Piled Raft Research Articles

Experimental and Numerical Analysis of Piled Raft Foundation with Different Length of Piles Under Static Loads

In order to understand the effect of (length of pile / diameter of pile) ratio on the load carrying capacity and settlement reduction behavior of piled raft resting on loose sand, laboratory model tests were conducted on small-scale models. The parameters studied were the effect of pile length and the number of piles. The load settlement behavior obtained from the tests has been validated by using 3-D finite element in ABAQUS program, was adopted to understand the load carrying response of piled raft and settlement reduction. The results of experimental work show that the increase in (Lp/dp) ratio led to increase in load carrying capacity by piled raft from (19.75 to 29.35%), (14.18 to 28.87%) and (0 to 16.49%) , the maximum load carried by piles decrease from(9.1 to 22.72%), (15.79 to 47.37%) and (44 to 81.05%) and the response of settlement piled raftdecrease from (16.67 to 23.33%), (9.09 to 39.39%) and (30%) with increase the number of piles from 4 to (6 and 9) and (length of pile / diameter of pile) ratio increase to (14.14 and 21.2), respectively. The numerical and model test results are found to be in a good agreement.

Experimental Evaluation of Load Distribution between Piles in Case of Block Failure

In this study, the load distribution of the piles in the pile group was investigated in the case of the pile raft foundations failure under the effect of vertical loads in cohesive soils. For this purpose, loading tests were carried out on model pile groups in the laboratory. In addition, the effects of these conditions on the load distribution between the piles were examined by changing the pile spacing and soil properties. As a result of the experiments, it was determined that the piles in the corner region carried more load than the ones in the center region in small settlement amounts as in the literature. However, it has been determined that as the amount of settlement in the pile foundation increases, the soil between piles behaves like a block and approaches failure, and the piles in the center region of the pile group carry more load. As a result of excessive settlement, a fracture surface formed around the block and therefore the corner piles of the block began to carry less load. It has been determined that failure analyses should be performed considering the load carried by the corner piles after failure due to excessive settlement.

Factors affecting the lateral contribution of a raft in a piled raft system

Contribution of the raft to lateral loads in a piled raft foundation system can be influenced by various factors including vertical contact pressure, number of piles, pile length, and spacing to diameter s/d ratios. Design of the piled raft system is highly dependent upon the contribution of the raft which should be thoroughly investigated. In this article, an experimental and numerical investigation was conducted in order to assess the effects of various parameters on contribution of raft to lateral loads. In experimental approach, small-scale models of the aluminium raft and galvanized iron (GI) hollow piles were used for testing under vertical and lateral loads. Vertical and lateral load cells on piles were installed for real-time load monitoring and LVDTs were used for displacement monitoring. In numerical approach, finite element software PLAXIS 3D was used. A raft was defined as a plate element, while piles were modelled as embedded beams. Based on experimental and numerical investigations, it was found that vertical contact of raft with soil has a direct relationship lateral raft contribution. Because, due to vertical load, the firm contact between the raft and the soil develops, resulting in a significant lateral contribution. Depending on the raft’s vertical contribution, the lateral contribution of the raft was found to be ranges from 14% to 55%. The numerical results were found fairly close to the experimental results. As a result, PLAXIS 3D was used to conduct a parametric analysis and It was determined that vertical pressure was directly related while the other three parameters were inversely related to the lateral raft resistance.

Experimental and Numerical Modeling of Connected and Disconnected Piled Raft

Piled raft foundations, wherein a small number of connected piles are utilized as settlement reducers, have been widely used for high-rise buildings. Recently, a new technique has been introduced by disconnecting the piles from the raft and inserting a cushion layer between them. In such cases, the piles are considered as soil reinforcement components rather than structural members. To explore the feasibility of utilizing the disconnected piled raft foundation, a series of experimental model tests were conducted to examine the load–settlement behavior and pile load sharing ratio of the connected piled raft (CPR) and disconnected piled raft (DCPR) foundation systems using cushions of different thicknesses and stiffnesses. In addition, ABAQUS 3D finite element analyses were performed to analyze the axial stress along the piles and the bending moments along the raft. The results indicated that the total settlement of the system was minimized significantly when disconnected piles were used. In addition, the disconnected technique provided a significant reduction in the axial stress along the piles and the bending moment along the raft compared with that of the CPR. These findings may be accompanied by economic and environmental benefits.

