Combined Pile Raft Foundation Research Articles

Reliability Analysis of Bearing Capacity of Soil of Combined Pile Raft Foundation (CPRF) of Historic Dharahara

When the raft foundation alone does not fulfil the design requirement, it may be possible to enhance the performance of the raft by economical addition of piles to transfer the heavy load to soft soil with a rather low total or differential settlement called a combined pile raft foundation (CPRF). The proposed twenty-one storey Dharahara tower at Sundhara, Kathmandu is being constructed with CPRF.This paper focuses on the reliability index, the probability of failure and reliability of Dharahara. Statistical analysis is carried out to determine the mean and variance of log-normally distributed geotechnical uncertainty parameters cohesion, unit weight, friction angle and the normally distributed loading on the foundation. First, the limit equilibrium performance function equation of bearing capacity is prepared using a First Order Reliability Method (FORM) and then solving the value of reliability index by coding on C Programming Language. Obtained value is validated with Euro Code-Basis of Structural Design. The results show that the reliability of the designed foundation system of Dharahara is 0.99999872% and the probability of failure is 0.00000128%. Sensitivity analysis is performed by varying uncertainty parameters and the result shows that friction angle is more sensitive than the other three parameters.

Soil Consolidation Analysis in the Context of Intermediate Foundation as a New Material Perspective in the Calibration of Numerical–Material Models

This paper presents the authors’ research results from an analysis of intermediate foundations as well as slab and pile foundations in the context of soil consolidation. Looking at soil as a building material that changes its properties over time is very important from the point of view of the safety of construction, implementation, and operation of building structures. In addition, soil can be parameterized in such a way as to accurately describe its possible behavior under service loading. Of great interest is the phenomenon of consolidation, which is based on the reduction of soil volume over time under constant loading. This study explores existing piles and replicates soil conditions to understand individual and grouped pile behavior in combined pile–raft foundations (CPRF). To assess pile settlement from primary and secondary consolidation phases, 13 field measurements on concrete columns in gyttja clay were conducted. Analyzing data from these tests allowed engineers to accurately calibrate a numerical model. This calibrated model was instrumental in designing high-rise buildings, ensuring stability and safety. This study emphasizes the importance of understanding soil behavior, particularly consolidation phenomena, in optimizing foundation design and construction practices.

Closed‐form solution for natural frequency of flexible combined pile‐raft foundation

AbstractDynamic attributes of flexible combined pile‐raft foundation (CPRF) hold paramount importance in assessing the behavior of it, especially in an earthquake prone area. The present study proposes a closed‐form solution for obtaining the natural frequency of flexible CPRF, where the piles and raft are represented as beam and thin plate respectively, lying on a two‐parameter Pasternak medium. Two simultaneous governing differential equations for the components are formulated and closed form solutions are obtained for finding the natural frequency and transient horizontal displacement of the foundation system. The complex soil‐structure interactions associated with the load‐bearing mechanism of CPRF is incorporated in the analysis scheme. The proposed method is validated with available centrifuge test result. Further parametric studies showcase that with decrease in raft thickness, the spectral acceleration response of the raft decreases. Also, for CPRF with high raft flexibility, the natural frequency of the foundation in dense sand is 34% and 53% higher compared to medium dense and loose sands, respectively. Thus, the present method can be reckoned as a notable contribution in comprehending the dynamic behavior of flexible CPRF.

