Increase In Pile Capacity Research Articles

Parametric Study of Helical Piles Subjected to Compression and Tension Loading Through Finite Element Analysis: A Case Study

AbstractThis study performed a parametric analysis on helical piles through finite element models. To carry out this study, the modeling of two case studies was carried out in the PLAXIS 2D software; the results of the models were compared with the results of experimental investigations of helical piles subjected to compression and tension. The calibrated models were used to carry out the parametric study in which the spacing between plates, the number of helical plates, and the diameter of the plates were modified. The results suggest that a spacing ratio s/D < 3 is not convenient since it generates a cylindrical shear failure, decreasing the capacity of the pile; furthermore, utilizing more than three plates in helical piles does not result in a substantial increase in pile capacity, while increasing the diameter of the helical plates provides a larger supporting surface, directly enhancing the pile's capacity.

Kajian Kapasitas Kelompok Tiang pada Tanah Lunak Menggunakan Skala Kecil Laboratorium

Soft clay is one type of subgrade that is problematic in infrastructure development. Pile reinforcement is an alternative to stabilize and increase the bearing capacity of soft clay soils. This study is expected to determine the capacity of the pile group from observations through a small-scale laboratory model and its comparison with the results of the analysis. Reinforcement of the piles used are concrete piles and wooden piles. The research was conducted through model testing on single piles and group piles in soft clay soil. The piles have a length of 20 cm and a diameter of 2 cm with the number of 1, 2, 4, and 9 piles. The study results show that the overall bearing capacity of the observed results is not much different from the results of the analysis. The average bearing capacity of the wooden and concrete piles is 20% and 18% greater than the analysis. The increase in pile capacity for each additional number of piles from the observations is not as high as the analysis. There is a tendency for the soil in the pile group to move down together, so that the capacity value of the pile group on soft clay is smaller than the single pile capacity value multiplied by the number of piles in the pile group.

An experimental study into the ultimate capacity of an ‘impression’ pile in clay

The ultimate capacity of a novel type of piled foundation called an ‘impression’ pile has been investigated using centrifuge modelling techniques. The name derives from the small discrete impressions created in the side walls of a bored cast in situ pile to increase the soil/pile friction such that the impressions form nodules on the shaft of the concreted pile. The technology is suitable for bored piles in overconsolidated clay, such as London Clay. The experiments explored the influence of geometrical parameters such as the vertical spacing of the impressions, their number at each level and their shape. The data show a consistent increase in pile capacity of 40% when the impressions extend over 85% of the pile length. The ultimate capacity of the pile is primarily affected by the length of the pile which is impressed, the number of nodules at a given cross-section and the spacing of the nodules in the vertical direction, as long as this is greater than a threshold value. According to the experimental evidence, a block failure occurs for a spacing lower than this threshold value. Plastic failure mechanisms for the impression pile were established. These were used successfully to calculate the ultimate capacity of the impression piles tested with an error of less than 10%.

Modified Time Factor to Estimate the Duration of Pile Setup

Pile setup is defined as the increase in pile capacity with time. This increase in pile capacity starts immediately after end of driving (EOD). Majority of the setup conducted during the consolidation period followed by a small amount during the post-consolidation period. However, there is no available methodology to estimate the duration of setup during the consolidation period, and to establish a theoretical upper time limit for pile setup capacity beyond which pile setup capacity can be neglected. This paper examines or focuses to develop a methodology to estimate the time duration of pile setup. Three instrumented test piles (TPs) were driven mainly in clayey soil in Louisiana besides regular bridge construction projects. The piles were instrumented with sister bar strain gages to measure the load transfer along the piles’ length, and four sets of pressure cells and piezometers were installed on each pile face to measure the total stress, effective stress and excess pore water pressure (PWP) with time. Dynamic and Static load tests were performed for a duration of 6 months after EOD and hence study setup. Piezocone dissipation tests were performed at each TP locations at different depths to measure the horizontal coefficient of consolidation (ch) besides the regular laboratory and in situ cone penetration test (CPT) to characterize the subsurface soil condition. The piezometers that were installed on the pile face showed that the dissipation continued from 2.5 h to 90 days after EOD depending on the soil properties. The equation that was proposed by Teh and Houlsby (Geotechnique 41(1):17–34, 1991) for dissipation test of piezocone was adopted to calibrate the time factor for pile. A new modified time factor for pile (Tpile*) was proposed in this study to estimate the duration of pile setup.

