical investigation on helical piles. It is observed that ML models like Decision Trees and Random Forests provided the better model with R-squared values of 0.92 and 0.96, respectively, for large diameters. The authors believe this study will permit engineers and state agencies to understand this prediction model's efficacy better, resulting in a more resilient approach to designing large-diameter helical piles for the compressive load. [ABSTRACT FROM AUTHOR]

Current research deals with the numerical investigation of the behavior of helical piles resting in cohesionless soil subjected to compressive load. The effect of the diameter of pile shaft (DS), the diameter of the helical plate (Dh), the depth of the pile (H), the inter-helical spacing ratio (S/Dh) the number of helix (m), and the type of sand on the load-displacement behaviour of helical pile was evaluated. The numerically determined compression capacity of the helical pile was compared with the existing theories. Apart from this, the artificial neural network technique was employed on the obtained results to develop the model equation. An increase in the compression capacity of single and double helical pile was observed with an increase in the pile depth, the friction angle of sand, and the diameter of pile shaft. For double helical pile, the optimum inter-helix spacing ratio was found to be 3. [ABSTRACT FROM AUTHOR]

Preparing uniform soil samples is essential for obtaining reliable results in the physical modeling of geotechnical structures, such as retaining walls and piles. The air pluviation method is a common approach adopted by researchers and engineers to reconstitute large-scale sand beds with high degrees of homogeneity and uniformity. This paper investigates the uniformity of sand beds prepared using the conventional manual sand pouring method, its potential impact on test results, and how creating a more uniform sand bed can enhance the reliability of the results. To achieve this goal, a novel portable sand pluviator capable of reconstituting sand beds inside a frustum confining vessel (FCV) apparatus at a high level of uniformity is adopted. The pluviator is utilised to prepare sand beds inside the FCV, and the uniformity of the resulting sand bed is assessed and compared with those prepared using the manual pouring method. Next, in order to investigate the influence of uniformity of sand beds on the outcomes of a large-scale test, a large-scale experiment involving the determination of the bearing capacity of a open-ended single-helix helical pile within the FCV is considered. Numerical analysis investigates how sand bed uniformity affects bearing capacity without extensive testing. It uses data from initial research and employs Latin Hypercube Sampling to determine the required simulations for accurate measurement of the impact of sand bed uniformity on bearing capacity. The results of this set of numerical simulations are then thoroughly analysed to assess the impact of the sand pouring method on the reliability of the experimental outcomes. Analysis of the results show that preparation of uniform sand beds at the desired relative density is one of the key elements in obtaining accurate and reliable experimental results in geotechnical physical modelling problems. [ABSTRACT FROM AUTHOR]

In this study, a large-scale laboratory testing apparatus was developed to investigate the effect of different helical shapes on the helical pile installation and the axial compression loading bearing pressure. The various helical shapes were designed using 3D printing technology. The high strength Polylactic acid Plus (PLA-Plus) material was used for printing different helical shapes. The helical pile's installation was accomplished by coupling the shaft of the helical pile to a torque engine that rotates the helical pile in the soil. After installing the pile to a predetermined depth, compression loading tests were conducted on helical piles under a constant strain rate. The results illustrate that the helix shape has a significant influence on the performance of the helical pile. The multi-pitch helix produces an 18% increase in bearing pressure compared to the traditional helix. The helix diameter has exhibited a significant influence on the helical pile bearing pressure. Moreover, an empirical equation was proposed to estimate the coefficient of torque ( K t ) using the helical pile embedment area for measuring the bearing capacity and installation torque of the helical pile. [ABSTRACT FROM AUTHOR]

The methods employed to calculate the axial bearing capacity of a helical pile depend on the shear failure model around the pile, which is also influenced by the spacing and diameter of the helical plates. However, studies on the transition of the failure mode and the load transfer mechanism with the change of helical plate spacing and diameter in cohesionless soil subjected to axial compressive load were limited. Thus, this paper investigated the effects of helix diameter and spacing on the axial compressive load-bearing capacity, shear failure model, and load transfer mechanism of helical piles with two helical plates embedded in the homogeneous medium and dense sands, as well as in the stratified medium to very dense sand. Axial loading tests on helical piles with various helix diameters and spacings were simulated using a two-dimensional finite element program with axisymmetric modeling to obtain the load-settlement curve, which was later used to estimate the ultimate bearing capacity of the helical piles. The ultimate bearing capacity of the helical piles was also computed using the conventional methods, i.e., the individual bearing and cylindrical shear methods, and then compared to the numerical-based axial bearing capacity. The stress-strain behaviors of pile and soil were modeled using the Linear Elastic and Mohr-Coulomb material models, respectively. The results show that the numerical-based ultimate bearing capacity of a helical pile increased with increasing the diameter and spacing of the helix. However, the ultimate bearing capacity computed using conventional methods did not show this trend. Then, the transition from the cylindrical shear to the individual bearing failure mechanism occurred at a spacing ratio (i.e., helical plate spacing divided by its diameter) of about two. Ultimately, the load transfer curves indicate that the helical plates mainly supported the applied load. [ABSTRACT FROM AUTHOR]

