Pile Tests Research Articles

On the installation effects of open ended piles in chalk

Chalk is a highly porous rock formed by cemented calcite grains. It covers areas of the UK and is widespread under the North Sea where offshore wind turbines (OWT) are currently being installed and where future offshore expansion will be sited (Figure 1(a)) [1]. Large piles are often driven in chalk to support OWT. The installation process causes the intact rock below the pile tip to crush into a putty characterised by a mechanical behaviour very different from the intact chalk. The difficulty to predict the final state of the putty and the stress around the pile after installation is the underlying reason for inadequate current design guidance for piles in chalk. Considering that for OWT, foundations account for 20-25% of the total development cost , pile design improvements in chalk, would be extremely beneficial from an economical and environmental perspective.
 Current guidelines for the design of piles in chalk (CIRIA C574) originate from the analysis of a limited number of pile tests [2]. These guidelines suggest crude average ultimate unit shaft friction (tsf) design values of between 20 and 120 kPa for low-medium density and high/very-high density chalk, respectively. tsf estimates are thought to be conservative hence introducing significant increases in cost and carbon footprint (Figure 1(b)). Reducing the level of conservatism (e.g. enabling more confident use of higher tsf) would reflect in significant savings of steel and consequent reduction of the cost and embodied carbon. Such design considerations are possible but require a better understanding of the long-term mechanical behaviour of the damage processes intact chalk experiences during dynamic installation in hydro-mechanical (HM) coupled conditions.
 In this work the coupled HM effects developing during pile installation in chalk are investigated numerically using a robust and mesh-independent implementation of an elasto-plastic constitutive model at large strains. The model, implemented into an open-source Geotechnical Particle Finite Element (G-PFEM) code [3], is shown to be able to capture the damage of the rock until the formation of a chalk putty layer around the shaft of a model piles jacked in chalk. In particular the complex flow processes occurring in the soil around both open and closed ended piles of variable shape jacked into saturated chalk are investigated. A fully coupled hydro-mechanical formulation, based on regularized, mixed low-order linear strain triangles is used [5]. To capture the relevant features of the mechanical response of chalk, a finite deformation, non-associative structured modified cam clay is used [6]. The model formulation is based on a multiplicative decomposition of the deformation gradient and on the adoption of an elastic response based on the existence of a suitable free energy function. Bonding-related internal variables, quantifying the effects of structure on the yield locus, are used to provide a macroscopic description of mechanical destructuration effects. To deal with strain localization phenomena, the model is equipped with a non-local version of the hardening laws [7]. The G-PFEM model is shown to be capable of capturing the destructuration associated with plastic deformations below and around the pile shoulder; the space and time evolution of pore water pressure as the pile advances; the effect of soil permeability on predicted excess pore water pressures, and the effect of chalk putty formation on predicted values of the load displacement curve. Installation effects are highlighted by comparing the axial performance between wished in place piles and piles which considered the full installation process.

Innovations in Micropile Testing Using Distributed Optical Fibre Sensors

Micropiles are primarily used in situations where traditional foundation methods, such as large-diameter bored piles or driven piles, are not feasible due to various constraints, including limited access, low headroom, or the need to work in environmentally sensitive areas. They are often employed in urban areas with restricted space or for underpinning existing structures. Two case studies of instrumented micropiles are presented in this paper, with the aim to showcase innovations in pile testing using Distributed Optical Fibre Sensing (DOFS). In the first project, micropiles were installed as part of an underpinning work where several existing piles were removed to allow safe passage of Tunnel Boring Machine (TBM) underneath the building. An experiment pile was instrumented with conventional and DOFS sensors to measure pile responses due to tunnelling in proximity. In the second project, the micropile was tested to determine its load carrying capacity for buildings constructed in hilly area. In both cases, API pipes were applied and socketed into bedrocks. Optical cables were attached on four sides of the steel pipes and subsequently grouted. During the load tests, DOFS data were able to display precisely the shaft distribution of micropiles when subjected to vertical loads as well as bending behaviour due to the close proximity tunnelling. Interpretations of distributed strain profiles from DOFS have contributed to a better understanding of API-reinforced micropile performance under complex geological and loading conditions.

