Piled Raft Foundation System Research Articles

The Influence of Raft Thickness on Settlement and Bending Moment of Pile Raft Foundation in Clay Soil

Geotechnical engineers often deal with shallow foundation designs to support relatively simple building structures. However, as the demand for construction grows over time, there is a need for larger and more complex building structures. This is closely related to the weight of the building that will be supported by the foundation as a substructure. Therefore, the development of knowledge about other types of foundations to support the structural loads above them is highly necessary. Pile foundations are a fairly good solution for supporting larger structural loads that cause significant settlement. In designing pile foundations as a structure, there will certainly be raft to combine each pile foundation into a unified group. Raft are often designed only for combining each pile, but in reality, It also provides additional influence on the foundation system besides just serving as pile heads and load distributors to the foundation system. The study will investigate the influence of raft thickness on settlement that occurs in piled raft foundations. The raft thickness varies, namely 0.25m; 0.40m; 0.80m; 1.50m; and 3.00m. The piled raft foundation will be modeled and analyzed using finite element program. This study shows that the raft thickness affects the differential settlement that occurs in the piled raft foundation system. The results indicate that the differential settlement can be addressed by designing the raft thickness.

Effect of asymmetry pile’s length on piled raft foundation system under earthquake load

In light of the global development of construction, particularly tall buildings and skyscrapers, researchers have spent years studying and developing types of foundations that are suitable in terms of endurance in heavily loaded and economic terms, particularly in weak soils under dynamic loading. In the event of high-rise buildings or weak soil media, the piled-raft foundation system is considered an efficient system that fits the design criteria. Several factors influence the performance of the piled-raft foundation system, among pile length, which was taken into account in the numerical study using the Plaxis 3D program. Four pile length scenarios are explored using 28 piles in a pattern 4 × 7 group with a rectangular raft of 10 × 20 and 1.5 m thickness. The central piles 3 × 2 are all the same length of 12 m or 20 m, but the others vary to 10 m, 8 m, and 6 m. In addition to the El-Centro seismic loading, a uniform static load of 300 kN/m2 was applied to the weak C- ϕ soil. The results reveal that the length of the asymmetric pile has a substantial effect on the behavior of the piled raft foundation. The symmetry pile’s length has greater static and dynamic load sharing than the asymmetry pile’s length. Static load sharing is greater than dynamic load sharing in all models, with static sharing values of 73.2% for the uniform pile’s length group L12 + 12 and 68.82%, 63.33%, and 55.96% for the asymmetry groups L12 + 10, L12 + 8, and L12 + 6, respectively. With decreasing external pile length, the dynamic load sharing values under 0.1 g intensity are 72.3%, 67.4%, 60.6%, and 55.2%, respectively. When the seismic intensity increases to 0.2 g, these values decrease by around 3%, and so on for 0.3 g. Vertical settlement increases as the exterior pile length decreases and the seismic energy increases. The maximum settlement values for L12 + 12, L12 + 10, L12 + 8, and L12 + 6 at 0.3 g seismic intensity are 137.9, 158.1, 169.7, and 171 mm, respectively. The differential settlement caused by seismic load is greater in the case of the asymmetry pile length group (L12 + 10). The differential settling increases with increasing seismic intensity in all models. The maximum lateral displacement increases as seismic intensity increases, whereas the length of the pile has no impact on the lateral displacement. When compared to a 12-m pile length, a 20-m pile will have a larger load sharing and less settlement, but exhibit a similar trend. The external pile’s length has a slight effect on the factor of safety where the reduction under the seismic load by about 18% for the both symmetry and asymmetry cases respectively in comparison to static case.

