THE BEHAVIOUR OF LATERALLY LOADED HELICAL PILES. A. Sanzeni1 1 ABSTRACT

Transcription

1 THE BEHAVIOUR OF LATERALLY LOADED HELICAL PILES A. Sanzeni1 1 ABSTRACT Helical piles offer a cost effective solution for the construction of many diverse structures in the fields of civil engineering and transportation due to their low material and production costs, fast installation in most soils and environmental sustainability. Steel helical piles can be used in the installation of road guard rails, noise barriers along motorways, street lightning, solar power plants, fences and construction of lightweight structures of sorts. For these applications the typical size range of such piles is 76 to 409 mm diameter ( to 16.0 inch), and to 5.0 m embedment length (8.3 to 16.7 foot). Moreover for such applications, in which axial forces are relatively small and lateral load capacity may constitute the limiting design condition, the pile can feature multiple sections, with large diameters near to the ground surface (where most of the lateral load is supported by the surrounding soil) and small diameters at the base of the pile to allow for easy installation (the tip of the pile is equipped with one or more helixes). Although a number of experimental and theoretical studies on the response of helical piles under axial loads is available in the literature (Das, 1978, 1980; Narasimha et al., 1991,1993; Merifield et al. 2003, 2011), there is a limited number of contributions on the behaviour of such short piles under lateral loads. The popular method developed by Broms (1964) for estimating the lateral capacity of a pile is based on the limit equilibrium analysis assuming rigid perfectly plastic soil and pile behaviour; thus it cannot be used to describe the response of the pile before failure and is limited to conditions of uniform soil along the pile shaft and constant pile section. The definition of the lateral capacity and the description of the response of the pile before failure are truly a matter of soil-pile interaction in which stiffness and strength properties of both pile and soil must be considered. The paper examines the lateral capacity of helical piles based on the finite difference method. The results of full-scale in-situ lateral load tests on prototype piles installed in natural soils and backfills at different locations in Northern Italy, were compared to the results of numerical simulations, performed with a software program that solves the differential equation for a beam-column using nonlinear lateral loadtransfer (p-y) curves, using soil strength and stiffness parameters obtained from routine geotechnical investigation campaigns. The results showed that the software program used for simulating the tests can reproduce the in-situ load-deflection curve and predict the ultimate lateral capacity of the pile-soil system as a result of a pushover analysis. Therefore, once the model is calibrated with experimental results, it can be used for improving and optimizing the pile design (upper and lower diameters, embedment length, 1 Research Assistant, DICATAM, University of Brescia, Brescia, Italy, alex.sanzeni@unibs.it

2 A. Sanzeni length of the pile with constant diameter, steel pipe thickness). In comparison the method developed by Broms, fed with input data selected to represent the soil stratigraphy and pile properties, always significantly underestimated the lateral capacity of the pile. The comparison between results of in-situ prototype tests and numerical simulations also demonstrated the significant influence of the installation procedure and soil disturbance. Keywords: helical piles, lateral load, pile foundation, numerical analysis

