Open-ended Steel Piles Research Articles

Experimental Study on the Time-Dependent Resistance of Open-Ended Steel Piles in Sand

Open-ended steel piles are commonly used as the foundation for offshore structures. Numerous model and field tests have demonstrated a time-dependent increase in the resistance of these piles, a phenomenon referred to as pile ageing or pile setup. Additionally, for open-ended steel piles with comparably small diameters, soil plugging enhances the resistance against axial compressive loads. Realistically predicting these effects is necessary for their reliable incorporation into design practice. This contribution presents static compression and tension pile load testing conducted in an experimental pit filled with wet, uniformly graded silica sand. In total, twelve piles (L= 5.5 m, Do= 325 mm) were driven into homogeneously compacted sand using a pneumatic impact hammer. Firstly, static compression pile load testing was executed at various times after installation. Subsequently, static tension pile load tests were carried out. The results of the static compression pile load tests indicate that the compressive resistance doubles over an ageing period of 64 weeks. The experimental investigations of the effect of soil plugging showed marginal soil plugging during pile installation, but a significant influence of the soil plug on the compressive resistance.

Soil plugging of open-ended steel piles during impact driving

During the driving process of an open-ended steel pile, soil is allowed to enter the pile’s core until partial or complete plugging occurs. Once this phenomenon is activated, soil resistance to driving (SRD) increases, which can prevent further soil introduction inside the pile, and in turn halt installation works under similar conditions. In this paper, we demonstrate the relationship between the plugging phenomenon, pile geometry, and driving acceleration by comparing literature with some conducted experiments (Paikowsky, 1990; Magroun, 2012), which show that the risk of plugging decreases mainly with an increase in both pile diameter and driving acceleration. A verification study of plugging occurrence in terms of depth is conducted on an open-ended driven pile at a Moroccan port terminal crossing predominantly sandy layers before anchoring in a lower marl layer. The pile’s bearing capacity is evaluated in both plugged and unplugged cases using the SRD method following the processes of Toolan & Fox (1977) and Alm & Hamre (2001), and based on cone penetration test (CPT) results. The pile is thus found unlikely to plug during driving. As a general rule, we found that the plugging phenomenon must be controlled to avoid any interruption in the pile driving process.

A simple continuum approach to predict the drained pull-out response of piles for offshore wind turbines

The article presents a continuum approach to predict the response of pile foundations for jacket-supported offshore wind turbines. Tensile loading conditions are examined, which may be critical for piles used in combination with this structure type, generally adopted to exploit wind energy in intermediate water depths. The approach is developed to guarantee a simple implementation with a limited number of input data easily attainable from cone penetration tests and laboratory tests, and to ensure computational cost-effectiveness. Data from technical-scale tests on open-ended steel piles driven in dense sand and subjected to drained pull-out are used to assess the performance of the approach. The results are shown to be accurate, approximating rather closely the experimental load–displacement curves. The accuracy of the approach is also compared to that obtained with a recently proposed design method, to investigate the predictive capacity of the approach and its potential to support preliminary design activities.

A Small-Scale Experimental Investigation on the Benefits of a Hybrid Pile/Footing System on Sand

This paper explores the capacity effects of a square steel plate welded at the ground surface on a driven open-ended steel pile (i.e. the plate would touch the ground surface after the pile achieves the required penetration). A series of strain-controlled, 1-g small-scale laboratory tests were undertaken on piles with and without a square steel plate attached. The piles were driven in dry, loosely packed, uniform sand. Two plates were used, one with a breadth equal to two times the diameter of the pile (2D) and the other with a breadth equal to three times the diameter of the piles (3D). A 20% increase in capacity was recorded for the 2D plate, and a 110% increase in capacity was recorded for the 3D plate when compared to the pile without an attached steel plate. The back-analysis of the results allowed the derivation of a new expression to calculate the capacity of bearing plates and plot its load-settlement profile, which accounted for the effects of sands compaction and dilation. By extrapolating the findings of these tests to a hypothetical scenario, a model design problem was described where the length of a pile can be reduced by 20% to 60% (depending on the load) by using a plate attached to the pile. The results of this study can help designers to minimise penetration depth; thus, achieving a more economical and sustainable design.