Influence of pile-raft connection on lateral performance of combined pile-raft foundations adjacent to tunnelling

The lateral response of combined pile-raft foundations (CPRFs) adjacent to tunnel excavation is a challenging problem owing to the complexity of the pile-raft connections. In current engineering practices, the impact of these connections on the lateral performance of CPRFs is frequently overlooked, despite their importance. To address this issue, this study conducted three-dimensional finite element analyses to evaluate the contribution of pile-raft connections to the tunnelling-induced lateral performance of CPRFs in saturated clay. In the analysis, both passive and active loading at the pile head could be considered by varying the tunnel depth. Several parameter studies, such as relative pile-raft modulus, pile embedded modulus, pile embedded depths, and pile shaft skin friction, were conducted to determine the optimal design parameters for CPRFs. The results indicate that pile-raft connections significantly affect the tunnelling-induced deflections and bending moments of pile groups. Inspired by the results, a simplified design method, the pile-raft connection coefficient Kc was proposed. Additionally, the pile-head restraint percentage was established to make a relationship with the pile-raft connection coefficient in order to assess the pile-raft connection and guide the pile-raft design. In this paper, the recommended range value of Kc is 10–200 and the range value of pile-head restraint percentage is 24%–42%.

Settlement Based Load-Bearing in a Combined Pile–Raft Foundation

Settlement Based Load-Bearing in a Combined Pile–Raft Foundation

Displacement-based finite element approach on analysing flexible combined pile–raft foundation in layered soil

The present study proposes a new finite element methodology to analyse the behaviour of flexible combined pile–raft foundation (CPRF) situated in layered soil, in a displacement-based framework. The soil medium is idealised as an advanced elastic Pasternak medium and the piles and raft are modelled as bar and plate element, respectively. The two components of CPRF are analysed simultaneously and displacement compatibility is satisfied at the pile–raft junctions. A number of soil–structure interaction factors, which govern the behaviour of CPRF, are suitably subsumed in the analysis scheme. The proposed method is validated with available analytical and experimental studies. Further parametric studies, investigating the effects of soil layering and raft flexibility on the behaviour of CPRF, are explored. It is observed that the load sharing proportion between the components and the raft deformation pattern depend upon the thickness and position of the soft soil layer in a multilayered soil system. The thickness of the flexible raft plays a pivotal role in determining the behaviour of CPRF, founded in a multilayered soil profile. Thus, this research manifests notable advancement in understanding the behaviour of flexible CPRF in layered soil.

Settlement of combined piled raft foundation of a nuclear power plant in non-liquefiable and liquefiable soils

The settlement analysis of a nuclear power plant (NPP) resting on the combined pile raft foundation (CPRF) under static and seismic loads is very important. However, it has not yet been reported in the literature as most NPPs are founded on raft foundation rather than on CPRF. In this paper, the settlement has been examined in detail. The geometrical nonlinearity at the interface of raft-soil and pile-soil is considered using the coulomb friction. The piles are rigidly connected to the raft. The nonlinear behaviour of the soil in the non-liquefiable soil is modelled using elasto-plastic Drucker Prager cap model and CycLiq model for the liquefiable soil. For this, user defined material (UDM) is implemented in the FLAC3D material library. Thus, a complete 3D finite difference model of an NPP considering raft-pile-soil-structure interaction (RPSSI) for nonlinear settlement analysis is developed. After successful validation of the present CPRF, numerical model with the literature, it has been used for further study under real earthquake acceleration–time history. Next, CPRF with NPP structure is studied under static loading, which is further extended for the nonlinear time domain settlement analysis. The effects of variation in various parameters i.e., spacing between piles, cohesion, friction angle, and Poisson's ratio of soil on the settlement, are examined and found to be significant. The dominant frequency of the earthquake significantly affects the response of the CPRF. It was inferred that the interface nonlinearity has greater significance in the liquefiable soil as compared to the non-liquefiable soil. Higher differential settlement in the liquefiable soil is attributed to the reduction in the stiffness of the soil. The findings of this study could be useful to frame the general guidelines for the economical design of NPP-CPRF systems.

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.

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.