Deriving a p-y curve from lateral pile load tests – Case studies in alluvial OC clays

Estimating lateral pile capacity is one of the most typical works for geotechnical engineers. Although it has been a routine calculation, obtaining reliable results is still challenging, especially when the ground condition is unfamiliar. Fortunately, the lateral pile capacity prediction can be performed by deriving a p-y curve from the load-deformation curve measured during pile testing. In cases where a preliminary test pile exists, fortunately, the p-y curve can be generated easily using a subgrade reaction approach suggested by Davisson and Gill (1963). Two lateral pile load test results were used to show the prospect of this method for practical applications. The field tests were on driven and bored piles with different diameters and lengths embedded in alluvial OC clays at two different sites in Jakarta, Indonesia, where both piles were categorised as long piles. Derivation of p-y curves was performed with a simple spreadsheet. Then, the obtained p-y response was used to estimate the load-deformation curve. Results showed that the back-calculated curve agree well with the measured data. This outcome might be because the p-y curves were directly derived from the field test where the pile experienced in-situ stress, resulting in a more representative of the actual condition.

Pile foundations problems in the Belarus geotechnical practice

Some problems arising in the design, construction and testing of piles in the geotechnical practice of Belarus are presented, and the possibilities of their solution are outlined. Recommendations are given to improve the efficiency of pile foundations.

Experimental study on axial compression monitoring of pile foundation based on sensor-enabled piezoelectric geocable

In concealed underground projects, pile foundations may cause severe engineering accidents owing to factors such as construction technology, geological conditions, and material properties. Accordingly, the construction industry has invariably focused on developing high-precision, distributed, and low-cost monitoring and disaster warning methods. Therefore, a new sensing material, sensor-enabled piezoelectric geocable (SPGC), was developed to monitor the deformation and failure of a pile foundation. Three model piles were constructed, and packaging and layout processes for the SPGC sensing material were proposed. Moreover, a fiber Bragg grating (FBG) sensor and resistance strain gauge were installed for comparison. Pile foundation failure and static load tests were performed using a large servo hydraulic press. The monitored effect was evaluated by obtaining the impedance and voltage signals of the SPGC. Pile foundation failure test results show that the mutation of impedance and voltage signals can be used as combined monitoring data, and the electrical signal of the SPGC can be used in three stages of early warning response. These findings ensure that pile foundation damage can be located, damage precursors can be identified, and timely warning can be provided. The static load test results indicate that the SPGC data are in good agreement with the FBG sensor and strain gauge data. The impedance of the SPGC increases linearly with the load. Accordingly, a linear calculation formula relating impedance and strain is derived. The research results are expected to provide a new technical method for pile foundation construction and long-term operation and maintenance monitoring.

Axial cyclic performance of a jacked pile in structured clays: A semi-analytical solution approach

Cyclic loads resulting from environmental factors such as wind, waves, trains, and construction activities pose a potential hazard to the stability of piled structures. These loads can cause the performance degradation of piles, including cumulative settlement and weakening of bearing capacity. This paper presents a semi-analytical solution for evaluating the axial cyclic behavior of jacked piles in structured clays. The solution employs enhanced nonlinear load-transfer models that effectively capture the stress-strain hysteresis and interfacial cyclic degradation characteristics. The key factors influencing pile behaviors are incorporated within the analytical approach, encompassing pile installation, subsequent soil equalization, and the static and cyclic loading encountered during service. The validity of the current solution is established through model pile tests conducted in artificially structured clays under static and cyclic loading conditions. Extensive parameter studies are performed using the proven theoretical approach to emphasize the influence of soil structure and cyclic loading on pile behavior. The results demonstrate a favorable agreement between the theoretical and measured values in static and cyclic pile response analyses. Compared with the response of jacked piles in reconstituted clays, the pile response in structured clays with a cement content of Aw = 4%, including penetration resistance, final ultimate bearing capacity, and the convergence speed of permanent settlement, has been substantially improved. The parameter studies indicate that soil structure and cyclic loading patterns significantly influence the axial cyclic performance of jacked piles in natural clays.