AbstractOffshore wind turbines, crucial for electricity generation worldwide, face challenges from strong winds, waves, and uneven ocean floors. To address these issues while remaining cost-effective, an innovative foundation is essential. The study introduces a helical monopile with a circular helix made of the same material as the shaft, representing a significant advancement. The study employs both 1 g model experimental tests and numerical simulations (PLAXIS 3D) to explore how changing slope conditions, pile positions, and the inclusion of the helix affects the response under monotonic lateral loading. Factors studied include ground slope angles (flat, 1 V:5H, 1 V:3H, 1 V:2H, and 1 V:1H), pile positions along the slope (c = 0Dp, 2.5Dp, 5Dp, and 7.5Dp), and variations in the number and positions of the helix (Hp) (0.25Lp, 0.5Lp, and 0.75Lp). The investigation involved analyzing the spacing between helices about both the length of the pile (Lp) and the diameter of the helix (dh). The eccentricity to the diameter of pile ratio (e/Dp) is maintained as 12. The main objective is to compare the effectiveness of large-diameter helical piles to conventional monopiles in resisting upward and lateral forces. Results show helical piles offer a 100% improvement in uplift resistance and a 53% increase in lateral resistance compared to monopiles. This significant enhancement in capacity, particularly on sloping surfaces where monopiles typically exhibit reduced performance, underscores the effectiveness of helical piles in enhancing lateral capacity compared to monopiles. [ABSTRACT FROM AUTHOR]

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AbstractAlthough helical piles are frequently installed at an angle to the vertical, there has been limited investigation into how this inclination affects the pile’s ability to resist pullout forces. This paper utilizes numerical analysis to examine the pullout capacities and failure envelopes of inclined helical piles under combined loading. Through the application of the coupled Eulerian-Lagrangian finite element analysis, the impact of load inclination and azimuth angle of the pullout plane on the ultimate holding capacity of inclined helical piles was assessed. It was found that the pullout plane that is in the opposite direction to the helical pile inclination is the most crucial. The parametric study showed that the crucial failure envelope is primarily influenced by the helical pile inclination angle, embedment depth ratio, helix diameter ratio, and normalized soil strength ratio. Several tables were provided, which contain fitting constants suitable for a range of helical pile inclination angles, embedment depth, helix diameter ratio, and normalized soil strength ratios. Based on these results, a straightforward design procedure was established to evaluate the holding capacity of an inclined installed helical pile. The research results are expected to assist engineers in accounting for helical pile inclination during the design phase. [ABSTRACT FROM AUTHOR]

The foundations of particular engineering structures, including marine and jetty structures, mooring systems for submerged platforms or those on the ocean surface, and transmission towers, are subjected to various external loads including compression, uplift, and lateral loads. In such cases, to improve the soil resistance below foundations, pile foundations such as helical piles, anchored piles, and batter piles are commonly preferred, depending on the in situ conditions. Helical piles, increasingly used as an alternative foundation to conventional piles, are placed in the soil body by rotating with torque. This paper deals with the contribution of a helical pile in improving loose sandy soil, and the main purpose is to study the effect of the helix-buried depth on the load-bearing capacity and failure mechanism. The investigated variables include the distance between helixes, the number of helixes, and the diameter of the upper helix. Physical model tests were conducted, and two- and three-dimensional numerical analyses were performed by using the finite element method with an advanced soil model to illustrate the failure mechanisms of helical piles. The aim was to reveal the efficiency of the finite element method in modelling helical piles placed in weak sandy soil. A simplified linear geometry for helixes was established in a two-dimensional finite element model whereas a real geometry for helixes, which was a more realistic approach, was created in a three-dimensional finite element model. The results show that the three-dimensional model indicates better agreement with the physical model compared to the two-dimensional model, and all investigated variables highly affect the load-bearing capacity of helical piles. [ABSTRACT FROM AUTHOR]

