Pile Toe Research Articles

Numerical investigations of soil plugging effect inside large-diameter, open-ended wind turbine monopiles driven by vibratory hammers

This study established a Couple Eulerian–Lagrange model to simulate monopile vibratory penetration for the investigation of soil plugging effect during high-frequency penetration of monopiles for wind turbine. Simulation analysis is focused particularly on soil plugging effect of a large diameter monopile during vibratory penetration into sand, clay, or layered soil. The results of the numerical simulation show that soil plugging effect is unlikely to occur during monopile penetration into the clay soil, while partial soil plugging may occur during the sand penetration. Penetration resistance at the pile toe is transferred to the radial stress around the pile wall. At a critical point penetration process, internal shaft friction becomes larger than external shaft friction. Moreover, radial pressure factors increase during partial soil plugging effect. For layered soil, the topsoil not only has great influence on the soil plugging effect, but also affects shaft friction in the subsoil during monopile penetration.

3D finite element analysis of pile responses to adjacent excavation in soft clay: Effects of different excavation depths systems relative to a floating pile

To gain new insights into single pile responses to adjacent excavations in soft ground, three dimensional numerical parametric studies are carried out. An advanced hypoplastic (clay) constitutive model which takes account of small-strain stiffness is adopted. The effects of excavation depths (He) relative to pile (Lp) were investigated by simulating the excavation near the pile shaft (i.e., He/Lp = 0.67), next to (He/Lp = 1.00) and below the pile toe (He/Lp = 1.33). In addition, the effect of pile head boundary conditions and different working loads with FOS = 3.0 and 1.5 were also studied. The model parameters are calibrated and validated against measured results in centrifuge reported in literature. It is found that the pile responses to excavation depend upon formation level of the excavation as well as the embedded depth of the wall. With different wall depth in each case, the induced settlement, lateral displacement and bending moment in the pile at the same stage of the excavation was different in the three cases. Among the three cases, the excavation in case of He/Lp = 1.33 resulted in the largest pile settlement (i.e., 7.6% pile diameter). On the other hand, the largest pile deflection was induced in case of He/Lp = 0.67. On contrary, insignificant bending moment and changes in axial load distribution induced in the pile on completion of the excavation in each case. However, significant bending moment (60% of pile BM capacity of 800 kNm) induced in the pile with fixed head condition. The different working loads (with FOS = 3.0 and 1.5) influence induced pile settlement but have relatively minor effect on induced bending moment.

The interaction of long and compressible piles with the surrounding and underlying subgrade soils

The article specifies the formulation and solution of problems on the interaction of a long incompressible and compressible pile with the surrounding and underlying subgrade firm soil by an analytical method. It is indicated, that there is the significant effect of the compressibility of the pile shaft on the patterns of force distribution applied to the long pile top between the side surface of the pile and its lower end. The force that comes on the pile toe bulb substantially depends on the ratio of the stiffness of the surrounding and underlying soils (G1 / G2). With the decrease of this ratio, all things being equal, the force on the pile toe bulb increases. It grows with a decrease in the ratio of the radii of the pile and the surrounding thick-wall soil cylinder (G / a). The analysis of the problem solution showed that it is possible to find the optimal ratios between the parameters G1 / G2,b / a,l / a, at which the force on the pile toe bulb reaches the optimum value wherein the bearing capacity of the underlying soil is at the most used at a given degree of approximation to the limit value.

Applying several soft computing techniques for prediction of bearing capacity of driven piles

Pile as a type of foundation is a structure which can transfer heavy structural loads into the ground. Determination and proper prediction of pile bearing capacity are considered as a very important issue in preliminary design of geotechnical structures. This study attempts to develop several intelligent techniques for prediction of pile bearing capacity in cohesionless soil. To show the effects of fuzzy inference system and imperialism competitive algorithm (ICA) on a pre-developed artificial neural network (ANN), two hybrid ANN models namely ICA-ANN and adoptive neuro-fuzzy inference system (ANFIS) were considered and developed to estimate pile bearing capacity. Then, results of these techniques were compared with those of ANN model and the best one among them was chosen according to the results of performance indices. Several parameters (i.e., internal friction angle of soil located in shaft and tip, effective vertical stress at pile toe, pile area, and pile length) were set as model inputs, while the output is the total driven pile bearing capacity. As a result of the developed models, coefficient of determination (R2) values of (0.895, 0.905), (0.945, 0.958), and (0.967, 0.975) were obtained for training and testing data sets of ANN, ICA-ANN, and ANFIS models, respectively. The results showed that both hybrid models are able to predict bearing capacity with high degree of accuracy; however, ANFIS receives more applicable based on used performance indices and it can be utilized for further researchers and engineers in practice.