Load-Settlement Response of Cushioned Piled Rafts with Varying Pile Lengths to Retaining Wall Movement

Load-Settlement Response of Cushioned Piled Rafts with Varying Pile Lengths to Retaining Wall Movement

Analytical analysis of settlement and interaction - floating granular piled rafts, single and group of two units with the effect of partial stiffening

ABSTRACT Analysis of granular piled raft (GPR) foundations that involves interactions between the pile, the soil, and raft is very challenging and an interesting area of research in geotechnical engineering. The present study pertains to the analysis of a single and group of two partially stiffened floating GPRs where the material in the top region of the granular pile (GP) is stiffened by using geosynthetics, which has better engineering properties. Analysis has been conducted by assuming the soil to be linearly elastic continuum and using the solutions due to Mindlin and Boussinesq for a point load acting in the interior and on the surface of the soil medium for estimating the stresses and displacements at any point in the soil medium. Computations were performed to assess and report the normalized shear stresses (NSS) at the granular pile – soil interface, fractional load shared by the raft and the same shared by the granular pile along its shaft and base (FLSR, FLSP, and FLSB), the distribution of normalized contact pressure (NCP) beneath the raft. The results have been reported in a non-dimensional form suitable to be used for design.

Physical Modeling and Analysis of Cast -In-Situ Reinforced Cement Concrete Piled Raft in Clayey Soil

The combined piled raft foundation (PRF) arrangement has been frequently used for various constructions across the globe such as high-rise buildings, railways and varies structures imposing heavy loads etc. This combined system plays an important role in minimizing the settlement and more importantly the differential settlement without compromising with safety of the structure. However, strategically located piles improve the overall structural performance of the system by minimizing settlement and improving its load carrying capacity. The load carrying capacity of either the pile or the raft is considered in traditional design methods. Number of research papers has revealed that this method is uneconomical. Such design method is correct when soil support below raft may vanish like in offshore structure. The present paper investigates the settlement behavior, structural interaction between pile, soil and the raft and load sharing ratio using series of in-house experiments. For this, tests were conducted on physically modelled reinforced concrete piles and raft embedded in compacted soil under steady static loads with different pile configurations viz., free standing pile, pile group, only raft, pile group without raft and piled raft foundation etc. The results of this analysis show that the pile raft foundation's ultimate bearing capability is significantly higher than the sum of load bearing capacity of pile group without raft and load bearing capacity of individual raft. The innovative design method in which bearing capacity of raft and pile considered is compared with traditional design method where raft capacity is neglected. Then there is increase in load bearing capacity by 175% to 600%. This variation depends on size of raft and number of piles provide. Also the load sharing of between pile and raft dependents upon individual capacities and is self-compensating or self-healing.

Analysis of Foundation Settlement Interaction among Multiple High-Rise Buildings

This paper aims to develop a relatively simple approach to examining the interaction among multiple foundation systems for closely spaced high-rise structures, termed here “urban forests”, and to assess the extent to which this interaction can influence the foundation performance of a building within this “forest”. To simplify the analysis and avoid undue numerical complexity, each foundation system is modelled as an equivalent pier, representing the deep foundations, the connecting mat or raft, and the soil contained within the piled raft system. The behaviour of a single equivalent pier is considered first, with the surrounding foundations being represented by an axisymmetric smeared ring outside the pier. After examining some general behavioural characteristics, a simplified approach using the concept of interaction factors is developed to facilitate estimation of settlement interaction between multiple adjacent foundations. The accuracy of this simplified approach is assessed via comparison with the axisymmetric approach. The significance of the interaction among foundations is examined for some simple cases. It is found that the settlement interaction depends largely on the characteristics of the foundations surrounding the building being examined. When all buildings are supported on foundations extending to a relatively stiff layer, the interaction effects tend to be relatively small. However, if one or more of the buildings is founded on a relatively short foundation system, interaction effects may be more significant. An example is provided to illustrate the application of the approach.

Numerical modeling of connected piled raft foundation under seismic loading in layered soils

AbstractUntil recently, the behavior of connected piled raft foundation was not fully understood in the seismically active region due to the complex dynamic soil–pile–foundation structure interaction. This concern arises when the soil deposit-supported foundations are stratified or heterogynous and subjected to high ground motion intensity. In the current study, a series of numerical analyses using ABAQUS software have been conducted on a pile group of (3 × 3) arranged into a square pattern to investigate the seismic response of piled foundations embedded in dry sandy soil (homogenous and layered), and how the amplification of propagated waves affects the bending moment along piles. For mesh generation, an artificial boundary condition using the tied-nodes approach was adopted to simulate the free-field motion of soil under earthquake excitation. The structure used a single degree of freedom with a lumped mass. Moreover, Mohr–Coulomb and linear elastic models have been chosen for soil and pile–raft, respectively. The results demonstrate that the foundation rocking increases in stratified soil compared to homogenous soil, irrespective of the seismic intensity. The maximum bending moment was observed at the pile head in homogenous soil and shallow depths in layered soil because of the kinematic interaction at the soil interface. The results also indicated that the amplification factor (acceleration at a certain depth to the acceleration at bedrock) was found to be 203 and 189% in homogenous soil for PGA values of 0.1 and 0.33 g, respectively. Almost there were no effects of seismic intensity in layered soil on the amplified waves transmitted into the soil surface.

