Helical Piles Research Articles

Bearing capacity analysis of helical pile foundation on peat

Peat is a kind of soil with a very low bearing capacity and high compressibility. Generally, a building construction on peat is done by using a wooden pile foundation. However, the length of the wooden piles is sometimes limited and causes the friction strength between the soil and wooden piles to became suboptimal. In order to enhance the bearing capacity of the foundation, the cross-sectional area of the foundation needs to be enlarged. One of the solutions for this problem is through helical piles. There are two methods to determine the helical pile`s bearing capacity, i.e. individual bearing and cylindrical shear methods. In this paper, bearing capacity prediction was discussed. A foundation load test was thoroughly done by a constant rate of penetration. This test consisted of compression and tension tests. The result was analyzed by individual bearing and cylindrical shear methods and next compared to each other. The result of the analysis has shown that the individual bearing method was more suitable in predicting helical piles’ bearing capacity since it produced the lowest error rate, with a magnitude of 21,31%.

Study of compressive loading capacities of helical piles using torque method and induced settlements

Torque is a most widely used method to estimate the load capacities of helical piles because of its simplicity and minimal costs. This research attempts to investigate the torque method by incorporating settlements and involves its comparison with bearing equations which have also been used to estimate the capacities of helical piles. The test program consisted of installation of seven helical piles including a static axial compression test to estimate the corresponding settlements. The compression test was conducted on the seventh pile which is referred to as the test pile. The load movement curve of the test pile is used to predict the settlement of each pile corresponding to the ultimate load calculated using torque method. In addition, few methods to interpret the load movement curve have been discussed. This research would pave way to yield more reliable results using torque method and could lead to the large-scale usage of helical piles.

Numerical analysis of helical piles in cohesionless soil

The paper presents a numerical investigation carried out for helical pile installed in loose cohesionless soil with varying density and dimensions of the helical blade/shaft. The analyses are carried out using three-dimensional finite-element software, Plaxis 3D AE. The load–deformation behaviour under axial compression and tension is compared with the results from a case study of field investigation and numerical analysis reported. The model thus calibrated is then used for a parametric study and the results were used to arrive at a set of generic equations to estimate the axial capacities. The investigation shows that relative density of sand plays an important role in the response of ultimate axial capacities towards various geometrical variations. Presence of helical blade and its embedment ratio (H/Dh) were found to influence axial capacity significantly. However, the shaft diameter is found to play a relatively less important role compared to increase in helical blade diameter while considering ultimate axial load capacities.

Field Testing of Axial Performance of Large-Diameter Helical Piles at Two Soil Sites

AbstractHelical piles are being used extensively in engineering applications in western Canada. There are insufficient studies on the axial load-transfer mechanism (ALTM) for helical piles, which r...

Performance assessment of single and group of helical piles embedded in expansive soil

The present work deals with helical piles that extensively used in foundations for structures subjected to both compressive and tensile forces. The behavior of square helical piles models (5 × 5) mm, embedded in expansive soil overlaying a layer of well compacted sandy soil, (single and group of helical piles) were investigated and compared with ordinary piles. Model tests are performed with helical pile length 150, 200, and 300 mm and helix diameters 15 and 20 mm. Helical piles with one helix and double helix were considered. The pattern of helical piles group models consists of four piles in a square arrangement. Thirty-six models of helical pile groups embedded into the expansive soil were performed. It is found that the pile length, number of helix, and helix diameter have a great effect on the amount and rate of uplift movement of helical piles embedded in expansive soil. The deeper helical piles with higher L/D ratios showed greater pullout capacity comparing to the shallower piles. The maximum upward movement of helical piles group is less than that of single pile.

Axial and lateral performance of spun-cast ductile iron helical tapered piles in clay

A novel piling system is proposed to support solar energy panels: a spun-cast ductile iron (SCDI) tapered pile fitted with a single helical plate installed by mechanical torque. The proposed pile combines the efficiency of the tapered cross-section and the construction advantages of helical piles, as well as the competitive cost, effectiveness and durability of SCDI with a rough surface. This paper presents an investigation of its compressive, uplift and lateral performance in clay employing three-dimensional non-linear finite-element models. In addition, simulations of other pile shapes, including tapered and straight shafts, as well as conventional steel helical piles, were performed for comparison purposes. Effects of the pile length, installation technique and the presence of a top crust on the pile performance were investigated. The results demonstrated the feasibility of the proposed pile, especially for applications that impose high uplift and lateral loading. Moment–horizontal force interaction diagrams are provided to aid in design of the proposed pile when subjected to a combination of moment and horizontal loads.