Helical piles are used extensively for low and medium rise developments and are a popular foundation solution in cases where resistance to significant uplift loads is required. The ability to re-use helical piles and recent interest in their use as foundations in offshore wind and solar energy applications has renewed interest in improving existing design methods. This paper presents the results from field investigations in medium dense and dense sand that examine effects on the axial tension and compression capacity of varying the helix pitch, shaft diameter, advancement ratio and shaft tip geometry. The results from the 20 pile tests conducted (many of which included instrumentation) indicate low sensitivity to a range of the investigated parameters and a strong correlation of the capacity to the cone penetration test resistance. A design method based on these findings is proposed for typical helical pile geometries and is shown to provide predictions that are generally within 10 to 15% of the axial capacities reported for 30 other well documented pile tests reported in the literature. The method, referred to as UWASP-22, incorporates a simple means of estimating the load-displacement response of a helical pile as well as a formulation enabling prediction of the installation torque which is a common quality control measure for helical piles. [ABSTRACT FROM AUTHOR]

The methods employed to calculate the axial bearing capacity of a helical pile depends on the shear failure model around the pile, which is also influenced by the spacing and diameter of the helical plates. However, studies on the transition of the failure mode and the load transfer mechanism with the change of helical plate spacing and diameter in cohesionless soil subjected to axial compressive load have been limited. Thus, this paper investigated the effects of helix diameter and spacing on the axial compressive load-bearing capacity, shear failure model, and load transfer mechanism of helical piles with two helical plates embedded in the homogeneous medium and dense sands, as well as in the stratified medium to very dense sand. Axial loading tests on helical piles with various helix diameters and spacings were simulated using a two-dimensional finite element program with axisymmetric modeling to obtain the load-settlement curve, which was later used to estimate the ultimate bearing capacity of the helical piles. The ultimate bearing capacity of the helical piles was also computed using the conventional methods, i.e., the individual bearing and cylindrical shear methods, and then compared to the numerical-based axial bearing capacity. The stress-strain behaviors of pile and soil were modeled using the Linear Elastic and Mohr-Coulomb material models, respectively. The results show that the numerical-based ultimate bearing capacity of a helical pile increased with increasing the diameter and spacing of the helix. However, the ultimate bearing capacity computed using conventional methods did not show this trend. Then, the transition from the cylindrical shear to the individual bearing failure mechanism occurred at a spacing ratio (i.e., helical plate spacing divided by its diameter) of about two. Ultimately, the load transfer curves indicate that the helical plates mainly supported the applied load.

Helical piles are a new type of pile that has good application prospects, and researchers have carried out an in-depth investigation into their vertical uplift and compressive bearing capacity. However, there is relatively little research on the dynamic bearing characteristics of helical piles. Therefore, the lateral vibration of a helical pile embedded in the viscoelastic foundation is systematically studied in this article. Utilizing the equivalent stiffness method to transform a helical pile into a cylindrical pile of special diameter, the lateral vibration model of the helical pile considering shear deformation is established based on the Winkler foundation model and the Timoshenko beam theory. The analytical solutions for the lateral dynamic displacement, bending moment, and shear force of the helical pile are strictly derived, and the rationality of the present solutions is also verified by comparing them with existing solutions. Based on the present solutions, a parametric study is carried out to investigate the influence of the pile and soil properties on the lateral dynamic response of the helical pile. It is found that the load excitation frequency and pile–soil stiffness ratio have a significant influence on the lateral dynamic displacement, bending moment, and shear force of the helical pile with space and time response. [ABSTRACT FROM AUTHOR]

Helical piles are one promising foundation option in offshore industries to support wind turbines and floating structures. During service, the helical piles are subjected to combined loading originating from the superstructure and environmental factors. For the inclined pullout capacity of helical piles, several studies focused on the influence of helical plate configurations and soil properties. In contrast, this paper aims to assess the effect of the random distribution of soil strength. This is undertaken through the use of a large deformation random finite element analysis combing the random field theory, coupled Eulerian-Lagrangian method and Monte Carlo approach. Spatial variability and linearly increasing with depth of soil strength are modeled as a three-dimensional random field and a mean trend term. The probabilistic failure envelope as well as the failure mechanism can be then determined from each random realization. It is found that soil randomness has an insignificant influence on the failure mechanism. For different spatial soil configurations, the probabilistic failure envelopes vary widely from one another. An equation is established to fit the envelopes with different occurrence probabilities and probabilistic analyses are conducted. The present findings might help to improve the helical pile design practices to achieve better reliability. [ABSTRACT FROM AUTHOR]

In the construction sector, axial loads have traditionally been supported by piles. There hasn't been sufficient comparative study related to the economics and viability aspects of conventional and helical piles up to this point in natural soil condition. The current study was conducted to test the behavior of helical piles subjected to axial loading and the results were compared with the conventional pile counterpart. For this, a total of eighteen experiments were performed with varying number of helical piles either single or multi helical piles in a raft. Helical piles were penetrated into ground, vertical load of 512 kg was applied and the settlements were recorded with the gauges installed. The reduction in settlement was found to increase due to increasing the number of piles, inter distance between the piles and increasing the number of helices. Nine double helical piles were observed to reduce settlement by 80.5% when compared to nine conventional piles. Due to the smaller number of piles needed to support the applied axial load when employing helical piles as opposed to conventional piles, the cost was greatly lowered.

