Piled Raft Research Articles

Influence of lateral cyclic loading on the piled-raft foundation embedded in layered soil

The work includes an experimental analysis was performed to determine the impact of several variables on the behavior of laterally loaded piled raft model (2×2) having a ratio of length to diameter (L/D=40) and a diameter spacing (S / D) ratio of 3, 5 and 7 at frequency (0.2) Hz embedded in layered soil. The results are submitted as load-deflection curves and bending moment profiles. The effects of degree of saturation, number of cycles, level of cyclic loads, and the location of piles in a row are studied in this work. The results illustrated that increasing degree of saturation for clay soil (mid-layer) decreasing the lateral resistance of this model by about (25%, 31%, and 39%) for S/D=3, 5, and 7 respectively and bending moment by about (7%, 15%, and 17%) for S/D=3, 5, and 7 respectively. The piles in the front rows resist the soil with its full capacity, while in the trailing rows (i.e back) the shadowing effect tends to cause a reduction in soil resistance. The results clarified that maximum bending moment for trailing row less than the leading row by about 7%. The presence of vertical loads lead to an improvement in the lateral resistance of the model with dry clay soil about (28%, 26% and 12%) for S/D=3, 5, and7 respectively, whereas the increase in the horizontal resistance of the model with partially saturated clay soil about (40%, 45%, and 44%) for S/D=3, 5, and 7 respectively, compared with the absence of vertical loads.

A study of differential foundation settlement of piled raft and its effect on the long‐term vertical shortening of super‐tall buildings

SummaryIn this study, the long‐term differential foundation settlement and vertical shortening of super‐tall buildings are investigated considering the coupling effect. At first, a time‐dependent settlement model is proposed to predict the foundation settlement based on the construction sequence method, in which both soil compression and pile shortening are considered. The pile shortening including the elasticity, shrinkage, and creep contributes to the most settlement of a single pile. The distribution of foundation settlement of piled raft is pot shaped with a large value in the core while small values in the surroundings. A time‐dependent soil‐foundation‐structure system is then developed using a series of equivalent springs to represent the distributed foundation settlements, by which the vertical shortenings of shear wall and mega column are predicted considering the coupling effect and moisture distribution. The pot‐shaped distribution of foundation settlement of piled raft leads to the decrease of vertical shortening for shear wall while the increase of vertical shortening for mega column, especially in the low stories. Finally, some health monitoring records of the Shanghai Tower are applied to validate the proposed time‐dependent settlement model and soil‐foundation‐structure system. The predicted foundation settlements show reasonable agreement with the monitoring records. The predicted vertical shortenings with considering the differential foundation settlement agree better with measurements than those without considering the differential foundation settlement.

Thermal stress analysis of energy piles

This study is concerned with the problem of energy piles subjected to constant axial load and temperature variation uniform over the cross-section. In the first part of this paper, equations for the analysis of energy piles based on elastic theory and the load-transfer approach are described. Closed-form solutions are derived for a few simple cases. It is observed that temperature variation effects can be conveniently expressed by a factor of thermally imposed strain. In the second part of the paper, a study on the effect of the non-linearity of reinforced concrete on the performance of energy piles under tension is described. A newly developed finite-element model that is within the framework of the load-transfer approach is employed to simulate the response of a pile subjected to thermomechanical loads in isolated as well as in piled raft foundations. The results suggest that the performance of the structure is strongly affected by concrete post-cracking behaviour. Nevertheless, piles can accommodate thermally imposed strains if the ductility capacity is provided. The outcomes of this study may be combined with the knowledge of non-linear behaviour of soil–pile interfaces for a more rational performance-based design approach toward energy piles.

