Existing Pile Foundations Research Articles

Three-dimensional centrifuge modeling of the influence of side-by-side twin tunneling on a piled raft

Although the effects of tunnel excavation on nearby existing pile foundations have been extensively studied, previous studies mainly focused on response of single piles and elevated pile groups. In this study, three-dimensional centrifuge model tests were carried out to investigate the effects of side by side twin tunneling on an existing two by two piled raft in dry sand. Upon completion of twin tunnel excavation, the measured cumulative settlement of piled raft due to tunneling near the pile toe is 2.7 times of that due to tunneling near the mid-depth of the pile shaft. For twin tunneling near the pile toe, differences of piled raft settlements due to each tunnel excavation are negligible. In contrast, settlement of piled raft due to the second tunneling is about 55% of that due to the first tunneling when twin tunnels are excavated near the mid-depth of the pile shaft. As an advancement of tunnel excavation near the pile toe, the axial loads carried by the front piles are decreased, and the decreased loads are transferred to the rear piles and the raft. On the contrary, twin tunneling at the mid-depth of the pile shaft causes a reduction in the front piles and raft, and the decreased loads are transferred to the rear piles. After completing tunnel excavation, the axial load carried by the raft is increased by 22.0% for tunneling near the pile toe, while it is decreased by 21.7% for tunneling near the pile shaft.

Numerical Parametric Study of Settlement and Load Transfer Mechanism of Pile Group Due to Twin Stacked Tunnelling with Different Construction Sequences

Construction of tunnels in urban cities may induce excessive settlement and tilting of nearby existing pile foundations. In this study, numerical parametric study was conducted to investigate the influences of twin stacked tunnelling with different construction sequences on an existing (2 × 2) pile group. Twin stacked tunnelling was simulated one after the other with the first tunnel excavated near the mid-depth of the pile shaft and the second either next to the toe of the pile (case ST) or below the pile (case SB). In addition, two more analyses were performed with change of construction sequence of twin stacked tunnels (i.e. cases TS and BS). The first tunnel was excavated either near the pile toe (case TS) or below the pile (case BS) and second tunnel near the mid-depth of the pile shaft. The different construction sequence of twin stacked tunnels at different depth had a limited effect on induced pile group settlement, tilting, lateral deflection and load transfer mechanism. The computed cumulative pile group settlement was about 22% larger for tunnelling sequence TS than for tunnelling sequence TS.

Investigation of effects of different construction sequences on settlement and load transfer mechanism of single pile due to twin stacked tunnelling

Construction of tunnels in urban cities may induce excessive settlement and tilting of nearby existing pile foundations. Various studies reported in the literature have investigated the tunnel-soil-pile interaction by means of field monitoring, centrifuge and numerical modelling, but the influence of construction sequence of twin stacked tunnelling on an existing pile is seldom reported in the literature. This paper presents a 3D coupled-consolidation numerical parametric analysis to understand settlement and load transfer mechanism in single pile due to twin stacked tunnelling with different construction sequences in stiff saturated clay. To accurately capture the tunnelling induced ground deformation, path- and strain-dependent soil stiffness at small strains were taken into account in each numerical analysis, by adopting an advanced hypoplastic clay model coupled with the inter-granular strain formulation. Twin stacked tunnelling was simulated one after the other with the first tunnel excavated near the mid-depth of the pile shift and the second either next to the toe of the pile (case ST) or below the pile (case SB). In addition, two more analyses were performed with change of construction sequence of twin stacked tunnels (i.e. cases TS and BS). The first tunnel was excavated either near the pile toe (case TS) or below the pile (case BS) and second tunnel near the mid-depth of the pile shaft. The different construction sequence of twin stacked tunnels at different depth had a substantial effect on induced pile deflection and settlement. In contrast, no significant effect was found on load transfer mechanism in the pile due to the change in construction sequence of tunnels.