The rapid development of offshore wind power and the need to move to deeper sea areas while reducing costs per kilowatt necessitate the employment of a new jacket and helical pile combination. This new combination combines the advantages of both jacket structures and helical piles and provides a superior bearing capacity and installation efficiency compared to conventional pile foundations. Foundations account for 25%–34% of the overall cost of construction, but the use of this new foundation would be highly significant for the further development of offshore wind power. This study presents numerical results for the horizontal bearing capacity when horizontal displacement is applied, focusing on the bearing capacity and characteristics of the helical pile jacket foundation as well as the differences between the bearing mechanisms and failure modes of normal pile and helical pile types. ABAQUS model parameters are obtained through trial calculations based on actual engineering data, and the finite element model (FEM) is validated using data from a model experiment. Subsequently, different FEMs are established, and numerical results are compared and presented. Through a comparison between a normal pile jacket foundation and a helical pile jacket foundation with different helical blade numbers, the differences in the bearing mechanisms and failure modes are revealed. The failure of the normal pile jacket foundation is instantaneous and sudden, whereas that of the helical pile foundation is incremental and accumulative. These data highlight the most significant contributions and vulnerabilities of the one-pile side of the foundation and suggest that the addition of blades on the one-pile side is the most effective way of improving the foundation's bearing performance. In addition, the interaction between the compression side and tension side is analyzed in relation to differing the relative magnitudes of their bearing capacities. [ABSTRACT FROM AUTHOR]

A laboratory study for screw pile group driven in two clay layers soft and stiff under the influence of lateral cyclic load. The aim of this research is to investigate how screw piles behave laterally under peridic load because these types of piles increased their use in previous years, especially in wind turbines and energy towers, the scarcity of studies on their lateral behavior, and the spread of this soil profile in central and southern Iraq. This study investigated the behavior of screw pile group 2 × 2 with three spacing equal to 1.5, 3, and 4.5 Dh where Dh is the helix diameter, two embedded lengths of 400 and 330 mm, shaft diameter equal to 10 mm by using single, double helix, and ordinary pile (straight shaft without helix) under the effect of cyclic lateral load with frequency 0.2 Hz. Through the results it was found the increasing lateral resistance was significantly impacted by the increment spacing between the piles. For pile group with an embedded length of 400 mm, increasing the distance between the screw piles from 1.5 Dh to 3 and 4, 5 Dh leads to an increase in the side resistance at rates ranging between 25–39% and 49–64% respectively and also found that the embedded length of the pile has an influent effect on the side resistance, the decrease in the embedded length from 400 to 330 mm leads to an increase in lateral deflection by a rate ranging between 27 and 150% depending on the strength and amount of the cyclic lateral load. It was also found that for the same distance between piles, the screw piles gave better lateral resistance than the ordinary piles, as well as the double helix piles gave a higher side resistance than the single piles. [ABSTRACT FROM AUTHOR]

With global offshore wind power gradually moving to deep and distant waters, there is a clear trend towards larger-scale units, posing significant challenges for future offshore wind power foundations. In this paper, a helical pile jacket foundation type is proposed by combining the advantages of the current jacket foundation, which is suitable for deep and distant seas and offers high stiffness, with the excellent bearing performance of helical pile foundations. The influence of buried depth on the bearing characteristics of this foundation type is discussed through a physical model test. The results obtained from the study reveal the distinct bearing characteristics and damage modes exhibited by the foundation under deep and shallow burial conditions. These findings clearly indicate that the overall bearing characteristics and damage modes are superior in deep burial conditions compared to shallow burial conditions. Furthermore, it is observed that the damage and displacement of the foundation are more concentrated in localized areas when subjected to shallow burial. [ABSTRACT FROM AUTHOR]

The foundations of particular engineering structures, including marine and jetty structures, mooring systems for submerged platforms or those on the ocean surface, and transmission towers, are subjected to various external loads including compression, uplift, and lateral loads. In such cases, to improve the soil resistance below foundations, pile foundations such as helical piles, anchored piles, and batter piles are commonly preferred, depending on the in situ conditions. Helical piles, increasingly used as an alternative foundation to conventional piles, are placed in the soil body by rotating with torque. This paper deals with the contribution of a helical pile in improving loose sandy soil, and the main purpose is to study the effect of the helix-buried depth on the load-bearing capacity and failure mechanism. The investigated variables include the distance between helixes, the number of helixes, and the diameter of the upper helix. Physical model tests were conducted, and two- and three-dimensional numerical analyses were performed by using the finite element method with an advanced soil model to illustrate the failure mechanisms of helical piles. The aim was to reveal the efficiency of the finite element method in modelling helical piles placed in weak sandy soil. A simplified linear geometry for helixes was established in a two-dimensional finite element model whereas a real geometry for helixes, which was a more realistic approach, was created in a three-dimensional finite element model. The results show that the three-dimensional model indicates better agreement with the physical model compared to the two-dimensional model, and all investigated variables highly affect the load-bearing capacity of helical piles.

