Piled Raft Foundation Research Articles

3D numerical study of the piled raft behaviour due to groundwater level changes in the frictional soil

ABSTRACT The Piled raft foundations are one of the most useful structures to control the total and differential settlements. In the present study, a three-dimensional analysis is performed using the finite element method (FEM) to evaluate the behaviour of the piled raft foundations due to the groundwater level changes. The results show that the groundwater level has a significant impact up to twice the raft width from the ground surface and subsequently by reducing the groundwater level more than twice the raft width from the ground surface, no significant changes are observed. Also, by increasing the pile diameter, the effect of groundwater level changes on the values of the differential settlement is greatly decreased.

A simplified solution for piled-raft foundation analysis by using the two-phase approach

In common practice, the pile–soil–raft interaction still remains a challenging problem in the analysis of piled-raft foundations. In the present study, a simplified analytical approach is introduced to analyze a vertically-loaded piled-raft foundation by using a developed homogenization technique called the two-phase approach. In spite of classical and simplified methods in the literature, the proposed method considers the pile–soil interaction. The other major advantage is the ability to predict the axial pile load along the pile length. The problem is solved in the domain of elasticity and simple closed-form solutions are presented for the prediction of the settlement and the pile load sharing of a piled raft as well as the pile's axial force distribution along its length. The applicability of the proposed method is validated by considering case studies and field measurements. A comparison of the results indicates that the method can be utilized safely in a proper, quick, and effective manner with the least computational effort in comparison with sophisticated numerical approaches. The raft settlement can be accurately predicted while the pile load sharing might be over/under estimated. A parametric study is also carried out to investigate the response of piled-raft foundations including the influence of the parameters of the soil and the geometric characteristics of the piles.

Behavior of Piles with Raft Foundation using Safe Software

Piles with raft foundation are a hybrid type of foundation in which the loads coming from the super structure is partly shared by the raft foundation and partly by the piles. A G+20 Storey building is analysed and designed in ETABS software. The modelling and analysis of piles with raft foundation is done in SAFE Software by importing the building loads. The storey drift and story displacement of the structure has been studied for the superstructure loads using response spectrum analysis and time history analysis in both X and Y axes. Then behavior of piles with Raft foundation is studied in this work by considering different parameters. The Pile diameter, Spacing of piles, Number of piles, and raft thickness are varied and keeping height of the pile as constant, the behavior of pile with raft foundation is determined. Pile with Raft foundation is economical when compared to a single pile foundation or Raft foundation. In the present work of piled with raft foundation, the load is shared between pile and raft. Hence Soil settlement is gradually reduced when the piles are introduced below the raft. Finally the optimum dimensions of pile diameter and thickness of the raft are obtained for minimum settlement.

Estimation of axial stiffness on existing and reinforcing piles in vertical extension remodeled buildings

In this study, the load distribution ratio (LDR) and axial stiffness of reinforcing piles in vertical extension remodeling structure will be investigated through numerical calculation and case studies. By comparing the results between existing and reinforcing piles, the efficiency of the reinforcing piles will be observed. To further understand the LDR of piles, the effect of elastic modulus of raft, pile end-bearing condition, raft rigidity, and the relative position of reinforcing pile in a group were investigated by using 3D FE analysis. The numerical model was validated based on actual field measurements. In addition, the method in estimating the axial stiffness of reinforcing piles (Kvr) was presented which is based on (1) the axial stiffness of existing piles (Kve) by using the 3D computer-based method, YSPR (Yonsei Piled Raft), and (2) by reducing the axial stiffness of existing piles by considering the aging and the deterioration process. The results of this can justify the necessity for settlement-based design and improvement of piled-raft foundation of vertical extension remodeling. The conclusions in this paper will significantly contribute to ensuring the safety and integrity of the structure during vertical extension remodeling process and throughout the service period.

Numerical analysis of piled raft foundation under combined vertical and lateral loading

Due to lack of research regarding the response of a piled raft foundation under combined lateral and vertical loading, an attempt has been taken in this study to analyse the influence of vertical load on the lateral response of a piled raft foundation. In this study, numerical analyses are carried out on the piled raft subjected to combined vertical and lateral load through 3D finite element modelling. The soil profile used in this study is clayey soil underlain by a layer of sand and the effect of clay layer thickness to the embedded pile length is checked. The lateral load displacement curves for the piled raft and the pile group in the piled raft system are assessed and the effect of the pile positioning on the lateral response are observed. For the structural design aspect, the bending moment, shear force and lateral soil stress along the pile depth are estimated and the lateral load sharing between the piles and the raft are assessed. Prediction models are then developed for estimating the load sharing ratio, p-multiplier and the percent increase in the lateral capacity and the lateral soil stress.