Comparative study on the behaviour of granular pile anchors and helical pile anchors in expansive soils subjected to swelling

Several foundation techniques, such as belled piers, under-reamed piles, helical piles are adopted in expansive soils to avoid the problem posed by its swelling nature. In recent years, the granular pile anchor (GPA) technique has been in practice for resisting the uplift of foundation caused by expansive clay beds. Model and prototype studies on GPA revealed that swelling of expansive clay beds can be effectively controlled by the GPA technique. This paper presents a comparative study on the behaviour of GPA and helical pile anchors (HPAs) in expansive soil subjected to swelling. The data presented in this paper pertain to model tests of clay beds reinforced with varying number of GPAs (N = 0, 1, 2 and 3). Similarly, the model tests were conducted on clay bed reinforced with varying number of HPAs (N = 0, 1, 2 and 3). Tests were also conducted by varying the number of helix. Test results shows that the heave (mm) decreases with an increase in the number of GPAs. Similarly, the heave decreases with an increase in the number of HPA. Further, heave (mm) also decreased with increase in the number of helix. The resultant heave reduction is found to be more in case of expansive soil reinforced with GPA when compared to expansive soil reinforced with helical pile.

Numerical Analysis of Helical Pile–Soil Interaction under Compressive Loads

The results of the field tests of full-scale steel helical piles in clay soils intended for prefabricated temporary buildings foundations are presented in this article. The finite element modeling was used for the evaluation of stress distribution of the clay soil around helical piles. An approach of modeling of the screw-pile geometry has been proposed through the Finite Element Analysis. Steel helical piles with a length of 2.0 m, shaft diameter of 0.108 m and a blade diameter of 0.3 m were used in the experiments. The experiments have shown the efficiency of double-bladed helical piles in the clay soils compared to single-bladed piles. It has been experimentally established that the introduction of the second blade into the pile shaft provides an increase of the bearing capacity in clay soil up to 30% compared to a single-bladed helical pile with similar geometrical dimensions. The numerical results are compared with the measurements obtained by a large scale test and the bearing capacity has been estimated. It has been found that the model results fit the field results. For a double-bladed helical pile it was revealed that shear stresses upon pile loading are formed along the lateral surface forming a cylindrical failure surface.

Developing hybrid artificial neural network model for predicting uplift resistance of screw piles

The pull-out capacity of screw piles is affected by the complex underground and geological conditions, the helical pile’s configuration, and the penetration depth. Several experimental, theoretical, and neural network methods are available to predict the pull-out capacity of these piles. However, the weaknesses of ANN with regard to slow rates of convergence as well as in finding reliable testing outputs with reasonable errors are known to be major drawbacks of implementing ANN-based techniques. The present study aimed to develop an ICA-ANN-based model to estimate the pull-out capacity of screw piles in a simple way. A total of 36 experimental observations were collected and used to train, test, and optimize the ANN using an imperialist competitive algorithm (ICA). The developed ICA-ANN model can be considered an effective method for predicting the ultimate pull-out resistance of the helical screw piles since excellent agreement is obtained with respect to the reliability of the proposed model. The overall (training and testing) errors obtained for the proposed ICA-ANN model in comparison with the experimental data are 0.706, 0.17, and 0.996 for the mean absolute error (MAE), root-mean-square error (RMSE), and correlation factor (CF) respectively.

The performance of grouted and un-grouted helical piles in sand

Helical piles, which have been widely applied as a deep foundation system from small to large load ranges, are very suitable for different kinds of applications. Therefore, concerns about their axial qualification and quantification bearing capacities and performance characteristics seem to be warranted. Tests of full-scale axial compression and tension performed on large capacity helical piles sited in the sand deposit in Babolsar are reported in this study and considerations of design, procedures of installing and full-scale field load test results are discussed. The results in grouted and un-grouted conditions about axial static loading and tension on single-, double- and triple-helix helical piles indicated that in spite of unsuitable cylindrical performance of helical piles in sandy soil which was more confidence-based on resistance, axial compression resistance of the piles increased substantially after grouting, and grouting process had a slighter effect on tensile resistance.