Helical piles with small sizes are widely used in traditional civil engineering infrastructure application, especially in onshore electrical power industry. The floating offshore wind power industry has taken an interest in helical piles because of their rapid installation and ability to support immediate loading. Compared with onshore structures, wind power structures suffer larger external load and it becomes necessary to increase the overall size of helical piles, leading to considerable load capacity provided by the shaft, which is typically neglected for a light structure. In this paper, the undrained uplift capacity of helical piles was studied using finite-element limit analysis. The analysis explored various diameter ratios of the shaft to the plate and embedment depths, as well as different interface conditions. The local bearing capacity on the plate was also explored for attached and vented interface conditions. The failure mechanisms of a helical pile were presented to explain the mutual influence between plate anchor and shaft on the uplift capacity. Algebraic expressions for the uplift capacity of a helical pile were proposed. Engineers may use these expressions to improve the accuracy of their estimates for the uplift capacity of helical piles. [ABSTRACT FROM AUTHOR]

Helical piles are a kind of foundation that can withstand compression, tension, and lateral stresses. However, this kind of pile was utilized extensively globally for almost 25 years. Its behavior, particularly in Iraq, is uncertain and frightening. The current research used the finite element technique to analyze this kind of pile. The helical pile geometry was suggested to be modeled using the finite element method using the computer software Plaxis 3D The soil used is medium sandy soil. Additionally, parametric analyses were conducted. The primary parametric research examines the impact of helix number, helix spacing, helix diameter, and helix configuration under cyclic load. The main results are more helices in a pile, the lower the amplitude of settlement (in the direction of z) compared to a pile without helices. As a result, the amplitude of settlement in the case of two helices decreased by 81.6 %, while the amplitude of settlement in the case of three-helix decreased by 77.74 %. In the case of three helices, the higher spacing between the helices, the lower the value of the amplitude of settlement (in the direction of z). The effect of the number of helices in the hardening soil model is more than the effect of the number of helices in the Mohr-Coulomb. As a final result, increasing the number of helices in a pile has a more significant effect in reducing the amplitude of settlement than increasing the between the helices.

This paper investigated and compared the dynamic responses of driven and helical piles in cohesive soil using finite element analysis. The finite element models were calibrated based on large-scale field dynamic tests on single steel piles driven into a soft clay layer and embedded in a gravelly sand layer. The soft clay was improved using the cement deep soil mixing technique. The calculated results from the numerical models were in good agreement with the measured response in the field tests. The numerical models were then employed to investigate the dynamic response of helical piles in clay. Driven steel piles and single- and double-helix piles were subjected to lateral dynamic loads, and their responses were compared. The results demonstrated that the helical pile exhibited excellent lateral and axial responses, comparable to that of the driven piles. The lateral displacement of the helical piles was similar to or less than that of longer driven piles depending on the clay strength, number, and spacing of the helices. In addition, the double-helix pile with an inter-helix spacing of 3 times the helix diameter reduced the required width of soil treatment to 7.5 times the pile diameter while achieving the same performance as a driven pile with a treated area 12 times the pile diameter. The kinematic and inertial interaction effects were evaluated for helical piles. It was found that the kinematic interaction forces increased the bending moment at the interface of different layers, while the inertial forces increased the bending moment at the ground level. Finally, the helical pile exhibited lower lateral deformations than the driven pile during the seismic motion, and the maximum bending moment was relatively lower. [ABSTRACT FROM AUTHOR]

Expansive soils exist in many countries worldwide, and their characteristics make them exceedingly difficult to engineer. Due to its significant swelling and shrinkage characteristics, expansive soil defies many of the stabilization solutions available to engineers. Differential heave or settlement occurs when expansive soil swells or shrinks, causing severe damage to foundations, buildings, roadways, and retaining structures. In such soils, it is necessary to construct a foundation that avoids the adverse effects of settlement. As a result, building the structure's foundations on expansive soil necessitates special consideration. Helical piles provide resistance to uplift in light structures. However, they may not fully stabilize foundations in expansive soils. A granular anchor pile is another anchor technique that may provide the necessary resistance to uplift in expansive soils using simpler methods. This review and numerical study investigate the fundamental foundation treatments for expansive soils and the behavior of granular anchors and helical piles. Results indicate that granular anchor piles performed better than helical piles for uplift and settlement performance. For heave performance, the granular anchor and helical piles perform nearly identically. Both achieve heave reductions greater than 90% when L/H > 1.5 and D = 0.6 m. [ABSTRACT FROM AUTHOR]

