Piled Raft Foundation Research Articles

Seismic response of nuclear power station with disconnected pile-raft foundation using dynamic centrifuge tests

The typical foundation type for a nuclear power station is the raft foundation in rock areas. With the power demand, it is possible that a nuclear power station located in soft soil areas. Traditional connected pile raft foundation (CPRF) has been successfully used in soft soil areas. However, under earthquake loads, high horizontal shear stresses and bending moments are generated in the pile head. Accordingly, researchers have sought to develop an innovative disconnected pile raft foundation (DPRF), and the benefits of DPRF have simulated increasing researches on it in the last two decades. In this study, a series of dynamic centrifuge tests were carried out to identify the effect of cushion and its thickness on the dynamic characteristics of the nuclear power station with DPRF. A withe noise excitation and three earthquake waves with different seismic intensities were adopted as ground motion. The seismic responses of soil, structure, and bending moment of piles were analysed. A cushion thickness equal to the diameter of piles is recommended. Comparing with the CPRF, the cushion layer of DPRF has an isolation benefit. With the gravel cushion, attention must be paid to the horizontal displacement, inclination and rocking of the superstructure.

Foundation systems for sustainable revitalisation of spoil dump areas

The design of foundations systems is well established in the literature and codes of practice. However, these are generally applicable to well characterised, homogenous ground conditions. Spoil materials represent a significant challenge when implementing these design procedures due to their time-dependent characteristics, their spatial variability , and the likelihood that they will consist of low strength/stiffness materials. As such, careful consideration is required when designing foundation systems on spoil materials, especially for sensitive structures such as wind turbines. Physical modelling experiments using a geotechnical centrifuge were conducted to study the spoil-foundation system interaction. In these tests, an equivalent spoil was used that provided satisfactory reproduction of key physical and mechanical characteristics of a real spoil material. Three types of foundation systems were considered in the centrifuge tests: (1) raft foundation, (2) pile-group foundation, and (3) pile-raft foundation. The long-term cyclic loading of 87kN, 171kN and 460kN maximum cyclic load were used for the raft foundation. A higher cyclic load of 501kN was applied to the pile-group and pile-raft foundations. To evaluate the effect of climatic condition variations on spoil-foundation interactions, the centrifuge tests were carried out at different water table levels within the spoil material. The experimental results were analysed in terms of foundation rotation and foundation vertical settlement. The raft foundation rotated around 0.1-0.2 degrees and vertically displaced by 5-10 mm, whereas the pile-group rotated about 0.6-0.8 degrees and vertically displaced by 20-30 mm due to long-term cyclic loading. The pile-raft foundation displayed better response as compared to pile-group foundation.

Thermomechanical Behaviors of an Energy Pile–Raft Foundation under Intermittent Operation with Forced Heat Recharge

This paper presents an experimental investigation of the thermal and mechanical behaviors of an energy pile–raft foundation under intermittent heating operation with forced heat recharge, with 16 h of pile cooling and 8 h of forced heat recharge (referred to as 16F, where F indicates forced). The forced heat recharge was found to increase the daily heat exchange rate to 180 W/m, which was 50% higher than the value under general intermittent heating operation without forced heat recharge. The raft and building load at the pile head imposed a significant restraint on the upper part of the energy pile, leading to a minimum degrees of freedom (0.27) near the pile head. The mechanical behaviors of the pile, including strains, stresses, and pile end displacements and stresses, followed reversible paths when plotted against the daily temperature cycles. A slight residual pressure stress (10 kPa) was observed at the bottom of the raft during the initial thermal cycles.

Experimental and Numerical Behavior of Horizontally Loaded Piled Rafts with a Defective Pile

The present paper aims to investigate the effects of defective piles in horizontally loaded piled raft foundations. Close to real scale foundation models of defective and intact three-piled systems were submitted to horizontal load tests. The models used bored type piles drilled with 5 m in length and 0.25 m in diameter, connected to a concrete raft founded on a tropical soil profile at the University of Campinas research site. Based on the experimental data, a three-dimensional finite element analysis was initially calibrated and consequently used to study the effects of the defective element at the pile load distribution and the horizontal subgrade forces at the pile shaft. The results show that the presence of a defective pile increases the raft tilting, which affects both vertical and horizontal load distributions among the raft and the piles, and among trailing and leading piles.

