Piled Raft Design Research Articles

Dynamic behaviour of a piled raft resting on saturated Kasai River sand

ABSTRACT The present work intends to calibrate the cyclic triaxial test and validate the 1-g shake table test of piled raft on saturated Kasai River sand. The outcome of the cyclic tests on Kasai river sand is already reported in the literature, but the present paper attempts to identify and explain the failure modes of the cyclic triaxial test as well as to numerically calibrate the experimental data(s) for relative density of 40% and 60%. For numerical validation and calibration, a bounding surface plasticity model is used. The calibrated parameters obtained from the element tests are used for validation of 1-g shake table test of piled raft resting on saturated sand. The numerical predictions are in good agreement with the experimental observations (in terms of acceleration, pore pressure). In addition, the effect of superstructure loading on piled raft is investigated in terms of variation of pore pressure (or effective stress) near and away from the structure and mechanism for vertical settlement of piled raft is identified. The relevant parameters related to seismic design of piled raft are identified and design guidelines in terms of strength and serviceability has been proposed.

Behaviour of piled raft foundation in sand subjected to combined V-M-H loading

A combined piled raft foundation (CPRF) is generally used for massive and tall structures. The conventional design of piled raft does not include the capacity of piled raft under combined lateral and vertical load. However, vertical (V), horizontal (H) and moment (M) loading may act on the foundation simultaneously. The present study examines the behaviour of piled raft foundation in sandy soil under V-M-H interaction using three-dimensional finite element analysis. The horizontal and moment capacity of piled-raft is assessed due to various influential parameters, such as relative stiffness of pile and soil, raft-soil relative stiffness, spacing to diameter ratio of the pile, length to width ratio of the foundation and pile head connection with the raft. It is observed that piled-raft lateral capacity increases due to combined loading than that of independent capacity. Both failure and design envelopes are also developed considering V-M-H interaction. Finally, the design implication of the outcome of the present study is presented using a case study. It is revealed that the design of piled raft with lateral capacity obtained from the combined action of the V-M-H load may reduce the number of piles as compared to conventional design performed using individual capacity.

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.

Piled Raft Design Approach for a High Rise Building on Tithal Road, Valsad, Gujarat

ABSTRACT The piled raft foundation has been efficiently used for the foundation of high rise structures. A typical high rise building generally imparts higher load on central core area due to existence of lifts, elevators and shear walls, comparison to the edge area. The shallow foundation may not control the settlement and the pile foundation cost may be expansive for the foundation of high rise structures. The piled raft foundation can reduce the total settlement as well as differential settlement effectively with a good economic performance. The components of a piled raft foundation should be designed in such a way that it can withstand the non-uniform load of a typical high rise building. The parametric numerical study has been carried out for the different piled raft configurations to propose the foundation of a 14 storey high rise building under Gujarat housing board project on Tithal road, Valsad. The present numerical study is based on the preliminary design strategies of piled raft foundation serving the settlement criteria as well as economic performance of the chosen arrangement of piled raft configurations. All the configurations of piled raft foundation were studied in the terms of maximum settlement, differential settlement, bending moment and pile axial load. The study shows that the reduction in the pile length at specified position was found effective to reduce the overall cost of construction, whereas the permissible values of settlements were always obeyed. Plaxis 3D finite element tool has been adopted for the present numerical analysis. Keywords: Pile, Raft, Settlement, Differential settlement, Pile axial load, Optimization Cite this Article Uttam Kumar, Vasanwala SA. Piled Raft Design Approach for a High Rise Building on Tithal Road, Valsad, Gujarat. Journal of Geotechnical Engineering . 2018; 5(1): 1–10p.

Load sharing ratio of pile-raft system in loose sand: an experimental investigation

This technical note presents the results of a series of tests carried out on instrumented model pile-raft foundation to investigate the load sharing ratios between the foundation’s components where a loose sand layer is encountered near the ground surface. Four different pile-raft models with two different pile numbers and lengths were considered. In each model, the settlement of the raft and the load of each pile were monitored during the load application process. The experiments showed the effectiveness of pile length on reducing the overall settlement of the pile-raft system. The results showed also that the proportion of load transferred to any pile in a closely spaced pile group increases with its distance from the centre of the group. Unlike some previous studies that classified soil profiles containing loose sands near the surface as unfavourable situations for piled rafts, the experimental results herein represent an evidence of the efficiency of piled rafts even in soil profiles with loose sands immediately beneath the raft. Indeed, ignoring the contribution of the raft would lead to unnecessary over-conservatism in the design of piled rafts.