With respect to the design of helical pile foundations, reliable prediction of pile settlement is necessary to ensure appropriate structural and serviceability performance. At present, most research into the load-settlement response is focused on the regular displacement piles (precast/cast-in-place driven displacement piles) while there is little research on the prediction of the load-settlement curve of helical piles in sand. This paper presents a new modified method taking Randolph's analytical pile solution as the theoretical framework with the consideration of the nonlinear soil stiffness degradation corresponding to different loads and settlements. The proposed method for helical piles is verified by centrifuge tests of 9 helical piles in three types of sand (loose, medium dense and dense sand) and 4 helical piles in two field test cases with better agreement with test results compared with the existing method for driven piles. Also, the influence of different parameters of pile and soil on the load-settlement curve of single-blade piles is analyzed. A novel empirical formula is proposed to analyze the nonlinear behavior of the load-displacement curve of a single-helix pile based on the back-analysis method. The load-settlement curves predicted from the new modified method is in reasonably good agreement with measured ones from both centrifuge and field tests. The calculated load-settlement curves of 4 helical piles in 2 field tests from the new modified method are in significantly better agreement with those from exisiting method for driven piles in sand. [ABSTRACT FROM AUTHOR]

The tunnels located in the shallow depths of loose saturated sand are significantly prone to liquefaction-induced uplift. Research works are, therefore, in progress to propose efficient techniques for mitigating uplift. In this study, 1 g physical modeling was used to assess the performance of helical piles for decreasing liquefaction-induced uplift. The effects of pile length, number of pile helixes, and the pile spacing in plan view were investigated. The uplift mechanism of the tunnel and helical pile system was also analyzed. The results demonstrate that the penetration of the helical piles into the dense layer underlying the superficial liquefiable sand has decreased tunnel uplift significantly. However, excessive close pile spacing along the tunnel resulted in shear surface interference, and the efficiency of the excessive number of helical piles decreased significantly. The detailed view of the uplift mechanism showed that utilization of the piles extended the transition phase of uplift during shaking. Helical piles can efficiently restrict the possibility of rapid uplift of the tunnel and shorten the duration of the primary uplift phase. [ABSTRACT FROM AUTHOR]

Helical piles are a new type of pile that has good application prospects, and researchers have carried out an in-depth investigation into their vertical uplift and compressive bearing capacity. However, there is relatively little research on the dynamic bearing characteristics of helical piles. Therefore, the lateral vibration of a helical pile embedded in the viscoelastic foundation is systematically studied in this article. Utilizing the equivalent stiffness method to transform a helical pile into a cylindrical pile of special diameter, the lateral vibration model of the helical pile considering shear deformation is established based on the Winkler foundation model and the Timoshenko beam theory. The analytical solutions for the lateral dynamic displacement, bending moment, and shear force of the helical pile are strictly derived, and the rationality of the present solutions is also verified by comparing them with existing solutions. Based on the present solutions, a parametric study is carried out to investigate the influence of the pile and soil properties on the lateral dynamic response of the helical pile. It is found that the load excitation frequency and pile–soil stiffness ratio have a significant influence on the lateral dynamic displacement, bending moment, and shear force of the helical pile with space and time response.

The optimization of the inter-helix spacing is a key issue of the axial bearing capacity of helical piles. In this paper, based on the cavity expansion, an analytical approach considering the small-strain stiffness, strength, compressibility and stress level of sand around the helical pile was proposed to analyze the influence zone of the helices to determine the optimal inter-helix spacing in sand. The calculation results of the proposed method were verified using the centrifuge test data and finite element analysis for helical pile in Congleton HST95 sand. They were also compared with those using the Meyerhof pile foundation theory. The results show that the optimal inter-helix spacing based on Meyerhof pile foundation theory differs significantly from the measurement. The range of the influence zone for the helices in sand calculated by the cavity expansion theory matches with the data from literature. The calculation results with the proposed method are consistent with the range of the optimal spacing ratio inferred in the centrifuge tests. The results based on the two-dimensional (2D) finite element model (FEM) are also basically consistent with the calculated analytical solution.

