Design Of Piled Raft Foundations Research Articles

Numerical Study on Optimization of Piled Raft Foundation on Stratified Soil under Static Load

Piled raft foundation (PRF) is a combination of pile and raft that provides adequate bearing capacity under the allowable settlement. This research aims to optimize the PRF design for a 48-story Commercial Bank headquarters building in Addis Ababa, Ethiopia, by conducting parametric study on various pile configuration using FLAC3D (Fast Lagrangian Analysis of Continua in three Dimensions) software. We analyzed 28 piled raft configurations and found that increasing the distance between piles increases the raft load sharing distribution by 11.5% while reducing settlement. Despite variations in pile length and diameter, a close load sharing between the pile and raft is accomplished at a separation of seven times the pile diameter (7D). The largest settlement decrease achieved was 9% for a 2-m pile diameter by doubling the pile length from 10 to 20 m, indicating that increasing pile length is ineffective for settlement reduction. We also proposed two new pile configurations that share load equally among pile and raft, and slightly increase total settlement below 10-cm limit. The results show that the first configuration exceeds differential settlement limit, but the second configuration reduces it by 95% and saves 40% of piles. This study provides a useful reference for designing PRF for high-rise buildings in similar soil conditions.

Experimental Analysis of the Behaviour of Piled Raft Foundations in Loose Sand

This paper presents the experimental analysis that was conducted on small-scale 1g physical models of piled raft foundation structures with a group of 2 × 2 piles in loose sand. The purpose of the piles was to reduce the settlement of the raft. The test program included twelve experiments, three of which were conducted on a raft alone and nine on piled rafts at pile distances of 3d, 4d, and 5d and pile lengths of 10d, 20d, and 40d, where d is pile diameter. The test results show that the current conventional approach to design of piled raft foundations, at a high safety load factor in piles that assume to take the whole external applied load, is very conservative. Instead, it is more economical to apply a low bearing capacity factor for piles as settlement reducers and maximize use of raft bearing capacity to carry part of the external load.

Improving the Performance of Piled Raft Foundations Using Deformation Adjustors: A Case Study

Complicated soil conditions are direct difficulties for high-rise building projects. A new device called a deformation adjustor, which is used to optimize the stiffness distribution in the piled raft system, has achieved good results for this challenge. This paper presents a case study on the application of deformation adjustors to improve the performance of a piled raft foundation. This case study describes the preliminary design of pile-raft foundations with deformation adjustors, followed by numerical analysis. Based on the numerical study, the potential savings are demonstrated due to the good performance of soil bearing capacity. Comparing the numerical results with the monitoring results in raft settlements, earth pressures, deformation amount of deformation adjustors, pile top reactions, and load-sharing ratios between soils and piles, the accuracy of the design schemes with an aided numerical analysis is verified. Through a long-term monitoring, soils below the raft carried 63% of the total applied loads, while the piles bear 37% of the loads. This case study proved that a piled raft foundation with deformation adjustors was an effective and economical design scheme, which can make full use of the soil bearing capacity. It is of great significance to facilitate the design and construction of piled raft foundations in complicated soil conditions.

Experimental and Numerical Investigation for Vertical Stress Increments of Model Piled Raft Foundation in Sandy Soil

Piled raft foundations have become a prominent foundation system in recent years as they combine bearing capacities of piles and raft. For this reason, comprehensive analytical, numerical and experimental studies have been carried out for reliable and economic design of piled raft foundations. In this study, vertical stress increments were mainly investigated experimentally and numerically. In the experimental study, model composite (steel and concrete) piles with a diameter of 20 mm and lengths of 200 and 300 mm were used. The raft was made of steel plate with plan dimensions of 160 × 160 mm and a thickness of 6 mm. The pressure gauges were placed at 2.5D, 5D and 7.5D intervals axially below center of model piled raft in the sand. The tests were carried out at different relative densities with the model piled raft. After model tests, numerical analyses were carried out with the FEM-based software ABAQUS, and the results were compared with the experimental results. From the tests results, it is determined that the piled raft foundations transfer loads to the deep layers with the help of piles, and the vertical stress increment reduces downward from the end of the piles. However, it is seen that the increase in relative density decreases the vertical stress increments in sand for same load. Some design parameters are suggested for use in foundation engineering applications.