Modeling of Large Diameter Piles in Soil Formations Including Soft Clay

In Egypt, large diameter piles are used in some sites with diameters exceeding 2 m. In some cases, these piles carry foundations of heavy structures and/or large vertical and lateral loads. The analysis and design methodologies of large diameter piles are mainly controlled by the relatively smaller diameter piles, usually ranging between 60 and 120 cm. However, there is a special need for the analysis and design of larger diameter piles (i.e., larger than 120 cm) to assess their actual load carrying capacities. This paper presents numerical modeling of end bearing piles constructed in thick layer of soft to very soft clay underlain by a dense sand layer, typical to large zones in Northern and North Eastern zones in Egypt. The finite element analysis software ADINA (2018) [2] is used to simulate the behavior of the large diameter bored piles. Results showed that increasing the pile diameter cause a corresponding increase in pile capacity. It is also noticed that increasing pile embedded length in the end bearing sand layer has an enormous effect on pile capacity especially for larger diameter piles.

Estimation of long-term set-up of driven piles in sand

ABSTRACT Observations derived from updated field test databases indicate that total axial pile capacity of driven piles in sand continues to increase with time long after complete dissipation of excess pore pressure generated by pile driving. The increase in pile capacity with time is attributed to time-dependent increase in shaft capacity which is termed in the literature pile ‘set up’. Although, many hypotheses have been proposed in literature, the exact mechanism that governs the observed set-up is not yet fully understood and the estimation of the magnitude and rate of driven piles set-up remains relying on empirical relationships derived from widely scattered data. In this paper, a new hypothesis is introduced which attributes the observed set-up to the recovery of the soil surrounding the pile shaft from the friction fatigue processes induced during pile driving and a new methodology is formulated for the estimation of the magnitude and rate of driven pile set-up in cohessionless soil. Comparison of predicted and measured shaft friction capacities of some well-documented field cases comprising previously untested piles indicates acceptable results and suggests that the proposed approach can be used to estimate long time set-up of driven piles in sand.

Laboratory Study on Load Carrying Capacity of Pile Group in Unsaturated Clay

In this study, an attempt is made to investigate the load transfer in pile groups constructed in saturated and unsaturated soil using aluminum model piles of $$20\times 20$$ mm in cross section and 200 mm in length with six group configurations, single, $$2\times 1$$ , $$3\times 1$$ , $$2\times 2$$ , $$3\times 2$$ , and $$3\times 3$$ , groups in addition to pull-out test. The samples are prepared at three different soil properties: two models with the same degree of saturation and two with different dry unit weights and the third is unsaturated soil model with. A relationship between the soil suction and water content is introduced using a digital monitor, accompanied by eight suction probes planted in CBR molds. This relationship is used to predict the value of suction when conducting tests on models of the unsaturated soil. Further, the soil suction is measured using a filter paper method. Results of ultimate load capacity obtained from the load–displacement curves reveal an increase in the ultimate load with increasing the number of piles in the group for the same soil properties, and also an increase in the soil dry unit weight. Further, the results indicate an increase in pile capacity when the soil becomes in unsaturated state compared with saturated soil for the same pile group tested. On the other hand, the pull-out test gives a maximum load carrying capacity when the single pile tested was in saturated soil with high dry density, and minimum results are obtained when tested in unsaturated condition with the percentage of increase being about 463 and 34% for two saturated soils, compared with that for unsaturated soil.