3 THE BEHAVIOUR OF LATERALLY LOADED HELICAL PILES Alex Sanzeni, Research assistant, University of Brescia, DICATAM, ABSTRACT: The paper examines the lateral capacity of innovative models of steel helical piles ( mm diameter, 1.3- m embedment length). The results of full-scale in-situ lateral load tests on prototype piles installed at different locations in Northern Italy, were compared to the results of numerical simulations, performed with a software program that solves the differential equation for a beam-column using nonlinear lateral loadtransfer (p-y) curves. The comparison between observations and simulations demonstrated the capability of the numerical models to reproduce the in-situ load-deflection curve and to predict the ultimate lateral capacity of the pile-soil system, and highlighted the importance of the pile installation process. The computer simulation can be employed for improving and optimizing the pile design. INTRODUCTION Helical piles offer a cost effective solution for the construction and installation of many diverse structures in the fields of civil engineering and transportation due to their low material and production costs, fast installation in most soils and environmental sustainability. Steel helical piles can be used in the installation of road guard rails, noise barriers along motorways, street lightning, solar power plants, fences and construction of lightweight structures of sorts. For these applications in Europe and U.S. the typical size range of such piles is 76 to 409 mm diameter ( to 16.0 inch), and to 5.0 m embedment length (8.3 to 16.7 foot). Moreover for such applications, in which axial forces are relatively small and lateral load capacity may constitute the limiting design condition, the pile can feature multiple sections, with large diameters near to the ground surface (where most of the lateral load is supported by the surrounding soil) and small diameters at the base of the pile to allow for easy installation (the tip of the pile is equipped with one or more helixes). Although a number of experimental and theoretical studies on the response of helical piles under axial loads is available in the literature, there is a limited number of contributions on the behaviour of such short piles under lateral loads. The definition of the lateral ultimate capacity of the soil-pile system and the description of the response of the pile before failure are truly a matter of soil-pile interaction in which stiffness and strength properties of both the pile and the soil must be considered. HELICAL PILES UNDER AXIAL AND LATERAL LOAD Brief literature review A large number of experimental and theoretical studies in the literature is dedicated to the axial capacity of helical piles [1,2,3], so that bearing capacity and uplift resistance are well understood and can be easily evaluated. The calculation of the axial capacity can be conducted by assuming some pre-defined failure schemes, depending on two main factors: the ratio between helix diameter and distance between helixes along the pile shaft; and the ratio between the helix diameter and the depth of the most superficial helix from the ground surface. The first parameter allows to distinguish between individual versus global failure mechanisms, whereas the second parameter will allow to distinguish between shallow or deep failure mechanism, in case uplift force is applied to the pile. Regarding the response of such piles under lateral loads, only a few studies in the literature focus on

4 A. Sanzeni the contribution of the helixes on the pile lateral capacity [4,5,6,7]. In 1996 Prasad and Rao [4] presented the results of an experimental study to quantify the contribution of helixes to the lateral ultimate capacity of model piles in clay. The piles were 513 mm long and had a number (0, 2 and 4) of 33 mm diameter helixes installed on a 13.8 mm diameter steel pipe. The piles were slowly screwed into a laboratory cylindrical test tank (500 mm diameter, 600 mm height) filled with soft to medium consistency marine clay; lateral loading was achieved by means of adding cast-iron weights to a load hanger attached to a wire passing over a pulley and it was measured with a load cell. Test results indicated that the lateral capacity of the piles increases with the ratio L/d (L: length of the pile, d: diameter of the pile), and with the undrained strength of the clayey soil. The capacity of helical piles was 1.2- times greater than the capacity of the straight shaft (with no helix) and increasing with the number of plates. Fuelled by these encouraging results the authors also developed a theoretical model to compute lateral capacity of a helical pile in clay, based on suggestions by Poulos and Davis [8], including the bearing capacity, uplift resistance and frictional contributions of helixes as the pile rigidly rotates and moves horizontally. In 2009 and 2010 Sakr [5,6] presented the results of a comprehensive experimental campaign conducted on