Large-Scale Laboratory Testing of Helical Piles: Effect of the Shape

Large-Scale Laboratory Testing of Helical Piles: Effect of the Shape

Species Identification and Attack Patterns Analysis of Marine Borers in Shipwreck Members and Driftwood from the Korean Coast

In this study, we aimed to identify the patterns of perforation damage on shipwreck members excavated from the Korean coast and to estimate the organisms responsible. Two types of marine borers were collected from driftwood collected from Chupo Beach, Shinchang-ri Beach, and Biin Beach, and test piles installed at Shinjindo Beach, and species identification was conducted through visual observation and DNA analysis. The damage patterns of marine borers on the driftwood and test piles were identified and compared with the damage patterns of the shipwreck members. The marine borers found in the driftwood and test piles were identified as <i>Teredo navalis</i> and <i>Limnoria lignorum</i> through DNA analysis and visual observation. <i>T. navalis</i> bored 3-5 mm diameter tunnels in the cross-section of the wood towards the fiber direction, and the damage was severe enough to completely penetrate a 7 cm long piece of wood. The tunnels were lined with calcareous material extruded by <i>T. navalis</i>. In some driftwood, fiber-oriented as well as random-directional tunnels were observed in <i>T. navalis</i> damage scars. In shipwreck members, <i>T. navalis</i> damage was primarily seen in the upper members, which were exposed to seawater and not buried in mud on the shore at the time of excavation, where <i>L. lignorum</i> drilled and gnawed small holes of less than 1 mm diameter in the surface of the wood. Very little evidence of damage by <i>L. lignorum</i> was rarely observed on the topsides of shipwrecks or on the surfaces of members, presumably because the fragile damage was worn away by the movement of currents at sea.

Model Testing of Rock-Socketed Piles under Combined Vertical–Lateral Loading

Abstract This paper presents the findings from a model experimental setup designed and fabricated for conducting single gravity (1-g) model experiments on model aluminum instrumented piles embedded in synthetic rock subjected to both independent vertical–compressive and lateral loading and combined vertical–compressive and lateral loading. Synthetic rock was prepared based on a mix design that can simulate the strength and modulus of soft rocks. Model tests were carried out on single piles of different socketing lengths (L/D ratios: 6, 9, and 12). Combined loading tests were done such that the resultant of the vertical–compressive and lateral loads was at constant inclinations (30° and 60°). Piles with smooth and rough surfaces were simulated for examining the effect of the pile–rock roughness profile. The vertical load–settlement and the lateral load–deflection response were measured from the tests. The bending behavior of piles under both independent and combined loading was also measured. The deflection profile of the rock-socketed pile was obtained by tracing the tested/failed piles after extracting them. The axial and lateral resistance of the rock-socketed piles are interpreted and discussed. It is observed that the rock-socketed piles behave in a distinctly different manner under combined loading compared with independent loading.

A new p – y curve criterion for piles in proximity to undrained clay slopes

In this paper, a systematic study of the soil – pile interaction of a laterally loaded pile in proximity to undrained clay slope is conducted. The emphasis is on a modified actual displacement of the induced soil resistance in the p – y curve criterion by considering the displacement of the slope surface. On the above basic, the effect of slope on the ultimate soil resistance is also considered. A new p – y curve criterion, which divides the p – y curves in front of the pile into three cases, is proposed. Finally, the accuracy of the new model is verified through a pile test and two hyperbolic p - y curve criterions. The new criterion not only automatically reduces the stiffness of the p - y curve, but also reasonably reveals the effect of slope on the soil-pile interaction.

Evaluation of ASTM D 1143/D 1143M-07 and Chinese Code JGJ 106-2014 for Pile Load Testing using Finite Element Method

Pile foundation is a structural element utilized to transmit structural load into the soil mass. During design processes, many empirical equations used to estimate axial pile capacity, and a pile load test is conducted to validate the design. In Indonesia, it is common to adopt ASTM D 1143/D 1143M-07 for pile testing. Chinese Code JGJ 106-2014 is another viable option, which has gained popularity recently. This study investigated the load–settlement curves obtained using both codes. The analyses were simulated using the Mohr–Coulomb and the Hardening Soil models. The Hardening Soil model yielded more reasonable load–settlement and load–excess pore water pressure curves than the Mohr–Coulomb model. The reason due to the Mohr–Coulomb model unable to capture the non-linear behavior of soil properly. Furthermore, the results showed that ASTM D 1143/D 1143M-07 and JGJ 106-2014 yielded comparable results. Hence, both methods could be substituted each other.