Development of analytical models to estimate the increase in pile capacity with time (pile setup) from soil properties

This paper presents the analyses of twelve prestressed concrete (PSC) instrumented test piles that were driven in different bridge construction projects of Louisiana in order to develop analytical models to estimate the increase in pile capacity with time or pile setup. The twelve test piles were driven mainly in cohesive soils. Detailed soil characterizations including laboratory and in situ tests were conducted to determine the different soil properties. The test piles were instrumented with vibrating wire strain gauges, piezometers, pressure cells that were monitored during the whole testing period. Several static load tests (SLTs) and dynamic load tests were conducted on each test pile at different times after end of driving (EOD) to quantify the magnitude and rate of setup. Measurements of load tests confirmed that pile capacity increases almost linearly with the logarithm of time elapsed after EOD. Case pile wave analysis program was performed on the restrikes data and was used along with the load distribution plots from the SLTs to evaluate the increase in skin friction capacity of individual soil layers along the length of the piles. The logarithmic linear setup parameter “A” for unit skin friction was calculated of the 70 individual clayey soil layers and was correlated with different soil properties such as undrained shear strength (Su), plasticity index, vertical coefficient of consolidation (cv), over consolidation ratio and sensitivity (St). Nonlinear multivariable regression analyses were performed, and three different empirical models are proposed to predict the pile setup parameter “A” as a function of soil properties. For verification, the subsurface soil conditions and setup information for additional 18 PSC piles collected from local database were used to compare the measured versus predicted “A” parameters from the proposed models, which showed good agreement.

Pile capacity under oblique loads – evaluation from load–displacement curves

Evaluation of pile capacity from load–displacement curves under oblique loads is an intricate task. Because under oblique loads, load–displacement curves do not exhibit a distinct plunge point. Several methods have been adopted by different researchers to evaluate pile capacity from load–displacement curves under oblique loads. A summary of all these methods has been presented in this paper. To delineate the effect of load inclination on pile capacity, the method opted, always plays an important role. It is the main factor which significantly affects the pile capacity quantitatively and leads to establish a distinct trend of load inclination on pile capacity. To illustrate the significance of opted method, an experimental study was conducted on 20 mm diameter and 600 mm length piles under oblique pullout loads of different inclinations. Pile capacity at each load inclination has been evaluated from load–displacement curves using three different methods. Results of one method indicate that there is an increase in pile capacity up to 10° of load inclination and thereafter it decreases as load inclination increases. Whereas, results of second method show that pile capacity continuously decreases as load inclination increases with pile axis. Decrease in pile capacity with increase in load inclination was also observed in the results of third method, but pile capacity at certain loads' inclinations significantly differs as compared to the second method with minimum at 45°. The paper also deliberates on the applicability of adopted methods as per serviceability and functionality requirements of system or structure to be supported by pile.

Large‐scale testing facility for cyclic axially loaded piles

AbstractOffshore piles have to withstand predominantly cyclic axial loads when they are installed in multi‐pile configurations, as in jacket foundations. The dimensions of the pile are governed by both the internal capacity and the fatigue behaviour of the steel cross‐section as well as by its external capacity in the pile‐soil interaction. Owing to the large numbers of piled foundations required for current and future offshore wind farms, there is an urgent need to optimize the dimensions and related costs of single piles. With regard to the pile capacity, two major topics of research are the determination of possible capacity gains due to pile ageing effects and proper consideration of cyclic degradation. In order to investigate both effects, a large‐scale testing facility has been constructed at the BAM TTS site in Horstwalde near Berlin. This open‐air facility allows large tubular driven piles to be loaded cyclically in both tension and compression while studying the ageing effects by introducing delays between the testing campaigns. First results already show a moderate increase in pile capacity over time. Concerning the anticipated capacity degradation of cyclically loaded piles, preliminary results show an unexpected behaviour. Additional tests are currently being conducted for further clarification.

Setup of Driven Piles in Layered Soil

The load capacity of driven piles has been reported to sometimes increase or decrease with time after pile installation. An increase in pile capacity with time is known as setup, whereas a decrease in capacity is referred to as relaxation. In this paper, we review the current understanding of pile setup and discuss how to account for it in design. A total of forty-three dynamic tests were conducted over a period of five months on four H piles and four closed-ended pipe piles driven into layered soil. Test results show that the amount and rate of pile setup are quite different from those observed in other studies. Empirical formulas for predicting setup proposed by several researchers were compared with observations. Of the empirical formulas considered, the Svinkin (1996) lower-bound method predicted the rate of setup on these piles driven in layered soil relatively well. Additionally, some theoretical methods for prediction of evolution of static pile bearing capacity with time were tested against the dynamic pile test results. The bearing capacity from these theoretical methods was found to correspond to approximately 2 times the capacity measured at the end of initial driving by a dynamic test.