true scale helical piles (for residential and industrial facility constructions) subjected to lateral and axial loads. Tests were executed in winter and summer in several sites located in Alberta (Canada); piles were mm long, mm diameter, equipped with one or two mm diameter helixes installed mm above the pile base. The soil profile at the tests sites comprised dense sands as well as glacial till and stiff clay till. The results indicated only marginal differences (if not none) in terms of lateral capacity between the piles with one or two helixes. Another similar experience has been recently reported by Seider and Chisholm in 2012 [7]. The authors were interested in the application of helical piles for the construction of solar power plants and presented data obtained from three test sites in North America, with each soil condition being different. Piles were mm long, mm diameter, equipped with one mm diameter helix welded near to the base of the piles. In both true scale experimental studies the authors compared the results of load tests with numerical simulations executed with a commercial software neglecting the presence of the helixes. Based on the experimental observations one can conclude that the effect of helixes on the lateral resistance of true scale piles can be regarded as negligible, if the plates are located near the base of the pile where lateral movements and rotations are minimal. Analytical methods to study laterally loaded piles Among the most popular theories for estimating the response of piles under lateral loads, the method developed by Broms (1965) [9] is based on the limit equilibrium analysis, assuming the pile and the surrounding soil as perfectly plastic materials; the pile has constant diameter with depth and the soil around the shaft is supposed to be homogeneous. The method is intended to provide an estimate of the limiting value (at failure) of the pile-soil system resistance to lateral loads, it does not allow the description of the pile response before failure and, moreover, it is not applicable to conditions of layered soil deposit or changing diameter with depth. The definition of the lateral capacity and the description of the response of the pile before failure are truly a matter of soil-pile interaction in which stiffness and strength properties of both pile and soil must be taken into account. One of the most popular methods to study the behaviour of piles under lateral loads is the socalled p-y or load-transfer-curve method (p: pressure; y: lateral deflection), developed by Reese and co-workers, which enables to solve the governing equation for the deflection of a laterally loaded pile with a finite difference solution. The method is capable of describing the pile response to lateral loading in non-linear elastic soil and can incorporate the effects of axial loads and variations of pile stiffness with depth. A full description of the resulting equations was given by Reese in 1977

5 [10] who also described a computer program that solves these equations. PROTOTYPE PILES A comprehensive experimental campaign has been carried out, with the financial support of an italian company aiming to develop innovative models of helical piles, to investigate the behaviour of such foundations under axial and lateral loads. This paper focuses on the response of piles to lateral loads and the results of five in situ load tests performed at three test sites in Northern Italy are presented and discussed. The tested prototypes can be grouped in two classes, named lux models and cargo models respectively (see schematic views in Fig. 1 and Fig.2). Lux models were designed for applications such as street lightning. Each prototype consists of an upper shaft where the lamp post and the power cables are conveniently installed. The diameter and length of the upper shaft of the pile were selected to maximize the volume of resisting soil under lateral loads (generated by wind and eccentric loads of sort). The upper shaft is connected by means of a short conical element to the lower shaft of the pile which features a mm diameter, 3-4 mm thick steel pipe with one or more helixes welded near the base (Fig. 1). All pile elements (upper and lower shaft, connecting element and helixes) are welded in the controlled environment of the production facility and the pile is