Numerical Analysis of Piled Raft Foundations Designed for Settlement Control on Steel Grain Silos in Collapsible Soils

The present article assesses the potential use of piled raft foundations for settlement control in grain silos, by performing numerical analyses with instrumented structures built in a particular city of the Mato Grosso state, deep in the Brazilian agricultural frontier. In this city, eight steel grain silos were constructed, and settlements were topographically monitored. The silos were numerically back analyzed using a finite-element-based hybrid numerical tool to verify interactions influence between the isolated (unpiled) foundation rafts during the grain storage season. A parametric analysis was subsequently carried out to verify the potential use of piled rafts to decrease settlement, as well as to improve design in terms of a better configuration concerning the use of compacted soil layers beneath the raft, raft thickness and pile length, and disposition within the foundation system. The numerical analyses indicated the use of a piled raft arrangement with a geometric factor equals to 0.09 as a proposed optimized solution aiming to minimize differential and total settlements. From this arrangement onwards, changes in pile spacing, pile diameter or in the number of piles, leading to higher geometric factors, were not able to provide significant settlement reductions. The increase in raft thickness and the use of compacted soil layers do not contribute to major improvements in the foundation settlement. The alternative optimized solution was able to ensure the serviceability and safety requirements of the silos, which can serve as a design benchmark.

Large scale model testing to investigate the influence of granular cushion layer on the performance of disconnected piled raft system

This paper presents the results of 1g model tests performed on instrumented model foundations, i.e., unpiled raft, single piled raft, single disconnected piled raft (DPR) and 3 × 3 DPR in sand under vertical load by using different granular materials and thickness of the granular cushion layer to investigate the effects of granular cushion on the performance of DPR system. The Settlement Efficiency (η) of the DPR which is nonlinear in nature is found to increase as the cushion thickness decreases and the mean particle size (d50) of the cushion material increases. A cushion thickness of two times the pile diameter and cushion material having d50 more than or, equals to 2 mm might serve as the suitable cushion material for the optimum performance of 3 × 3 DPR. The neutral axis and the zone of occurrence of maximum axial load of the pile in DPR are found to move downward with increasing cushion thickness. The piles in DPR are found to carry 35% of the total superstructure load within the settlement range considered in the study.

A numerical analysis on the effect of pile head connections on piled raft foundation subjected to vertical and static horizontal load

A numerical analysis has been performed on the piled raft subjected to uniform vertical load and static horizontal load on piled raft foundation. The applied horizontal load was about 6%, 8%, 9% and 12% of vertical applied load. A constant spacing and uniform length to diameter ratio of 3x3 pile group of piled raft models have been adopted in the present study. These models were validated first and then analysis of adopted finite element models resting on cohesion less soil has been carried out. The rigid and hinged pile head connectivity with raft has been considered. The effect of raft-soil stiffness has been adopted in the study. Normalized axial forces, normalized bending moment and normalized lateral displacement of piles throughout its length have been studied. In the case of rigidly connected piles of piled raft, the maximum positive moment of the piles was reached near the top and the maximum negative moment was found at the middle one-third section of the pile depth. The bending moment at the top was zero in case of hinged connected piles of piled raft and the highest positive moment was reached at one-third portion of pile depth from the top. Furthermore, at the bottom end of the pile it decreased to about zero. The influence of increment of horizontal load on vertically loaded piled raft has been examined too. With an increase in horizontal load from 6% to 12% of applied vertical load, the piled raft with rigid connected piles displaced 8 percent −20 percent lower than the piled raft with hinged connected piles at a constant value of relative raft rigidity.

Experimental Analysis on Dynamic Response of X‐Section Piled Raft Composite Foundation under Cyclic Axial Load for Ballastless Track in Soft Soil