Experimental Modelling of Helical Piles under Axial Loading


 Due to the rapid increase in population, the demand for high-rise buildings is also increasing daily. Researchers have performed various numerical and experimental studies to find out the best foundation type for structure. A helical foundation is one of the most suitable foundation types to be incorporated in the construction of high-rise buildings. In a helical pile, some fraction of axial load is taken by the helix, which increases the load-carrying capacity of the foundation. This study is carried out to determine the settlement and load carrying capacity of the helical pile and helical pile raft cohesion-less soil. For this study, a small-scale model of a helical pile and helical pile raft foundation is prepared and fitted in the rectangular shear box filled with cohesion-less soil (sand). Testing on single helix piles, double helix piles, two helical pile rafts, and four helical pile rafts has been conducted. Instrumentation of the model is carried out using a settlement transducer and load cell to note down the settlement and load applied, respectively. Results provided that the maximum axial load carrying capacity of a single helix pile is almost 1200N at a settlement of 25mm while that is 1750N, corresponding to a 25mm settlement for a double helix pile. Similarly, the maximum load carrying capacity for two helical pile rafts and four helical pile raft are 2700N and 4400N, respectively, at 25mm. These results were compared, and a double helix pile has a greater load-carrying capacity than a single helix pile. Similarly, a pile raft with four helical piles has a greater load-carrying capacity than a pile raft consisting of two helical piles.

Two-Dimensional Finite Element Analysis of Piled Raft Coefficient Settlement Ratio on Clays

Nowadays, designing a piled raft foundation is challenging because the behavior is three-dimensional. For some engineers, a three-dimensional analysis might not be affordable due to more costly than a two-dimensional analysis. In this study, 2D Finite element analysis – axisymmetric was used to study the piled raft foundation. The pile diameter and pile length were varied to investigate the relation between piled raft coefficient and load-settlement. In addition, the load transfer mechanism between the raft part and the pile part in clayey soils is also examined. The results show the longer the pile and the larger the diameter, the greater the load carried by the pile and the smaller the settlement. Increase in pile length by 5 m, resulting in a load transfer of 2% to 6% from a raft to pile, and reduced settlement by 2% to 3%. Furthermore, A 0.5 m increase in pile diameter results in an 8% to 25% load transfer from a raft to pile, and a 2% to 7% reduction in a settlement. The soil consistency affects the load distribution and settlement of the pile-raft foundation system. The higher the soil consistency, the smaller the amount transferred to the pile, and the higher the effectiveness of the pile in reducing the settlement that occurs.

A parametric study of large piled raft foundations on clay soil

In the present study, three-dimensional numerical analyses have been conducted to understand the behavior of large piled rafts on stiff clay. The effects of pile number, pile spacing, pile length and loading intensity have been examined. For the same soil, the raft thickness is varied such that the its stiffness changes from flexible to intermediate flexible and to rigid. The load-settlement behavior, load-sharing ratio, raft bending moment and pile axial load distribution are presented and discussed in detail. For a higher pile number, locating piles over a larger central raft area is necessary for the effective reduction of differential settlement. For the intermediate flexible raft, addition of piles covering the maximum raft area is not beneficial in decreasing the differential settlement or the raft bending moment. With the intermediate flexible raft, decreasing the pile length can prove to be advantageous in removing hogging differential settlement. Based on the loading intensity and serviceability limit considerations, the appropriate pile configuration can be determined from the results of the parametric study in the cyclical sequence of firstly the selection of pile number, secondly the adjustment of pile spacing, and finally the reduction of pile length.