Evaluation of lateral response of single piles to adjacent excavation using data from cone penetration tests

Excavation inevitably induces stress changes in the surrounding soil, which causes significant lateral movements, additional forces, and bending moments in the existing pile foundations. To prevent or minimize damage to adjacent piles, this paper presents an actual full-scale instrumented study to examine the lateral response of existing piles to an adjacent test pit excavation. Cone penetration tests (CPTs) near the test piles before and after excavation are conducted and compared. A simple p–y evolution model for lateral piles during excavation is proposed. The proposed model is defined by the pre-excavation and post-excavation p–y curves from CPT data, which can provide better predictions by comparison with the field measured results. Additionally, for analysis of the pile behaviour after excavation, the observed lateral bearing capacity of full-scale tests are compared with those computed by the pre-excavation and post-excavation p–y curves. The post-excavation p–y curve can generally give a satisfactory prediction of the residual pile bearing capacity after excavation. The calculated results from the free-field cone parameters have a serious overestimation and are detrimental to the service design of pile foundations.

Three-dimensional numerical simulation of pile-twin tunnels interaction – Case of the Shiraz subway line

The use of an underground transportation network to solve urban traffic problems is a practical solution. Shiraz, one of the most famous touristic cities of Iran, is developing its own urban railway network using mechanized twin tunnels. It is almost inevitable for tunnels to pass nearby an existing pile foundations. In this study, a three-dimensional numerical model was developed to investigate the pile-tunnel interaction. All the mechanized excavation phases of an EPB-TBM (Earth pressure balance, Tunnel boring machine) and the segmental lining were modeled using a finite difference code. The role of the Constitutive soil’s model is also investigated by using both a linear elastic-perfectly plastic one (failure criteria of Mohr-Coulomb type) named as MC and a nonlinear perfectly plastic one the Cap-Yield (CYsoil) constitutive model. Two different scenarios were considered: the influence of tunneling on existing piles and the impact of installing bridge piles nearby existing twin tunnels. The influence of the soil/structure interaction is presented in terms of surface settlements, forces induced in the tunnel lining and forces along the piles. It should be noted that on the base of the obtained results, the MC model cannot accurately estimate the ground settlements and the forces in tunnel and piles, therefore it is necessary to use advanced constitutive models like CYsoil for design purposes.

Explicit Form of Exact Analytical Solution for Calculating Ground Displacement and Stress Induced by Shallow Tunneling and Its Application

In urban environment, it is often unavoidable for shallow tunnels to be constructed adjacent to existing pile foundations. To obtain the ground displacements and stresses induced by shallow tunneling and existing pile foundation loads, the key procedure involves superimposing the analytical solution for shallow tunneling in green‐field with the analytical solution for existing structure loads. In green‐field, the complex variable method provides exact analytical solutions of ground displacements and stresses caused by shallow tunneling. However, the exact analytical solutions are not directly expressed as explicit functions of the coordinates (x, y) in the physical plane (called implicit form of exact analytical solutions), whereas the displacements and stresses induced by existing structure loads are explicit functions of the coordinates (x, y) in the physical plane, which makes it difficult to superpose the displacements and stresses induced by existing structure loads. In this paper, explicit form of exact analytical solutions of displacements and stresses induced by shallow tunneling in green‐field is obtained by using the inverse conformal transformation and the Cauchy–Riemann equations. Comparison with implicit form of exact analytical solutions shows that the explicit form of exact analytical solutions is intuitional and easily used by engineers, and moreover, the calculation amount is much smaller than that for the implicit form of exact analytical solutions. Then, an application involving superimposing the explicit form of exact analytical solutions with Mindlin’s solution is implemented to analyze the secondary stress field and the related potential plastic zone caused by shallow tunneling adjacent to pile foundations. Moreover, the influences of pile foundation parameters on the ranges and shapes of the potential plastic zones induced by nearby tunneling are analyzed.

