Helical Piles Research Articles

Analysis and design methods of helical piles: a critical review with emphasis on finite element method

Analysis and design methods of helical piles: a critical review with emphasis on finite element method

Dynamic performance of driven and helical piles in cohesive soil

Dynamic performance of driven and helical piles in cohesive soil

Research on Lateral Bearing Behavior of Spliced Helical Piles with the SPH Method

The length of a spliced pile is 2 m assembled from an original spiral pile using a connector. The whole pile is the structure of the upper straight pipe and the lower spiral. The pile–soil model is established with FEM-SPH by LS-DYNA to simulate and analyze the characteristics of the spliced piles. When the helical pile is subjected to a horizontal load, the pile rotates around the point of rotation, and the contact force position of the soil in the model is as expected. During the process of pile driving, the soil forms an inverted cone stress-area, and the maximum particle stress area near the pile tip and the ground surface is 400 Kpa, which is highly concentrated. When loaded laterally, the area of the interaction stress of the soil particles is divided into three regions: the stress effect region; the transition region; and the critical region. Then, 7° is defined as the ultimate horizontal bearing-capacity of the spliced pile, and the numerical simulation of the horizontal bearing-capacity fundamentally matches the test results. The simulation model realizes the transition from the pile installation to the lateral loading, predicts the ultimate horizontal bearing-capacity, and analyzes the stress distribution of the soil particles and the time-development of the soil displacement.

Analysis of the Installation Effect on the Axial Performance of Pressure-Grouted Helical Piles in Clay by Small-Scale Model Tests

A pressure-grouted helical pile (PGHP) is a stiffened helical pile installed by using the simultaneous drilling and grouting technique. The formation of the soil-cement column surrounded by the helical pile is influenced by the multiple installation parameters, including helix number, helix size, and drilling speed. A series of small-scale model tests was carried out in clay to investigate the influence of installation parameters on axial behavior of PGHP and the load transfer mechanism. The model piles were pulled out after the loading tests, and the dimensions of the soil-cement columns were measured. The ultimate compressive bearing capacities of the PGHPs were 260% to 293% higher than the un-grouted helical piles. The ultimate bearing capacities of the PGHPs were proportional to the increase of the helix number and helix size and decreased with the drilling speed. The average bond diameter of the soil-cement column was 1.26 to 1.35 times the helix size. The adhesion between the soil-cement column and the surrounding clay was in the range of 0.8 to 1.2. The overall results examine the feasibility of the simultaneous drilling and grouting technique in clay and the improvement of the axial bearing performance compared to un-grouted helical piles.

Mitigating liquefaction-induced displacements of shallow foundation using helical piles

During previous earthquakes, displacements of shallow foundations on liquefiable sites caused significant damage to overlying super-structures leading to casualties and catastrophic economic loss. Various countermeasures have been developed to lessen liquefaction-related damages while minimising cost and environmental impacts. This study aims to evaluate the use of helical piles as a possible technique to mitigate the settlement, tilting, and sliding of shallow foundations during seismic liquefaction. For this purpose, 12 1g shaking table tests were conducted on loose saturated sand under harmonic base input for (a) free-field condition, (b) a model foundation on the soil surface and (c) a model foundation underpinned by helical piles. The effects of input amplitudes and the number of helical piles were investigated in terms of acceleration response, excess pore-water pressure and foundation displacements. The results confirmed the satisfactory performance of helical piles in reducing shallow foundation displacements. In particular, the mean permanent settlements were reduced by 45 and 75% when using four and eight helical piles, respectively. Similar trends were observed for the shallow foundation permanent tilting and sliding: permanent tilting was reduced by 30 and 59% when using four and eight helical piles, respectively, while these results were 45 and 68% for the sliding.

Behavior of Mono Helical Pile Foundation in Clays under Combined Uplift and Lateral Loading Conditions

Offshore wind energy development is driven by ongoing research and development to determine technological issues connected with offshore wind turbines. A fundamental design difficulty is the offshore wind turbine foundations. By far, one of the most commonly practiced foundation systems is mono helical pile foundations because of their simplicity and resilience. Most of the studies on mono helical piles were focused on granular soils and few studies were on clayey soils. However, mono helical piles in clayey soils under combined loading remain to be examined. Hence, a series of three-dimensional finite element analyses were carried out to predict the uplift and lateral capacity of the helical pile embedded in soft clay. The numerical model is verified using the results from the analytical results and a small-scale laboratory test. The verified model is used to simulate the helical pile behavior under the combined uplift and lateral loading condition considering the pile with a different number of helical plates. The results of the combined loading analyses reveal that the lateral capacity decreased when the pile was subjected to uplift load, and a maximum reduction of 12% is observed in mono helical piles provided with a single plate for an applied 50% of the ultimate uplift load. The influence of uplift load also increased the bending moment of the helical pile. Finally, it is suggested that the helical piles should be designed considering the aspects for combined loading conditions.