Three-Dimensional Analyses of Long-Term Settlement of Storage Tanks Supported by a Large Piled-Raft Foundation System

Three-Dimensional Analyses of Long-Term Settlement of Storage Tanks Supported by a Large Piled-Raft Foundation System

3D FE modeling of a complex multi-stage foundation underpinning for a high-rise building

ABSTRACT The piled foundation of an existing high-rise building in Cairo was assessed as inadequate. The building foundation consists of 370 piles arranged below the columns and walls. The building was constructed in 1977, and up to the start of this investigation in 2014, it was just a concrete skeleton without finishing. The existing foundation was subjected to underpinning in 1985 using micropiles to support new lateral load-resisting elements proposed by the project structural consultant. According to the new Egyptian codes of practice, underpinned foundations are still considered inadequate and it was proposed to add additional micropiles with a raft connecting the different pile groups. This paper presents the 3D FE modeling for this complex multi-stage foundation underpinning considering the construction sequence and the different interactions between the original piles, existing and proposed groups of micropiles, supporting soil and the capping raft. The load transferred by direct contact between the raft and the soil is also investigated. Available detailed geotechnical investigation and evaluation of the available micropiles loading tests were used to estimate the parameters of soil and pile−soil interface. The advanced 3D FE model is used to ensure that the proposed number of additional micropiles satisfies the required safety and stability. The outcome of the analysis proved to conservatively satisfy safety requirements. The raft transferred a considerable part of the total load directly to the soil, hence increasing the capacity of the combined piled raft foundation system.

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.

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.

Numerical Analysis of the Carrying Capacity of a Piled Raft Foundation in Soft Clayey Soils

Piled raft foundations are a common type of foundation for high-rise buildings. Unlike shallow foundations, deep foundations (piles) pass through weak or soft soil deposits and can reach stiff soil or bedrock to support the weight of the structure. In this paper, the performance of a medium embedment depth piled raft foundation in soft soil is presented. A numerical model was developed and a parametric study was conducted in order to simulate the case of such a foundation system and to investigate its performance in soft clay. This parametric study investigated the effect of the geometry of a piled raft foundation and the stiffness ratio between the pile material and clay on the performance of the foundation system in soft soil. Additionally, the failure mechanism of such a foundation system under load was examined. An analytical model was developed to predict the ultimate carrying capacity based on the observed failure mechanism. A semi-empirical model is proposed for determining the Improvement Factor (IF) of a given soil, pile, and geometric condition. Findings of the study indicate that the performance of piled raft foundations on soft soils is significantly affected by the piles’ spacing. As the ratio S/D increases, the ultimate carrying capacity of a piled raft foundation decreases. However, when this ratio exceeds 10 (S/D> 10), piles have little or no effect on the ultimate carrying capacity of this foundation system. A piled raft foundation system fails by bearing at the base of the piles and also by shear at the side of the pile group on hyperbolic plans. Doi: 10.28991/CEJ-2022-08-04-01 Full Text: PDF

Physical 1g modelling of defective small-scale piled-raft systems founded in sand

The need for understanding the interaction between the elements of a piled-raft foundation system becomes relevant when one of the piles collapses, presents a defect related to the installation procedure, or when it is sought to optimise the number of piles to meet the criteria of load capacity and admissible settlement. In the paper, 20 small-scale 1g load tests were performed in model piled-raft foundations placed within a cylindrical container. Pluviated sand under vertical loading was adopted to evaluate the cross-correlated qualitative behaviour of piled rafts with and without the presence of a single defective pile in systems of four, nine and 16 piles. The defects were simulated by the variation of the length of the pile, considering it shorter or absent, at distinct positions on the system – that is, corner, edge and centre of the raft locations. The results indicate that the position of the defective pile may be more detrimental to the foundation stiffness and load capacity than the level of damage of the pile, such as the cases evolving the presence of defective piles in the corner position.

Study on Parametric Analysis of Piled Raft Foundation System Using Finite Element Approach

To accommodate the shear requirement and settlement requirement of high rise construction, the concept of piled-raft foundation has been developed. This research deals with successive analysis of parameters of piled-raft foundation system using PLAXIS-2D as a FEM tool. Plain strain analysis of piled raft foundation system has been conducted out by successive fixing up of parameters. For the analysis two cases has been studied for piled-raft lying on silty soil deposit and on clayey deposit with respect to uniform static loading from superstructure. The result of successive variation of parameters showed that variation has limiting effect on stress and displacement behavior. The analysis is also performed for raft of different relative stiffness and pile of different relative compressibility and load sharing between plain strain pile and raft has been analyzed.