Study on three-dimensional penetration mechanism of snowflake pile

The indoor model tests of snowflake pile are carried out in this paper, the finite element method based on the coupled Euler-Lagrange (CEL) method is used, the effects of the cross-sectional shape of snowflake pile and the friction coefficient of the pile-soil action surface on the distribution of the plastic zone, displacement field and stress of the soil are considered. The results show that (1) the CEL method can better simulate the large deformation of the soil around the pile during the penetration of the snowflake pile; (2) the range of the plastic zone increases when friction exists at the pile-soil action surface, the shape effect on the plastic zone is negligible; (3) In the stress analysis, the friction coefficient has little effect on the peak strength of radial stress as well as vertical stress, there is a certain effect on the change of radial stress with vertical distance after the completion of pile compression, the shape effect has an effect on the change of radial stress with vertical distance (when μ = 0) and the change of vertical stress along the vertical distance, the change of radial stress with radial distance is negligible.

Results of the “DPDT-Auger” Research Project on Screw Displacement Piles

Abstract The main objective of the “DPDT-Auger” research project was to test the prototype DPDT auger for forming screw displacement piles in the ground (patented in Poland in 2020). An additional aim was to develop design methods and rules for the making of such piles. The augers and piles were first tested on a model scale, and then more extensively in the real scale on experimental field plots. The results found the overall functionality of the DPDT auger to be good, and in several aspects better than that of the SDP auger. The load-bearing capacities and Q-s characteristics of piles made with both augers were considered comparable. All the conducted tests and their derived dependencies together with the results of in situ subsoil tests allowed for the development of empirical calculation methods and prognostic procedures, useful for designing and producing piles with DPDT and SDP augers. FEM numerical simulation rules for the considered piles were also developed, verified and calibrated by the results of real pile tests. This article describes only the most important final results of the research project but not the detailed results of the numerous tests and analyses that were carried out. Also omitted are the results of model tests and numerical simulations, as well as the implementation and acceptance recommendations, as they have already been or will be the subject of separate publications.

Vertical compressive bearing performance and optimization design method of large-diameter manually-excavated rock-socketed cast-in-place piles

To study the vertical compressive bearing characteristics of large-diameter rock-socketed cast-in-place piles, eight manually-excavated rock-socketed cast-in-place piles were subjected to vertical compressive on-site load and pile stress tests. The test results showed that the load–displacement (Q-s) curves of the eight test piles were all slow-varying, and the settlement of the piles was less than 11 mm, which met the minimum engineering requirements. The unloading rebound rate was between 55 and 75%, and the elastic working properties of the piles were apparent. The pile axial force gradually decreased with depth, and the slope of the axial force distribution curve reached a minimum in the moderately weathered muddy siltstone layer while the pile side friction resistance reached its maximum value. Pile end friction increases with the increase of load. But the pile end resistance was inversely proportional to the single pile length-to-diameter (L/D) ratio and the depth of rock embedment for the pile. The percentage of pile side friction resistance under maximum load was 86%, indicating that these were characteristic friction piles. Based on the test results and the current Chinese code, the friction coefficient of the pile side soil layer η and the total resistance coefficient of the rock-socketed section ζ were introduced. A revision to the calculation equation for the vertical bearing capacity of the rock-socketed cast-in-place pile in the code was proposed, together with an optimization design method for large-diameter rock-socketed cast-in-place piles.

Theoretical model for investigating three-dimensional effect in integrity test of open-end pipe piles

An innovative theoretical model of non-axisymmetric vibration of pipe pile (NVPP) is proposed in this paper to investigate the three-dimensional effect in the pile integrity test (PIT) for open-end pipe piles. A new dynamic equilibrium equation of the pipe pile is established based on the stress simplification assumption. It overcomes the inaccuracies of existing three-dimensional models in predicting the longitudinal wave velocity of pile material. The surrounding soil and soil plug conform to the classical plane strain assumption, while the pile end soil is simulated by the fictitious soil annular pile developed from the traditional fictitious soil pile. Theoretical solutions for the velocity response of each point on the pipe pile top are deduced and validated against existing solutions and FEM results. The NVPP model is then applied to analyze the characteristics of testing signals of PITs for thick-wall and thin-wall pipe piles. Problems concerned in actual PITs for pipe piles are settled and practical recommendations for acquiring high-quality testing signals with minimal interference are provided.