Determination of Subgrade Reaction Coefficients and Spring Stiffnesses for a Combined Pile Raft Foundation (CPRF) by Means of a Cluster Analysis

Abstract An analysis of combined piled raft foundations could be performed by means of a special geotechnical finite element software that utilizes 3D modeling and advanced constitutive models for the soil. However, a foundation is generally included in the structural models in a structural design program. The least advanced part of finite element software that has been developed for structural FEM modeling are the tools used for modeling behaviour. A method is required with which the input parameters of the structural design software that are used for modeling the soil structure interaction as well could be determined so that the results calculated by means of a geo-technical software that approximates realistic soil behaviour the best can be reproduced by structural design software as well. The procedure outlined in this paper by means of a cluster analysis provides a tool for a substantial and innovative improvement in subsoil modeling, to which not enough attention is generally paid in the structural FEM software, thereby resulting in a reduced amount of work associated with the task of inputting data.

Steady-state response of flexible combined pile-raft foundation under dynamic loading

The present study proposes a closed-form solution of flexible combined pile-raft foundation (CPRF) resting on an elastic medium under dynamic loading. Two components of the foundation system, i.e. pile and raft are idealized as beam and plate, respectively, resting on an elastic two-parameter Pasternak soil medium. The governing differential equations of these components under horizontal harmonic loading is formulated and the subsequent closed-form solutions of the same are obtained by using Fourier transformation technique. Two simultaneous analyses are performed for piles and raft and displacement compatibility is satisfied at the junction of the components. The proposed method is validated with dynamic centrifuge test results, available in the literature. In further parametric studies, the effect of loading frequency on the dynamic displacement response of CPRF is investigated. Also it is perceived that with an increase in raft flexibility by 60%, the horizontal load sharing proportion of raft reduces by 75%–65% for loose to dense sands, correspondingly. This study contributes significantly in understanding the steady-state response of CPRF under dynamic loading from an analytical perspective.

PRaFULL: A method for the analysis of piled raft foundation under lateral load

A new code, called PRaFULL (Piled Raft Foundation Under Lateral Load), was developed for the analysis of laterally loaded Combined Pile Raft Foundation (CPRF). The proposed code considers the contribution offered by the raft-soil contact and the interactions between all the CPRF system components. The nonlinear behaviour of the reinforced concrete pile and the soil are accounted. As shallower soil layers are of great relevance in the lateral response of a pile foundation, PRaFULL includes the possibility to consider layered soil profiles with appropriate properties. The shadowing effect on the ultimate soil pressure is accounted, when dealing with pile groups, as proposed by the Strain Wedge Model. PRaFULL BEM code obviously requires less computational resources compared to FEM (Finite Element Method) or FDM (Finite Difference Method) codes. The proposed code was validated in the linear elastic range by comparisons with the code APRAF (Analysis of Piled Raft Foundations). The reliability of the procedure to predict piled raft performance was then verified in nonlinear range by comparisons with both centrifuge tests and computer code PRAB.

Pseudostatic Approach to Analyze Combined Pile–Raft Foundation

Abstract A new simplified approach based on the pseudostatic method of analysis is proposed to investigate the response of a combined pile–raft foundation (CPRF) and a group pile considering the co...

Analysis of Combined Pile Raft Foundations Based on a Static Load Test

Combined pile raft foundations (CPRF) take into account the synergy of a raft with a group of piles in transferring the load to the ground. In many ground conditions, such foundations are optimal in both utilitarian and economic terms. Based on a literature review, it can be concluded that, despite the numerous extant calculation methods for the analysis of piled raft foundations, there is lack of research related to the possibility of using a load-settlement curve from a static pile test in the full range, in order to analyse the load-settlement curve of a pile working in a group under the raft. Furthermore, the current trend in the design of pile foundations, based on the static load test databases, encourages the formulation of methods using the results of the static pile test. These methods allow for analyses closer to the physical model. In this paper, the above research postulate was answered, formulating a mathematical model in which a particular emphasis was placed on the actual work of a single pile in the ground, taking into account the interaction among raft, piles and subsoil. The developed mathematical model was verified on the basis of the results of field research. For this purpose, the results of settlement of two tanks were used.

