Piled Raft Foundation Research Articles

Interactive iterative methodology for seismic analysis of pile-raft supported nuclear power plant structure in a conventional framework

Due to the fast growth of the nuclear-powered industry, nuclear power plants (NPP) are being planned on soil sites, especially in active seismic zones. In such cases, a combined pile-raft foundation (CPRF) is considered a preferred choice for designers to support massive and stiff nuclear structures. As such, no study has been stated on the seismic design approach for NPP supported on the CPRF system. In the present study, an interactive iterative practical seismic design approach is proposed to estimate the dynamic response of a NPP resting on the CPRF considering continuum-based geotechnical and spring-supported structural models. The pseudo-static response of the continuum-based model is used to determine the soil-pile system stiffness for subsequent seismic analysis in a spring-supported structural model. A step-by-step algorithm is proposed to arrive at the final converged state of the combined system through an iterative procedure. A reasonably good agreement is achieved between the outcomes obtained from the suggested approach and the results of the experimental testing. The proposed approach is subsequently implemented on a typical NPP structure, and a good match is obtained with the results of the coupled time domain direct analysis, thus showing the reliability and practical applicability of the suggested approach. The suggested procedure can be used for the seismic analysis of NPP structures supported by the CPRF system.

Field Studies of the Stressed State of the «soil base – combined pile raft foundation» System of the Multifunctional Complex in Kharkiv City

In the projects of buildings and structures, in the last two to twenty-five years, in world practice, the so-called combined pile raft foundation (CPRF) is increasingly used, for which, in the design and calculations, it is assumed that the calculated load from the building is distributed between the piles and the raft, allowing up to 50% inclusion in the work (resistance) of the raft. As part of the scientific and technical support [1-8] of the construction object, instrumental automated monitoring of the stress state of the «soil foundation - combined pile raft foundation» system of the multifunctional complex on St. Pushkinska, 2 in Kharkiv (Ukraine) is carried out. The building has 7 floors and a mezzanine floor, including 2 floors of underground parking, 5 floors of shopping and entertainment area. There is a parking lot on the first 2 floors. The building system is a monolithic reinforced concrete frame with beamless floor slabs. The foundation of the building is a combined pile raft foundation with the centering of bored piles under the load-bearing elements of the frame. The soil base is represented by Quaternary alluvial-deluvial sediments – loams, sands and clays. As part of the implementation of the project, 23 sensors of two types were installed during the zero-cycle construction works: strain gauges (Geosense, England) to determine the stresses in the shafts of bored piles and in the upper and lower grids of the raft reinforcement and soil pressure sensors (Geokon, USA), which were placed under the sole of the raft. The results of the monitoring showed that the actual pressure under the raft does not exceed the pressure from its own weight of the raft (≈25kN/m2), which indicates the absence of inclusion in the work of the raft due to insignificant relative settlements of the combined pile raft foundation. Taking this fact into account when modeling the "base - foundation - structure" system and calculation in the SOFiSTiK showed a qualitative and quantitative convergence of effort values in reinforced concrete structures of a combined pile raft foundation.

Numerical study on settlement-dependent variation of raft-soil-pile interactions for large piled raft in clay soil

In a piled raft foundation, the stresses applied on the soil from the raft and piles overlap, which leads to changes in the settlement and bearing capacity of the raft and each individual pile. In this numerical study, settlement-dependent variation of three interaction factors, namely piles-to-piles (P-P), raft-to-piles (R-P), and piles-to-raft (P-R) have been investigated for large piled raft in stiff clay. The unpiled raft is flexible, the piles are friction piles and the effects of pile number, spacing and length on these interaction factors are presented. The P-P factor increases and then tends to become constant at a certain settlement. In contrast, the R-P factor always shows an increasing trend with settlement. However, the P-R interaction factor initially decreases and then increases with settlement. At the final settlement equal to 0.35% of raft width, the P-P, R-P, and P-R interaction factors range from 0.95 to 1.10, 0.86–1.04, and 0.25–0.92, respectively. Predictive equations have been proposed for these interaction factors and thereby determine the load-sharing ratio and mobilized safety factor. Within the allowable settlement range satisfying the serviceability limit, a good agreement between the results obtained from predictive equations and the finite element analyses is demonstrated for different piled raft configurations.

