Piled Raft Research Articles

Experimental Study of Lateral Loading on Piled Raft Foundations on Sandy Soil

A Shallow foundation on cohesionless soil cannot support greater weights; piled raft foundations are recommended because they combine the load-bearing qualities of piles and raft. Combined Piled Raft Foundations (CRPF) are efficient for tall buildings because they account for both vertical and lateral loads. In a pile raft foundation, the raft’s load-resistance is disregarded due to soil-structure interaction. Simplification may lead to an uneconomical design. While study on raft’s vertical resistance is extensive, its horizontal resistance is limited. In the present study, 160 mm x 160 mm pile-raft model with different pile spacing and pile length was tested. Studies showed that pile length and spacing of pile improve bearing capacity and reduce settlement of raft. The pile raft system rests 65 percentage of the lateral load, depending on pile spacing and its length. Pile spacing and pile length lessen the raft’s lateral load contribution. Furthermore, as increasing in pile spacing reduces raft overturning by 60 percentage. Upgrade pile raft system design may make a cheaper and more efficient option for skyscrapers and make this foundation system more economical design.

Experimental Analysis of the Behaviour of Piled Raft Foundations in Loose Sand

This paper presents the experimental analysis that was conducted on small-scale 1g physical models of piled raft foundation structures with a group of 2 × 2 piles in loose sand. The purpose of the piles was to reduce the settlement of the raft. The test program included twelve experiments, three of which were conducted on a raft alone and nine on piled rafts at pile distances of 3d, 4d, and 5d and pile lengths of 10d, 20d, and 40d, where d is pile diameter. The test results show that the current conventional approach to design of piled raft foundations, at a high safety load factor in piles that assume to take the whole external applied load, is very conservative. Instead, it is more economical to apply a low bearing capacity factor for piles as settlement reducers and maximize use of raft bearing capacity to carry part of the external load.

Nonlocal FEM modeling of piles as beam elements embedded within 3D continuum

Finite element analysis of piled rafts and other foundation systems, in which concrete columns/piles are used to strengthen weak subsoil, is a commonly accepted tool in modern geotechnical engineering. It enables one to analyze all important aspects of soil–structure interaction (SSI), including nonlinear stress–strain constitutive relations for soils, strong displacement discontinuity between rigid inclusions (piles/barettes/columns) and subsoil, but also coupling soil deformation with groundwater flow. In practice, the number of designed inclusions is usually large. Therefore, the standard approach in which the subsoil, and rigid inclusion as well, are discretized using continuum finite elements, may yield huge computational models, which are not acceptable from the practical point of view. To remedy this serious drawback a nonlocal method of modeling rigid inclusions as beam elements embedded within the 3D continuum is proposed and verified. Moreover, it removes another pathological mesh dependency effect when modeling beam–shell connection.

A new nonlinear spring‐dashpot model of CPRF of NPP structure based on coupled BEM‐FEM approach

AbstractIn this paper, a BEM‐FEM coupled model of combined piled raft foundation (CPRF) is proposed for both the frequency domain and the time domain dynamic analyses of a NPP structure in the linear, equivalent linear and nonlinear soil domain. The piles are modeled using the finite element method as a beam element, while the raft and internal domain of soil are modeled using the eight nodded brick element. It is assumed that the piles are rigidly connected with the raft. The nonlinearity of soil is modeled using the hyperbolic stress–strain relation and Modified Drucker–Prager Model. The boundary of the layered soil domain is modeled using the boundary element method. For simplification, it is assumed that the raft and piles are rigidly connected with the soil that means there are no slip and separation conditions at the interface of pile‐soil and raft‐soil. In order to validate the proposed approach, numerical results, that is, impedances in the horizontal, vertical and rocking directions, are compared with the published literature. Further, dynamic analyses of a lumped structure founded on CPRF is carried out in the time domain and frequency domain using these impedances and results are compared with the FEM. It is found that the spring‐dashpot models give the reasonably good agreement with finite element method for linear, equivalent linear and nonlinear models for both frequency domain and time domain analyses. Moreover, practical application of proposed model is shown for a very large problem of raft‐pile‐soil‐structure interaction such as a NPP structure on the layered soil. It was found that the nonlinearity significantly increases the response of the NPP structure. The proposed model has many advantage over the finite element model, for example, first it is very simple and second it reduced the computation time significantly with the good accuracy.