Large-scale model tests of a single pile and two-pile groups for an offshore platform in sand

Estimating pile group efficiency for open-ended steel piles in small group arrangements is a challenging task for designers. This paper reports on the large-scale experimental campaign performed for the BorWin gamma offshore converter platform, which involved single piles and two-pile group systems on a scale of 1:10. The experimental works included installation, dynamic end-of-driving tests, dynamic restrike tests, and static load tests of a single pile and a pair of two-pile groups in densely compacted, artificially prepared homogeneous sand. The cone penetration test (CPT) profiles and the blow counts confirmed that the foundation systems are comparable to each other. The experimental results of the single pile system were compared with conventional design methods. Such comparison indicated that CPT-based methods and load-transfer methods are applicable at the considered model scale. The bearing capacity prediction obtained via the Case Pile Wave Analysis Program (CAPWAP) method is conservative with respect to the static capacity. A consistent setup effect can be detected by analyzing the complete dynamic loading session. The pile group efficiency for the given foundation system was found to be less than 1.0 at both very small and very large soil strains, while it equaled 1.0 at failure.

Influence of Vertical Loading on Behavior of Laterally Loaded Foundation Piles: A Review

The majority of installed offshore wind turbines are supported on large-diameter, open-ended steel pile foundations, known as monopiles. These piles are subjected to vertical and lateral loads while in service. In current design practice, interaction of vertical and lateral loads are not considered, rather piles are designed to resist vertical and lateral loads independently. Whilst interaction effects are widely studied for shallow foundations, the limited research on this topic for pile foundations often produces conflicting results. This paper reviews the research of the influence of vertical loading on the lateral response of pile foundations under combined loads, from the perspective of analytical research, numerical research, and experimental research from tests performed on 1-g (gravitational acceleration) model, centrifuge, and full-scale piles. The potential reasons for the differences among the results of previous research are discussed. Some guidance for future research on the effect of vertical loads on the lateral response of piles is provided.

Factors Influencing the Prediction of Pile Driveability Using CPT-Based Approaches

This paper investigates the applicability of Cone Penetration Test (CPT)-based axial capacity approaches, used for estimating pile static capacity, to the prediction of pile driveability. An investigation of the influence of various operational parameters in a driveability study is conducted. A variety of axial capacity approaches (IC-05, UWA-05 and Fugro-05) are assessed in unmodified and modified form to appraise their ability to be used in estimating the driveability of open-ended steel piles used to support, for example, offshore jackets or bridge piers. Modifications to the CPT-based design approaches include alterations to the proposed base resistance to account for the resistance mobilized under discrete hammer impacts and the presence of residual stresses, as well as accounting for the effects of static capacity increases over time, namely ageing. Furthermore, a study on the influence of various operational parameters within a wave equation solver is conducted to ascertain the relative impact of uncertain data in this respect. The purpose of the paper is not to suggest a new design procedure for estimating pile driveability, rather to investigate the influence of the various operating parameters in a driveability analysis and how they affect the magnitude of the resulting predictions. The study will be of interest to geotechnical design of piles using CPT data.

Field experiments at three sites to investigate the effects of age on steel piles driven in sand

This paper investigates the influences that steel type, in situ soil properties, water table depth, pile diameter, roughness and driving procedures have on the ageing behaviour of piles driven in sand. Tension tests have been performed on 51 open-ended steel micro-piles, with 48 to 60 mm outside diameter, driven at well-established research sites at Larvik in Norway, Dunkirk in France and Blessington in Ireland to better understand the processes that control axial capacity set-up trends in the field. Mild steel, stainless and galvanised steel micro-piles were driven and left to age undisturbed for periods of between 2 h and 696 days before being subjected to first-time axial tension load tests. In addition to reporting and interpreting these experiments, further investigations of the sites’ geotechnical profiles are reported, including new piezocone and seismic cone penetration soundings as well as laboratory tests. Integration with earlier ageing studies at the same sites with larger (340 to 508 mm outside diameter) open-ended steel piles driven to 7 to 20 m embedments and experiments that varied the piles’ initial surface roughness shows that corrosion, pile scale, roughness, the bonding of soil particles and the driving process can all be highly significant. New insights are gained into the mechanisms that control the axial capacity of piles driven in sand.