Structural analysis of piled raft foundation in soft soil: An experimental simulation and parametric study with numerical method

The high infrastructural demand and the unavailability of proper foundation soil have increased the evolution of combined piled raft foundation (CPRF) in the current generation. The use of CPRF in soft soils has increased immensely in the last few decades. But there is a lack of laboratory experiments to understand the behaviour of CPRF in soft/clayey soil, and hence, this study considers an innovative approach to understand different behaviour of CPRF using clayey soil through laboratory experiments. Two different types of small-scaled CPRF models are prepared with 2 × 2 and 3 × 3 pile groups, and the spacing among the piles as well as the thickness of clayey soil are also altered. Several aspects are studied through the experiments such as load-settlement behaviour, load improvement ratio, behaviour of piles in CPRF, load sharing behaviour, axial forces, raft-soil contact behaviour, bending moment in the raft, etc. Based on the experimental test results, a numerical model is also prepared, and then a parametric study is performed numerically considering a 10-storey residential building. The effect of different pile lengths in CPRF is also checked and the parametric study is conducted with several types of pile arrangements.

Long-term effect of vertical and lateral loads on piled raft foundations: a case study

In this study, numerical analyses of connected and disconnected piled-raft foundations (PRFs) comprising a single pile, 2 × 1, 3 × 1, 2 × 2, 3 × 2, 3 × 3 and 5 × 5 pile groups subjected to different lateral loads, with a constant vertical load, have been carried out by three-dimensional finite-element analysis. Results from the analysis have been validated with reported field studies considering vertical load and reported experimental studies considering lateral load. The reported multilayered sub-soil profile has been considered for the analysis. Parametric studies in terms of the effect of variation in pile length (L/d = 20, 30, 39), spacing of piles in the pile group (S/d = 3, 5, 6), number of piles, thickness of raft (0.5, 1.2 and 2 m), variation of axial force, excess pore water pressure, bending moment and shear force along the depth of the piles for connected and disconnected PRFs have been discussed for a time period of 20 years. The horizontal connected piled-raft coefficients have been determined corresponding to the lateral deflections. A generalised equation has been predicted for the horizontal connected PRF coefficient.

Evaluation of Coupled Interaction Effects Caused by the Long-Term Performance of Energy Piles in a Piled Raft Foundation

Evaluation of Coupled Interaction Effects Caused by the Long-Term Performance of Energy Piles in a Piled Raft Foundation

Investigation on the seismic response of nuclear power stations with a pile-raft foundation using centrifuge tests

Investigation on the seismic response of nuclear power stations with a pile-raft foundation using centrifuge tests

A Simplified Method for the Nonlinear Analysis of Composite Piled Raft Foundation

A Simplified Method for the Nonlinear Analysis of Composite Piled Raft Foundation

Study on dynamic safety distance of multi-storey buildings on the top of loess slope

In view of the frequent occurrence of strong earthquakes and the actual characteristics of buildings built on the top of slopes in the loess region of Western China, the dynamic response and stability law of loess slope under the building load on the top of the slope are discussed by using the three-dimensional finite element numerical calculation method. Taking the slope safety factor, acceleration amplification coefficient, building permanent displacement and slope sliding range as evaluation indexes, the dynamic safety of buildings on the top of the slope under earthquake is analyzed, and the recommended value of dynamic safety distance between top building and slope shoulder is put forward. The results show that: firstly, affected by the amplification effect of slope elevation, the dynamic response of buildings on the top of slope is stronger than that of buildings under slope. At the same time, affected by the amplification effect of the free surface of the slope, the dynamic response amplitude of the building is negatively correlated with the distance from the slope shoulder. Secondly, the horizontal acceleration ratio between the top building and the under building is the largest at the height of 3–6 m, indicating that the multi-storey building with pile raft foundation is most prone to damage at this height. Thirdly, the dynamic safety distance of multi-storey buildings with pile raft foundation located on the top of loess slope with slope height H ≤ 30 m and slope gradient 25° ≤ α ≤ 70° can be 24 m.