Prediction of pile set-up in clays and sands

Increase in pile capacity after initial driving has been well observed in clays and sands over decades. The phenomenon is referred to as pile set-up by geotechnical engineers. More economical pile design may benefit from this time-dependent increase subject to a reliable prediction. Simple empirical relations of the current capacity with the initial capacity and elapse time after driving are available in the literature with different model parameters being suggested for clays and sands, respectively. Nevertheless, appropriateness of the relations and confidence interval of the model parameters are rarely investigated and this hinders the application of these formulae. In this study, a revised single-parameter empirical relation is proposed based on the existing formulae. A comprehensive database from pile field tests data in clayey and sandy ground in the literature is compiled and Bayesian analysis is conducted on both these empirical formulae independently for clays and sands. Bayesian inference allows not only the estimation of the uncertain parameter but also the quantification of the associated uncertainty in the form of probability distribution. This study sheds lights on the confidence interval of the model parameter and it provides designers more reliable prediction of the additional capacity due to pile set-up.

A Neural Network Model for Prediction of Pile Setup

The time-dependent increase in pile capacity after its driving may be significant. A reliable prediction of this increase (setup) may lead to a significant saving in pile design. A neural network model to predict pile setup was developed. A database derived from field tests reported in the literature showing setup of driven piles was compiled, with six variables selected as input parameters: soil type, pile type, pile diameter, pile length, time after pile installation, and initial effective stress at tip. Ultimate pile capacity at beginning of restrike (QBOR) is the sum of pile capacity (QEOD) at end of drive and increased in capacity (ΔQBOR) caused by setup, which in this study is predicted by a backpropagation neural network. The results demonstrate that the neural network model provides a better prediction than predictions by the available empirical methods. A neural network model can serve as a reliable tool for the prediction of pile setup, and further training with additional data will lead to additional improvement in the quality of prediction.

Visco-elastic consolidation subsequent to pile installation

Driven piles normally generate excess pore pressures in the surrounding soil. Dissipation of the pore pressures following driving is predicted currently by available elastic theory. However, viscosity is pronounced for many soft clays, therefore, its eAect should be suitably accounted for. The gradual increase in pile capacity is dominated by the dissipation of excess pore pressure as has been widely explored both experimentally and theoretically. To predict the load-settlement response, the variation of pile-soil stiAness with the dissipation of pore pressure must also be quantified. This paper presents closed form solutions for the radial consolidation of the soil around a driven pile, assuming that the soil skeleton deforms viscoelastically. The solutions are theoretically valid for any initial distribution of excess pore pressure. However, the current study is focussed on the logarithmic variation of the initial pore pressure with radius, due to the expansion of a cylindrical cavity in an ideal elastic, perfectly plastic soil. The overall pile response measured from three cases has been used to backfigure the time-dependent variation of the two key facets: shear modulus and strength. The back analysis shows that variation of the two key facets can be well predicted with the presented radial consolidation theory. # 2000 Elsevier Science Ltd. All rights reserved.

Influence of loading rate on the capacity of a model pile in clay

Loading tests were carried out on a model pile embedded in clay to examine the influence of rate of loading on the capacity of the pile. The pile was loaded to failure using constant rate of penetration (CRP), quick maintained loading (QML), and quick continuous loading (QCL) methods of loading. The QCL test models the Statnamic loading test, which has been recently developed in Canada. The CRP tests were used as a reference, and the results were normalized using the CRP test results. The durations of the QML and QCL tests varied from approximately 0.1 s to 17 min, which are significantly faster than normal loading rates. Applied loads and point load were measured using load cells, and top displacement was measured using a displacement transducer. The test results showed an increase in pile capacity with increased rate of loading. Damping was found to be significant for the QCL tests (duration = 0.1 s) and negligible for the QML tests (duration ≥ 10 s). Correcting the results of the QCL tests for damping, using the equilibrium point method developed for Statnamic testing, greatly improved the correlation of the QCL and QML test results. Key words : model piles, axial loading, loading rate, clay, laboratory study, test methods.