Evaluation of the existing piled foundation based on piled–raft design philosophy

Abstract Many well-documented case histories for conventionally designed large pile groups proved that this design approach is conservative and dramatically increases the cost of foundation without almost any benefit neither to geotechnical capacity of foundation nor to serviceability. Monitoring results of these case histories, especially in the case of floating piles, have shown low efficiency of pile capacity usage due to direct transfer of considerable part of the load to the supporting soil via the raft. In this paper, a proposed methodology for the combined-piled–raft design based on the conventional philosophy is applied to evaluate existing conventionally designed piled foundations of two identical residential towers located in Cairo, Egypt. The proposed methodology considers a conventional factor of safety for piles, and consequently, it does not violate the existing building codes. The three-dimensional finite-element analyses are performed to evaluate the load sharing between raft and piles. A detailed geotechnical investigation is carried out to obtain soil stratification and material parameters. The pile–soil interface parameters are obtained through back analyses of the available pile load tests. The results of the analyses show that the predicted geotechnical capacity of the combined piled–raft foundation system considering the conventional factor of safety for piles considerably exceeds the design capacity of the original conventional pile group.

Effect of in-situ variability of soil on seismic design of piled raft supported structure incorporating dynamic soil-structure-interaction

Inherent variability of soil considerably affects the seismic design of piled raft supported structures. Conventional design of such structure adopts fixity at base level of superstructure and pile head. However, soil–pile–superstructure interaction largely affects the fundamental frequency and design forces in columns and piles. In contrast, fixed base assumption cannot capture soil structure interaction (SSI) effect. In addition, uncertainty in soil may further leads to a change in the dynamic behavior of the system. This study examines the effect of inherent variability of undrained shear strength of soil in seismic design of structures supported by piled raft foundation embedded in soft clay. Superstructure is modeled as lumped mass stick model and piled raft slab is modeled as rigid plate. Pile is modeled as Euler–Bernoulli beam element and soil resistance is modeled using linear Winkler springs attached to the pile. Dynamic analysis is carried out in time domain to estimate the responses. Monte Carlo simulation technique is used for probabilistic assessment of the fundamental frequency and forces at column and pile attributing a wide range of parametric variation of a representative soil–piled raft–superstructure system. The study shows that the fundamental frequency and forces in column and pile changes significantly due to soil variability.

Analysis of piled raft coefficient and load-settlement on sandy soil

A piled raft foundation is a combined foundation, which is developed to utilize the load-carrying capabilities of both raft and piles. To obtain an optimum piled raft design, it is important to properly evaluate and consider the load-sharing behavior between the raft and piles, which changes according to the settlement level of the piled raft. In this study, 27 three-dimensional finite element models were analyzed to investigate the piled raft coefficient with linear and nonlinear load-settlement behaviors. The length of piles was varied between 10, 15, and 20 m. The spacing between pile centers was varied between 3D, 5D, and 7D, and the pile diameter was kept constant. The number of piles and the distance between the exterior piles and the edge of the raft were maintained at 9 and 1 m, respectively. The sand conditions varied between dense, medium, and loose. The results indicated that the piled raft coefficient increases when the load-settlement curve is linear and decreases when the load-settlement curve is nonlinear. The influence of the incremental increase in pile length on the piled raft coefficient is more pronounced in short piles than in longer piles. The raft thickness has a negligible effect on the piled raft coefficient.

Piled Raft Design Strategies for High Rise Buildings

Piled rafts have been widely adopted as an effective foundation for designing high-rise buildings because of their efficiency in controlling the total and differential settlements and improving bearing capacity. In many cases the piled rafts settlements are likely to be large, which leads to an increase of the pile length and/or number of piles required to reduce the settlements. However, this increase does not satisfy the design requirements or an economical design. The majority of piled raft foundations has been designed with a uniform pile length and configuration. This paper describes the process of optimizing the design of a piled raft foundation for a high rise building in the Mazandaran province in Iran by considering an economical design methodology in which piles are placed more densely beneath the maximum load positions when the piled raft is subjected to non-uniform loads. By using the ELPLA software in the analysis process, the validity of the software is examined through the results of a report prepared on behalf of Technical Committee TC18 on piled foundations. The study shows that the pile arrangement method can help to considerably reduce the total and differential settlements with similar total pile length as well as the induced bending moments and shear forces of the raft. This study can help practicing engineers to choose pile and raft parameters with the pile arrangement method to produce an economical design.