Field trials on monotonic behaviour of a driven pile at an offshore wind farm in China

The existing database of the loading response of offshore pile foundations needs to be enriched, so that safety and economic considerations can be optimally balanced in design. This paper details a programme of field tests on an open-ended steel pile. The pile with a diameter of 2 m was driven to a depth of 72.8 m in a stratified soil site and then subjected to monotonic compressive, tensile and lateral loadings. Resting periods were allowed before each test to enable strength recovery, and a period of about one month was found to be sufficient (i.e., the axial bearing capacity increases from about 10 MN to 25 MN). The test results were interpreted in terms of the internal forces and displacements/deformations of the pile, and the more detailed pile-soil interactions (i.e., qs-s and p-y curves). It is found that, for cohesive soils, the unit shaft resistance qs remains constant through each soil layer regardless of the almost linearly varying undrained shear strength su obtained from CPTu tests, and the soil layer with overall higher su does not guarantee greater qs owing to different proportions of silty and clayey components. Upon reaching the ultimate uplift load, the shaft resistance within the cohesive soils shows a clear softening response. Extensive comparisons were made between these results and the predictions from existing design methods wherever applicable to discuss the accuracy of each.

Direct interface shear tests on Dunkirk sand

After a field test campaign conducted in Dunkirk (north of France) on open-ended steel piles aiming to study the ageing phenomenon, laboratory scale shear tests were designed to study the behaviour of the sand-steel interface. In order to carry out this laboratory investigation, the direct interface shear apparatus was used for characterizing Dunkirk sand (in dry or unsaturated conditions with about 6% water content as in the field) consolidated on initially smooth mild steel plates at different consolidation time intervals (0, 1 and 7 days) and different consolidation stresses (50, 100, 200 and 300 kPa). The test program also included two normal boundary conditions (Constant normal load CNL and constant volume CV) so that they can be compared to the field results and determine the most approaching configuration. More, the unsaturated condition induced a corrosion of the mild steel plates, causing a layer of sand remaining glued to the plate after removing the shear box. Traces of corrosion were also observed on the lower part of the sand samples (in contact with the plate). These observations lead to the interpretation of an increase of the mechanical properties (local cohesion and increase of the friction angle). In order to follow the evolution of the corrosion for each plate, thickness measurements of the sand layer stuck on the plates were carried out.

Artificial neural network (ANN) approach for modelling of pile settlement of open-ended steel piles subjected to compression load

This study was devoted to examine pile bearing capacity and to provide a reliable model to simulate pile load-settlement behaviour using a new artificial neural network (ANN) method. To achieve the planned aim, experimental pile load test were carried out on model open-ended steel piles, with pile aspect ratios of 12, 17, and 25. An optimised second-order Levenberg-Marquardt (LM) training algorithm has been used in this process. The piles were driven in three sand densities; dense, medium, and loose. A statistical analysis test was conducted to explore the relative importance and the statistical contribution (Beta and Sig) values of the independent variables on the model output. Pile effective length, pile flexural rigidity, applied load, sand-pile friction angle and pile aspect ratio have been identified to be the most effective parameters on model output. To demonstrate the effectiveness of the proposed algorithm, a graphical comparison was performed between the implemented algorithm and the most conventional pile capacity design approaches. The proficiency metric indicators demonstrated an outstanding agreement between the measured and predicted pile-load settlement, thus yielding a correlation coefficient (R) and root mean square error (RMSE) of 0.99, 0.043 respectively, with a relatively insignificant mean square error level (MSE) of 0.0019.