Nonlinear Dynamic Soil–Foundation–Superstructure Interaction Analysis for a Reactor Building Supported on a Combined Piled–Raft System

There has been a rapid development of the nuclear industry on account of climatic change caused by global warming. The most favorable rocky sites for nuclear power plants are exhausted worldwide, and now nuclear facilities are proposed on soil sites. In a soil site, a combined piled raft foundation (CPRF) system can provide the desired level of performance for a nuclear structure under seismic loading conditions. The combined system of the nuclear structure with the CPRF involves complex dynamic interactions between the pile, soil, raft, and the superstructure. In the existing literature, the dynamic behavior of the CPRF system supporting massive and stiff structures is not well studied and investigated. Due to the highly nonlinear dynamic characteristic of the CPRF system supporting massive and stiff reactor buildings, this paper adopts interface nonlinearities (separation and sliding) through the master and slave concept, concrete and reinforcement material nonlinearities through the concrete damage plasticity model for piles and superstructures, and an equivalent linear approach considering soil degradation and damping with Mohr–Coulomb postyielding hardening behavior for the soil. After the successful validation of the modeling approach with the results of the experimental testing, the different aspects of the CPRF of reactor building are investigated, such as amplification, pile and raft behavior, pile–raft coefficient, separation and sliding, contact pressure, nonlinearity in the piles, and the response of the superstructure. This study indicates that the peripheral piles of the CPRF system could prevent the separation of the raft from the soil. The numerical result shows that damage in the CPRF system occurs near the top portion of the pile. The presented procedure to develop the combined model for the seismic analysis in the time domain could help to formulate guidelines for safety-related nuclear structures resting on the CPRF system.

Seismic Response of Pile-raft Foundations of a Building on Layered Soil-based Design in South India

Abstract This study aims to assess the seismic effects of a mid-rise building on the layered soil medium and to estimate the dynamic impacts of pile raft performance that have been studied using PLAXIS 3D-based finite element analysis, optimization of BSDPT, and economical building design. Further, to understand the stiffness of the raft and the effect of the piles on the surface and inside an excavation. The geotechnical analysis of raft with piles PLAXIS 3D simulation, Poulos-Davis-Randolph (PDR), and Stress Distribution on Pile Top (BSDPT) optimization method indicate the displacements have been reduced by 60% based on superstructure design. The column reactions were estimated by Indian standard code using STAAD-Pro. The study reveals that when the building is as-built on the ground, piled-raft foundations have a considerable favourable influence on the seismic behaviours of the building structure, such as a 20% reduction in load and 2.36 cubic meters (m³) pile volume reduction. This study provides insight into the impacts of pile-raft foundations on the seismic response of pile-raft foundations of the superstructures, helping to fill a knowledge gap and better understanding the dynamic assessment of the layered soil settings.

Static analysis of prestressed micropile-raft foundation with varying lengths resting on sandy soil

The micropile-raft system is used in the foundation system of buildings due to the combination of the advantages of raft foundations and micropiles. In this paper, the numerical model was calibrated and validated with a field test, and then several parametric studies were performed. The overarching aim of the study is to determine the efficiency of different arrangements of micropiles and the effects of these parameters on the behavior of piled raft foundations in sandy soil. Parameters including diameter, length, and distance of micropiles from each other were considered in the analyses. Prestressed rebars have been used to increase the load-bearing capacity and prevent premature buckling under severe compressive load. Therefore, to compensate for the weak bearing capacity of the raft foundation, a micropile system equipped with prestressed rebars was considered to increase the bearing capacity of the foundation. The results revealed that using a set of micropiles with a diameter of 30 cm and a length of 10 m had the maximum efficiency among other models. Furthermore, an innovative arrangement of micropiles was proposed in which a set of micropiles with a length of 30 m was placed at the middle region of the foundation, and in the peripheral area of the foundation, several short micropiles were used.