1g model test of piled-raft foundation subjected to vibration load and its simulation considering small confining stress

The numerical simulations of 1 g model tests are presented to evaluate the feasibility of the proposed elasto-plastic constitutive model, which can precisely describe the less compressibility of clean sand under small confining stress. The dynamic responses of piled-raft foundations subjected to cyclic train loads were investigated by comparing the results of model tests and simulations. Different frequencies of dynamic loading and different grounds including dry loose sand, dry/saturated medium dense sand, were taken into consideration. It is confirmed that for all types of grounds and input vibration, the numerical results showed a good match with model tests data, especially the settlement and acceleration of piled-raft foundation. Both numerical simulations and experiments showed that the higher the frequency of the dynamic load is, the greater the peak acceleration will be. The proposed constitutive model has greatly improved the accuracy of the numerical simulations on the small-scale 1 g model tests because this model can accurately describe the mechanical characteristics of sand from very small confining stress to normal confining stress, especially the less compressibility and more dilative behavior of sand under small confining stress. • The proposed constitutive model can precisely describe the less compressibility of sand under small confining stress (CS). • The model tests were well reproduced by the numerical analysis with proposed constitutive model in the different grounds. • The proposed model has greatly improved the accuracy of the numerical simulation on the small-scale 1 g model test. • Excess pore water pressure was small both in the tests and simulations, because of more dilative behavior of sand at small CS.

Effect of CPRF on nonlinear seismic response of an NPP structure considering raft-pile-soil-structure-interaction

A number of Nuclear Power Plants (NPPs) in India are coming up in soft soil condition particularly on alluvium in active seismic zone on Combined Piled-Raft Foundation (CPRF). So far, no study has been reported considering nonlinearity of soil for the NPP structures founded on CPRF considering earthquake loads.In the present study, the dynamic responses of raft, pile group and CPRF of a NPP are investigated by simulating the nonlinear finite element model. The effects of nonreflecting boundary condition, pile-soil-interaction by the embedded constraint, raft-pile interaction by the tie constraint and raft-soil-interaction by the penalty constraint, were considered. The material nonlinearity of soil was taken into account using an advanced constitutive model Drucker-Prager with cap plasticity. The geometrical nonlinearity is also considered using the concept of the contact surface. Thus, a complete 3D finite element numerical model of a nuclear structure considering raft-pile-soil-structure interaction (RPSSI) for nonlinear seismic analysis is developed. The validation is carried out by comparing the results from the present study with experimental and numerical results available in the literature. Nonlinear analyses of raft, pile group and CPRF in the time domain finite element formulation carried out and results are compared with that using the linear analysis.A comparison of two analyses shows that linear analysis underestimates the response of soil, piles and structure. The effect of type of boundary used in the analyses is significant. It was also observed that the response of NPP structure is significantly affected by the type of foundation used under the superstructure. The effect of SSI is examined with two major novelty i.e. (a) Effect of CPRF (b) Consideration of Nonlinearity of Soil. Effect of frequency of excitation and PGA of earthquake motion is examined. This study indicates the importance of CPRF over the raft foundation and pile group foundation for the design of nuclear power plants. Also, it examines the effects of nonlinearity on the response.

A simplified settlement prediction method for piled rafts in clay

A method for the estimation of the non-linear, time-dependent settlement of piled raft foundations (PRFs) embedded in saturated clayey soils subjected to uniformly distributed load under the single-drainage condition was developed. A total of 20 different rectangular PRFs resting on four different types of normally consolidated to moderately overconsolidated clays (overconsolidation ratios of 1, 2 and 5) were analysed to quantify their short- and long-term settlements. Rigorous three-dimensional, elasto-plastic finite-element (FE) analyses were performed with the critical-state-based modified Cam Clay soil constitutive model. Biot's coupled consolidation theory was used to model the time-dependent dissipation of excess pore pressure under the single-drainage condition. The FE analysis results were used to investigate systematically the influence of time and various soil and PRF parameters on the settlement responses of piled rafts. Fitted equations correlating PRF settlements and the degree of consolidation with various influencing parameters were developed. The proposed equations predicted the immediate and primary consolidation settlements and the degree of consolidation with reasonable accuracy and can be used in initial design stages.