3D Numerical Analysis of the Effects of an Advancing Tunnel on an Existing Loaded Pile Group

Tunnels are often preferred for underground transportation in densely populated areas. In these areas, it is almost inevitable for tunnels to run close to some existing pile foundations. Since tunnelling activities induce stress relief and soil movement in the ground, existing piles may suffer from additional axial and lateral forces, bending moments, settlements and lateral deflections. Most of the previous researches on the responses of pile foundations due to tunnel construction were carried out under the plane strain condition. In this paper, a three-dimensional, elasto-plastic and coupled-consolidation finite element parametric study has been carried out to investigate the effects of a 6 m open-face advancing tunnel on a two by two pile group in saturated stiff clay. The influence of different cover-to-diameter (C/D) tunnel ratios (namely 2.0, 2.5 & 3.0) was studied. The objectives of this study are to determine the changes in axial load distribution, changes in shaft resistance along the shaft of pile group and settlement of pile cap due to an advancing open-face tunnel.

既設杭基礎の地盤改良併設による耐震補強技術

既設杭基礎の地盤改良併設による耐震補強技術

Effects of Multipropped Excavation on an Adjacent Floating Pile

AbstractDeep excavations inevitably induce stress relief and soil movement, which may affect the safety and serviceability of nearby existing pile foundations. Although the effects of excavation on...

New Technology on Seismic Reinforcement of Pile Foundation for Long Life Bridges

Nowadays, based on the experiences in the recent severe earthquakes, a certain level of seismic performance is required for the foundation design of newly-built bridges. Moreover, there are so many existing pile foundations with serious deteriorations and damages. In this report, new technique to improve seismic performance of pile foundation, in which ground improvement is provided around the existing piles in very soft or liquefiable soil, is proposed. It enables existing bridges to be sustainable and long life structures. The patent to this technique named the Composite-Pile Method has been granted in Japan in 2012. Strength and deformation of improved soil test pieces having different levels of strength were fully investigated through various types of material tests. Seismic performance of pile foundation with improved soil subjected to Level 2 Earthquake wave was then verified by large-scale shaking table tests for model piles. The bending strain in the existing piles surrounded by improved soil with a uni-axial strength of greater than 300kN/m2, when subjected to severe earthquake, was reduced by half of the strain generated in the piles without soil improvement, which leads to the reduction of response displacement of the piles. It was clarified that, based on the test results, seismic performance of pile foundation could be drastically improved by the proposed method. Practical application of the proposed method, when needed in the future, will be undertaken for higher safety of public structures.

3D dynamic finite element analyses and 1 g shaking table tests on seismic performance of existing group-pile foundation in partially improved grounds under dry condition

Pile foundations are commonly used for heavy superstructures in soft grounds. The failures of pile foundations have been reported in past major earthquakes. An economical and effective method for improving the seismic performance of existing pile foundations is the partial-ground-improvement method (PGI). The main concern of this method is how to determine the size and the location of the areas around a pile foundation to be improved. In this paper, numerical analyses and shaking table tests are conducted to find the optimum pattern for the PGI method. In the numerical analysis, the three-dimensional (3D) dynamic finite element method (FEM) is used with a unified system consisting of a ground, a group-pile foundation and a superstructure. Furthermore, the mechanical behaviors of the ground, the group-pile foundation and the superstructure are modeled by the Cyclic Mobility model, the axial force-dependent model, and the tri-linear model, respectively. With the above-mentioned numerical method, four different patterns of PGI are investigated and then an optimum pattern for the PGI method is proposed. All the numerical analyses are also confirmed by 1g shaking table tests. All the tests and the numerical analyses are conducted under dry condition.

A seismic reinforcement method for an existing pile foundation in soft ground and liquefiable ground

A seismic reinforcement method for an existing pile foundation in soft ground and liquefiable ground

Influence of Tunneling on Adjacent Existing Pile Foundation

Influence of Tunneling on Adjacent Existing Pile Foundation

Load transfer mechanism in pile group due to single tunnel advancement in stiff clay