Comparative analysis of Piled Raft Foundation System (PRFS) settlements placed on soft soils via geotechnical centrifuge

The use of Piled Raft Foundations Systems (PRFS) has been extended to different types of soils, including soft clay soils. In this type of soil it is possible that, in addition to the consolidation process due to the presence of loads, a subsidence process is generated, associated with variations in pore pressure with depth. In many cases, these variations are associated with the loss of recharge of the aquifers or with the extraction of water from deep soil layers. In this work, the behaviour of some PRFS built on soft clay soils, which are subjected to the double consolidation process, are evaluated, both by loading and by the extraction of water from deep soil layers. The research is based on the implementation of reduced-scale models in a geotechnical centrifuge; the influence of the separation and number of piles on the deformation or settlement of the system is analysed. It is shown that, normally, groups of piles with greater separation control settlement more effectively. However, the settlements are greater when the soil is subjected to the weight of the structure in addition to a process of depletion of the pore pressure, because the settlement depends on the distribution of the piles, which is described using the Filling Factor (FF).

3D numerical analysis of piled raft interaction in drained soft clay conditions

Raft and pile foundations are combined with the twofold aim of increasing the overall bearing capacity and reduce the average/differential settlements. This results in a complex system and gives rise to significant soil–structure interaction mechanisms. In the last years, both bearing capacity and settlements of piled raft have been usually expected on the basis of empirical formulas, analytical relationships, and numerical analyses. The conventional design methods currently used in practice are based upon relatively conservative assumptions of raft–soil–pile interaction; as a result, they are in many cases inaccurate and unlikely to result in optimum design. Thus, resorting to numerical analyses seems to be the most reliable method, mainly because of the multiple interactions and the three-dimensional geometry that must be considered in most cases. In this context, 3D finite difference analyses were performed to investigate the behavior of piled raft foundation system in drained conditions of soft clays. The overall objective of the present paper focuses on the load response and effects of multiple interactions of piled raft subjected to vertical loading. For this purpose, several types of foundations were considered including piled raft, group of piles and unpiled raft. For optimization purposes, the behavior of the piled raft was also investigated by varying some parameters as pile number and piling configuration. The concluded observations from the parametric study provide further insight into the mechanical response of piled raft and aim at helping the engineers in taking a logical path in an iterative design process for a piled raft foundation.

Three-dimensional numerical analysis on seismic behavior of soil-piled raft-structure system

Increasing demand for high-rise buildings and other important structures in urban areas and unfavorable land has resulted to opt for piled raft foundation by the designers. The present study is an attempt to investigate the seismic behavior of piled raft foundation supporting structural system embedded in very soft clay encompassing variations of different influential parameters. The piled raft system is modeled using three-dimensional (3D) finite element analysis. At first, the 3D numerical model is validated with past numerical studies. This is followed by a detailed parametric study on the effect of different influential parameters, such as flexibility of raft, pile spacing to diameter ratios, and asymmetry in pile length on dynamic characteristics and seismic response of the piled raft foundation system considering dynamic soil-structure interaction (DSSI). It is observed that the fundamental period of the structure gets significantly lengthened with increasing raft flexibility whereas pile head shear considerably reduced. Further, non-identical pile lengths attributing asymmetric group configuration results in a significant change in the fundamental period of the structure and pile head shear.