A Parametric Study of Effect On Single Pile Integrity Due to An Adjacent Excavation Induced Stress Release in Soft Clay

To gain new insights into single pile responses to adjacent excavations in soft ground, numerical parametric studies are carried out. An advanced hypoplastic (clay) constitutive model which takes account of small-strain stiffness is adopted. The effects of excavation depths (i.e. formation level) relative to pile were investigated by simulating the excavation near the pile shaft (i.e., He/Lp=0.67), next to (He/Lp=1.00) and below the pile toe (He/Lp=1.33). Among the three cases, the excavation in case of He/Lp=1.33 resulted in the largest pile settlement (i.e. 7.6%dp). On the other hand, the largest pile bending moment was induced in case of He/Lp=0.67.

Results of Static Loading Tests on Single Piles and on Pile-Supported LPG Tanks

Full-scale static loading tests on eight single test piles and hydrotests on two 50-m diameter tanks supported on 849 piles intended for storing refrigerated gas were performed at Cai Mep Industrial Park approximately 90 km southeast of Ho Chi Minh City, Vietnam. The test piles were precast concrete piles installed through 21 m of soft clay into dense sand to about 45 m depth below ground surface. Four piles were installed by driving and four by jacking. The maximum pile test loads were 3000 kN, which was well below any ultimate resistance, but showed that the tank-foundation piles (to be installed by jacking) could expect to shorten about 3 mm for the 750-kN working load. Each tank was hydro-tested to 636-MN maximum service load, which load was held constant for 1 week, while monitoring the tank settlement at benchmarks placed along the tank perimeters. The records showed the tank perimeters to settle about 15 mm in addition to an about 3-mm pile shortening. Back-analysis of the tank foundation modeled as flexible equivalent rafts at the pile toe level showed that the settlement of the tank center was about three times larger than that of the perimeter. Analysis of long-term settlements indicated that, under service loading, the tank perimeters and centers will settle an additional about 100 mm and 300 mm, respectively. Due to a 3 m thick fill placed over the site causing the clay to consolidate, the ground surface is expected to settle more than 1 m over the long-term. The general subsidence will affect the perimeter piles and transfer load to the interior piles. However, because the pile neutral plane is located in the sand below the clay, downdrag is not an issue for the piled foundation. The drag force will be well below the limit of the pile axial structural strength.

Settlement and bearing capacity of the circular foundation

In process of designing foundations of circular form in plane, as well as bored piles of circular cross section, there is important to quantify the stress-strain state and the bearing capacity of the soils interacted with the foundations, as well as under the toe of piles. The article presents the formulation and solution of the problem of determining the initial critical load at the base of the circular foundation, which is also relevant for the design case of the soil under the toe of a circular bored pile. It is shown that the condition of limiting equilibrium arises both under the center of the loaded area and on the contour of the circular loading area, and primarily on the contour. A comparative estimate showed that the ratio of the initial critical load to the base soil of the circular foundation under the center of the site is 2 times greater than the corresponding load for the point at the edge of the site.

The Influence of Water-Jet Modification of Soil Conditions on Reduction of Vibrations during Sheet Pile Driving in the Subsoil

Vibratory driving is the most efficient method of sheet pile installation. The elimination or at least reduction of harmful consequences of sheet pile vibratory driving becomes an increasingly important and often the decisive factor in the selection of the excavation protection technology. In difficult soil conditions with strength soil parameters, pressure water jetting precedes sheet pile driving. This technique changes the soil properties, destroying its existing structure near the installed sheet pile. Unfortunately, the results and achievements of contractors using this technique are very often held confidential or simply remain in the records and it is difficult to find papers on this subject. This article features a detailed description of jet-assisted sheet pile driving, including a prefabrication description for the sheet pile toe used to jet water through, water pressure and quantity data and the results with regard to reduce vibrations and noise, as well as to the increase in sheet pile vibratory driving performance.