The behavior of micropiled raft foundations subjected to combined vertical and lateral loading: numerical study

The micropiled raft offers a foundation system that combines the advantages of the conventional piled raft and the efficient installation of micropiles. In this paper, a comprehensive numerical analysis was conducted on the behavior of micropiled rafts under combined vertical and lateral loading using finite element modeling. A numerical model calibrated against full-scale axial and lateral field tests is used to conduct the analysis. The soil profile was soft clay soil underlain by a layer of dense sand. Numerous cases were analyzed to assess the lateral performance of micropiled rafts while considering several factors that may affect the performance, such as micropiles spacing, raft thickness, the magnitude of vertical loading, micropiles length, and their arrangement. The difference between the lateral performance of a primary micropiled raft and that of an existing raft enhanced by underpinning micropiles was also investigated. The lateral load capacity of the micropiled raft, the bending moment, and the shear force along the micropiles depth were estimated. Moreover, the load-sharing ratio between the micropiles and the raft was assessed. Based on the parametric study results, significant guidelines for choosing suitable micropiled raft configurations for initial design purposes were provided.

The Effect of Jet Grouting on Enhancing the Lateral Behavior of Piled Raft Foundation in Soft Clay (Numerical Investigation)

Soft clay soils cannot usually support large lateral loads, so clay soils must be improved to increase lateral resistance. The jet grouting method is one of the methods used to improve weak soils. In this paper, a series of 3D finite element studies were conducted using Plaxis 3D software to investigate the lateral behavior of piled rafts in improved soft clay utilizing the jet grouting method. Parametric models were analyzed to explore the influence of the width, depth, and location of the grouted clay on the lateral resistance. Additionally, the effect of vertical loads on the lateral behavior of piled rafts in grouted clay was also investigated. The numerical results indicate that the lateral resistance increases by increasing the dimensions of the jet grouting beneath and around the piled raft. Typical increases in lateral resistance are 11.2%, 65%, 177%, and 35% for applying jet grouting beside the raft, below the raft, below and around the raft, and grouted strips parallel to lateral loads, respectively. It was also found that increasing the depth of grouted clay enhances lateral resistance up to a certain depth, about 6 to 10 times the pile diameter (6 to 10D). In contrast, the improvement ratio is limited beyond 10D. Furthermore, the results demonstrate that the presence of vertical loads has a significant impact on sideward resistance.

Parametric Investigations into the Analysis of Piled Raft for Multi-Storeyed Building

This paper presents the analysis of the piled raft for a 50-story building using an approximate method to estimate the settlement and load distribution of the foundation. The pile and soils are considered to be interacting springs, and the raft is represented as a thin plate. The model takes into account both the resistance of the piles and the resistance of the raft foundation. It is calculated how the raft, soil, and pile interact. The suggested technique enables the use of the finite element based program ETABS to quickly address the issues of small, non-uniformly arranged rafts and big, non-uniformly ordered rafts. The effect of different pile length and diameter is evaluated on the behaviour of piled raft. With an increase in pile lengths, the moments in the raft are found to increase while the settlement of the pile decreases. Further, increases in pile diameter are found to increase the moments in the raft while decreasing the settlement of the raft. The parameters such as pile diameter and pile length have a considerable effect on the response of a foundation considered in the present study

Effect of Infiltration on Single-Pile and Monopile–Raft Foundation Embedded in Unsaturated Sand

The present study discusses the effect of hydraulic loading in terms of water infiltration caused by rainfall on load-mobilization mechanism of single-pile (SP) and monopile–raft foundation (MPRF) embedded in unsaturated sand by means of the finite element-based computer program Plaxis3D. Soil was modeled using the conventional Mohr–Coulomb model incorporating Bishop’s effective stress, and the hydromechanical behavior was incorporated using the van Genuchten model. A fully coupled flow deformation analysis based on Biot’s theory of consolidation was employed to model the behavior of soil during water infiltration. Water infiltration at different rates ranging from 5 to 864 mm/day was simulated using the time-dependent flow boundary condition at the ground surface. After successful validation of the developed numerical model using the experimental study available in literature, the evolution of suction, effective saturation, and suction stress during the different rates of infiltration was investigated and the responses of SP and MPRF in terms of settlement and axial forces were obtained. The results indicated the contribution of suction stress in increasing the stiffness of soil to an infiltration rate of 240 mm/day, thereby increasing the capacities of both SP and MPRF. The mobilization of shaft resistance was greatly dependent on the suction stress, and a reduction in the suction stress upon infiltration led to a lesser mobilization of shaft resistances (reduced around 6%). This caused a higher pile deformation, leading to further mobilization of pile base resistance. The rate of infiltration and the raft dimension has significant effect on mobilization of raft and pile resistance within MPRF due to raft–soil and pile–raft interactions. A higher rate of infiltration led to a greater mobilization of pile resistance due to positive pile–raft interaction brought by reduction in the contribution of raft base resistance that was associated with reduction in Bishop’s stress. The outcome of the present study would help in better understanding the effect of hydraulic loading on the behavior of SP and MPRF and can be extended to pile group and combined pile–raft foundation.

