Raft Foundation Research Articles

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.

Geotechnical Considerations for Foundation Design in Parts of Yenagoa, Bayelsa State, Niger Delta Nigeria

This research was to determine geotechnical properties of the subsoils in some part of Yenagoa and environs to obtain proper foundation design parameters, six towns in Yenagoa local government area, Bayelsa state was investigated. Six geotechnical boreholes was drilled and laboratory studies of soils samples were obtained from 0-20.25m deep. Subsurface soil profiles were delineated followed by determination of their index and mechanical properties, including Atterberg limits, particle sizes distribution, undrained shear strength, shear box test and consolidation coefficient. The general soil profile consists of (from top to bottom), , upper Silty clay horizon (0.0-5.25m thickness) soft to firm for Yenagoa study areas, Medium silty clay horizon (0.75 to 1.5m thickness) soft to firm Yenagoa study areas, low clayey sand horizon (0.75 to 1.5m thickness) soft Yenagoa study areas, peaty clay (1.0m thickness between 3.0-4.0m) soft Igbogene Yenagoa, upper sand horizon (3.0m thickness) silty sand Etegwe town Yenagoa, lower sand horizon (13.5 to 18.0m) silty sand to fine to medium and medium coarse appear in all the boreholes in Yenagoa. Yenagoa sub-soil show clay of high plasticity, silt of intermediate to high plasticity (CH, MI and MH) according to unified soil classification system from the results it shows that pad foundation is more economical in the study areas. Raft foundation is more economical in the six towns study areas of Yenagoa with Allowable bearing capacity of the upper clay layer ranges from 23-128KN/m2 In view of the significant variations in the stratification and engineering geological index properties of the soil in the six towns in Yenagoa while geotechnical data of one location cannot be used as a basis for design of foundation in a nearby land. Axial load carrying capacity for 305, 306mm, 356, 360mm, 406mm, 600and 610mm diameter for bore pile and tubular driven steel cased piles respectively were calculated for all the studies areas. Where high rise building is required in the various study areas. The study shows that the frequent causes of building collapse in Yenagoa are as a result of inadequate geotechnical investigations of the subsoil, poor quality materials, and poor work supervision.

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.

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.

Engineering Geological Properties of Subsurface Soils for Foundation Purposes in Parts of Bayelsa State, Niger Delta

The geotechnical properties of soil samples obtained from eight (8) boreholes from some parts of Bayelsa State were determined to assess their suitability as foundation materials. The geotechnical characteristics of the soils were determined from the laboratory and field works. The Atterberg limit results reveal that the liquid limit ranges from 46.5% to 98.3%, the plastic limit ranges from 23.5% to 56.3% while the plasticity Index values range from 17.1% to 51.3%. The clays are highly plastic (CH) in the USCS designation. The natural moisture content ranges from 35.9% to 91.3%, the moisture content is relatively high, this could be attributed to the wet season period of sampling. The particle size analysis disclosed that the cohesionless samples are predominantly fine to medium and medium dense sands. The triaxial test results shows that values of Cohesion (C) ranges between 16 – 40KN/m2 and the friction angle (ϕ) ranges between 3 – 6o. The result of the undrained shear strength of the clay ranges between 20Kpa and 24Kpa. The coefficient of consolidation (Cv) of the clay soil samples varies between 1.13 m2/yr and 2.89 m2/yr, the coefficient of volume compressibility, (Mv) for these same materials varies between 0.215 m2/MN and 6.338 m2/MN. Generally, indicating clay layers of high to very high compressibility. The calculated values for ultimate bearing capacity (qu) and allowable bearing capacity (qa) varies between 100.29KN/m2 to 151.49 KN/m2 and 33.42 KN/m2 to 50.50 KN/m2 respectively. The analyses showed that the values of ultimate bearing capacity increases with depth. The calculated settlement ranges between 69mm to 653mm with the thickness of clay of 7.5m and 30.0m respectively. Raft foundation is best suited for these weak, soft foundation materials for light structural loads but for heavy structural load, a pile foundation is recommended.

Parametric Analysis of Blast Load Effect On Diaphragm Wall In Basement

There are various kinds of loads acting on the basement structure of a building, blast load for an example. In this study, the effect of variation of the structural and soil parameter values on the response of the basement structure due to blast load will be analyzed. Thus, it can be seen the influence of the tested parameters. The deformation of the diaphragm wall will be the focus of this research. This research will be using Midas GTS NX program in modeling the basement structure of the building and the surrounding soil using the finite element method. The analyzed basement structure consists of flat slabs, diaphragm wall, king posts, raft, and pile foundation. The analyzed blast load is a surface blast load of 40000 liters of gasoline that occurs at a distance of 20m from the building and will be analyzed using the time history method. The analysis was carried out on 12 parameters: the void ratio of soil, the density of soil, cohesion, undrained shear strength, elasticity modulus of soil, poisson ratio of soil, porosity, structural damping ratio, damping ratio of soil, shear angle of soil, diaphragm wall’s thickness, and concrete’s quality of the diaphragm wall. From the results of the analysis, it is found that the parameters that affect the deformation of the diaphragm wall are the modulus of elasticity of the soil, density of soil, the poisson ratio of the soil, diaphragm wall’s thickness, and the concrete’s quality of the diaphragm wall. The soil modulus of elasticity has greatest influence on deformation changes, which is 282.27%, while concrete compressive strength of diaphragm wall has smallest influence, which is 0.87%.
 

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

Restoration of Tilted Buildings via Micropile Underpinning: A Case Study of a Multistory Building Supported by a Raft Foundation

This paper presents a real case study of a micropiling process that was developed to stop the continuous tilting of a 9-story residential building in Dakahlia, Egypt. Shortly after the construction of the building, the surface raft foundation exhibited severe settlement problems. In order to carry out a geotechnical investigation, boreholes were drilled around the constructed building. It was discovered that in addition to a thick, soft clay layer in the soil profile, there was also a crucial eccentricity between the centroid of the total building loads and the centroid of the raft. The issue needed to be addressed immediately, and a micropiling system was proposed to satisfy the geotechnical and structural conditions associated with the case history. In addition to describing the field measurements, detailed methodology, and micropile installation process, this paper also presents three different design approaches for determining the number and location of the micropiles. Although the underpinning process itself initially induced some settlement, micropiling the raft proved to be an efficient solution to stop the continuous tilting of the building. A micropile load test confirmed the advantageous effect of the grouting technique used for Type B micropiles, where the grout is injected under high pressure.

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

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.

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.

A multiaxial inertial macroelement for deep foundations

A hyperplastic macroelement is proposed as an efficient means for simulating nonlinear and multi-directional soil-piles interaction in the nonlinear analysis of structures. The macroelement relates the generalised forces exchanged between the superstructure and the foundation to the corresponding displacements and rotations of the foundation raft. The inertial effects developing under dynamic loading can be reproduced by coupling the macroelement with the participating masses of the soil-foundation system, obtainable with a modal identification of the pile group. Failure conditions of the foundation are described by a hyper-ovoidal ultimate limit state surface in the force space, which is calibrated through standardised procedures. The plastic behaviour, controlled by a series of yield surfaces with kinematic hardening, produces the desired directional coupling and allows to model the cyclic response and the irreversible deformation of the foundation. The macroelement is implemented in the analysis framework OpenSees for a prompt use in earthquake engineering applications. Its predictions under monotonic loads are validated against experimental data and advanced, fully coupled numerical modelling, while its dynamic response is tested on a reference case study.

Analysis of piled raft foundation behaviors in sandy soils using a numerical approach

Analysis of piled raft foundation behaviors in sandy soils using a numerical approach