Optimizing an ANN model with ICA for estimating bearing capacity of driven pile in cohesionless soil

The application of models provided by artificial neural network (ANN) in predicting bearing capacity of driven pile is underlined in several investigations. However, weakness of ANN in slow rate of convergence as well as finding reliable testing output is known to be the major drawbacks of implementing ANN-based techniques. The present study aims to introduce and evaluate an optimized ANN with imperialism competitive algorithm (ICA) model based to estimate bearing capacity of driven pile in cohesionless soil. The training data for optimizing the ICA-ANN structure are based on the in situ study. To develop the ICA-ANN model, the input parameters are internal friction angle of soil located in shaft (φ shaft), and tip (φ tip), pile length (L), effective vertical stress at pile toe (σ v), and pile area (A) while the output is the total driven pile bearing capacity in cohesionless soil. The predicted results are compared with a pre-developed ANN model to demonstrate the ability of the hybrid model. As a result, coefficient of determination (R 2) values of (0.885 and 0.894) and (0.964 and 0.974) was obtained for testing and training datasets of ANN and ICA-ANN models, respectively. In addition, values of variance account for (VAF) of (88.212 for training and 89.215 for testing) and (96.369 for training and 97.369 for testing, respectively) were obtained for ANN and ICA-ANN models, respectively. The obtained results declare high reliability of the developed ICA-ANN model. This model can be introduced as a new model in field of deep foundation engineering.

3D Centrifuge Modeling of the Effect of Twin Tunneling to an Existing Pile Group

In densely built urban areas, it is inevitable that tunnels will be constructed near existing pile groups. The bearing capacity of a pile group depends on shear stress along the soil-pile interface and normal stress underneath the pile toe while the two would be adversely affected by the unloading process of tunneling. Although extensive studies have been conducted to investigate the effects of tunnel construction on existing single piles, the influence of twin tunnel advancement on an existing pile group is merely reported in the literature. In this study, a series of three-dimensional centrifuge tests were carried out to investigate the response of an existing pile group under working load subjected to twin tunneling at various locations in dry Toyoura sand. In each twin tunneling test, the first tunnel is constructed near the mid-depth of the pile shaft, while the second tunnel is subsequently constructed either next to, below or right underneath the pile toe (Tests G_ST, G_SB and G_SU, respectively). Among the three tests, the 2nd tunnel excavated near the pile toe (Test G_ST) results in the smallest settlement but the largest transverse tilting (0.2%) of pile group. Significant bending moment was induced at the pile head (1.4 times of its bending moment capacity) due to the 2nd tunnel T. On the contrary, tunneling right underneath the toe of pile (i.e., Test G_SU) results in the smallest tilting but largest settlement of the pile group (4.6% of pile diameter) and incremental mobilisation of shaft resistance (13%). Due to stress release by the twin tunneling, the axial force taken by the front piles close to tunnels was reduced and partially transferred to the rear piles. This load transfer can increase the axial force in rear piles by 24%.

CPT-Based Investigation for Pile Toe and Shaft Resistances Distribution

The true distribution of both pile shaft and toe resistances along depth, has been an important issue in geotechnical engineering. Twenty four pile load test case records of deep foundation projects, where CPT, Cone Penetration Test, soundings were also advanced close to piles locations, throughout the world have been compiled for investigations and analyses. The geomaterial at these sites are homogenous, including sandy, clayey and silty soils. Four different methods for determining pile bearing capacity comprising the method based on critical depth concept, recommended approaches by CFEM, Canadian Foundation Engineering Manual 2006 (Unified method), American Petroleum Institute (API), and CPT-based methods are presented, compared and discussed. In addition, the influence of factors such as soil friction angle variations, calculated from qc, cone tip resistance, and overburden stress, are taken into account to perform indirect calculations. Direct and indirect CPT-based calculations on the pile capacity are studied to investigate the actual pile toe and shaft resistances distribution along depth. The comparisons reveal that in practice and in observations from full scale tests, shaft and toe resistances follow a nonlinear and gradual trend beyond the certain depth. However, it is not in agreement with conventional linear-constant or relatively linear distribution concept and some recommended codes.

Evaluation of static capacity of piles from rapid load testing

The unloading point method (ULM) is a simple technique for determining equivalent static load-settlement behaviour of piles from rapid load tests. Since the ULM assumes piles to be rigid, it does not consider the effects of elastic shortening and the lag in response along pile shaft. This assumption can be improved by increasing measurement points, e.g. adding an accelerometer at pile toe. In this article, the stress-wave theory was used instead of the additional sensor for estimating the response at pile toe. By considering the lag in response along pile shaft, a better estimate of the stationary point can be obtained and used in the ULM. The refined ULM was verified by comparing with the non-linear model using rapid load test results in literatures. Predictions by the proposed method agreed well with the results from static pile load tests and were better than other methods in this study.