Analysis and effect of piles on raft foundation for High-Rise framed structure under seismic loading

In recent years, the usage of piled raft foundations has grown in popularity because the collective response of the raft with piles may enhance soil load carrying capacity, minimize the settling of soil, and the piles may be positioned for preventing unequal deformation in the mat. A stacked raft foundation is less expensive than a piled foundation. As piles are not required to reach the whole thickness of the clayey soil, they may be ended at upper altitudes. Raft foundation with piles is a novel idea by which the complete weight of the structure above is carried partially by the mat slab via soil interaction and partially carried through piles via surface friction. The seismic analysis of a stacked raft foundation is performed in this work utilizing CSI Safe software and response spectrum analysis load cases obtained from ETABS. For the analysis, the G + 10 story structure is taken and designed in ETABS and support reactions were transferred from ETABS to safe software. The analysis had been performed for different values of soil bearing capacities and different thicknesses, the diameter of the pile, and the length of the pile. The effect of different parameters had been analyzed on the foundation design. The results demonstrate that changing the diameter of piles from 200 mm to 400 mm and raft thickness from 300 mm to 500 mm, leads to a 68.66 % reduction in the deformation of the raft slab. After performing the study, it had been concluded that piles provided with raft foundation improve the performance and capacity of the raft foundation under seismic loading and this type of foundation can be used where soil bearing capacity is low.

Optimum Design of Oil Tank Foundation on Different Soil Conditions

Tank foundation is a base for the tank which is designed to support the weight of oil tank in order to ensure its stability. Failure of tank foundation usually occurred when the foundation system cannot support the weight of the tank or the soil bearing capacity of the area failed to resist the imposed stress from oil tank inclusive of foundation’s self-weight. Failure of tank foundation can lead to pivoting effect and cracking on the foundation slab. Hence, the right choice of foundation system with reference to the bearing capacity of foundation soil is important to ensure the stability of tank foundation. Thus, this study was carried out in order to determine the deformation characteristics of foundation slab of oil tank foundation on different soil conditions with the aim to determine optimum design of oil tank foundation. The dimensions and design of the models were based on a published case study. Series of analyses with finite element models were conducted using STAAD Foundation CONNECT Edition V9. The models were aimed to analyze the stability of foundation structure, and deformation characteristics of the foundation slabs on different soil conditions. Three types of foundation system (raft, pile raft and pile foundation) were modelled in STAAD Foundation CONNECT Edition V9. The findings showed that the most optimum design for foundation structure on stiff residual soil is raft foundation. Whereas pile raft foundation structure is the most appropriate to construct on unconsolidated marine sediment deposit while pile foundation structure is highly recommended on soft peaty soil.

Deviation rectification of a high-rise building with piled raft foundation: a case study

Deviation rectification of a high-rise building with a piled raft foundation was successfully carried out by vertically digging holes. Through site investigation and finite-element analyses of pile foundation settlement, the building tilt was attributed to shortening of the pile length, large post-construction settlement and cancellation of the setting of the post-cast strip of concrete between the main building and the skirt of an underground garage. A comprehensive rectification scheme was conducted, including drilling settlement-resisting micropiles on the side with a greater subsidence, cutting off the raft between the main building and the garage, vertically digging holes and disturbing soil on the side with less subsidence. When the rectified inclination reached the proposed value, the foundation was strengthened by the micropiles for settlement reduction and bearing capacity enhancement. During construction, close monitoring was carried out to guide dynamically the soil extraction operation and monitor building settlement. By analysing the monitoring data over time, the effectiveness of soil extraction for building rectification was verified, and the rectification mechanism of high-rise buildings supported on piled raft foundations was clarified. Finally, the rectification scheme was evaluated and discussed.