Construction of tunnels in urban cities may induce excessive settlement and tilting of nearby existing pile foundations. Various studies reported in the literature have investigated the tunnel–soil–pile interaction by means of field monitoring, centrifuge and numerical modelling. However, the load transfer mechanism between piles in a group, the induced settlement and the tilting of a pile group due to tunnel advancement has not been investigated systematically and is not well understood. This study conducts three-dimensional, coupled-consolidation finite element analyses to investigate tunnelling effects on an existing 2×2 pile group. The construction of a 6m diameter (D) tunnel in saturated stiff clay is simulated. Responses of the pile group located at a clear distance of 2.1m (0.35D) from a tunnel constructed at three different cover-to-diameter-of-tunnel ratios (C/D) of 1.5, 2.5 and 3.5 are investigated. The computed results are compared to published data based on field monitoring. It is found that the most critical stage for settlement, tilting and induced bending moment of pile group due to tunnelling is when the tunnel face is close to the pile group rather than at the end of tunnel excavation. The depth of the tunnel relative to the pile group has a vital influence on the settlement, tilting of pile group and the load transfer mechanism between piles in pile group induced by tunnel excavation. Tunnelling near the mid-depth of the pile group (i.e. C/D=1.5) induces the largest bending moment in the piles, but the settlement and tilting of the pile group are relatively small. Based on a settlement criterion, apparent loss of capacity of the pile group is 14% and 23% for tunnels constructed at depths of C/D=1.5 and at both C/D=2.5 and 3.5, respectively. The largest load redistribution between the front and rear piles in the group and the largest tilting of the pile cap towards the tunnel occurs when tunnel excavated at C/D=2.5.

Effects of tunnelling on pile foundations

An efficient analysis method is presented for estimating the effects induced by tunnelling on existing pile foundations. The method is based on a two-stage procedure: (1) an estimate of the free-field ground movements caused by the tunnel excavation, and (2) an analysis of the pile group subjected to the computed free-field ground movements. The first step may be carried out using alternative approaches, ranging from empirical methods to 3D numerical analyses. The second step is performed by PGROUPN, a computer program for pile-group analysis based on a non-linear boundary element solution. The validity of the approach is assessed by comparing it with alternative numerical solutions and field measurements. The results indicate that the method is capable of generating reasonable predictions of pile response for many cases of practical interest, thus offering substantial cost savings over a complete 3D analysis of tunnel–soil–pile interaction.

Numerical Simulation of the Influence of Bored Pile Construction on Adjacent Existing Pile Foundation

AbstractThe procedure of constructing the bored piles of a new bridge is simulated by using the three-dimensional finite element method. The variance of soil body stress, the deformation in each step of the construction, and the stress variance of the existing pile foundation are predicted. The following analysis results are obtained: (1) The construction of a new abutment affects the displacement and structure stress of existing bridge piles; (2) Every step of the construction process affects the displacement of the lower part of the foundation pit; (3) The influence of the first and third steps on the stress of the existing bridge piles is larger compared with other steps; (4) Given the surrounding soil disturbance it causes, the construction of a new pile foundation has little influence on the settlement of existing bridge piles. In view of these findings, the following guidelines are recommended: (1) The first and third steps should be given more attention as they are the key sources of risk; (2) The ...

Numerical Modeling of Piles in Collapsible Soil Subjected to Inundation

Collapsible soil exhibits considerable strength when it is dry, but when it is inundated, it loses its strength and experiences significant volume reduction. Foundations on collapsible soils subjected to inundation may experience sudden settlement without any increase in the in situ stress level. Pile foundations are often used to penetrate layers of collapsible soil to transfer the load to a lower, stronger soil layer. Nevertheless, because of indentation, these piles may experience negative skin friction, which may lead to significant reduction in the pile capacity, or perhaps separation of the pile from its cap. In the literature, although several theories can be found dealing with the design of pile foundations and the prediction of negative skin friction on these piles, little to none was found to estimate the negative skin friction because of the sudden collapse of the surrounding soils. This is mainly because of the difficulties associated in modeling collapsible soil experimentally or numerically. This paper presents a numerical model, which is capable of incorporating the effects of inundation of collapsible soil, and the negative skin friction on axially loaded vertical piles. The model uses the theories of the unsaturated soil and soil-water characteristic curve to estimate the effect of soil suction because of progressive inundation. The procedure takes place by estimating the change in the soil properties and the irrecoverable soil-volume change during inundation. The paper also presents the results of laboratory tests conducted on a single pile driven in collapsible soil subjected to inundation. The measured values of the negative skin friction compared well with the predicted values from the numerical model presented. The pile design guideline is also presented to assist designers in predicting negative skin friction because of inundation. Furthermore, the theory developed can be used to evaluate the condition of an existing pile foundation driven in collapsible soil and to establish the remedies needed to avoid catastrophic failure.