Retraction: Relationship between installation torque and axial capacities of helical piles in cohesionless soils

Retraction: Relationship between installation torque and axial capacities of helical piles in cohesionless soils

Numerical modelling of pullout of helical soil nail

An investigation into the pullout response of helical soil nail using finite element subroutine Plaxis 2D is presented. The numerical modelling of actual pullout response is achieved by axisymmetric and horizontal loading condition. The effect of varying number of helical plates, helical plate spacing and helical plate diameter is studied to understand the pullout capacity behaviour. The failure surfaces for various helical soil nail configurations and their pullout mechanisms are also analysed and discussed. The pullout capacity is found to increase with increase in number of helical plates. The helical plate spacing ratio (s/Dh) and diameter ratio (Dh/Ds) are found to increase the pullout only up to a critical value. The response of helical soil nail using axisymmetric finite element simulation is found similar to the uplift behaviour of helical piles and helical soil anchors. In the absence of literature regarding numerical modelling of helical soil nail, simulation results are validated with uplift responses of helical piles and soil anchors. A good agreement in their comparative study for pullout response is also observed.

Large-diameter helical pile capacity – torque correlations

Large-diameter helical piles are utilized increasingly to support heavy structures. Both the magnitude of the required installation torque and the pile capacity can be directly attributed to the soil shearing resistance developed over the embedded area of the pile including the shaft and helical plates. Hence, the pile capacity can be correlated to installation torque. Such correlations are widely used in the helical pile industry as a means for quality control and quality assurance. In the current study, a total of 10 test piles were installed while monitoring the installation torque continuously with depth. The recorded installation torque profiles were demonstrated to be accurate and repeatable. Field pile load tests were conducted and their results were analyzed to determine the interpreted ultimate capacity of the test piles. The results demonstrate that the ultimate capacity of large-diameter helical piles can be interpreted from pile load test data employing the failure criteria proposed by Elkasabgy and El Naggar in 2015 and Fuller and Hoy in 1970. The measured installation torque and corresponding ultimate capacity values were employed to define torque–capacity correlation (Kt) based on embedded pile area. It was demonstrated that the proposed Kt is suitable for large-diameter helical piles.

Probabilidade de ruína de estacas helicoidais nas fundações de torres de linhas de transmissão

Nos projetos de linhas de transmissão são inúmeras as variabilidades devido ao fato da linha de transmissão consistir em uma obra que pode se estender por vários quilômetros, passando por diversas unidades geológicas e diferentes microclimas. As mudanças de unidade geológicas causam variabilidade na resistência e as mudanças de microclimas inserem variabilidades nas solicitações. Desta forma, neste trabalho faz-se uma avaliação da probabilidade de ruína e da segurança de um trecho da Linha de Transmissão de alta tensão 230 kV executada em sua maioria com torres do tipo estaiadas e fundações em estacas helicoidais. O cálculo das solicitações das torres foi feito pelo software Tower considerando os mapas de vento da região. As resistências foram determinadas com base no torque de instalação das estacas, calibrados a partir de ensaios de arrancamento. Obteve-se como resultados a probabilidade de ruína e os coeficientes de segurança que permitiram concluir que quando as estacas helicoidais são executadas tendo como referência na instalação o torque de projeto a probabilidade de ruína é praticamente nula; porém, quando se toma como referência durante a instalação a profundidade mínima prevista em projeto, a probabilidade de ruína da Linha de Transmissão aumenta consideravelmente.

Soil and ribbed concrete slab interface modeling using large shear box and 3D FEM

Cast in situ and grouted concrete helical piles with 150-200 mm diameter half cylindrical ribs have become an economical and effective choice in Shanghai, China for uplift piles in deep soft soils. Though this type of pile has been successful used in practice, the reinforcing mechanism and the contribution of the ribs to the total resistance is not clear, and there is no clear guideline for the design of such piles. To study the inclusion of ribs to the contribution of shear resistance, the shear behaviour between silty sand and concrete slabs with parallel ribs at different spacing and angles were tested in a large direct shear box (600 mm × 400 mm × 200 mm). The front panels of the shear box are detachable to observe the soil deformation after the test. The tests were modelled with threedimensional finite element method in ABAQUS. It was found that, passive zones can be developed ahead of the ribs to form undulated failure surfaces. The shear resistance and failure mode are affected by the ratio of rib spacing to rib diameter. Based on the shape and continuity of the failure zones at the interface, the failure modes at the interface can be classified as punching, local or general shear failure respectively. With the inclusion of the ribs, the pull out resistance can increase up to 17%. The optimum rib spacing to rib diameter ratio was found to be around 7 based on the observed experimental results and the numerical modelling.