Piled raft with hollow auger piles founded in a Brazilian granular deposit

Geotechnical projects typically achieve load transfer to the ground using shallow or deep foundations. The conventional design approach does not provide for the combination of these two types of foundation. The piled raft philosophy allows the association of the soil elements, raft, and piles to obtain technical and economic advantages over conventional design. The city of João Pessoa, in northeastern Brazil, has developed foundation practices with hollow auger piles in piled raft design. The coastal area of the city has topsoil layers with favorable conditions for using such a technique. This paper addresses the results of a research project with instrumented load tests on foundation systems of hollow auger piles and a piled raft. The analysis is based on the load–settlement curve through extrapolation criteria. The Poulos–Davis–Randolph (PDR) method is applied according to a trilinear and hyperbolic approach to simulate the load–settlement curve of piled rafts. The results indicate that the raft absorbs most of the load, and the raft–soil contact significantly increases the load capacity of the foundation. The PDR hyperbolic method could apply to practical use in the foundations of the region, as it allows a more detailed assessment of the behavior of the foundation and can forecast the behavior of the (locally nontraditional) piled raft foundation system.

Non-linear analysis of vertically loaded piled rafts

While the availability of 3D FEM or FDM analyses has greatly contributed to the increasing use of piled rafts for high rise structures, there is a need in industry for practical analysis methods which would also allow the adoption of piled rafts for more ordinary structures. For this purpose, a 3D boundary element solution is proposed for computing the non-linear response of piled rafts to vertical loads. The validity of the analysis is demonstrated through comparison with alternative numerical solutions and field measurements. Examples are given to demonstrate the basic importance of considering soil nonlinearity effects in design, thereby leading to more realistic predictions of the raft and pile response. The key feature of the proposed approach lies in its computational efficiency which makes the analysis economically viable not only for the design of piled rafts supporting high rise buildings (generally based on complex and expensive 3D FEM or FDM analyses) but also for that of bridges, viaducts and normal buildings.

Automatic optimization of piled raft design based on branch and bound method

SummaryPiled rafts are foundation systems that combine a shallow raft with the minimum number of piles beneath the raft, in order to achieve the necessary bearing capacity and the acceptable settlement restrictions. Its use is increasing in many countries, especially in skyscrapers in Europe and Asia, for which the foundation design can result in hundreds of piles due to the high load magnitude. However, controlling the most influencing variables on the foundation behaviour, while achieving an acceptable cost, is a challenge for designers. This paper presents an optimization procedure to determine the best raft thickness, number of piles and their location. The branch and bound optimization method was combined with a specific computational programme for piled raft analysis (GARP) providing an automatic technique to obtain the lower cost design. A building in Brazil and theoretical examples of a piled raft with different load sets are presented and analysed using this numerical approach. The results show that the presented automatic procedure provides interesting solutions, economically enhancing the design of piled raft foundation systems. The optimization process provides arrangements using lower number of piles while satisfying the constraints in both analysed cases. Copyright © 2014 John Wiley & Sons, Ltd.

Analysis of load sharing behavior for piled rafts using normalized load response model

The behavior of piled rafts is complex because of the combined nature of raft and piles that behave as a unit. For the optimized design of piled rafts, it is important to properly evaluate and consider the load sharing behavior between raft and piles, which varies non-linearly with settlement level. In the present study, the load response and load carrying capacity of piled rafts are investigated focusing on the non-linear load sharing behavior of piled rafts. A settlement-based load sharing model for piled rafts is proposed using the normalized non-linear load–settlement relationship with consideration of piled–raft interaction effect. According to the proposed load sharing model, the load sharing ratio αp decreases as settlement increases with rates depending on the load capacity ratio. The values of αp are higher for higher load capacity ratios. To check the validity of the proposed load sharing model, centrifuge load tests are conducted. The values of αp calculated from the proposed load sharing model are in close agreement with measured values.

Sustainable Design for Unpiled-Raft Foundation Structure

This abstract forms Part 1 of 4 of full report on research work: “Bearing and settlement behaviour of piled-raft in soft clay.” The convention approach on piled-raft design tends to ignore load bearing and settlement contribution from the raft slab. Thus, selecting an effective raft size taking into effect of the soil-structure interaction environment is often neglected and this resulted with conservative design, expensive piled-raft foundation structure, depleting of resources and ineffective construction – in all, expensive and non-sustainable work This report evaluates on the conventional design approach, problems and limitations faced and propose possible alternative design approach to derive an optimum raft size which are both effective and practical based on the load bearing and settlement criteria. The model would then be used on piled-raft foundation in the next phase of research work to study any significance contribution from the raft and it level of contribution through similar parametrics. Analyses work and charts done would be used to support the selection of the most efficiency raft size model through the use of FEM geotechnical software in both short and long terms design consideration with structure founded on homogeneous normal consolidated soft clay overlaying a thickness of firm clay soil strata. As such, all presentations in this paper would only be encompassing solely on unpiled-raft foundation design under undrained condition since clay soil is expected to consolidate with times and gets firmer and stronger in long term. A sustainable design chart together with self-explained flowchart to serve as quick-reference design guide is developed for completeness.