Behavior of Different Configuration of Piled Raft Foundation for a High-Rise Building by using FEM

Abstract In this paper, the numerical simulation is performed on two piled raft configurations having uniform loading on a piled raft foundation. To investigate the interaction between various parameters of pile soil foundation with varying components of its length of pile, diameter and raft thickness. These components are critical for increasing the foundation’s bearing capacity. It’s important aspect to achieve a reliable design to optimize piled raft foundations subjected to uniformed load- settlement behaviors of the curves. These load – settlements are basically depending on the changing of their components piled raft foundation. These component of piled raft foundations to choose an optimal selection of embedded length of pile (L/dP), normalized diameter of pile (L/dP) and normalized raft thickness (tR/dP). The effect of load-settlement on distributions of shear force and bending moments is also investigated. In this study, a finite elements methods based on numerical tools ELPLA software is used for numerical simulation. The study’s findings validate the numerical analysis for the calculation of proper pile configuration arrangement, as a result of settlements. It will be generated as formation of contours patterns as shear stress and bending moments on the rafts, may be minimized with the total pile length. The conclusion drawn from the study validates the numerical analysis for the computation of proper pile arrangement can results in total and differential settlements, as well as generated shear stress and bending moments on the rafts, are all being reduced, with identical total pile length. Moreover, it captures the contours patterns of different behaviors of piled raft settlements, maximum shear force and maximum moments.

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

Scaled Modeled Tests and Finite Element Numerical Study on Lateral Responses of PRF System under V-H-M Loading

ABSTRACT Considerable research has been endeavoured to examine the responses of piled raft foundation (PRF) under gravity load. In reality, PRF often encounters combined vertical (V), horizontal (H) and moment (M) load simultaneously due to wind, earthquake and wave load along with dead load of the structure, and vertical soil pressure. Present study aims at a scaled-model test on the PRF subjected to V-H-M loads to investigate the lateral responses of PRF due to the interaction of loadings. The scaled model tests are conducted in Agartala sand with a relative density of 30%. Effects of relative stiffness raft with respect to soil, the length and spacing to diameter ratio of the pile, number of piles in a group and non-uniform pile length configuration on the lateral response of PRF are studied. Results indicate that the V load in V-H-M loading has a significant influence on the lateral response of PRF. Further, a substantial influence of other parametric variations on lateral response is also observed. Furthermore, a numerical model is also developed and validated based on experimental results that substantiate investigation on prototype foundation. Finally, the load-sharing behaviour of prototype PRF is also studied through the validated numerical model.

Numerical Analysis of Piled-Raft Foundations on Multi-Layer Soil Considering Settlement and Swelling

Numerical modelling can simulate the interaction between structural elements and the soil continuum in a piled-raft foundation. The present work utilized a two-dimensional finite element Plaxis 2D software to investigate the settlement, swelling, and structural behavior of foundations during the settlement and swelling of soil on various soil profiles under various load combinations and geometry conditions. The field and laboratory testing have been performed to determine the behavior soil parameters necessary for numerical modelling. The Mohr–Coulomb model is utilized to simulate the behavior of soil, as this model requires very few input parameters, which is important for the practical geotechnical behavior of soil. From this study, it was observed that, as soil is soft and has less stiffness, the un-piled raft was not sufficient to resists and higher loads and exceeds the limits of settlement. Piled raft increases the load carrying capacity of soil, and the lower soil layer has a higher stiffness where the pile rests, decreasing the significant settlement. Further, the effects of (L/d) and (s/d) of the pile and Krs on the settlement are also discussed, detailed numerically under different scenarios. The swelling of expansive soil was also simulated in Plaxis 2D with an application of positive volumetric strain. The above-mentioned parametric study was similarly implemented for the heaving of foundation on expansive soil.