Large‐scale model tests were established at a scale of 1/5 using a 7 m deep model tank with cross‐sectional dimension of 5 m × 4 m, to study the vibration response characteristics of ballastless track, embankment, and X‐section piled raft foundation under cyclic axial load, including the vibration displacement, velocity, dynamic soil, and pile stress. Cyclic dynamic loading can be achieved by controlling the loading frequency and cycles through the vibration servo control loading system. The test results are presented in the variation of dynamic displacement, velocity, and stress of X‐section piled raft composite foundation. The vibration displacement, velocity, and stress of the track, embankment, and pile foundation follow a pattern of vibration characteristics of loading sine wave. The vibration characteristics of loading waves can be identified easily from the peaks and troughs in the dynamic response of displacement, velocity, and stress at many locations of track slab, embankment, cushion, and underlying soil, at which the vibration response presents almost monotonically increasing tendency with the loading frequencies. With the increase of loading frequency, the vibration responses at the track structure and embankment have higher increasing rates than those at substructure (raft, cushion, and subsoil). The piled raft bears more dynamic load than cushion and subsoils, to ensure long‐term dynamic stability and safety of the foundation soils. The model testing results provide a better understanding of the dynamic response characteristics of ballastless track, embankment, and X‐section piled raft foundation under cyclic axial load in soft soil.

Reliability Analysis of Piled Raft Foundation Using a Novel Hybrid Approach of ANN and Equilibrium Optimizer

In many civil engineering projects, Piled Raft Foundations (PRFs) are usually preferred where the incoming load from the superstructures is very high. In geotechnical engineering practice, the settlement of soil layers is a critical issue for the serviceability of the structures. Thus, assessment of risk associated with the structures corresponding to the maximum allowable settlement of soils needs to be carried out in the design phase. In this study, reliability analysis of PRF based on settlement criteria is performed using a high-performance hybrid soft computing model. The new approach is an integration of the artificial neural network (ANN) and a recently developed meta-heuristic algorithm called equilibrium optimizer (EO). The concept of reliability index was used to explore the feasibility of a newly constructed hybrid model of ANN and EO (i.e., ANN-EO) against the conventional approach of calculating the probability of failure of PRF. Experimental results show that the proposed ANN-EO attained the most accurate prediction with R2 = 0.9914 and RMSE = 0.0518 in the testing phase, which are significantly better than those obtained from conventional ANN, multivariate adaptive regression splines, and genetic programming, including the ANN optimized with particle swarm optimization developed in this study. Based on the experimental results of different settlement values, the newly constructed ANN-EO is very potential to analyze the risk associated with civil engineering structures. Also, the present study would significantly contribute to the knowledge pool of reliability studies related to piled raft systems because the works of literature on reliability analysis of piled raft systems are relatively scarce.

Investigating the behavior of piled raft foundation in core wall foundation of high-rise buildings

Core structure is an indispensable part of high buildings. Normally, the foundation of the core structure has a raft of larger size than the other foundations in the same project; therefore, the foundation of the core structure can be viewed as a small piled raft foundation. Currently, when calculating the piled foundation of the core, it is mostly assumed that the piles system will bear all the project loads. But this calculation method is not suitable for the actual constructions as well as does not make full use of the bearing capacity of the structure and the ground, leading to using more materials and causing more waste. Core structure aims to increase both stiffness and horizontal load capacity in high-rise buildings, so the moment inside the core transmitted to the foundation is very large. One of the shortcomings of the Plaxis 3D Foundation software is its inability to declare the moment affecting on the foundation due to the loads in this program just includes distributed loads, line loads and point loads in the geometry model. Consequently, when using Plaxis 3D Foundation software to calculate the core wall foundation of high-rise buildings, this moment is converted into an equivalent pair of moments. The research shows that when the core foundation of a high-rise building is placed on soft soil, the pile carries 96%, most of the load transmitted to the foundation. However, when the foundation is placed on hard soil, the soil surrounding the raft will bear about 10% of the load transmitted to the foundation. If this matter is skipped, there will be a large error in calculation and design.

Improvement Separation Layer of Disconnected Piled Raft Foundations using Geogrid and Geocell Layers

To control the problems of high stresses and applied moments in the piled raft foundations disconnected piled were used and the piles considered as reinforcement to the soil rather than as structural members, the gap (separation layer) between the piles and the raft can be filled with stabilized granular material which distributes the stresses below the raft and decreases the concentration stresses at the pile heads. Fifteen model tests were performed using three precast concrete pile models embedded within soft clayey soil of undrained shear strength 30kPa. Flexi-Force sensors were used fixed on piles head and the surrounding soil to measure the sharing load transmitted to piles and the surrounding soil. Geogrid and geocell materials were used in different depths and in multilayers to reinforce the separation layer and increase its stiffness. The results show that when reinforcing the separation layer with geogrid at different depths, an increase in depth over the top of the piles reduced the bearing pressure. When used, geocell reinforcement significantly increases in load carrying capacity, and a significant reduction in footing settlement occurs.