Seismic Helical Pile Response in Nonliquefiable and Liquefiable Soil

Seismic Helical Pile Response in Nonliquefiable and Liquefiable Soil

Bearing Characteristics of Helical Pile Foundations for Offshore Wind Turbines in Sandy Soil

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.

Optimization of inter-helix spacing for helical piles in sand

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.

Effects of helix shapes and locations on compression capacity of helical piles for offshore foundations

Piles are widely used in marine projects where inappropriate soils are present and soil replacement is not justified economically. Among various pile types, helical piles have been considered as a potential substitution for driven piles in offshore applications. The present paper investigates numerically the effects of tapered helices and inter-helix spaces to achieve the maximum compression capacity in homogeneous and in-homogeneous soil profiles. Both individual and cylindrical failure mechanisms were studied. Tapered helices with decreasing diameters from the top to bottom instead of the same number of identical helices were also studied. Results show that the best inter-helix spacing to helix diameter ratio is two to achieve the greatest pile capacity. Also, the compression load corresponding to 10% of the pile head settlement was found to be reasonable for computing the pile capacity. The results show that tapered helix piles offer better compression capacity than uniform helix piles with the same number of identical helices. In addition, tapered helix piles have almost similar load-settlement variation to solid frustum shape helix piles while in the latter more steel is consumed. In general, the present research highlights that using tapered helices due to their technical and economic performance is advantageous.

Finite element modelling of helical pile installation and its influence on uplift capacity in strain softening clay

This study investigated the effect of helical pile installation in undrained softening clay using a coupled Eulerian-Lagrangian (CEL) finite element modelling approach. A previously published strain softening soil constitutive model was used to evaluate soil disturbance. The influence of two key strain softening parameters was considered to assess installation-induced soil disturbance. The CEL-predicted remoulded soil strength field was then imported into a finite element limit analysis (FELA) to evaluate the subsequent influence on uplift capacity. The CEL results showed that helical pile installation caused a significantly disturbed zone of soil that measured approximately 1.4 helix diameters in plan and extended the full length of the pile. When the disturbed soil strength field was imported into FELA calculation, it was found that the solutions reported in previous literature had significantly overestimated the ultimate uplift capacity for softening clay. The numerical output informed the development of a new closed-form analytical approach to predict uplift capacity considering the influence of the installation process. The design method was shown to provide a high-fidelity representation of the numerical results.

Centrifuge modeling of the behaviour of helical piles in cohesive soils from installation and axial loading

Centrifuge modeling offers a viable tool for research in the soil-pile interaction, but this technique has not been used for helical piles in cohesive soils. A centrifuge model test program of helical piles in cohesive soils was carried out to investigate the axial soil-pile interaction and pile failure mechanism. Helical piles were installed while the centrifuge was spinning, which enabled the determination and interpretation of installation torque and pore pressure response of the soil. An analytical model for calculating the installation torque of helical piles screwed into cohesive soils was proposed and verified by test results. The pore pressure response to pile installation was monitored near two piles at two depths. Comparing the measured dissipation curves with the analytical curves for driven piles suggested that the excess pore pressure was primarily induced by the helical pile shaft. The model piles were axially loaded under 20 g condition. The present research may be considered as the first centrifuge test program that measured the axial load distribution along helical piles. The shaft internal loads were recorded using an innovative strain gauging method. The results show that the axial failure modes of helical piles depend on the strength of soil and inter-helix spacing. In general, it may be easier for a stiffer clay to form an inter-helix soil cylinder during axial pile movement.