Blast-induced ground motion effect on dynamic response of a cylindrical vertical water tank with piled raft foundation

This paper studies to estimate the dynamic behavior of a demineralized water tank with a piled raft foundation system considering soil-pile-structure-fluid interaction to shock-ground motion. A three-dimensional finite element model of a coupled system is constituted in ANSYS software. Interaction between pile and soil is represented with the frictional contact element. The frictionless contact elements are utilized to model between the water and tank shell to allow for displacement of the free surface adjacent to the tank wall. Shell elements are used for the tank body and its vault. The dynamic analyses of the tank including soil-pile-structure-fluid interaction are presented by using shock response spectra. Ground shock acceleration time histories, generated by using a developed computer program based on Fortran programming language, produce shock response spectra. The effects of the different charge weights and distances from the charge center are examined in the analyses. Also, the effect of the water fill level in the tank and the number of piles is also investigated. The results of the research are presented with the directional displacements and equivalent stresses. It seen from the analyses that the dynamic responses of the tank increase with the charge weight, while decreasing with the charge center distance. Moreover, the water fill level and the number of piles extremely affect the displacement and stress values of the coupled interaction system.

Optimizing the Unconnected Piled Raft Foundation for Soft Clay Soils: Numerical Study

Over the past two decades, a number of researchers studied different aspects of the unconnected piled raft foundation (UPRF) system. In this system, a structural fill cushion is inserted between the raft and the concrete piles (PC) where the cushion transfers the loads from the superstructure to the piles. They showed that UPRF could increase the load-bearing share of the raft relative to that of the concrete piles, which leads to a favourable economic impact. Stone columns (SC) and deep-mixed columns (DMC) have also gained substantial popularity in the last few decades, and have shown pronounced success in substituting concrete pile foundations in numerous projects. This paper is an attempt to explore the viability of using SC and DMC in the UPRF system. Different column parameters such as material, stiffness, spacing, embedment length, configuration and raft thickness may affect the design considerations of UPRF systems. The current study investigates the effect of such parameters on the performance of the UPRF. In addition, two cushion alternatives were also studied: structural fill and EPS Geofoam. 3D finite element models of a 16-storey building on soft clay were used to compare the behaviour of different types of UPRF foundations embedded in different types of soft clays. A coupled hydraulic and mechanical model using the Modified Cam Clay soil model was used to model the soft soil.

Analysis of piled raft foundation in sandy soil using full scale models

The conduct of the piled raft foundation system is measured by using different methods. Full scale loading tests were conducted on 18 full scale models consistent with ASTM D1143-07, so as to get actual behavior. The experiment study included carrying out still field load testing for piles fixed in sand soils in Karbala province, these piles are viewed as pioneers within this area. Testing was conducted for Six groups of piles (1×1, 1×2, 3 heaps, 2×2, 5 pile and 2×3). Every group comprised of three states; pile group, just rafts as well as piled rafts, accordingly, a sum of 18 still load testing were done. It was inferred that the piled raft basis diminishes the sum settling as well as prompts increment of the bearing limit more than the piles’ groups along with the raft basis. The loading that was shared via the raft basis is approximately (46.3, 28.5, 27.6, 26.4, 25.7 and 25%) regarding the groups of piled raft (1×1, 1×2, 3 piles, 2×2, 5 piles as well as 2×3), individually. The instance of pilled raft basis can be taken as the basic state on the grounds that the raft basis has no loading share until the piles’ group achieves the completely preparing state. It was seen that the pile achieves this state as the connected loading is in the scope of (40-60%) of the sum burden, after which the raft participates loading share, and afterward it will be failed before pile.