Measurements and analysis of load sharing between piles and raft in a pile foundation in clay

This research presents the monitoring results and their interpretation on load sharing of the pile foundation during the construction of a high-rise (124 m in height) building in Bangkok, in soft clayey ground. Axial forces in several piles, pore water pressure and earth pressures beneath the raft in a tributary area were monitored through the construction period of the building. The raft of the pile foundation in soft clayey ground can share the load up to 10-20% even though the foundation was designed using the conventional approach in which the raft resistance is ignored. The benefit from the return of ground water table as the uplift pressure is recognized. A series of parametric study by 3D-FEA were carried out. The potential of utilizing the piled raft system for the high-rise building with underground basement in soft clayey ground was preliminarily confirmed.

EVALUATION OF DYNAMIC INTERACTION FACTOR OF RECTANGULAR PILED RAFT FOUNDATION FOR HORIZONTAL AND ROTATIONAL MOTION

The interaction between a building and the soil its foundation is built into is important in evaluating the seismic response of the building. The dynamic impedance of a piled raft foundation depends on a dynamic interaction factor between the raft and pile group. In this paper, numerical analyses were performed to investigate the frequency characteristics of the dynamic interaction factor of a rectangular piled raft foundation with various aspect ratios, subjected to a building inertial force caused by an earthquake. Based on the analytical results, a formula was proposed that considers the aspect ratio of the foundation in evaluatiing the dynamic interaction factor for a rectangular piled raft foundation. It was confirmed that the dynamic impedance calculated by a simplified calculation using the dynamic interaction factor and the dynamic impedances of a spread foundation and pile foundation, corresponds well with the results of the numerical analysis.

Experimental Investigation of Optimum Piles Spacing for Piled Raft Foundation in Sandy Soils

Kazıklı temeller, yapı yüklerinin yüzeysel temellerle güvenle taşıtılamadığı durumlarda kullanılmaktadır. Kazıklı radye temeller, kazık ve radyenin yük taşıma kapasitelerini birleştiren bir temel sistemi olması sebebiyle son yıllarda ön plana çıkmıştır. Bu sebeple söz konusu temel sistemlerinin güvenilir ve ekonomik tasarımı için kapsamlı teorik ve deneysel çalışmalar yapılmaktadır. Bu çalışmada, kum zemin içerisinde yer alan kazıklı radye temellerin tasarım parametrelerinin etkileri (kazıklar arası mesafe, kazık çapı, kazık uzunluğu ve rölatif sıkılık, vb.) deneysel olarak araştırılmıştır. Laboratuvar deneylerinde, çapı 20 ve 30 mm, uzunluğu 200 ve 300 mm olan model kompozit (çelik ve beton) kazıklar kullanılmıştır. Plan boyutları 160x160 mm, 240x240 mm ve kalınlığı 6 mm olan model radyeler, çelik plakadan imal edilmiştir. Farklı tasarım parametrelerine sahip model kazıklı radye temeller test edilmiştir. Deney sonuçlarından, kazıklı radye temellerde kazıklar arası mesafe arttıkça taşıma gücünün bir sınır değer kadar arttığı ve kazıklar arası optimum mesafenin 4D olduğu belirlenmiştir. Temel mühendisliği uygulamalarında kullanılmak üzere tasarım parametreleri önerilmiştir.