Prediction of axial capacity of piles driven in non-cohesive soils based on neural networks approach

This paper presents an application of two advanced approaches, Artificial Neural Networks (ANN) and Princi­pal Component Analysis (PCA) in predicting the axial pile capacity. The combination of these two approaches allowed the development of an ANN model that provides more accurate axial capacity predictions. The model makes use of Back-Propagation Multi-Layer Perceptron (BPMLP) with Bayesian Regularization (BR), and it is established through the incorporation of approximately 415 data sets obtained from data published in the literature for a wide range of un-cemented soils and pile configurations. The compiled database includes, respectively 247 and 168 loading tests on large-and low-displacement driven piles. The contributions of the soil above and below pile toe to the pile base resistance are pre-evaluated using separate finite element (FE) analyses. The assessment of the predictive performance of the new method against a number of traditional SPT-based approaches indicates that the developed model has attractive capabili­ties and advantages that render it a promising tool. To facilitate its use, the developed model is translated into simple design equations based on statistical approaches.

Dynamic response of a pile considering the interaction of pile variable cross section with the surrounding layered soil

SummaryAssuming that the pile variable cross section interacts with the surrounding soil in the same way as the pile toe does with the bearing stratus, the interaction of pile variable cross section with the surrounding soil is represented by a Voigt model, which consists of a spring and a damper connected in parallel, and the spring constant and damper coefficient are derived. Thus, a more rigid pile–soil interaction model is proposed. The surrounding soil layers are modeled as axisymmetric continuum in which its vertical displacements are taken into account and the pile is assumed to be a Rayleigh–Love rod with material damping. Allowing for soil properties and pile defects, the pile–soil system is divided into several layers. By means of Laplace transform, the governing equations of soil layers are solved in frequency domain, and a new relationship linking the impedance functions at the variable‐section interface between the adjacent pile segments is derived using a Heaviside step function, which is called amended impedance function transfer method. On this basis, the impedance function at pile top is derived by amended impedance function transfer method proposed in this paper. Then, the velocity response at pile top can be obtained by means of inverse Fourier transform and convolution theorem. The effects of pile–soil system parameters are studied, and some conclusions are proposed. Then, an engineering example is given to confirm the rationality of the solution proposed in this paper. Copyright © 2017 John Wiley & Sons, Ltd.

Pile responses to side-by-side twin tunnelling in stiff clay: Effects of different tunnel depths relative to pile

In densely built areas, the development of underground transportation systems often involves twin tunnels, which are sometimes unavoidably constructed adjacent to existing piled foundations. Because soil stiffness degrades with induced stress release and shear strain during tunnelling, it is vital to investigate the pile responses to subsequent tunnels after the first tunnel in a twin-tunnel transportation system. To gain new insights into single pile responses to side-by-side twin tunnelling in saturated stiff clay, a three-dimensional coupled-consolidation numerical parametric study is carried out. An advanced hypoplasticity (clay) constitutive model with small-strain stiffness is adopted. The effects of each tunnel depth relative to pile are investigated by simulating the twin tunnels either near the mid-depth of the pile shaft or adjacent to or below the pile toe. The model parameters are calibrated against centrifuge test results in stiff clay reported in literature. It is found the second tunnelling in each case resulted in larger settlement than that due to the first tunnelling with a maximum percentage difference of 175% in the case of twin tunnelling near the mid-depth of the shaft. This is because of the degradation of clay stiffness around the pile during the first tunnelling. Conversely, the first tunnelling-induced bending moment was reduced substantially during the second tunnelling. The most critical location of twin tunnels relative to the pile was found to be the tunnels below the pile toe. This is because the entire pile was located within the major influence zone of the twin tunnelling. Two distinct load transfer mechanisms can be identified in the pile, namely downward load transfer in case of tunnels near mid-depth of the pile shaft and next to the pile toe and upward load transfer in case of twin-tunnelling below the pile toe. These two transfer mechanisms can be useful for practitioner to assess the pile performance due to twin tunnelling.