Comparative Analysis of Raft, Pile & Piled Raft Foundation using Designing Software

Raft foundations are essentially a slab that extends the entire length of the building, sustaining and dispersing its mass to the earth. A pile foundation is a collection of columns that are erected or inserted into the ground to distribute weight to the subsoil underneath. A pile is a long, cylindrical construction made made of a sturdy substance such as concrete. Building loads are transferred to hard strata, rocks, or high-bearing-capacity soil using piles. Pinned raft foundations are a combination of a pile and a raft slab. They’re frequently used for large structures and when the soil is insufficient to avoid excessive settling. As a result of this, the soil is less strained. Pile foundations are necessary in areas where buildings are large and heavy, yet the soil beneath them is weak. In areas where settlement. Deep foundations with pile rafts can help move strata Adding piles to a raft increases the effective size of the foundation and can help it sustain horizontal loads.On a G+20 residential structure, the research of raft, pile, and stacked raft foundations was conducted using structural software safe 16. For a zone factor II earthquake, a building’s seismic study is completed. The pile raft foundation has less upward soil carrying load and less settling than the raft foundation, according to a study of the G+20 structure for pile, raft, and piled raft foundations.

Investigation of the Load-settlement Behavior of Box-shaped Deep Foundations

The load-settlement and monolithic behaviors of a new type of deep foundation in sand named Box-Shaped Deep Foundation (BSDF) were studied, and a comparison to Conventional Piled Raft Foundations (CPRF) was made by carrying out extensive numerical analysis. Physical model tests were also conducted to validate the numerical approach presented in this study, and it turned out to be a reasonable agreement. In the scope of this paper, the results of the parametric study are presented, and design strategies for an optimized design of BSDFs are discussed.

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.

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.

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.

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.

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.

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.

Analysis of the Influence of Pile-Raft Foundation Reinforcement on an Adjacent Existing Line Foundation

Pile-raft foundation reinforcement will have a certain impact on the adjacent existing line foundation, which will affect the normal service of the subgrade, and even lead to the instability of the subgrade. Until now, there have been few studies on adjacent construction problems, and there are few field experimental data available for us to consult. Therefore, this study relies on the construction project of Shanghai–Nanjing intercity high-speed railway close to the existing line, using in situ monitoring methods, such as stress shovels, horizontal strain gauges, and inclinometers combined with finite-element calculation and rail-inspection vehicle-data analysis. The stress, displacement, and geometric linearity of the adjacent existing line foundation during the reinforcement construction of a pile-raft foundation were studied. Our aim was to measure the impact and optimize the existing roadbed-protection measures employed during the construction period. We found that the cumulative horizontal displacement of the existing line foundation slope toe during the construction period was 24.25 mm, and the lateral displacement rate was less than 0.59 mm/d. The distance between the two lines was 9 m. The horizontal stress of the foundation soil in the depth range varied according to extrusion and retraction with the different construction stages, and the extrusion stress was less than 10 kPa. The horizontal stress changes in different construction stages were not obvious; the track quality index (TQI) in the existing track inspection during the construction period increased by 129.58%, and the existing track geometric linearity fluctuated greatly. According to the test results, the excavation stage of the subgrade foundation pit was the most dangerous stage of the existing subgrade during the construction of the new line pile-raft foundation. Although the change of the horizontal stress in different construction stages was not obvious, the horizontal displacement of the slope toe was sensitive to the construction process. Therefore, it could be used as a key indicator to monitor the deformation and stability of the existing subgrade. The correction coefficient was obtained by combining the detection data of the track-inspection vehicle with the finite element calculation data, based on which the accurate estimation of the horizontal displacement of the slope toe after the excavation of the foundation pit was realized. The monitoring and evaluation method of the stability of the existing line foundation under the influence of the pile-raft foundation reinforcement construction was preliminarily established through field monitoring and the analysis of the track-inspection data. Based on this method, relevant early warning values were proposed. The test results and engineering measures ensured the safe operation of the existing line foundation. This has important theoretical significance for guiding the construction of a new subgrade of adjacent existing lines.

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.

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

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.

Adaptive Optimization Method for Piled Raft Foundations Based on Variable Pile Spacing

Stricter control of the differential settlement is always required in important buildings more than in ordinary buildings. It becomes a necessity to find a simple and efficient optimum design method for pile foundations in terms of performance and economy. In this paper, an adaptive optimization method (AOM) is proposed to reduce the differential settlement of the pile group and piled raft, in which the piles are located in appropriate locations according to the settlement characteristic of the raft. Piled raft foundations with different types of load and different raft shapes are optimized using this method; soil inhomogeneity and nonlinear characteristic are considered during this process. The optimization results show that the reductions of the differential settlement are more than 80%. The overall foundation performances are improved as the maximum settlements of the foundations are reduced. The maximum bearing capacity of the pile group is no more than its ultimate bearing capacity after optimization design, and part of the excess bearing capacity can be translated into economic savings (AOM-ES). By keeping a good optimization effect of the differential settlement, the number of piles can be reduced by AOM-ES compared with the initial design. The AOM is robust and can be applied to the piled foundations of various raft shapes in layered soils under complex vertical loads with no significant impact on optimization efficiency.