Comparative Analysis of Axial Load Capacity for Piled-Raft Foundation with Changes in Groundwater Level

Groundwater affects the load carrying behavior of foundations as the effective stress changes with the location of groundwater level (GWL) where the non-linear variations in stiffness and strength and the GWL influence zones below the foundation are involved. In this study, the load-settlement curve and the axial load capacity of piled-raft foundation with changes in GWL was investigated based on the results obtained from the finite element (FE) analyses. The full depth range of GWL from the top soil surface to the depth of well-below pile base was considered in the FE analyses. Changes in the effective stress and state-dependent soil stiffness with GWL were quantitatively evaluated and considered. It was found that the axial load capacity changed most significantly for GWL depths from 0 to 1.0 times the raft width, indicating that the GWL influence depth for piled rafts is controlled by raft size. The GWL effect factors for the load capacity (Cw) and settlement (Sw) of piled rafts were proposed. It was found that the values of Sw were larger than for unpiled rafts. The maximum value of Sw for piled rafts was 1.55 for GWL at the top surface, which decreased logarithmically with increases in the depth of GWL. The correlation of Sw to GWL was proposed.

Comparative study of load distribution ratio of various piled raft foundations based on coupled stiffness matrix

SummaryLoad distribution ratio of piled raft foundation was investigated based on an interactive foundation analytical method that considers the coupling effect between the stiffness of the superstructure, the piled raft, and the soil. Series of numerical analysis was performed to verify the proposed analytical routine in comparison with various methods and field measurements. Through the comparative studies between various piled rafts with different aspects, it was found that the proposed analytical method derived accurate results in structural response compared with actual field data, in settlement, load distribution ratio, and raft bending moment. And it was also found that the proposed analytical method was capable of considering interaction between superstructure and foundation in predicting the behavior of building structures. As a result, it was found that in most cases, the pile in the piled raft foundation supports a majority portion of the structural load. However, for cases where the length of the pile was short and acts as a short column, the load distribution ratio between raft and pile may be overturned and shows significantly different behavior and load distribution ratio.

Seismic performance of end-bearing piled raft with countermeasure strategy against liquefaction using centrifuge model tests

This paper presents the results of centrifuge model tests conducted on piled raft foundations in liquefiable soils to investigate the performance of drainage techniques as a liquefaction countermeasure method. Two series of centrifuge tests were conducted on 2 × 2 end-bearing piled raft foundations with and without drainage wells in multilayer soil deposits. The results indicate that the drainage well systems significantly reduce the foundation settlement, the excess pore water pressure ratio, and the time of dissipation under seismic loading by providing shortened water flow paths. Variations of the acceleration response, bending moment, and axial force of the piled raft are shown to be dependent on the type of surface layer. In Test-1-E with the liquefiable surface layer, the acceleration response, bending moment, and axial force of the piled raft with drainage systems were significantly larger than those observed in the piled raft without drainage systems during shaking. In contrary, in Test-2-E including a non-liquefiable surface layer, bending moment and axial force of piled raft in the absence of drainage wells were larger than those of the piled raft system with drainage wells. However, the acceleration response of raft in both systems was almost the same during shaking. The recorded data of the piled raft systems with/without vertical drains and the surface gravel layer (as a horizontal drainage system) are compared in details.