강관 부착 PHC파일로 시공된 매입말뚝의 선단지지력 산정

강관 부착 PHC파일로 시공된 매입말뚝의 선단지지력 산정

Study of soil disturbance effect on bearing capacity of helical pile by experimental modelling in FCV

Helical piles as deep foundations are a traditional solution for supporting structures such as residential construction, communication tower installations, static or seismic structural retrofitting and reconstruction. Two mechanisms of failure during helical pile loading occurred depending on pile geometry and soil strength as individual or cylindrical failure. Commonly, soil disturbance effects during pile driving by torque mechanism are not considered in conventional static analysis methods for determine bearing capacity of the helical pile toe or helices plates, while this disturbance can influence the bearing resistance of the pile. Present study considered the behaviour of three models of instrumented helical piles in two densities of Babolsar Sand by the frustum confining vessel developed at Amirkabir University of Technology (FCV-AUT) as the physical modelling apparatus. Bearing capacity of the pile toe and helices plates in addition to axial capacity of the pile are measured and controlled during pile compression load test. Two static analysis methods are used in order to verify the experimental results and deriving the coefficient of bearing capacity for promotion of the conventional static analysis to predict axial capacity of helical piles with more accuracy. The axial capacities of 21 full-scale helical piles are predicted by the new approach and compared with measured capacity from static pile load test. The results of comparison have shown that using the new approach errors in helical pile capacity estimation have been lowered almost 43%. Therefore, it can be realised as an alternative in static analysis of helical pile design.

Theoretical Estimation of the Drilling Rates Comparing the Evans and Nishimatsu Models in Relation to the Offshore Piles

AbstractDriven piles are the main foundation system for offshore oil and gas facilities. They are installed through the jacket legs or at the bottom of the structure. Driven piles can, however, experience premature refusal. In this case, drilling solutions may be needed, such as drilling to remove the plug ahead and below the pile toe. Because time is important during offshore operations, offshore bored piles are rarely used. Unlike pile drivability analysis, which is based on wave analysis and hence reliable, there is no established pile-drillability assessment. This paper extends the findings of previous research in which the specific excavation energy (SEE) was estimated in subsoils from Turin, Italy, at a small scale. Two subsoil classes were considered and compared in this study. The SEE was linked to different friction-angle and cohesion values, and using the Nishimatsu and Evans cutting models, preliminary theoretical net drilling penetrations were estimated. The results should be considered as the...

Non-uniform thermal strains and stresses in energy piles

This paper presents the non-uniform thermally induced strains and stresses that develop in the cross-sections of energy piles when subjected to operational energy demands. The thermal operation of energy piles depends on the thermal demands of the supported building. These thermal demands vary seasonally, daily and even hourly, which generates non-uniform temperature changes over the pile’s cross-section throughout the lifetime of the system. Therefore, non-uniform thermal strains and stresses develop over the cross-section of energy piles. Numerical analyses performed in this study indicate that the maximum temperature changes and the thermally induced strains and stresses occur near the location of the ground loops. Further, contradicting the current understanding that heating the piles creates only thermal compressive stresses, while cooling them creates only thermal tensile stresses; tensile and compressive thermal stresses were found to coexist during heating and cooling over the same pile cross-section due to the non-uniform temperature changes. Furthermore, the magnitude of the tensile stresses that develop due to non-uniform temperature changes is crucial when the reinforcing cage is cut short of the pile toe. Finally, ignoring the non-uniform temperature changes while processing the temperatures and strains measured in full-scale tests will result in errors in approximated thermal stresses.

Investigation of Peak Particle Velocity Variations during Impact Pile Driving Process

Impact pile driving is a multi-component problem which is associated to multi-directional ground vibrations. At first, vibration is transferred from the hammer to the pile and then to the common interface of pile and soil. This is then transferred to the environment and has great impact on the adjacent structures, causing disturbance to residents and also damage to the buildings. It is of high importance to have sufficient estimation of pile driving vibration level in order to maintain the comfort of residents near the site and also to prevent the structural damage to buildings. In this study, a finite element model, using ABAQUS, with the ability of simulating continuous pile driving process from the ground surface, was introduced. The model was verified by comparing the computed peak particle velocities with those measured in the field. Parameters affecting the peak particle velocity (PPV), for example elastic modulus, shear strength parameters, impact force, pile diameter, etc. were considered, and variations of PPV was investigated. Results of present study indicated that PPV at the ground surface does not occur when the pile toe is located on the ground surface; as the pile penetrates into the ground, PPV reaches a maximum value at a critical depth of penetration. Moreover, the amplitude of vibration on the ground surface reduced logarithmically with increasing distance to the pile. Also, on the ground surface and radial distances of 3 to 20 m, maximum particle velocity occurred between 1 to 5 m depths of pile penetration. The results showed PPV as being directly proportional to the hammer impact force, pile diameter, friction angle and cohesion intercept and inversely proportional to the elastic modulus of the soil.