Geosynthetics reinforced interposed layer to protect structures on deep foundations against strike-slip fault rupture

In the present study, the interaction mechanism of a 10-story moment-resisting building frame sitting on the conventional piled raft foundation with a strike-slip fault rupture with a dip angle of 90̊ is studied via three-dimensional finite element numerical simulation using ABAQUS. In addition, an alternative composite foundation system with geosynthetics reinforced interposed layer between piles and raft is proposed to improve the safety and performance of foundation under strike-slip fault ruptures. The interposed layer is reinforced with two high tensile strength of the geotextile layer. The inelastic behaviour of piles under large ground deformations is simulated using moment-curvature relationships of the real reinforced concrete section of piles and ductility concepts. The performance of both composite and conventional piled raft foundations are evaluated in terms of the geotechnical and structural responses of foundations including rotational and translational displacements and shear forces of the raft, as well as shear forces and ductility capacity of piles. The obtained results show the superior performance of composite foundation with geotextile reinforced interposed layer in terms of a significant reduction in shear forces in the raft and piles, as well as ductility demand in the piles.

Estimation of the response of piled raft using nonlinear soil and interface model

ABSTRACT In this research paper, the complex interaction mechanism between different components of CPRF was simulated through 3D finite element analyses using the nonlinear interface and soil model for three case histories. The interface behaviour between concrete and soil was accounted using combined adhesion and friction model. The nonlinear soil behaviour was included using cap plasticity model, which has stress-dependant shear and compression yield surface, and captures the effect of stress history and stress path. The computed response parameters for the three case histories resulted in reasonably close agreement with the field measurements and led to significant improvement in the estimation of the load sharing of piles. The comparison of the results with the different methods showed that the stiffer interface results in a higher pile-raft coefficient and lesser settlement. The pile and soil stiffness found to vary with their position below the raft due to the different interactions.

Investigating the seismic isolation effect of the cushioned pile raft foundation in soft clay through dynamic centrifuge tests

Cushioned pile-raft foundation (Cushioned-PR) is a new type of foundation for sea-crossing bridges in deep water (more than 50 m in depth). Cushioned-PR is able to tolerate a certain amount of slide that occurs between the raft and the piles under strong earthquakes. This characteristic could reduce the seismic motion transmitted to the superstructure to maintain the integrity of the bridge. Thus, Cushioned-PR has become popular for bridges under the threat of strong earthquakes. However, the dynamic responses of the Cushioned-PR under various earthquake intensities in soft clay have not been thoroughly understood. It is essential to better evaluate the isolation effect of Cushioned-PR to improve the design. In this study, the non-linear behavior of Cushioned-PR and the isolation effect of its interposed layer are evaluated using centrifuge tests. A series of dynamic centrifuge tests are performed with a 3 × 3 pile group foundation embedded in soft clay under different earthquake motions. Both the cushioned pile raft foundation system (Cushioned-PR) and the connected pile raft foundation system (Connected-PR) are tested in the centrifuge tests. Acceleration and residual displacement of the super-structure, and bending moment of columns and piles, are monitored during the experiments. The results show that the interposed layer in Cushioned-PR has a significant impact on the bending moment and residual displacement of the pile-raft system. Based on the observation from this study, the maximum bending moment of the column in the Cushioned-PR is around 28%–54% of the values from the Connected-PR. On the other side, the maximum horizontal displacement of Cushioned-PR is about 2–10 times larger than that of the Connected-PR. The results show that Cushioned-PR can effectively reduce the seismic excitation transmitted to the super-structure if the seismic intensity is high. However, the benefit of Cushioned-PR is marginal under low-intensity earthquakes.

3D finite element analysis of effects of twin stacked tunnels at different depths and with different construction sequence on a piled raft

Underground transport infrastructure (tunnelling) plays a vital role to accommodate the rapid increase in urban population. Those tunnels are unavoidably constructed adjacent to high rising building supported by piled raft foundations which reduces differential settlements in the buildings. In this study, 3D numerical modelling using an advanced hypoplasticity soil model was carried out to investigate the responses of an existing (2 × 2) piled raft to twin stacked tunnelling at various depths. In each simulation, the first tunnel was constructed near the mid-depth of the pile shaft, the second tunnel was constructed either next to the toe of the piled raft (case ST), below but to one side of the piled raft (case SB) or directly beneath the pile toe (case SU). In addition, three more analyses were carried out to investigate the effects of change of construction sequence of twin stacked tunnels (i.e. cases TS, BS and US) on piled raft. The twin stacked tunnelling in case of SU caused the largest settlement but the smallest induced bending moment of the piled raft. On the contrary, the largest tilting of the pile raft took place in case SB. The first tunnel in each case caused the load transfer from the raft to the piles. On the contrary, the second tunnelling caused the increment in the load taken by the raft due to penetration of the raft into the ground. The different construction sequence of twin stacked tunnels had a substantial effect on settlement and tilting of the piled raft.