Innovative piled raft foundations design using artificial neural network

Studying the piled raft behavior has been the subject of many types of research in the field of geotechnical engineering. Several studies have been conducted to understand the behavior of these types of foundations, which are often used for uniform loading on the raft and piles with the same length, while generally the transition load from the upper structure to the foundation is non-uniform and the choice of uniform length for piles in the above model will not be optimally economic and practical. The most common method in identifying the behavior of piled rafts is the use of theoretical relationships and software analyses. More precise identification of this type of foundation behavior can be very difficult due to several influential parameters and interaction of set behavior, and it will be done by doing time-consuming computer analyses or costly full-scale physical modeling. In the meantime, the technique of artificial neural networks can be used to achieve this goal with minimum time consumption, in which data from physical and numerical modeling can be used for network learning. One of the advantages of this method is the speed and simplicity of using it. In this paper, a model is presented based on multi-layer perceptron artificial neural network. In this model pile diameter, pile length, and pile spacing is considered as an input parameter that can be used to estimate maximum settlement, maximum differential settlement, and maximum raft moment. By this model, we can create an extensive domain of results for optimum system selection in the desired piled raft foundation. Results of neural network indicate its proper ability in identifying the piled raft behavior. The presented procedure provides an interesting solution and economically enhancing the design of the piled raft foundation system. This innovative design method reduces the time spent on software analyses.

An Evaluation of Pile-Raft Interaction in Cohesive Soils using 3D Finite Element Method

This paper presents the results of a numerical study of soil-structure interaction in a piled-raft foundation system in clay soil by reviewing the deformation and load transfer mechanism of the piled-raft foundation system. ABAQUS was used to evaluate the interaction in the system, while a Mohr-Coulomb constitutive model was chosen to model the clay soil. Verification of the model was conducted by comparing the simulation result to an experimental laboratory result. The verification result showed that the model used in this research agreed well with the experimental laboratory research. Subsequently, a parametric study was performed by varying the pile spacing, raft size, pile length, and raft thickness. A parametric study was conducted on very stiff and hard clays. This study concludes that the load transfer mechanism in a piled-raft foundation system between the pile and raft foundation occurs after the pile reaches its ultimate capacity and is in the plastic zone.

Performance Analysis of a Piled Raft Foundation System of Varying Pile Lengths in Controlling Angular Distortion

The performance of a vertically-loaded floating piled raft foundation in very soft and deep layers of compressible subsoil, in terms of its ability to control key factors such as bending moment and pile-raft coefficient, was presented and discussed in previous research. The other critical factor that needs to be evaluated is the angular distortion of the foundation. This paper presents the effectiveness of the piled raft foundation system in controlling the angular distortion of the foundation. A comprehensive analysis of this proposed foundation system for various pile lengths in soft compressible subsoil was carried out. Different pile groups with 9 × 9 arrangement of piles were loaded vertically. A total combination of 45 pile group cases with different pile lengths, pile spacing, and raft thicknesses was analyzed. Results showed lower angular distortion for the proposed foundation system, which indicated that a piled raft with varying pile lengths is most efficient in controlling angular distortion as compared to piled rafts with equal pile lengths.

Natural frequencies of piled raft foundation including superstructure effect

Dynamic characteristics of piled raft foundation system plays an important role in the safety of high-rise buildings subjected to seismic loadings though the analytical study considering the effect of both foundation and superstructure is very few in literature. The present study first proposes an exact analytical solution for piled raft foundation subjected to harmonic excitation and resting on an elastic Winkler foundation to obtain its natural radial frequency. After successful validation through available centrifuge test results, a series of parametric study has been carried out investigating the influence of various geometrical and geotechnical parameters of the foundation and the soils respectively. It is observed that the pile length and the pile diameter has significant effect on the natural radial frequency of the foundation system whereas soil density and spacing between piles have minimal effect. The importance of stiffness of the superstructure is also considered in the proposed methodology. It is found that the natural radial frequency of piled raft foundation including superstructure stiffness decreases by 12% to 28% when compared with the computation of the natural radial frequency excluding effect of superstructure stiffness. Hence, this study provides a new analytical methodology to obtain the dynamics characteristics of piled raft foundation considering the superstructure effect which can be used for the design.

Issues on Design of Piled Raft Foundation

A piled raft foundation consists of a thick concrete slab reinforced with steel which covers the entire contact area of the structure, in which the raft is supported by a group of piles or a number of individual piles. Bending moment on raft, differential and average settlement, pile and raft geometries are the influencing parameters of the piled raft foundation system. In this paper, a detailed review has been carried out on the issues on the raft foundation design. Also, the existing design procedure was explained.