How to reduce carbon dioxide emissions on infrastructure projects in the Middle East

A systematic approach is proposed for civil engineers in the Middle East to minimise the impact of their infrastructure projects on climate change. Based on value engineering principles and a carbon dioxide emissions calculator, the approach was applied to three case studies: temporary retaining walls for a deep tunnel; reuse of site-won material from an open-cut tunnel; and design of piled raft foundations. The results showed the approach was effective in reducing carbon dioxide emissions and could be beneficially adopted on future earthworks projects in the Middle East.

Nonlinear analysis of lateral load sharing response of piled raft subjected to combined V-L loading

Piled raft is the most desirable foundation option in the recent era for offshore and onshore structures which are often exposed to combined vertical and lateral (V-L) loads. Due to the scarcity of literature, this study aims to comprehend the response of the piled raft foundation (PRF) under combined vertical and lateral loads. A series of laboratory experiments and finite element analysis has been executed on a two layered soil profile. The lateral response of PRF and the piles attached to PRF are evaluated under combined V-L loading action and the consequences of the pile positioning on lateral load capacity are observed. From the structural point of view, bending moment and shear force are also estimated and the lateral load sharing between the raft and the piles are evaluated. The results attained from the tests indicate that the presence of vertical load has both positive and negative impact on the lateral load capacity of PRF depending up on the soil profile. A nonlinear multiple regression (NMR) analysis is also performed for the lateral load sharing behaviour and some empirical formulae are proposed to identify the lateral load sharing ratio which will enhance the design of PRF under combined V-L loading.

Influence of Variable Rigidity Design of Piled Raft Foundation on Seismic Performance of Buildings

In order to reduce the costs and improve the overall performance of building systems, the static optimized design with variable rigidity of piled raft foundations has been widely used in recent years. Variable rigidity design of piled raft foundations that support midrise buildings in high-risk seismic zones can alter the dynamic characteristics of the soil-pile-structure system during an earthquake due to soil-pile-structure interaction. To investigate these aspects, a nuclear power plant sitting on multilayered soil is simulated numerically. The paper describes a numerical modeling technique for the simulation of complex seismic soil-pile-structure interaction phenomena. It was observed that the total shear force on top of the piles and the rocking of the raft are reduced after optimization, whereas the displacement of the superstructure is nearly unaffected. The findings of this study can help engineers select a correct pile arrangement when considering the seismic performance of a building sitting on soft soil.

A Few Critical Aspects to Rational Design of Piled Raft Foundation for Oil Storage Tanks

Some level of settlement is allowed in the design of oil tanks if uneven settlement is controlled within allowable values. Considering the critical condition of piled raft foundation (PRF), that is, secure contact of raft base to the ground surface, PRF is considered as one of the rational foundations for the oil tanks. However, PRF has a complicated interaction with soil under horizontal seismic loading, especially if the tank rests on a liquefiable soil which may cause an extreme change of the soil stiffness under the tank. Regarding this complexity, the main concern in use of PRF for oil tanks is proper design of this foundation system. In this study, a series of centrifuge tests were performed to investigate the mechanical behavior of oil tanks supported by PRF on non-liquefiable and liquefiable sand. Using the observed results, such as accelerations of the tank and ground, displacements of the foundation and excess pore water pressures of the ground, some practical hints for reasonable design of piled raft foundation for oil tanks on non-liquefiable and liquefiable sand are discussed. In case of PRF of oil tank on non-liquefiable sand, the main concern in the rational design of the foundation is piles’ design and their punching effect on the raft, while the major critical issue in case of PRF of oil tank on liquefiable sand, is reduction of piles bearing capacity during the liquefaction that absolutely affects the performance of piled raft foundation.

Optimization Method for Irregular Piled Raft Foundation on Layered Soil Media

Important buildings such as nuclear power plants always require stricter control of differential settlement than ordinary buildings. Therefore, it is necessary to provide an optimized design for the piled raft foundations of important buildings. In this paper, a new optimization method (using different pile diameters and different pile spacing) was proposed for the design of piled raft foundations. This method adjusts the pile diameters and pile spacing according to the stress distribution at the pile top of the initial design to achieve a more uniform settlement of the raft and stress distribution on top of piles, which can solve the differential settlement problems caused by uneven loads of the superstructure. After optimized design, the differential settlement and integral bending moment of the raft decreased more than 64% and 52%, respectively, and the differential stress on top of piles decreased by at least 63%. The new method proposed in this paper could be applied to large‐scale piled raft foundations with complex superstructure loads.