subjected to a galvanizing process to improve durability. Two prototypes of lux pile were tested, with 1.3 and 1.8 m total embedment length, upper shaft diameter 219 and 406 mm, and 500 and 800 mm length respectively (Table 1). The cargo model is intended as general purpose helical pile for the construction of lightweight structures and sound barriers along motorways. The cargo pile is made of two main elements: the upper shaft is m long and it is made of a 4 mm thick, 219 mm diameter steel pipe; the lower shaft is made of a 0.85 long, 4 mm thick, 76 mm diameter steel pipe. Upper and lower pipes are connected by a m long steel conical element and one or two helixes are welded at the tip of the pile and at the base of the conical connection element (Fig. 2, Table 1). Fig. 1 lux prototype pile, schematic view and lamp post installation Fig. 2 cargo prototype pile, schematic view and loading configuration

6 A. Sanzeni Table 1 Relevant geometrical features of prototype piles (as indicated in Fig. 1) Measurement (mm) Lux medium Lux short Cargo L D L L d d p EXPERIMENTAL CAMPAIGN AND NUMERICAL SIMULATIONS Test sites and procedures The experimental testing campaign (Table 2) took place in three different test sites near the cities of Verona and Padua, in Northern Italy: a) Gazzo Veronese (2 lux piles tested; b) Bovolone (2 cargo pile tested); c) Ospedaletto Euganeo (1 cargo pile tested). At the test site of Gazzo Veronese the soil profile consisted of a superficial m thick layer of clayey soil with undrained strength of approximately 70 kpa (obtained by TxUU and unconfined compression tests), followed by a medium dense predominantly sandy layer with (CPT estimated) friction angle of 36 ; the water table is located -1.2 m below the ground surface. The test site of Bovolone is characterized by the presence of a 0.8 m thick layer of compacted artificial sand with friction angle 35-38, followed by layers of sandy silts and silty clay with low plasticity and undrained strength of kpa, no water table was found to a depth of 3,0 m below ground surface. Test #3 was executed on a pile driven into a m thick embankment of homogeneous medium-loose silty sand (3% gravel, 70% sand, 27% silt) with average friction angle of 33. Ospedaletto Euganeo test site was investigated by means of a mechanical CPT test and the soil profile consisted of 0.6- m of stiff clay (undrained strength kpa) followed by a m thick layer of sand (con resistance qc = 5 MPa) and another m thick layer of medium stiff clay (with undrained strength of 80 kpa); the water table was located 3,0 m below ground level. Regarding the installation procedures it must be noted that, due to the dimensions and limited experience of the operator the lux pile prototypes tested in Gazzo Veronese were installed (before the author s involvement) at the bottom of a previously excavated trench, -0.8 m deep and 0.6 m by 0.6 m large. After driving the pile to the required depth, the space between the shaft and the excavated soil was manually filled and poorly compacted; therefore the soil around the upper shaft of the pile must be regarded as highly disturbed. In the light of the results of the first loading tests, all other prototypes were installed after the execution of a -0.6 m deep pre-boring, drilled by an auger with approximately the same diameter of the pile, in order to minimize soil disturbance. Pile testing was executed 24 hours after installation following the procedure suggested by the Italian National Research Council (CNR) bulletin n. 191 (issued in 1989) (in turn similar to ASTM D3966 Standard test method for piles under lateral load). Lateral load increments were provided by hydraulic jacks, load tests were executed with two or three loading cycles and horizontal deflection was measured at the head of the pile and at the top of a m steel beam connected to the pile head (for cargo piles, when installed, Fig. 2). Table 2 summarizes the experimental campaign, indicating where each prototype pile was tested and how the lateral load was applied (at the pile head or at the top of a m HEA160 steel beam extension). Table 2 Experimental campaign Test # Test site Prototype Load application 1 Gazzo V. Lux M. Pile head 2 Gazzo V. Lux S. 3 Bovolone Cargo 4 Bovolone Cargo Steel beam 5 Ospedaletto E. Cargo Steel beam Numerical simulations To improve understanding of the prototypes performance, the lateral load tests were simulated with the LPILE computer program (version 2013) developed by Ensoft Inc. (Austin, Texas, US), a special-purpose software based on rational procedures for analysing a pile under lateral loading using the p-y method [11]. LPILE solves