An elasto-plastic numerical analysis of THM responses of floating energy pile foundations subjected to asymmetrical thermal cycles

Although end-bearing energy pile foundations subjected to symmetrical thermal cycles have been extensively studied in the laboratory and in the field, the mechanisms underlying the thermo-hydro-mechanical (THM) interactions in floating energy pile groups and rafts, especially when subjected to asymmetrical thermal loads, are not well understood. In this study, an advanced thermo-mechanical bounding surface model was implemented in finite-element (FE) code to investigate the THM interactions of a two-by-two floating energy pile group and pile raft, focusing on asymmetrical thermal cycles. Computed results are compared with published centrifuge model test results in soft clay. It is revealed that the irreversible volumetric contraction of the soil adjacent to the energy piles accumulates with each thermal cycle, resulting in a decrease in the horizontal stress and hence shaft resistance of the floating piles. During thermal cycles, the stress states of the soil around the energy pile shaft and the soil beneath the pile toe approach the critical state line along different paths. The induced temperature in the soil adjacent to the non-energy pile (NEP) is 5°C lower than that in the soil at the energy pile EP1, which is flanked by the other two energy piles EP2 and EP3. Consequently, the induced excess pore pressure in the soil at the NEP is approximately 20% smaller than that in the soil at EP1. The irreversible volumetric soil contraction at the NEP is about half that at EP1, resulting in approximately 45% less toe settlement. The thermally induced ratcheting settlements of the head and toe of the NEP are less than those of the energy piles, resulting in unacceptable ratcheting tilting of the floating energy pile group. However, the excessive tilting can be reduced by the use of a pile raft.

On the AVISA model for clay – Recommendations for calibration and verification based on the back analysis of a piled raft

This paper provides a manual for the thorough calibration of material parameters of the anisotropic visco ISA (AVISA) model and discusses some of the pitfalls that can arise when performing numerical analysis of geotechnical problems using this constitutive model. Considering element test simulations as well as finite element calculations, clear recommendations are extracted for the initialization of state variables based on comparative studies. Restrictions of parameter values to avoid numerical instabilities are revealed, and recommendations are provided. In addition, simulations of various laboratory tests on Frankfurt clay are analyzed and discussed. The model’s performance is further validated by means of 3D finite element analysis of the piled raft of the Messeturm in Frankfurt, for which extensive measurement data are available. The simulations comprise both the construction phase and the subsequent long-term deformation due to creep.

Experimental Study on Vibration Velocity of Piled Raft Supported Embankment and Foundation for Ballastless High Speed Railway

In recent years, the high development of high-speed railway lines cross through areas with poor geological conditions, such as soft soil, offshore and low-lying marsh areas, resulting geotechnical problems, such as large settlements and reduction of bearing capacity. As a new soil reinforcement method in high speed railway lines, the piled raft structure has been used to improve soil conditions and control excess settlement. In order to study the dynamic behavior of piled raft supported ballastless track system in soft soil, an experimental study on vibration velocities of piled raft supported embankment and foundations is presented in soft soil with different underground water levels. Vibration velocities at specified positions of the piled raft supported embankment and foundations are obtained and discussed. The vibration velocity curves on various testing locations of piled raft foundations are clearly visible and have sharp impulse and relaxation pattern, corresponding to loading from train wheels, bogies, and passages. Vibration velocity distribution in the horizontal direction at three train speeds clearly follows an exponential curves. Most of the power spectrums of vibration velocity at various locations are mainly concentrated at harmonic frequencies. The change in water level has slight impaction on the peak spectrum of vibration velocity at harmonic frequencies. The vibration power induced by train loads are transmitted, absorbed, and weakened to a certain extent through embankment and piled raft structure. The dynamic response character of embankments are affected by their self-vibration characteristics and the dynamic bearing capacity of the piled raft structure.

Parametric study of unconnected piled rafts in clayey soil using finite element method

Parametric study of unconnected piled rafts in clayey soil using finite element method

Seismic Analysis of RC High-Rise Buildings Rested on Cellular Raft

This paper includes the investigation of the soil–structure interaction (SSI) effect on the seismic response of 20 and 30-story reinforced concrete moment resisting frames (MRFs) rested on a piled raft foundation using the direct approach. After that, a study is conducted to show the impact of using a cellular raft instead of the designed solid raft on the dynamic response of the building. A study is introduced to select the best gap size for the cellular raft. The soil model is assumed as a single layer of sandy clay. Time history analysis by the direct integration method is performed under seven earthquake records (El-Centro, Northridge, Kobe, Chichi, Friuli, Kocaeli, and Loma), which are scaled to the Egyptian Code for Loads (ECP-201) response spectrum using a full 3D model by a finite element software named (Midas GTS NX). It is concluded that considering SSI significantly affects the dynamic response of high-rise buildings, and using cellular rafts generally leads to a decrease in their dynamic response.