Economic design optimization of piled raft foundations

Most conventional piled raft design methods focus on ultimate limit state and/or serviceability limit state, with economics being evaluated afterward. Economics is a basic requirement of the piled raft foundation design. The piled raft foundation cost estimation consists of two tasks: the material quantities and the construction labor, and its assembly include three tasks: installation, reinforcement and concrete. In this paper, piled raft construction cost is the objective function and the piled raft design parameters such as: allowable settlement, allowable differential settlement and maximum moment of the raft, are the performance indices. The design procedure in this study was obtained through two stages. At the first stage, different pile configurations were considered and the pile length, diameter and the space were changed for obtaining equal construction cost in all pile configurations. At the second stage, performance indices for each pile configuration were calculated. Each pile configuration has a maximum performance index, which is the optimum pile configuration. Sensitivity of the studies on the pile raft cost shows that, for typical pile raft foundation design, pile spacing and configuration are the key parameters in economic design. In general, it is recommended that, according to a given applied load (amount of load and load type), the optimum pile configuration method should be used in combination with a reasonable increasing of the pile length and/or pile positions to achieve the best possible behavior of the piled raft foundation.

Analysis of load sharing characteristics for a piled raft foundation

The load sharing ratio (apr) of piles is one of the most common problems in the preliminary design of piled raft foundations. A series of 3D numerical analysis are conducted so that special attentions are given to load sharing characteristics under varying conditions, such as pile configuration, pile diameter, pile length, raft thickness, and settlement level. Based on the 3D FE analysis, influencing factors on load sharing behavior of piled raft are investigated. As a result, it is shown that the load sharing ratio of piled raft decreases with increasing settlement level. The load sharing ratio is not only highly dependent on the system geometries of the foundation but also on the settlement level. Based on the results of parametric studies, the load sharing ratio is proposed as a function of the various influencing factors. In addition, the parametric analyses suggest that the load sharing ratios to minimize the differential settlement of piled raft are ranging from 15 to 48% for friction pile and from 15 to 54% for end-bearing pile. The recommendations can provide a basis for an optimum design that would be applicable to piled rafts taking into account the load sharing characteristics.

Effect of interactions between piled raft components and soil on behavior of piled raft foundation

The use of piled raft foundations has been considered as an economical alternative to conventional deep foundations because a portion of the applied load is thought to be transferred to the raft. In analyzing the behavior of a piled raft as a settlement reducer, an accurate estimate of settlement of the system should be made, based on the adequate understanding of nonlinear behavior of piles in layered soils as well as the interaction between the raft, soils, and piles. This study developed a computer program incorporating an approximate hybrid analysis method for the practical design of piled raft foundations. In the presented method of analysis, the interactions between pile-soil, raft-soil, pile-soil-pile and raft-soil-pile are considered, and the effect of nonlinear behavior of piles and the interactions between piled raft components and soil are investigated by comparing with 3-dimensional FEM program analysis results.

Settlement of Thick Clay Deposits under Piled-Raft Foundation and Design Considerations (Pile Dimensions)

Characteristics and histories of the deltaic deposits in geotechnical perspective are studied. Geotechnical issues of clay deposits under floating foundation systems also analyzed. Theoretical expressions and parameters were examined by an experimental study and numerical analysis on the laboratory scales and field measurement in this study. Also, piled raft foundation on thick clay deposits is designated to optimize pile configuration. The predictions of settlements of piled rafts foundation are proposed based on pile dimensions by utilizing a normalized Ap/nL and Bg/Br. Practical design of piled raft foundations is made for the light bridges and five story buildings on thick clay deposits to discuss the long-term settlement, and it is found that the piled raft is well applicable and effective on thick clay deposits, and that differential settlements of the foundation should be managed by designing the configuration of pile lengths and spacing.

A simplified algorithm for the design of piled raft foundations applied for the case study of a building erected within Żoliborz-Szczęśliwice glacial tunnel valley

Abstract A common engineering solution for excessive settlement with raft foundation (s) is the use of piles in order to reduce the vertical displacements, in this method, the whole structural load is transferred to the piles. This is an overly cautious approach, and there remains a need to find an optimal design method for a building’s foundations. Such a solution may be the piled raft foundation, which allows a reduction of the number of piles due to the integration of the raft in the bearing capacity of the foundation. The aim of the article is to estimate the contribution of foundation elements such as the raft and the piles in the bearing capacity of a residential building located in Warsaw, where the geological conditions are characterized by organic soil layers, principally of gyttja.