Helical piles are getting increasingly popular for lightweight engineering applications. Currently, the helical pile is mostly designed with torque capacity; however, it cannot explain the foundation's anticipated settlement. The current study developed a settlement-based design technique to determine the capacity of a single helix in small diameter helical piles. A three-dimensional nonlinear finite element analysis (FEA) was conducted using PLAXIS 3D. At first, the numerical models were calibrated and verified using load testing data from representative soil properties and realistic modeling assumptions. Then a parametric study was extended to determine the load settlement curve in different c- ϕ soil types, along with varying helix and shaft configurations. The load-deformation curves were grouped to develop multiple load capacity curves. Finally, the load capacity curve was validated by load test data. The range of calculated vs measured (from load tests) capacities (Qc/Qm) is 0.68 to 2.67, where the mean Qc/Qm is 1.10.

In this paper, the compressive and tensile ultimate bearing capacity of helical piles, in the load range required for designing 63-kV transmission towers, are studied by conducting analytical and numerical investigations. In this light, an explicit easy-to-use analytical framework was developed by considering the unsaturated soil condition that was validated against experimental records. Also, finite-element models were constructed for fully saturated and fully dry soil conditions. Then, to validate the analytical and numerical solutions in fully saturated and fully dry soil conditions, three compressive and one tensile field tests were carried out, and the results were compared quantitatively with analytical and numerical calculations. Comparative studies showed that the analytical, numerical, and experimental results have a high level of conformity. In accordance with the load ranges of transmission towers, analytical parametric studies were conducted for different geometrical aspects of the helical piles in different soils. Also, for two typical unsaturated sand and clay soils, the ultimate load of helical piles was calculated versus matric suction. Moreover, a vast numerical parametric study was performed to investigate the role of the geometrical aspects of helical piles and soil characteristics on the load-transfer mechanism and ultimate bearing-capacity values for helical piles driven in sandy and undrained saturated clayey soils. Results showed that the maximum load capacity of helical piles occurred in a matric suction less than that of a fully dry soil. In addition, the soil type and the geometrical aspects of the helical pile were shown to have significant effects on the load–displacement behavior and ultimate bearing-capacity values. [ABSTRACT FROM AUTHOR]

With global offshore wind power gradually moving to deep and distant waters, there is a clear trend towards larger-scale units, posing significant challenges for future offshore wind power foundations. In this paper, a helical pile jacket foundation type is proposed by combining the advantages of the current jacket foundation, which is suitable for deep and distant seas and offers high stiffness, with the excellent bearing performance of helical pile foundations. The influence of buried depth on the bearing characteristics of this foundation type is discussed through a physical model test. The results obtained from the study reveal the distinct bearing characteristics and damage modes exhibited by the foundation under deep and shallow burial conditions. These findings clearly indicate that the overall bearing characteristics and damage modes are superior in deep burial conditions compared to shallow burial conditions. Furthermore, it is observed that the damage and displacement of the foundation are more concentrated in localized areas when subjected to shallow burial.

Bearing behaviour of helical piles under axial loading Helical piles are rotating installed into the ground using hydraulic equipment. This method of construction offers time and cost advantages over bored piles. Small‐scale 1g tests using the PIV method were carried out to investigate the soil mechanical behavior that occurs due to axial loading. The results are presented and compared with analytical approaches for the calculation of the bearing capacity from literature. The results of the model tests are compared with observations from large scale laboratory tests and field tests. Recommendations for dimensioning and quality assurance are given in conclusion. [ABSTRACT FROM AUTHOR]

This paper examines the seismic performance of a bridge–helical pile foundation based on the seismic fragility analysis, considering the element fragility of a coupled bridge–soil–foundation system. Nonlinear time history analyses were conducted using a finite-element modeling scheme that was validated using the results of large-scale shaking table tests of soil–piles–structure systems involving both liquefiable and nonliquefiable soils. Incremental dynamic analysis was conducted to generate the fragility curves for a two-span bridge supported on helical piles in nonliquefiable and liquefiable sites considering a suite of ground motions. The damage limit states were defined to describe the capacity of the bridge components. In total, 440 nonlinear time-history analyses were performed to evaluate the seismic demand of the helical piles and the bridge-reinforced concrete pier components, and the results were used to establish their fragility curves. The results revealed that the helical piles were the most fragile component in the nonliquefiable and liquefiable tests. The liquefiable soil could decrease the seismic demand on the column lateral deformation and increase the demand dispersion. On the other hand, the reinforced concrete pier exhibited a large drift response in the nonliquefiable soil, causing it to be more vulnerable to seismic hazards than in the case of liquefiable soil. [ABSTRACT FROM AUTHOR]