Experimental Studies on Settlement Search of Piled Raft System

Present study is an experimental work to analyse the effect of size of raft, length, number of piles on the settlement of the foundation system. In this work, study has been carried out to understand the load sharing ratio of the pile and raft with different patterns of pile and of different lengths. It was observed that, the load carrying capacity increases and settlement ratio reduces for the piled raft foundation tested on dry sandy soils deposits with relative density 70%. Total 39 tests were carriedout for the different piled raft models of different raft sizes (15 x 15 cm, 17.5 x 17.5 cm, 20 x 20 cm), number of piles (4 nos., 5 nos., 9 nos.) and of different pile lengths (10 cm, 20 cm, 30 cm, 40 cm). With the increase in raft size, load carrying capacity increased from 8.9 kN (for 15 cm raft) to 10.54 kN (for 17.5 cm raft), 17.22 kN (for 20 cm raft) for 25 mm settlement. Length of pile considerably effect on the foundation settlement, for 5 kN applied load, raft settlement (15 x 15 cm) without pile was 10 mm which reduces to 3 mm, 2.4 mm, 1.8 mm and 1.4 mm for 10 cm, 20 cm, 30 cm and 40 cm pile lengths respectively. Similar results were observed for other pile dimensions. Additionally, it is also observed that, pile number has considerable effect on the load carrying capacity and settlement of foundation system.

Performance of piled raft foundation supported by either connected or unconnected piles

Piled raft is commonly used as foundation for high rise buildings. The design concept of piled raft foundation is to minimize the number of piles, and to utilize the entire bearing capacity. High axial stresses are therefore, concentrated at the region of connection between the piles and raft. Recently, an alternative technique is proposed to disconnect the piles from the raft in a so called unconnected piled raft foundation UCPR, in which a compacted soil layer (cushion) beneath the raft, is usually introduced. The piles of the new system are considered as reinforcement members for the subsoil. In the current study, the behavior of both connected and unconnected piled rafts systems has been studied experimentally. The effects of different factors, such as; the piles number and configuration, on the load settlement behaviour and pile load sharing are also investigated. The results indicate that when unconnected piles are used, the total settlement of the piled raft system is significantly reduced, and the pile load sharing considerably decrease, e.g., for piled raft with nine piles the total settlement has decreased by 35.6% and pile load sharing has decreased by 20.9%.

Coupled horizontal and rocking vibration, and seismic shear-wave scattering of a piled plate on a transversely isotropic half-space

A boundary element model of a piled raft foundation under horizontal and rocking excitations is presented. Excitations are time-harmonic and can be either horizontal loads and rocking moments applied to the raft, or a vertically-propagating seismic shear wave. The raft is modeled as a rigid circular plate bonded to the surface of the soil, and the pile is an elastic body embedded in the soil. The soil is a three-dimensional, transversely isotropic half-space. Contact tractions at the plate–soil and pile–soil interfaces are obtained by a numerical integration of Green’s functions for the aforementioned half-space at discrete points. The equation of motion for the discretized piled raft is obtained upon establishing equilibrium and continuity conditions between foundation and soil throughout those interfaces. The paper investigates the effect of foundation and soil parameters in the response of the system to both external and seismic excitations.

An Analytical Approach for Piled-Raft Foundation Design Based on Equivalent Pier and Raft Analyses by Using 2D Finite Element Method

The piled-raft foundation systems have started to be a very popular design method that is commonly used for especially high-rise buildings. In this foundation system, both raft and piles share the applied loads coming from the supper structures. There are several methods proposed by researchers in order to solve piled-raft foundation problems by simplified methods, approximate methods, and more sophisticated computer-based methods. These sophisticated computer-based solutions are not simply adapted to geotechnical works, and accessing these programs by everyone is not easy. The main purpose of this study is to present an easy and new technique for solving piled-raft problems with using 2D-FEM analysis combined with equivalent pier. In this study, design concepts of a pile-raft system prepared by 2D-FEM analysis combined with equivalent pier were presented. Furthermore, the test results obtained from this method were compared with the results that were taken from equivalent raft method and equivalent pier with single pile response in a sandy soil. A generic example of a piled-raft system including 3 × 3 number piles was designed, and its some properties such as length of pile (10–28 m), pier-soil stiffness ratio (1000–5000), and spacing between the piles (2–4 m) were evaluated and compared as variables. It was concluded from this study that equivalent pier with 2D-FEM analysis can make very similar predictions for calculating settlements of piled-raft system regarding on equivalent pier with single pile response method. As a result of this study: (1) as aspect ratio of piled-raft system increases, raft to pile stiffness ratio gradually increases for all s/d ratios, (2) the efficiency exponents obtained from this study ranges between 0.46 and 0.58 values, (3) stiffness of the piled-raft system gradually increases with increase of slenderness ratio for both two equivalent pier methods, and (4) as slenderness ratio increases, stiffness ratio of the piled-raft system contrarily decreases.