Settlement of Piled Rafts with Different Pile Arrangement Schemes via Centrifuge Tests

In the design of piled raft foundations, the control of total and differential settlements is crucial. On the basis of centrifuge tests, this study presents the feasibility of a fairly optimal pile arrangement scheme for reducing total and differential settlements. Two models of piled raft foundations having the same flexible raft and number of piles, the uniform pile arrangement model and the concentrated pile arrangement model, and one model having a rigid raft with a concentrated pile arrangement were designed for centrifuge tests. The settlements of three rafts of the piled raft models and the induced bending moments of two flexible rafts were measured during the load application process. The measured results were compared between the tests to illustrate the ability of the concentrated pile arrangement scheme in reducing total and differential settlements, as well as induced bending moment. Moreover, three piled rafts were simulated by the Plaxis 3D Foundation software package to calculate the total and differential settlement, as well as the induced bending moments, after which a comparison was made with the centrifuge tests to verify the reliability of these tests. The results show that the piled raft model with a concentrated pile arrangement can effectively decrease the total and differential settlements in comparison with the pile raft model with a uniform pile arrangement.

Application Study of Long-Short-Piled Raft Foundation

The long-short-piled raft foundation is an unusual type of building base. This kind of foundation is usually applied for pile foundation reinforcement of existing buildings when shortage of bearing capacity of piles occurs. The bearing capacity of pile foundation is improved and less settlement is expected. Since this method has so many obvious advantages, it is recommended in the reinforcement design of piled raft foundation of an existing building in Tianjin. Longer reinforced concrete bored piles are adopted as the supplementary ones. The bearing capacity of this kind of piled raft foundation was studied. The settlement was also analyzed with the National standard method together with the finite element numerical method. According to the study, the bearing capacity of piled raft foundation is enhanced effectively after adding piles. Also, the results show that the total settlement and differential settlement during the construction is in control respectively.

말뚝지지 전면기초의 3차원 근사해석기법 개발

철지반의 비선형성을 고려한 말뚝지지 전면기초의 3차원 해석기법(YSPR)을 개발하였다. 전면기초는 6개의 자유도를 가진 평면쉘 요소로, 말뚝은 보-기둥 요소로 모델링하여 전면기초와 결합하였다. 또한 말뚝두부 및 지반의 강성은 <TEX>$6{\times}6$</TEX> 강성행렬로 모델링 하였으며, 전면기초-말뚝-지반의 상호작용은 비선형 하중전이함수를 이용하여 선형/비선형거동의 모사가 가능하도록 하였다. 기존의 단순해석기법, 유한요소해석 및 현장계측값과의 비교 분석 결과, 본 해석기법이 대단면 말뚝지지전면기초에서 말뚝의 축하중 분포와 침하량을 비교적 정확히 산정하는 것으로 판단되며, 이러한 검증을 토대로 실제 대단면 기초설계에 대한 적용 가능함을 확인할 수 있었다. A three-dimensional approximate computer-based method, YSPR (Yonsei Piled Raft), was developed for analysis of behavior of piled raft foundations. The raft was modeled as a flat shell element having 6 degrees of freedom at each node and the pile was modeled as a beam-column element. The behaviors of pile head and soil were controlled by using <TEX>$6{\times}6$</TEX> stiffness matrix. To model the non-linear behavior, the soil-structure interaction between soil and pile was modeled by using nonlinear load-transfer curves (t-z, q-z and p-y curves). Comparison with previous model and FEM analysis showed that YSPR gave similar load-displacement behaviors. Comparison with field measurement also indicated that YSPR gave a reasonable result. It was concluded that YSPR could be effectively used in analysis and design of piled raft foundations.

Closure to “Contribution to Piled Raft Foundation Design” by Widjojo A. Prakoso and Fred H. Kulhawy

Closure to “Contribution to Piled Raft Foundation Design” by Widjojo A. Prakoso and Fred H. Kulhawy

Discussion of “Contribution to Piled Raft Foundation Design” by Widjojo A. Prakoso and Fred H. Kulhawy

Discussion of “Contribution to Piled Raft Foundation Design” by Widjojo A. Prakoso and Fred H. Kulhawy

Contribution to Piled Raft Foundation Design

Raft foundations enhanced with deep foundation elements (typically piles), simply known as piled rafts, were examined to develop a more integrated, displacement-based, design methodology. Illustrative piled rafts were analyzed using simplified linear elastic and nonlinear plane strain finite element models. The effects of raft and pile group system geometries and pile group compression capacity were evaluated on the average and differential displacements, raft bending moments, and pile butt load ratio of the piled rafts. The results were synthesized into an updated, displacement-based, design methodology for piled rafts.