Nonlinear Interactive Analysis of Concrete Piled-Raft.(Dept.C)

The piled raft foundation has shown growing applicability in the last two decades as an economic foundation type, where the structural loads are carried partly by the piles and partly by the contact stresses under the raft. The ratio of the load carried by piles to that carried directly by the contact stresses under the raft depends on many factors. Among these factors are the relative rigidity of the raft, the shear strength of the soil under the raft, the shear strength of the soil at the piles tin, the frictional stresses along the piles, and the piles settlement. The investigation of the effect of these factors requires a reliable finite element model that takes into consideration the nonlinear soil behaviour and the interaction between soil, piles, and all. In this article, two 3-D nonlinear (finite element models of piled raft foundation are adopted. In the first model, the interface elements are placed between the piles and soil, as well as between the raft and soil. In the second model, the interface elements are placed between the piles and the soil only and a gap is formed between the raft and the soil (free standing cap). The soil is modeled using Drucker Prager yield criteria while the piles and the raft are treated as a linear elastic material. The proposed models are applied to piled rafts resting on medium clay while the piles are rested on medium dense sand. The results reflect, the interaction among the piles, raft and soil. The study shows the effect of the contact between the soil and the raft on the pile loads and raft moment and deflection.

Deep Excavation in Very Soft Soil Using Cross Wall as Lateral Support

Deep excavation in areas with very soft clay deposits need a good soil retaining system. Using diapraghm wall as soil retaining system for deep excavation is a good choice. Diapraghm wall is expected to limit the movement that occurs in the walls and avoid leaks that may occur in retaining system, this is needed to minimize damage to adjacent buildings. The top down excavation method by utilizing the basement floor as lateral resistance can be carried out to reduce the movement that occurs on the ground. In very soft soil areas with excavation distances to neighbors very close, the movement on the ground must be limited to minimize damage to adjacent buildings. Cross walls can be used to reduce the movement that occurs on the ground. Analysis finite element using the 2D Plaxis program was carried out to investigate the performance of the retaining wall. From the results of the analysis conducted shows that using a cross wall at a location below the raft pile can reduce the movement that occurs in the retaining wall and the excavation stage can be reduced. The results of this study can be used as a reference in the development of solution lateral support of soil retaining system in deep exaction especially needed in very soft soil condition.

Modeling the Response of a Piled-Raft Footing: Ultimate Bearing Capacity

The design of pile–raft foundation is a complex problem due to pile-to-pile, pile-to-soil, pile-to-raft and raft-to-soil interaction effect; hence, a simplified and standard procedure could not be evolved so far; in spite of the fact that this type of footing finds versatile applications in the field and its use as a foundation system helps to transfer a heavy load satisfactorily and that comes with a reduced settlement, and thus, it offers an economical design without compromising the safety of the structure. The present paper presents an analytical model to predict the ultimate bearing capacity of piled-raft footings. The study has been derived using an extensive set of experimental data obtained from the studies carried out in the laboratory. Finally, the response of a full-sized raft footing, designed using the proposed model, was simulated on PLAXIS-3D for the validation purpose. The results from the model are in good agreement with the values obtained during the simulation study carried out on the PLAXIS-3D. Sensitivity analysis carried out with a soil stratum of loose and dense sand indicates that there is a significant effect of the soil properties on the load-carrying capacity of the piled-raft footing. Accordingly, a relationship is proposed to take into account the effect of the sand relative density during the analysis.