Numerical Analysis of Settlement of a Piled Raft Foundation on Coastal Soil

There is a growing demand for multi-story buildings for residence and commercial purposes in coastal areas of Sindh, Pakistan. Such types of soils are generally considered more compressible with high groundwater levels, which may cause lower shear strength and higher settlement. The computation of the settlement of foundations requires the use of advanced constitutive models, which are not commonly used due to a lack of field or experimental data. This study is carried out to illustrate the use of an advanced soil model, i.e., Hardening Soil Model for the computation of settlement. For this purpose, numerical modeling was carried out using Finite Element Program PLAXIS 2D. Initially, the MC Model was utilized for the calculation of the settlement of a 10-story building in the coastal soil. In addition, parametric analyses for the effects of modulus of elasticity, permeability, and dilatancy angle were carried out. The results mainly suggest that the settlement of the building constructed on a piled raft foundation, predicted with the MC model, was 40% higher than that of the HS model. For prediction of settlement of the piled raft foundation, the results suggest that the HS model can be given preference as compared to the MC model. Doi: 10.28991/CEJ-2023-09-02-05 Full Text: PDF

Numerical investigation of rigidity and flexibility parameters effect on superstructure foundation behavior using three-dimensional finite element method

A comprehensive three-dimensional finite element of the superstructure foundation model includes the effect of lots of important parameters on the structure performance. Among these parameters, rigidity and flexibility of raft foundations and soil interactions become an important aspect of design due to their efficient, practical and economic performance compared to the isolated foundation for buildings and heavy projects. In this study, the elastoplastic constitutive model based on the three-dimensional finite element (FE) method is employed to investigate rigidity and flexibility parameters' effect on superstructure foundation behavior. The model is developed and calibrated using SAFE 12 and ABAQUS programs by comparing two case studies. Utilizing the Winkler approach, flexible soil foundations are designed. In addition, technics like turning the raft into a boxed-raft or piled raft are evaluated to improve the performance of the raft from a rigidity and flexibility point of view. Then, the effects of various factors and conditions which affect design such as geometry, modulus of subgrade reaction, stiffness of the foundation, the effect of soil and thickness of foundation, piled raft foundations (PRF) and boxed raft foundations are studied. The results demonstrate that the superstructure components such as shear walls and basement walls can significantly affect the deflection, shear stress, and bending moment of the raft and its structural design subsequently. Also, shear walls, as structural parts which transmit an important part of vertical and lateral loads to the foundation, can play an important role in the rigidity of the raft. It is concluded that using PRF instead of the simple raft, results in a much rigid foundation with more bearing capacity and less differential deflection. The results of this study have the potential to be utilized in the design of advanced superstructure, in which rigidity and flexibility act as the basic elements.

Effect of cushion types on the seismic response of structure with disconnected pile raft foundation

In recent years, the benefits of reducing the impacts of the earthquake on the seismic response of disconnected pile raft foundation (DPRF) have simulated increasing research on this type of foundation to provide references for foundation design. One effective way to study the seismic response of soil-structures is to investigate the scaled models in 1-g shaking table tests. In this paper, a series of scaled 1-g shaking table tests were carried out to study the seismic response of scaled nuclear power stations with DPRF under earthquake excitations founded in clay. Three different cushion types were adopted to study their different effects on the seismic response of the structure and the foundation. The fundamental structural frequency, horizontal displacement, and acceleration results showed that cushion A with well-graded gravel and cushion B with a mixture of two gravel sizes were better than cushion C with a single gravel size. Although the cushion type had an insignificant influence on the bending moments of the disconnected piles, the maximum bending moments in all cases were found to be proportional to the near-pile acceleration. Furthermore, design engineers should pay more attention to the rocking of a structure with DPRF under earthquake loads.

3D Numerical Analysis of Single Pile and Pile–Raft Subjected to Vertical Loading

3D Numerical Analysis of Single Pile and Pile–Raft Subjected to Vertical Loading