7 the differential equation for a beam-column using nonlinear lateral load-transfer (p-y) curves. The program computes deflection, bending moment, shear force and soil response over the length of the pile. Nonlinear lateral load-transfer from the pile to the soil is modelled using p-y curves generated internally using published recommendations for various types of soils. Special procedures are programmed for computing p-y curves for layered soils and for rocks (alternatively, externally generated p-y curves can be employed). A number of pile-head boundary conditions may be selected and the structural properties of the pile can vary as a function of depth. LPILE has analytical features to compute the nonlinear moment-curvature relationships and nominal moment capacity of a pile s section based on specified pile dimensions and nonlinear material properties. The program is capable of simulating the execution of a load test (only monotonic load) and optionally performs a push-over analysis to evaluate ultimate lateral capacity. TEST RESULTS AND SIMULATIONS Figures 3-12 present the results of in situ lateral load tests, the comparison between measurements and numerical simulations in terms of pile head lateral deflection, and computed deflection and bending moment versus pile depth for each loading step. Table 3 summarises the most relevant test results in terms of lateral pile resistance and lateral deflection. It is worth noting that ultimate lateral load is usually specified to satisfy a limiting lateral deflection criterion. In the absence of such a criterion, Table 3 reports values of lateral resistance defined as the load that corresponds to a lateral deflection of 1 mm (arbitrarily chosen value). Table 4 reports estimated values of ultimate lateral capacity obtained by push-over analyses (LPILE) and by Broms theory (in this case, regardless of the amount of lateral deflection involved). To overcome Broms theory main limitations some simplifying assumption were adopted, namely: a) upper shaft diameter was assumed as the reference diameter for computing soil resistance upon lateral loading; b) soil around the pile was supposed homogenous, with mechanical properties of the soil surrounding the upper shaft; c) pile moment capacity was assumed to be governed by the structural properties of the lower shaft. The lateral resistance of test piles #1 and #2 measured at 1 mm deflection was 7.8 kn and 5.0 kn (for the medium and short model respectively) and the observed behaviour of piles appeared to be highly affected by the installation process. Fig. 3 and Fig. 5 show the comparison between measured and numerical load-deflection curves computed using undisturbed and remoulded soil parameters: the simulations indicated that in order to reproduce the observed behaviour the resistance and stiffness parameters of the superficial 0.6 m soil layer had to be reduced by 70% approximately, a value that is considered realistically consistent with the mode of installation adopted. Fig. 4 and Fig. 6 show the piles deflection and bending moment versus depth (computed with remoulded soil parameters) and indicate that structural failure at the highest lateral loads may occur in the lower shaft (60.3 and 76.1 mm diameter respectively). Test #3 and #4 were conducted on cargo prototypes in Bovolone test site; pile #3 was installed in a homogenous embankment of medium-loose silty sand, pile #4 was installed in a 0.8 m thick layer of compacted sand, followed by layers of sandy silts and silty clay. The lateral capacity at 1 mm deflection was 1 kn and 25.0 kn respectively, and the comparison between the observed loading curves demonstrate the relevance of the mechanical properties of the soil around the upper shaft. Also, a different installation procedure was adopted here (preboring), therefore numerical simulations were capable of reproducing the observed behaviour without reducing soil parameters. As Fig. 8 and Fig. 10 suggest, the structural properties of the steel pipe adopted for the lower shaft (between and m below ground level) may contribute to the failure mechanism at large deflections. Test #5 was conducted on another cargo prototype, installed in a natural, predominantly