Axial Loading Effect on the Behavior of Large Helical Pile Groups in Sandy Soil

Axial Loading Effect on the Behavior of Large Helical Pile Groups in Sandy Soil

The Performance of Taper Helical Pile Embedded in Loose Sand Under Uplift Static and Cyclic Load Using 3D-Finite Element Analysis

Piles with helices are a kind of foundation that is capable of withstanding compression, tension, and lateral loads. However, for almost 25 years, this kind of Pile was widely used across the world. Its behaviour is unpredictable and terrifying, especially in Iraq. The present study analysed this kind of Pile using the finite element method. It was recommended that the helical pile geometry be modeled by numerical model technique and the computer program Plaxis 3D. The plaxis 3D software is a well-known geotechnical engineering tool that numerically analyses soil and simulates experimental work in terms of curve matching and outcomes. Furthermore, an analysis of variables was conducted. The primary variable research investigates the influence of the number of helices and the tapered helix distance under static and cyclic load. The final finding is that the more helices in a pile, the smaller the displacement (or amplitude) in comparison to one helix under the effect of uplift static and cyclic load. As a result that the effect of helix number on soil behaviour is more than the effect of changing the distances between helix.

A new modified method of predicting load-settlement behavior for large-diameter helical piles in sand considering nonlinear degradation of soil stiffness

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. Highlights 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.

Correction to: Assessment of SSI effects on stiffness of single and grouped helical piles in dry sand from large shake table tests

Correction to: Assessment of SSI effects on stiffness of single and grouped helical piles in dry sand from large shake table tests

Seismic response of helical pile groups from shake table experiments

This paper presents and discusses the measured responses of single and grouped helical piles to strong ground motions during a large-scale shake table testing program. The effects of earthquake characteristics (i.e., intensity and frequency content) on the seismic performance of single and grouped helical piles are evaluated from the measured responses. In addition, the performance characteristics of helical pile groups are discussed in terms of the interaction between piles within a group and the contributions of vertical and lateral stiffness of individual piles to the rocking stiffness and the overall capacity of the pile group. Furthermore, the effect of pile head connection to the pile cap (fixed or pinned) on the pile group's response is evaluated and the responses of fixed pile groups (2-bolts connection) and pinned pile groups (1-bolt connection) are compared. Finally, the behaviour of a single pile and a pile within a group is compared in terms of their normalized responses.

Assessment of SSI effects on stiffness of single and grouped helical piles in dry sand from large shake table tests

Assessment of SSI effects on stiffness of single and grouped helical piles in dry sand from large shake table tests

Effect of model scale on helical piles response established from shake table tests

This paper investigates the effect of model scale on seismic response of prototype helical pile (HP) groups employing finite element modelling approach verified by the results of single gravity shaking table models of piles installed in dry and saturated sands. The prototype shaking test configuration was established from model shake table tests by adopting a set of similarity and dimensional laws. The results demonstrated that the scaled models exhibited generally similar responses. The HPs' response in the saturated sand bed was dominated by the rocking behaviour, while the flexural behaviour dominated the response in the dry tests. In addition, the HPs' lateral response followed the lateral soil uniform deformation in the dry tests, while they did not follow the nonuniform soil deformation in the saturated tests. However, the prototype models' axial and lateral responses were different from the results of the shaking table scale. The HP exhibited less scaled lateral deformation, bending moments, and settlement in the prototype models than those at the shaking table scale in the dry and saturated tests. Meanwhile, the prototype helices’ stress was higher than the shaking table helices in the saturated tests and less than them in the dry tests. A set of scaling factors was suggested for extrapolating the soil and HP responses from 1 g shake table model tests. Based on the obtained results, the soil and HP responses may be extrapolated from 1 g shake table model tests employing the following scaling factors: soil displacement increase factor, SDIF = 0.8n in the saturated tests, and SDIF = 0.6n in the dry tests, while the pile displacement increase factor, PDIF = 0.7n in the saturated test and 0.5n in the dry test.

An experimental evaluation of helical piles as a liquefaction-induced building settlement mitigation measure

This study evaluates the application of helical piles to reduce liquefaction-induced foundation settlement and investigates their seismic performance in liquefiable grounds. Two large-scale shake table test series, one without mitigation and one using helical piles, were conducted using the shake table facility at the University of California, San Diego. Each model was extensively instrumented and subjected to two consistently applied shaking sequences. The experimental results indicated reduced excess pore-water pressure generation around the helical pile group, attributed mainly to the densification around the piles during installation. The foundation supported on helical piles underwent almost no foundation differential settlement and tilt. The post-shaking liquefaction-induced settlement mechanisms did not affect the helical pile foundation settlement. Although this study introduced helical piles as a reliable and highly efficient measure to mitigate liquefaction-induced foundation tilt and settlement, the proper design and application of helical piles still need thorough investigation due to possible amplified superstructure response.