A rigorous analytical approach to the pile-soil interaction of torsionally loaded piles in saturated SANICLAY soils

The paper proposes a rigorous analytical approach to predict interaction between torsionally loaded piles and saturated clay soils. The soil deformation mechanism around the pile is determined using the non-associated and anisotropic constitutive model, SANICLAY, along with the axisymmetric condition, equilibrium equation, and boundary conditions. By solving the derived governing equations, the hoop shear stress against rotation curve is obtained, including elastic, elastoplastic, and critical state processes. Taking advantage of the finite-difference method, the torque-rotation behavior of the pile and the stress-strain state of the soil around the pile are figured out. The proposed approach is compared with a FEM simulation and a parametric analysis is conducted to investigate the effect of overconsolidation ratio and in-situ coefficient. Model pile tests and field tests are used to manifest the validity and capacity of the model. The results demonstrate that the present approach can well predict the torque-rotation response of the pile and can reflect some important phenomena observed in pile tests.

Modeling Lifetime Performance of Monopile Foundations for Offshore Wind Applications

This paper explores the application of a numerical method for modeling pseudorandom cyclic loading, at very large cycle numbers, to the design of offshore wind turbine foundations. The work expands the development of a novel constitutive modeling framework, the hyper-plastic accelerated ratcheting model (HARM), for which the key constitutive equations and the calibration method are presented. HARM captures both the nonlinear hysteretic behavior during cycling and the accumulation of permanent deformation (ratcheting) with large cycle numbers in a rigorous, yet computationally efficient manner, enabling the computation of foundation response over a lifetime of loading. This paper demonstrates how the approach can be applied to the cyclic pile field testing from the pile soil analysis (PISA) project. Following calibration, the model is used to assess pile response to three load signals representative of operational and extreme loads throughout the lifetime of a full-scale wind turbine foundation: (1) a short storm, (2) a 35-h storm, and (3) lifetime loading. The paper discusses how computational efficiency can be achieved while maintaining a high level of calculation accuracy.

Modeling of Grout Returned Height for Postgrouting at Pile Tip Considering the Soil Unloading and Time-Dependent Behavior of Cement Viscosity Effects

Postgrouting at the pile tip can improve the behavior of pile shaft resistance through the grout returned along the pile shaft. The soil radial unloading effect that existed in the bored pile shaft soil and the cement viscosity with time-dependent behavior affect the grout diffusion. A grout returned height model for postgrouting at the bored pile tip considering the soil unloading effect and time-dependent behavior of cement viscosity was proposed to analyze the grout returned diffusion behaviors. The reliability of the grout returned height model was compared and verified with a series of model bored pile tests and the measured results of the field case. The analysis results showed that considering the influence of pile shaft soil unloading for bored piles and the time-dependent behavior of cement viscosity in the grout returned height model is essential for analyzing the grout returned diffusions. A parametric study with the proposed model was performed, and the effects of soil unloading degree, the grout flow rate, the grouting pressure, and the pile scaling effect on the grout returned height were analyzed.

Prediction of the End-Bearing Capacity of Axially Loaded Piles in Saturated and Unsaturated Soils Based on the Stress Characteristics Method

This study proposed an analytical approach for predicting the end-bearing capacity of driven piles that are subjected to axial loads in saturated and unsaturated soils. This is a generalized approach in which the stress characteristic method is employed successfully for both saturated and unsaturated soils. An iterative technique computer code was developed for the proposed analytical approach extending the finite difference method to develop solutions with the aid of MATLAB (version 2019a) that provides graphical output to visualize the results. The results from the proposed approach were compared against measurements for 13 pile load tests that include 11 in saturated soils and 2 in unsaturated soils, with good agreement. In addition, numerical analyses were performed using ABAQUS (version 6.14) to simulate the driven pile penetration and pile loading by employing the arbitrary Lagrangian–Eulerian adaptive mesh methods. The comparisons between the numerical predictions and measurements from a published model pile test suggested that the ultimate bearing capacity is well predicted by the finite-element model in comparison to the proposed analytical method. However, the proposed analytical method was simple for use in engineering practice applications to estimate the end-bearing capacity of pile foundations in both saturated and unsaturated soils.

DEM study of the stress fields around the closed-ended displacement pile driven in sand

Three-dimensional discrete element method (DEM) is employed to model the published calibration chamber test of a closed-ended displacement pile driven in sand. To achieve large chamber-to-pile and pile-to-particle size ratios for unbiased predictions, a high-performance graphics processing unit-powered DEM code is utilised in conjunction with the axisymmetric assumption and particle refinement method. The DEM parameters are first calibrated against the drained triaxial compression test data on Fontainebleau sand. Then, the DEM modelling of pile driving demonstrates quantitative agreement with the experimental counterpart in terms of cone resistance, stress contours, and distributions.