8 A. Sanzeni clayey soil. The lateral capacity at 1 mm deflection was 27.6 kn and the loading curve presents a smooth transition from nearly elastic to plastic response (Fig. 11). Again, the installation procedure allowed for a fairly faithful reproduction of the test by numerical simulation (performed with undisturbed soil parameters) (Fig. 11 and Fig. 12). Table 4 reports the results of analyses performed to evaluate the ultimate lateral capacity of the pilesoil system; the results were obtained by push-over analyses and by Broms theory, regardless of the lateral deflection involved (the analyses of pile test #1 and # were performed with remoulded soil parameters, derived by the comparison presented in Fig. 3 and Fig. 5, to take into account the effect of the pile installation). The comparison between lateral resistances measured at the end of the tests and ultimate resistances (by push-over analysis) confirm that the tests have mobilized almost all the capacity of the piles. In contrast, Broms method (applied with the over-simplifying assumptions previously described) greatly underestimated the capacity of the piles to support lateral loads and the results confirmed the non-applicability of the method to such unusual loading conditions and pile features. Lateral load (kn) load test, "lux medium", Gazzo Veronese (VR) LPILE, undisturbed soil parameters LPILE, reduced (disturbed) soil parameters Pile head lateral deflection (mm) Fig. 3 Load test #1: measurements and numerical simulations, prototype lux medium, Gazzo Veronese test site a) Pile deflection (mm) H = kn H = kn H = kn H = 4.5 kn H = 6.0 kn H = 7.5 kn H = 9.0 kn H = 1 kn H = 1 kn Bending moment (knm) H = kn H = kn H = kn H = 4.5 kn H = 6.0 kn H = 7.5 kn H = 9.0 kn H = 1 kn H = 1 kn b) Fig. 4 Test #1: computed lateral pile deflection (a) and bending moment (b) versus depth for each loading step Lateral load (kn) load test, "lux short", Gazzo Veronese (VR) LPILE, undisturbed soil parameters LPILE, reduced (disturbed) soil parameters Pile head lateral deflection (mm) Fig. 5 Load test #2: measurements and numerical simulations, prototype lux short, Gazzo Veronese test site

9 a) Pile deflection (mm) H = kn H = kn H = kn H = kn H = kn H = 4.0 kn H = 5.0 kn H = 6.0 kn Bending moment (knm) H = kn H = kn H = kn H = kn H = kn H = 4.0 kn H = 5.0 kn H = 6.0 kn b) Fig. 6 Test #2: computed lateral pile deflection (a) and bending moment (b) versus depth for each loading step Lateral load (kn) load test, "cargo" on embankment, Bovolone (VR) LPILE, undisturbed soil parameters Pile head lateral deflection (mm) Fig. 7 Load test #3: measurements and numerical simulation, prototype cargo (driven into a homogeneous silty sand embankment), Bovolone a) Pile deflection (mm) H = kn H = 4.2 kn H = 8.4 kn H = 12.6 kn H = 16.9 kn H = 21.3 kn H = 25.7 kn H = 30.2 kn Bending moment (knm) H = kn H = 4.2 kn H = 8.4 kn H = 12.6 kn H = 16.9 kn H = 21.3 kn H = 25.7 kn H = 30.2 kn b) Fig. 8 Test #3: computed lateral pile deflection (a) and bending moment (b) versus depth for each loading step Lateral load (kn) load test, "cargo", Bovolone (VR) LPILE, undisturbed soil parameters Pile head lateral deflection (mm) Fig. 9 Test #4: measurement and numerical simulation, cargo (driven into natural soil covered by 0.8 m compacted sand layer), Bovolone

10 A. Sanzeni a) Pile deflection (mm) H = kn; M = knm H = 4.2 kn; M = 4.3 knm H = 8.4 kn; M = 8.7 knm H = 12.6 kn; M = 1 knm H = 16.9 kn; M = 17.4 knm H = 21.3 kn; M = 2 knm H = 25.7 kn; M = 26.5 knm H = 30.2 kn; M = 31.1 knm H = 34.6 kn; M = 35.6 knm H = 39.0 kn; M = 40.2 knm Bending moment (knm) H = kn; M = knm H = 8.4 kn; M = 8.7 knm H = 16.9 kn; M = 17.4 knm H = 25.7 kn; M = 26.5 knm H = 34.6 kn; M = 35.6 knm H = 4.2 kn; M = 4.3 knm H = 12.6 kn; M = 1 knm H = 21.3 kn; M = 2 knm H = 30.2 kn; M = 31.1 knm H = 39.0 kn; M = 40.2 knm b) Fig. 10 Test #4: computed lateral pile deflection (a) and bending moment (b) versus pile depth for each loading step Lateral load (kn) load test, "cargo", Ospedaletto Euganeo (PD) LPILE, undisturbed soil parameters Pile head lateral deflection (mm) Fig. 11 Load test #5: measurement