Rational Design of Piled Raft

Piles have been traditionally designed to ensure the transfer of the whole structural load to the soil in which they are embedded. It is now largely accepted that this way of doing is unduly conservative, especially in those cases in which an unpiled raft is sufficient to guarantee a satisfactory safety level. Nevertheless, it is still very common in the daily practice, sometimes for the restrictions imposed by local codes and regulations.Much research efforts have been dedicated to a more rational design of piled foundations, essentially based on a proper collaboration among raft and piles not only for increasing the overall stiffness of an unpiled raft but, if required, also for increasing its capacity, for controlling differential settlements, for reducing the state of stress into the raft. With no claim to cover any single aspects, some design options will be presented, providing some insight into a rational approach to piled raft design.

The settlement behavior of piled raft in clay soils

The settlement behavior of a square piled raft in clay soil was investigated using numerical analysis. The emphasis was on quantifying the reduction of the average and differential settlements in soft and stiff clay soils. To obtain the detailed information on the piled raft, a nonlinear three-dimensional finite element analysis with pile–soil slip interface model was performed for various pile positions, pile numbers, pile lengths under the raft and different loading types. Based on the results obtained, the settlement aspects for an efficient design of piled rafts subjected to vertical loading are discussed for soft and stiff clay soils. It is found that the variation of reduction ratio (sPR,avg/sUR,avg) of soft clay was relatively greater than that of stiff clay, whereas the reduction ratio of soft clay was relatively smaller than that of stiff clays. It is also found that the required pile group-raft area ratio (Ag/Ar) for minimizing differential settlement in soft clay was slightly larger than that of stiff clay in the same pile array.

Elasto-Plastic Design of Piled Raft Foundation

When the loading sustained by the foundation is large, employing piled raft foundation is one of the best solutions. In the elasto-plastic design of piled raft, piles could improve the differential settlement and reduce raft thickness. As the raft sustains high earth and water pressures, by reducing the span length of raft and excavation depth, one can get economic design. Using elasto-plastic property of the pile is a better method to avoid increasing the pile length and pile diameter and making full capacity of the piled raft foundation in design compared to normal piled raft. With adoption of few small diameter piles, the raft thickness could be reduced significantly. This makes it much better than raft foundation.

Elasto-Plastic Design of Piled Raft Foundation

When the loading sustained by the foundation is large, employing piled raft foundation is one of the best solutions. In the elasto-plastic design of piled raft, piles could improve the differential settlement and reduce raft thickness. As the raft sustains high earth and water pressures, by reducing the span length of raft and excavation depth, one can get economic design. Using elasto-plastic property of the pile is a better method to avoid increasing the pile length and pile diameter and making full capacity of the piled raft foundation in design compared to normal piled raft. With adoption of few small diameter piles, the raft thickness could be reduced significantly. This makes it much better than raft foundation.

EXPERIMENTAL OBSERVATIONS ON THE BEHAVIOR OF A PILED RAFT FOUNDATION

The piled raft is a geotechnical composite construction consisting of three elements: piles, raft and soil.In the design of piled rafts, the load shared between the piles and the raft, and the piles are used up to aload level that can be of the same order of magnitude as the bearing capacity of a comparable singlepile or even greater. Therefore, the piled raft foundation allows reduction of settlements in a veryeconomic way as compared to traditional foundation concepts.This paper presents experimental study to investigate the behavior of piled raft system in sandysoil. A small scale “prototype” model was tested in a sand box with load applied to the system througha compression machine. The settlement was measured at the center of the raft, strain gages were usedto measure the strains and calculate the total load carried by piles. Four configurations of piles (2x1,3x1, 2x2 and 3x2) were tested in the laboratory, in addition to rafts with different sizes. The effects ofpile length, pile diameter, and raft thickness on the load carrying capacity of the piled raft system areincluded in the load-settlement presentation.It was found that the percentage of the load carried by piles to the total applied load of thegroups (2x1, 3x1, 2x2, 3x2) with raft thickness of 5 mm, pile diameter of 9 mm, and pile length of 200mm was 28% , 38% , 56% , 79% , respectively. The percent of the load carried by piles increases withthe increase of number of piles.

Design Strategies for Piled Rafts Subjected to Nonuniform Vertical Loading

The piled raft is a geotechnical composite construction, consisting of the three elements piles, raft, and soil, which is applied for the foundation of tall buildings in an increasing number. In a parametric study, 259 different piled raft configurations have been analyzed by means of three-dimensional elastoplastic finite element analyses. In the study, the pile positions, the pile number, the pile length, and the raft-soil stiffness ratio as well as the load distribution on the raft has been varied. In the scope of this paper, the results of the parametric study are presented and design strategies for an optimized design of piled rafts subjected to nonuniform vertical loading are discussed.