Axial Performance of Helical Tapered Piles in Sand

The axial capacity of novel spun-cast ductile iron (SCDI) tapered pile fitted with a lower helical plate is investigated. Seven instrumented piles, five SCDI tapered and two steel straight shafts, were installed in sand soil using mechanical torque. The piles were tested under axial compressive loading and their ultimate capacities were determined. To assess the cyclic loading effect on the piles performance, two load sequences were adopted: four piles were subjected to monotonic loading, and three were subjected to initial cyclic loading followed by monotonic loading. The installation torque was monitored and the resulting capacity-to-torque ratio was compared to the literature reported values. Tapered helical piles displayed a stiffer response and yielded higher capacities compared to the straight ones. Strain gauges were used to evaluate the piles load transfer mechanism, and demonstrated increased shaft resistance due to the pile taper. The taper helped compact the sand within the zone adjacent to the pile, originally disturbed by the helix penetration, hence increased the soil strength and stiffness. These effects were prominent for larger ratio of shaft/helix diameter. Finally, 3D finite element analyses were conducted to evaluate the axial performance of the system and demonstrated its enhanced frictional resistance. The experimental and numerical results confirmed the superior performance of the proposed system in sands.

Physical and numerical pull-out modelling of ribbed piles

In situ cast concrete piles with helical ribs have been used in Shanghai, China for uplift piles. The piles are about 600 mm in diameter with half-cylindrical ribs of 75 to 100 mm in radius. Pull-out tests on model helical ribbed nylon piles of different rib spacing have been carried out in a large pull-out box under different confining pressures. Pull-out resistance and displacement of the piles revealed that the resistance varies with rib spacing and is at a maximum when the ratio of rib spacing to pile diameter is 1, and the resistance is twice that of a smooth pile with the same outer diameter. Three-dimensional finite-element analysis of the model piles revealed that the inclusion of ribs changed the soil and pile interaction mechanism. Shear wedges may form ahead of the ribs. The size of the wedges varies with rib spacing, and is the largest when the ratio of rib spacing to pile diameter is about 1. The wedges may connect to each other to form larger shear zones around the piles to increase the pile resistance. It is suggested that rib spacing is one of the most important factors that affect the pull-out resistance of the piles with the designed geometry.

A critical review: State of knowledge in seismic behaviour of helical piles

Helical piles are being used in seismic regions of the U.S. and other countries, yet there remains much confusion regarding the state of practice and building codes for this pile type. Nonetheless, it is anticipated that piles with comparatively small cross-section and high anchoring capacity, such as helical piles, could be beneficial for seismic resistance due to their slenderness, higher damping ratios, ductility, and resistance to tip uplift. In addition, helical piles can be easily implemented as a retrofitting solution for foundations that are found to be deficient according to updated seismic codes. This paper is part of three phase investigation on the use of helical piles for earthquake mitigation. The results of an extensive literature and industry search for previous seismic tests performed on helical piles are highlighted as well as the current design standards used in seismic regions. Existing seismic testing results and current design standards are analysed to make recommendations about how ...

FEATURES OF CALCULATION OF UNEVEN SWELLING SCREW PILES OF OIL AND GAS PIPELINES IN FROZEN GROUND

The article is devoted to the problem of buckling of helical piles and deformation of the superstructure of oil and gas pipelines under the action of forces of frost heaving of soils. Analyzed the characteristics of the nonuniform buckling of the piles. Revealed and substantiated the necessity of development of methods for the assessment of uniformity of buckling of piles on the distribution of frozen soils. On the basis of this study offers the calculation formula, taking into account the helical blade of the piles, the results of the calculations, built the graphical dependence, formulates relevant conclusions.

Field Study of Capacity Helical Piles in Sand and Silty Clay

Helical piles have been increasingly used as a deep foundation system for small to large load ranges. They are suitable for various applications, including residential housing, solar farms, utilities, retrofitting projects, power transmission lines, oil facilities, and industrial applications. Hence, many researches have been conducted recently on qualifying and quantifying axial capacities and performance characteristics of helical piles. This paper discusses design considerations, installation procedures, and results of full-scale field load tests. In this study, axial static loading tests on single, double, and triple helix helical piles under grouted and un-grouted conditions have been conducted. The field study has been performed on two different soil types including sand and silty clay soil to investigate the behavior of helical piles. Results showed that in the silty clay soil, grouted and un-grouted helical piles had a similar performance while grouted piles showed greater axial compressive strength. In the sandy soil, cylindrical performance in helical piles was not satisfactory and in triple-helix piles, the axial compressive strength was less than that of double-helix piles even though the number of helixes has been increased.