A Combo Foundation Does the Job: Piled-Raft Foundation Design for a Supertall Skyscraper

Lotte World Tower is a 123-story, 555-m supertall skyscraper with a 6-level basement for retail and car parking located at Jamsil-dong in Seoul, South Korea. (Editor’s Note: For additional information, see “Supertall in Seoul” in the July 2018 issue of CE Magazine, pp. 44-53.) The tower’s footprint is approximately 72 m x 72 m, and its structural form consists of a concrete core wall with eight peripheral 3.5 m x 3.5 m square-shaped mega-columns to resist both the gravity and lateral forces. To enhance structural rigidity, two belt trusses are positioned at levels 218 m and 347 m from the base of the foundation raft. The total serviceability gravity load is around 6,700 MN. The skyscraper opened to the public in April 2017 and is currently the tallest building in South Korea and the fifth tallest building in the world. A structure of such proportions demanded a foundation to resist immense loads and problematic subgrade.

Load-sharing mechanism of non-displacement piled-raft foundation in sand

Piled-raft foundations are recognised for sharing the total load by both the piles and the raft. The economic benefits of this system lead to its popularity in practice; however, research in this field has continued to lag behind owing to the difficulties associated with modelling and availability of field data. This is due to the fact that the load-sharing mechanism of the piled-raft foundation is a complex soil–structure interaction problem, which is governed by the pile/raft/geometry/soil condition and by their by-product parameters. This paper presents the results of an experimental investigation on a prototype model of a piled-raft foundation in sand. The effects of the governing parameters on the load-sharing mechanism are examined. The results of this investigation show that pile spacing and settlement govern the load-sharing mechanism between the piles and the raft. A widely accepted analytical model in the literature was modified in this study to take into account the effect of settlements and pile spacing on the load-sharing mechanism in a piled-raft system. The theory developed is validated by the current experimental results and those available in the literature. The design procedure is presented as a guide in the design of the piled-raft foundation in sand.

Experimental Study of a Square Foundation with Connected and Non-Connected Piled Raft Foundation Under Eccentrically Loaded

In the recent years, non-connected piled raft foundation has been considered as an economical and practical deep foundation in the situation that high shear and concentrated loads may occur at the connection of the raft and pile head. This paper was presented an experimental study of a square foundation on the effects of parameters such as S/D, L/D and etc. in two cases of connected or non-connected piled raft system under the eccentrically loaded raft. The results was showed that square raft in the case of S/D = 3 and L/D = 8, the bearing capacity of the non-connected piles is more than that of the connected piles. The results of the experiments was showed pile length is more effective than the pile spacing in connected pile raft system. However by decreasing pile spacing, bearing capacity is increased in non-connected pile raft and pile spacing is more effective than the pile spacing in non-connected pile raft system. Comparison of bearing capacity and settlement indicated in the non-connected piled raft system, the longer piles not only has not much effect in increasing bearing capacity significantly, but also has lower effect on the reduction of the settlement. Also in non-connected piled raft system by increasing the pile spacing reduced BPI (bearing pile index) wile in connected piled raft system increased.

Analysis of Load Sharing Response and Prediction of Interaction Behaviour in Piled Raft Foundation

Piled raft foundation is anticipated to show high load carrying capacity through partial load sharing between the pile and the raft components. But this complex load sharing behaviour due to the presence of interaction effects is not well known, and there is a scarcity of proper illustration and correlation among the interaction and load sharing factors. In the present study, prediction models are developed to estimate and correlate the load sharing factors and the interaction factors. 3D finite element modelling is performed for different foundation types in clayey soil underlain by sandy soil taking various design parameters. The nonlinear behaviour of the piled raft foundation, the load sharing factors and the interaction factors for every foundation types are evaluated, and a predicted model is suggested. The predicted expressions are also validated with the calculated results from the numerical analysis to a better goodness of fit. A proposed model is then suggested for the design of the piled raft foundation considering both the safety and serviceability conditions. The proposed simplified model provides an easy solution for the design of the piled raft foundation subjected to vertical loading.