Impact of Shield Tunneling on Adjacent Pile Foundation in Sandy Cobble Strata

The assessment of the impact of urban area shield tunneling on adjacent pile foundations is essential to ensure the stability and integrity of the piled buildings. Sandy cobble strata has force transmission feature of strong dispersion; therefore the equilibrium state of original strata would be easily destroyed by shield tunneling. In this study, model tests referring to the Chengdu Metro Line 1 and using the Φ520 mm EPB model shield machine are carried out to investigate the impact of shield tunneling on the existing pile foundation deformation and internal forces. Meanwhile, particle distinct element method is applied to analyze the soil particle movement feature surrounding the pile foundation from the micro view. Results showed that soil disturbance surrounding the pile foundation caused by shield tunneling makes the ground surface subsidence and pile foundation tilt, leading to the occurrence of stress concentration at the pile foundation base and nearby regions. Cutter head positive resistance and friction effect between shield machine and surrounding strata lead to the pile foundation three-dimensional movement and deformation during the shield tunneling.

Shaking table test on reinforcement effect of partial ground improvement for group-pile foundation and its numericalsimulation

In this paper, particular attention was paid to the seismic enhancement effect of group-pile foundation with partial ground improvement method that is used for existing pile foundations in practical engineering. A model test on a full system with a superstructure, a nine-pile foundation and a sandy ground was conducted with the shaking table test device. The model pile is made from aluminum and the model ground is made from Toyoura Sand. The shaking table test device is 120cm in width and 160cm in length. The maximum acceleration is 1g and the maximum displacement is 5cm. The maximum payload is 16kN and the highest frequency is 10Hz. The model ground is carefully prepared to obtain a ground with controllable unified density. Before the shaking table test, the pattern of the partial ground improvement for an existed group-pile foundation is carefully selected using numerical tests with a 3D elastoplastic static finite element analysis. In the analysis, the nonlinear behavior of ground and piles are described by the cyclic mobility model (Zhang et al., 2007) and the axial force dependent model (AFD model) proposed by Zhang and Kimura (2002) can take into consideration of axial-force dependency in the nonlinear moment–curvature relations. The applicability of the numerical analysis has been verified in previous works by comparing the numerical results with a real-scale field tests (Kosa et al., 1998). Based on the results from the numerical tests on seismic enhancement effect of group-pile foundation with ground improvement, an optimum pattern of partial ground improvement of an existing pile foundations has been picked out for shaking table test. A numerical analysis using the program DBLEAVES (Ye, 2007) is also conducted for the same optimum pattern for comparison purposes. The effectiveness of the partial ground improvement method has been proved by both the shaking table test and the numerical analysis.

Three-dimensional centrifuge modelling of the effects of twin tunnelling on an existing pile

Tunnelling activity inevitably induces soil stress changes and ground deformation, which may affect nearby existing pile foundations. Although a number of studies have been carried out to investigate the effects of tunnelling on existing piles, the excavation of only one tunnel is often considered. The fundamental interaction between twin tunnel construction and an existing pile foundation has not been thoroughly studied. In this study, a series of three-dimensional centrifuge model tests investigating the effects of twin tunnel construction on an existing single pile in dry sand were conducted. The influence of the depth of each tunnel relative to the pile was investigated by constructing the twin tunnels either close to the mid-depth of the pile shaft or near the pile toe. The pile settlement induced by the excavation of the twin tunnels is found to be closely related to the depth of each tunnel relative to the pile. The measured cumulative pile settlement due to tunnelling near the toe is about 2.2 times of that due to tunnelling near the mid-depth of the pile shaft. Apparent losses of pile capacity of 36% and 20% are identified due to the construction of twin tunnels near the pile toe and at the mid-depth of the pile, respectively. Although there is an increase in the axial force induced in the pile when a tunnel is constructed at the mid-depth of the pile, significant increases in bending moment is not observed in any of the tests.