and numerical simulation, prototype cargo, Ospedaletto Euganeo (PD) test site a) Pile deflection (mm) H = kn; M = knm H = 4.2 kn; M = 4.3 knm H = 8.4 kn; M = 8.6 knm H = 12.6 kn; M = 12.9 knm H = 16.9 kn; M = 17.0 knm H = 21.3 kn; M = 21.8 knm H = 25.7 kn; M = 26.3 knm H = 30.2 kn; M = 30.8 knm H = 34.6 kn; M = 35.3 knm H = 39.0 kn; M = 39.8 knm H = 43.4 kn; M = 44.3 knm H = 47.9 kn; M = 48.8 knm Bending moment (knm) H = kn; M = knm H = 8.4 kn; M = 8.6 knm H = 16.9 kn; M = 17.0 knm H = 25.7 kn; M = 26.3 knm H = 34.6 kn; M = 35.3 knm H = 43.4 kn; M = 44.3 knm H = 4.2 kn; M = 4.3 knm H = 12.6 kn; M = 12.9 knm H = 21.3 kn; M = 21.8 knm H = 30.2 kn; M = 30.8 knm H = 39.0 kn; M = 39.8 knm H = 47.9 kn; M = 48.8 knm b) Fig. 12 Test #5: computed lateral pile deflection (a) and bending moment (b) versus pile depth for each loading step Table 3 Lateral load test results Lateral resistance (kn) Lateral Test # At deflection 1 mm At end of test at end of deflection test (mm) Table 4 Estimated ultimate lateral capacity of prototype piles Test H ultimate (kn) # Pile type Push-over Broms analysis theory 1 Lux M Lux S Cargo Cargo Cargo 5 2

11 CONCLUSIONS The experimental campaign and the numerical simulations provided a valuable insight on the behaviour of these innovative piles under lateral loading. The observation of in situ, full scale, loading tests and the comparison between measured and computed pile response highlighted the relevance of the soil condition around the upper shaft and of the mode of installation. Once validated by the comparison with the observed behaviour, the numerical models provided further understanding of the response of the prototypes under lateral loading below the ground level. All tests (supported by the results of numerical analyses) indicated a flexible behaviour of the piles under lateral loading; computed deflection and bending moment versus depth suggested that the structural properties of the steel pipe adopted for the lower shaft may contribute to the failure mechanism at large deflections. The comparison between lateral resistance measured at the end of the tests and the ultimate resistance estimated with Broms theory (fed with over-simplifying assumptions) confirmed the non-applicability of the method to such unusual loading conditions and pile features. The simple numerical models developed in this research to support the experimental evidence of in situ loading tests represented a useful tool and can be employed to help the design optimization of prototypes. Acknowledgments The research was conducted with the financial support of the Ecobuilding srl company. The Author wishes to thank Giuliano Usvardi and Pier Luigi Subitoni for providing the test results, Paolo Crescini for the fruitful collaboration and Diego Panizza for performing the numerical simulations. 2. Narasimha Rao, S., Prasad, Y.V.S.N., Veeresh, C. (1993), Behaviour of embedded model screw anchor in soft clays, Géotechnique, 43(4), Merifield, R.S (2011), Ultimate uplift capacity of multiplate helical type anchors in clay, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 137, Prasad, Y.V.S.N. and Rao, S. (1996), Lateral capacity of helical piles in clay, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 122, Sakr, M. (2009), Performance of helical piles in oil sand, Canadian Geotechnical Journal, 46, Sakr, M. (2010), Lateral resistance of high capacity helical piles case study, Proc. 63 th Canadian Geotech. and 6 th Canadian Permafrost Conference, Calgary, Alberta, Seider, G. and Chisholm, J. (2012), Lateral capacity of helical piles actual vs. theoretical foundations for solar power plants, GeoCongress 2012, Oakland, California, Poulos, H.G. and Davis, E.H. (1980), Pile Foundation Analysis and Design, John Wiley and Sons, New York. 9. Broms, B.B. (1965), Design of laterally loaded piles, Proc. ASCE, Journ. Soil Mech. Found. Div., 91(SMM), Reese, L.C. (1977), Laterally loaded piles: Program documentation, Journal Geotech. Eng. Division, ASCE, 103(GT4), Ensoft Inc. (2013), LPile 2013: A program for analysing stress and deformation of individual piles or drilled shafts under lateral load, Version [computer program], Ensoft Inc., Austin, Texas. REFERENCES 1. Das B.M. (1980), A procedure for estimation of ultimate uplift capacity of foundations in clay, Soils and Foundations, 20(1),