Physical modelling for pile performance combined with ground improvement using frustum confining vessel

Piles were physically modelled using a frustum confining vessel device made in Amirkabir University of Technology, to examine the effect of density and soil improvement from grouting on piles. Piles scales were ranged from ∼13 to 19. The results of pile tests on Babolsar sand under different compaction (density) states were used to study the effects of soil improvement on pile performance. Increasing the soil–pile interaction by post-grouting of the base and sides of the piles also improved the soil capacity. Post-grouting of the base was applied for bored piles and post-grouting of the side was applied for precast concrete piles. According to the results by soil densification, capacity increase from loose to medium state is more than from medium to dense soils. Bored concrete piles showed a greater increase in bearing capacity than steel piles under compression testing as the capacity reduction factors due to installation were minimised. In tensile tests, the capacity increase of open-ended steel piles is more than that of concrete piles because of more surfaces of soil–pile interaction. Post-grouting increased bored and precast concrete pile performance by enhancing soil–pile interaction and soil densification around the pile.

A full-scale field study for performance evaluation of axially loaded large-diameter cylinder piles with pipe piles and PSC piles

This paper presents the results from a pile load testing program for a bridge construction project in Louisiana. The testing includes two 54-in. open-ended spun cast concrete cylinder piles, one 30-in. open-ended steel pile and two (30- and 16-in.) square prestressed concrete (PSC) piles driven at two locations with very similar soil conditions. Both cone penetration tests (CPTs) and soil borings/laboratory testing were used to characterize the subsurface soil conditions. All the test piles were instrumented with vibrating wire strain gauges to measure the load distribution along the length of the test piles and measure the skin friction and end-bearing capacity, separately. Dynamic load tests were performed on all test piles at different times after pile installations to quantify the amount of setup with time. Static load tests were also performed on the PSC and open-ended steel piles. Due to expected large pile capacities, the statnamic test method was used on the two open-ended cylinder piles. The pile capacities of these piles were evaluated using various CPT methods (such as Schmertmann, De Ruiter and Beringen, LCPC, Lehane et al. methods). The result showed that all the methods can estimate the skin friction with good accuracy, but not the end-bearing capacity. The normalized cumulative blow counts during pile installation showed that the blow count was always higher for the PSC piles compared to the large-diameter open-ended cylinder pile, regardless of pile size and hammer size. Setup was observed for all the piles, which was mainly attributed to increase in skin frictions. The setup parameters “A” were back-calculated for all the test piles and the values were between 0.31 and 0.41.

Field validation of fibre Bragg grating sensors for measuring strain on driven steel piles

In recent years, fibre Bragg grating (FBG) sensors have emerged as a relatively new strain sensing technology for civil engineering applications. This paper presents a field trial to assess the feasibility of using FBG sensor arrays to measure strain in driven steel piles. Two FBG arrays were installed in grooves within the wall of an open-ended steel pile such that the finished profile was completely flush with the pile shaft. The pile was then driven into a dense sand deposit using an impact hammer to provide the required installation energy. The FBG gauges were monitored throughout driving in conjunction with accelerometers to quantify the scale of the hammer impacts. The FBG sensors were subjected to hammer blows that yielded pile accelerations between 500 g and 1400 g during installation. The fibre optic sensors were measured throughout driving, where they were observed to respond to the hammer impacts, showing a rapid increase in strain and a return to their initial values between hammer strikes. After installation, a lateral load test was performed with independent load measuring devices. Excellent agreement was observed between the measured moments and those inferred from the FBG strain output. The output of this trial demonstrates that FBG strain sensors are a viable means of measuring load transfer in foundation systems and are suitably robust to withstand high pile driving accelerations.

The effect of ageing on the axial capacity of piles in sand

Open-ended steel pipe piles are used to support jacket structures in the offshore wind sector. These piles experience significant tensile axial loading in-service and the tensile shaft resistance thus governs their design. Although pile ageing (increased shaft resistance with time) has been noted by a number of workers and incorporation in design could lead to significant efficiencies, there is a dearth of high-quality field test data that measure its effects. This paper presents the results from an experimental investigation designed to examine the effect of ageing on the tension shaft resistance developed by open-ended piles in sand. As part of this investigation, four 340 mm diameter open-ended steel piles are driven 7 m into a dense sand deposit in Blessington, Ireland. Each pile is subjected to a series of static axial tension load tests at different time intervals after driving and the effects of ageing are assessed. The tension capacity of the piles is seen to increase by as much as 185% over a period of 7 months after driving.