Experimental Investigation for Effects of Mini-piles on the Structural Response of Raft Foundations

Mini-piles made their debut as a cost-effective way to stabilize the historical structures. Recently, mini-piles have increased in popularity all over the world and are being used for bridges, buildings, slope stability, antenna towers, and residential construction. This paper presents the preparing, executing, data acquisition, and result presentation for an experimental work concerns with five scale-down mini-piled raft foundation models. All models were prepared to study the effectiveness of the mini-piled raft foundation in reducing the settlement and the bending moments. Five tests have been achieved. The reference first test includes a raft foundation with 15mm thickness. Second, third, and fourth tests are mini-piled raft foundations with five mini-piles and with thicknesses of 15 mm, 10 mm, and 8mm respectively. Finally, the fifth test dealt with a single mini-pile 178mm in length and 6mm in diameter. It has been adopted to investigate the reference behavior of the single mini-pile. When they were used, the piles have 42 mm center to center distances. A scale-down factor of , a sandy soil with with of , and relative density of 60% have been considered in all tests. Test results indicated a 45% decrease in settlement for 15mm mini-piled raft foundation comparing with the reference 15mm raft foundation. Moreover, there is no significant difference in settlement between 15mm mini-piled raft foundation comparing with the 10mm and 8mm thick mini-piled raft foundations. Regarding to the bending moments, they decrease at the mid and edge of the 15mm mini-piled raft foundation comparing to those of the reference raft foundation. It has also been noted that the moments are inversely proportional to the thickness of the piled raft foundations. With respect to the mini-piles, it has been found that most of the pile axial loads are transferred to the underneath soil through friction and this friction increases as the raft thickness decreases.

IBEM-FEM model of a piled plate within a transversely isotropic half-space

This paper presents a numerical model of the vertical time-harmonic response of a piled-raft foundation under external and seismic excitations. The surface raft is a circular, rigid plate on the surface of the soil, while the pile is represented by a classical finite element bar model embedded within the soil. The soil is modeled as a homogeneous, transversely isotropic, viscoelastic half-space. The fully coupled response of the plate–pile–half-space system is modeled through a superposition of Green’s functions, in the framework of the indirect boundary element method. The article investigates the response of the coupled system for different sets of constitutive parameters, especially with regards to the effectiveness of the pile in reducing vibration amplitudes of the surface plate.

Seismic Response Characteristics of a Piled Raft in Clay

This paper presents a study on three-dimensional seismic soil–structure interaction analysis of a piled-raft (PR) foundation in clay by the substructure approach. Two different pile modeling techniques were adopted and compared with centrifuge shaking table test results reported in the literature. The effect of pile spacing on dynamic impedances, kinematic response parameters, axial pile forces and bending moments of PRs is studied. It is found that the presence of a rigid raft on a pile group results in significant variation in vertical and horizontal dynamic stiffness after a dimensionless frequency ([Formula: see text] value of around 0.22. The influence of raft on kinematic response parameters at the top of raft is found to be significantly influenced by pile spacing beyond a dimensionless frequency ([Formula: see text] value of 0.4. The axial forces generated at the pile heads of a PR–structure system during seismic shaking are quantified using a new dimensionless factor, and are found to be significantly influenced by the pile spacing. Pile layout in the PR system is found to play an influential role in seismic response as well as pile bending moment for a structure–PR–soil system studied.

Statistical modelling of impacts on a pile-raft foundation

This paper reviews performance of combined pile-raft foundations (CPRF). The goal of the study is obtaining the ability to design solid, reliable and cost-effective foundations allowing for minimum settlements. The task is to develop methods for calculating CPRF for optimum choice of the foundation raft size, dimensions and number of piles. To achieve these results, an analytical method was used, curves of the ratio of CPRF settlement to raft foundation settlement vs. total load N on the piles were plotted. The scientific significance of the study is in the method of engineering calculation of CPRF, which is better correlated to the experimental data and allows optimal distribution of loads between the raft and piles, ensuring reliability and operational integrity of buildings, reduction of settlements and the number of piles, which would lead to cost savings and reduced consumption of materials for foundations. A new coefficient showing the residual stress has been introduced, and a method for its calculation has been proposed. A new equation for determination of the share of total load N on the piles has been obtained. This equation is more general in comparison with the previously generated equations. It allows excluding the assumption that the stresses arising under the raft completely dissipate at the level of the pile end. Application of the developed methods will increase reliability and reduce settlements and the number of piles, which ultimately will lead to cost savings and reduced consumption of materials for foundations.