Numerical Simulation of Combined Pile Raft Foundation under Horizontal Loading

Raft foundations generally have the ability to reduce differential settlement, contrarily causing excessive settlement. In order to overcome this, piles are used along with the raft termed as combined pile-raft foundation system. Due to the lack of availability of simplified tools and intricacy of work, the combined pile raft foundation remains as an untouched area in research. This paper explores the performance of 2x2 numerically simulated combined pile-raft foundation embedded in sand exposed to pure horizontal load using ABAQUS 3D. The effect of horizontal loading in terms of displacement is studied by varying the raft thickness, length and spacing to diameter ratios respectively for the combined pile-raft foundation having a pile diameter of 500mm. The model is validated using the existing findings. Based on this study, it is inferred that the raft thickness effect remains inconsequential whereas the length to diameter ratio and spacing to diameter ratio has an impact on the static response of the combined pile-raft foundation system.

Dynamic Load Sharing of Combined Pile Raft Foundation (CPRF) for Reinforced Concrete Structures

Combined pile raft foundation (CPRF) is one of the emerging concepts for providing a cost-effective and efficient solutions for heavily-loaded structures. However, predicting the behaviour of such foundations, especially the load sharing between raft and pile is a challenge due to its inherent complex interactions. Existing analysis methods are either bound within a range of simplifying assumptions or sometimes computationally demanding. Hence, an attempt has been made to evolve a simple and easily implementable methodology, considering non-linear degrading behaviour of soil in a rational manner. The approach has first been validated with measured response during an experimental centrifuge testing of a CPRF in soft Malaysian kaolin clay and an instrumented bridge (Impulsora) founded in soft clayey soil. Subsequently, a range of comparative parametric evaluation of load sharing and settlement characterstics has been carried out which has indicated the importance of pile layout, length and numbers to arrive at a safe and economic design.

Serviceability-Based Finite-Element Approach on Analyzing Combined Pile–Raft Foundation

AbstractThe present study proposes a finite-element approach to focus on the load sharing and deformation aspects of combined pile–raft foundation (CPRF) in a serviceability-based framework. Soil n...

Statistical modelling of impacts on a pile-raft foundation

This paper reviews performance of combined pile-raft foundations (CPRF). The goal of the study is obtaining the ability to design solid, reliable and cost-effective foundations allowing for minimum settlements. The task is to develop methods for calculating CPRF for optimum choice of the foundation raft size, dimensions and number of piles. To achieve these results, an analytical method was used, curves of the ratio of CPRF settlement to raft foundation settlement vs. total load N on the piles were plotted. The scientific significance of the study is in the method of engineering calculation of CPRF, which is better correlated to the experimental data and allows optimal distribution of loads between the raft and piles, ensuring reliability and operational integrity of buildings, reduction of settlements and the number of piles, which would lead to cost savings and reduced consumption of materials for foundations. A new coefficient showing the residual stress has been introduced, and a method for its calculation has been proposed. A new equation for determination of the share of total load N on the piles has been obtained. This equation is more general in comparison with the previously generated equations. It allows excluding the assumption that the stresses arising under the raft completely dissipate at the level of the pile end. Application of the developed methods will increase reliability and reduce settlements and the number of piles, which ultimately will lead to cost savings and reduced consumption of materials for foundations.

Seismic Response of Multistoried Building with Different Foundations Considering Interaction Effects

Current seismic design practice assumes the base of the building to be fixed and does not consider the flexibility of foundation and soil. This assumption is realistic only when the structure is founded on solid rock or when the relative stiffness of the foundation soil compared to the superstructure is high. Whereas, in reality due to natural ability of soil to deform, supporting soil medium modifies the response of the structure during earthquake to some extent. In this work the effect of soil structure interaction on seismic response of building resting on different types of foundation was studied. Present work aims to study the effect of soil structure interaction on seismic response of building resting on fixed base, pile foundation, raft foundation and combined pile-raft foundation. G+9 RCC building is analyzed for earthquake loads considered in zone III by response spectrum method and storey displacement and base shear force of building by considering and without considering SSI effect is found out by using MIDAS GEN software.