Improving the Performance of Piled Raft Foundations Using Deformation Adjustors: A Case Study

Complicated soil conditions are direct difficulties for high-rise building projects. A new device called a deformation adjustor, which is used to optimize the stiffness distribution in the piled raft system, has achieved good results for this challenge. This paper presents a case study on the application of deformation adjustors to improve the performance of a piled raft foundation. This case study describes the preliminary design of pile-raft foundations with deformation adjustors, followed by numerical analysis. Based on the numerical study, the potential savings are demonstrated due to the good performance of soil bearing capacity. Comparing the numerical results with the monitoring results in raft settlements, earth pressures, deformation amount of deformation adjustors, pile top reactions, and load-sharing ratios between soils and piles, the accuracy of the design schemes with an aided numerical analysis is verified. Through a long-term monitoring, soils below the raft carried 63% of the total applied loads, while the piles bear 37% of the loads. This case study proved that a piled raft foundation with deformation adjustors was an effective and economical design scheme, which can make full use of the soil bearing capacity. It is of great significance to facilitate the design and construction of piled raft foundations in complicated soil conditions.

Studying the damping characteristics of the system of building and infrastructure considering the Soil-Structure Interaction

This paper studies the damping characteristics of the soil foundation under a building concerning the Soil-Structure Interaction (SSI). By considering a multi-story building resting on a piled raft foundation as a typical SSI system, subjected to an earthquake (i.e., Chi Chi earthquake, Taiwan 1999), the time-dependent responses of specified locations in the foundation are recorded and analyzed, both in-time domain and frequency domain. Referring to prior knowledge about system identification and damping, this study suggests an overwhelming approach to determining the damping ratio for analyzing more thoroughly a system of SSI systems. Results are a general procedure to establish the damping matrix, namely the Caughey damping matrix which takes several frequencies into account. A different viewpoint that the connection of superstructure and infrastructure in series could help to estimate the damping, and contribute to a wide range of system identification. All these procedures are used to predict more properly the damping characteristics of a system of structures. Suggestions over the findings would be expected to contribute a more rigorous component to the analysis of high-rise buildings in the future.

Interaction analysis of two floating granular piled raft units

ABSTRACT The pile–soil–raft interactions remain a daunting problem in the study of piled-raft foundations. A simple analytical continuum approach is implemented in the present research to analyse the interaction between two granular floating piled-rafts. The formulation uses the equations of Mindlin and Boussinesq for the estimation of the displacements for forces in the interior and on the surface of the continuum. Settlement influence factor, interaction factor, the normalised shear stresses along the granular pile (GP)-soil interface, the percentage load shared by GP, the percentage load transferred to the GP base, and the normalised contact pressure distribution underneath the raft are evaluated and presented for design.

The applicability and load-sharing behavior of piled rafts in soft clay: experimental study

The applicability and load-sharing behavior of piled rafts in soft clay: experimental study

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.

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.

Effect of Contact Pressure on the Lateral Capacity of Piles

This study aims to determine how to contact pressure affects pile capacity. First, a single pile model was subjected to a total of six tests, five conducted under varying surface loads and one conducted on a single pile raft. Plaxis- 3D was utilized in the second step of numerical modelling to imitate experimental activity. The galvanized iron pipe was used as a model pipe, and for confining pressure, a rectangular plate made of aluminum had a slot in the center for placing a pile. For numerical study in Plaxis 3-D pile was used as a model pile surrounding the soil to find the relative displacement between the piles and surrounding the soil. In the proposed research work, the axial and lateral capacity of the pile increases due to the contact pressure of the soil. As a result of the interaction between the pile and the raft, when the results were compared, the pile tightly connected to the raft took less axial force than the other pile that was not. Similar outcomes emerged from the trials when they were conducted in Plaxis 3-D.

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.