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Helical pile foundations are a new foundation type for offshore wind power applications with high bearing capacity and good recovery that can be quickly and easily constructed. In this study, the finite element method was used to simulate the bearing characteristics of helical pile foundations after installation. For simulations, a blade, deeply buried in a single layer of sand, was selected. Through numerical simulations, the vertical bearing characteristics of a single helical screw pile and an ordinary pile without blades were compared, and the compression and uplift characteristics of the helical pile were revealed. In addition, the effects of pitch, blade diameter, inclination angle, number of blades, and blade spacing on the bearing characteristics of a single helical pile were analyzed. The results show that the single helical pile has the highest bearing capacity and bearing efficiency when the pitch is 0.02 times the blade buried depth, the blade diameter is 2.5 times the pile diameter, the multi-blade spacing is more than two times the blade diameter, and the number of blades is less than or equal to three. However, compared with the straight pile, the vertical bearing capacity of the single inclined helical pile did not improve significantly. [ABSTRACT FROM AUTHOR]

Helical piles with small sizes are widely used in traditional civil engineering infrastructure application, especially in onshore electrical power industry. The floating offshore wind power industry has taken an interest in helical piles because of their rapid installation and ability to support immediate loading. Compared with onshore structures, wind power structures suffer larger external load and it becomes necessary to increase the overall size of helical piles, leading to considerable load capacity provided by the shaft, which is typically neglected for a light structure. In this paper, the undrained uplift capacity of helical piles was studied using finite-element limit analysis. The analysis explored various diameter ratios of the shaft to the plate and embedment depths, as well as different interface conditions. The local bearing capacity on the plate was also explored for attached and vented interface conditions. The failure mechanisms of a helical pile were presented to explain the mutual influence between plate anchor and shaft on the uplift capacity. Algebraic expressions for the uplift capacity of a helical pile were proposed. Engineers may use these expressions to improve the accuracy of their estimates for the uplift capacity of helical piles.

The use of helical piles as a group to sustain large compressive loads has increased in the recent decade, and an appropriate evaluation of their behavior is essential for a robust design. Therefore, a sequence of analyses via numerical modeling was conducted to evaluate the behavior of helical pile groups of different geometries and configurations installed in sandy soil. The 3D nature of the problem was modeled using Midas GTS NX and was calibrated and validated against field data. In this study, 2 × 1, 2 × 2, 5-piles, and 3 × 3 double- and triple-helix free-standing pile groups in a symmetrical arrangement were considered. The group effect on the ultimate load capacity and the settlement of each group were expressed in terms of group efficiency and settlement ratio. The load-transfer mechanism and its variation with group configuration were also explored and presented. The results indicated that center-to-center pile spacing, number of piles, helix-to-shaft diameter ratio, number of helices, and the method used to interpret the test data significantly influenced the group efficiency and settlement ratio, being more significant in the latter compared with the former. Moreover, center-to-center pile spacing and embedment depth contributed to the shape of the load-transfer mechanism of helical pile groups. The results of this study can be used to better predict the behavior of helical pile groups in sandy soil. [ABSTRACT FROM AUTHOR]

Vast land areas in northeastern Iran are covered with loess soils being susceptible to collapse and excessive settlement after moistening. Different improvement techniques were developed for loess soils, including compaction, immersion, or reinforcement. However, the shortcomings, such as limited improvement depth and lack of control over the treated area restrict these techniques' effectiveness. The present study suggests the installation and application of helical piles as a combination of improvement techniques. About 13 uniform triple-helix piles of 3 m length, 250 mm blade diameter, and 750 mm blade spacing were installed via two different dry and wet installation methods. Static pile load tests were performed for all the helical piles. The results showed that the pile load tests were highly dependent on the moisture content at the testing time, the crowd force and the injected water pressure. Eventually, wet installed piles led to a more stable structure of loess soils than dry installed piles subjected to water. [ABSTRACT FROM AUTHOR]

Designing structures to be the least vulnerable within earthquake-prone areas is a serious challenge for structural engineers. One common and useful tool that structural engineers use to predict the vulnerability of a structure during an earthquake is a fragility curve. However, most structural fragility curves do not take into consideration the contribution of pile foundation systems in the structural vulnerability. Therefore, this study aims to modify existing fragility curves of a six-story fixed-base steel frame hospital building with buckling-restrained braces, to incorporate the effect of helical pile group behavior on the fragility of the structure. To that end, a finite element model of the investigated structure was modified with results from a full-scale shake table test performed on two groups of helical piles embedded in dense sand supporting a superstructure. The primary results show that fixed-base design may not be conservative for all conditions and soil–foundation interaction should be considered when creating fragility curves, especially for a stiff structure on soft soils where a high-intensity earthquake is anticipated. [ABSTRACT FROM AUTHOR]

An investigation into axial compression capacity of single helical piles placed in dry sand through laboratory model tests and numerical analyses is presented. The compressive bearing capacities were compared with existing theoretical results given in the literature. Laboratory model tests were performed to determine some design parameters of helical piles such as the plate number, plate diameter, and plate spacing. A good correspondence between experimental, numerical, and theoretical results was established.