Piled Raft Foundation System Research Articles

Settlement Analysis of Piled Raft Foundation System (PRFS) in Soft Clay

Settlement Analysis of Piled Raft Foundation System (PRFS) in Soft Clay

Evaluation of Piled Raft Performance Using a Verified 3D Nonlinear Numerical Model

The piled raft foundation system presents a feasible foundation option for high rise buildings. Piled raft foundations have a complex three-dimensional soil-structure interaction scheme including the pile-soil interaction, pile–pile interaction, raft-soil interaction, and finally the piles-raft interaction. Consequently, comprehensive 3D numerical models are sought to investigate this complex interaction. In this paper, a 3D finite element model was verified using published geotechnical centrifuge test results to further enhance the accuracy of the model. The study considered piled rafts installed in sand with stiffness that varies linearly with depth. A comprehensive parametric study was conducted to evaluate the effect of different foundation parameters including: pile diameter, pile spacing, raft width, and raft thickness on the overall behavior of the piled raft. The load carrying capacity of the piled raft and the load sharing mechanism between the raft and piles are evaluated. It was found that the load carried by piles is higher for rigid rafts (Kf > 600) due to the minimal interaction between the raft and subsoil compared to the relatively flexible rafts (Kf < 150). Moreover, the load transmitted by the raft increased by about 75% as the raft width increased from 4 to 7 m.

3-D NUMERICAL ANALYSIS OF CONSOLIDATION EFFECT ON PILED RAFT FOUNDATION IN BANGKOK SUBSOIL CONDITION

This study focused on the investigation of influencing factors on behaviour of piled raftfoundation in Bangkok subsoil. To evaluate the possibility of implementing this system in Bangkok subsoilcondition, this research performed the consolidation analysis of piled raft foundation systems for low-rise (8-storey) and high-rise (25-storey) buildings with basement levels in clay soil, using coupled three-dimensional(3D) mechanical and hydraulic numerical model. The soils are modelled with Hardening Soil model andMohr-Coulomb model. Evaluations of piled raft foundation, i.e., the load sharing ratio of piles, settlementbehaviours in the foundation system are performed. The parametric study on the effect of raft depth, and loadcarried by piles of piled raft was done. The consolidation had a strong influence on the load carried by pilesof the piled raft foundation in Bangkok. The load shared by piles can increase by up to 12% and 6% for lowrise and high-rise buildings, respectively due to the consolidation effect. Therefore, the design of the piledraft foundation system in Bangkok subsoil essentially consider the consolidation effect.

Pile Spacing Optimization of Short Piled Raft Foundation System for Obtaining Minimum Settlement on Peat

Short Piled Raft is a modified piled raft foundation system, which represents combination between raft foundation and pile foundation, but the length of pile is relatively shorter. The basic concept of the Short Piled Raft foundation system considers the passive soil pressure creating a stiff condition of slab-pile system. This means that the thin concrete slab floats on the supporting soil, while the piles serve as stiffeners concrete slab and also to reduce settlement of the foundation. Slab to pile ratio of such system has been mentioned by several researchers, however the optimum pile spacing of stability performance for obtaining minimum settlement on peat haven't been clearly discussed. In this study, finite element method to simulate the stability performance related to settlement of Short Piled Raft foundation system was used. Short Piled Raft foundation system with concrete slab of 7.0 m x 7.0 m square was assumed to be built on peat with the thickness of 3.5 m. The material properties of pile and raft were constant. The outer diameter of galvanized steel pipe as pile was 0.30 m; raft thickness was considered to be constant of 0.15 m and the length of pile was 3.0 m, while the pile spacing varied from 0.50 to 3.00 m. Point load varied from 0 to 100 kN with increment of 20 kN was also considered as a static load, acted on the centre of the concrete slab. Optimization was done by comparing each numerical result of simulations, thus conclusion can easily be drawn. The optimum pile spacing was 1.00 m which produced minimum settlement of 30.11 mm under the load of 100 kN.

Evaluation of the existing piled foundation based on piled–raft design philosophy

Abstract Many well-documented case histories for conventionally designed large pile groups proved that this design approach is conservative and dramatically increases the cost of foundation without almost any benefit neither to geotechnical capacity of foundation nor to serviceability. Monitoring results of these case histories, especially in the case of floating piles, have shown low efficiency of pile capacity usage due to direct transfer of considerable part of the load to the supporting soil via the raft. In this paper, a proposed methodology for the combined-piled–raft design based on the conventional philosophy is applied to evaluate existing conventionally designed piled foundations of two identical residential towers located in Cairo, Egypt. The proposed methodology considers a conventional factor of safety for piles, and consequently, it does not violate the existing building codes. The three-dimensional finite-element analyses are performed to evaluate the load sharing between raft and piles. A detailed geotechnical investigation is carried out to obtain soil stratification and material parameters. The pile–soil interface parameters are obtained through back analyses of the available pile load tests. The results of the analyses show that the predicted geotechnical capacity of the combined piled–raft foundation system considering the conventional factor of safety for piles considerably exceeds the design capacity of the original conventional pile group.

Numerical investigation of piled raft foundation in mitigating embankment vibrations induced by high-speed trains

A three-dimensional dynamic finite element model of track-ballast-embankment and piled raft foundation system is established. Dynamic response of a railway embankment to a high-speed train is simulated for two cases: soft ground improved by piled raft foundation, and untreated soft ground. The obtained results are compared both in time domain and frequency domain to evaluate the effectiveness of the ground improvement in mitigating the embankment vibrations induced by high-speed trains. The results show that ground improving methods can significantly reduce the embankment vibrations at all considered train speeds (36- 432 km/h). The ground response to a moving load is dictated largely by the relationship between load speed and characteristic value of wave velocities of the ground medium. At low speeds, the ground response from a moving load is essentially quasi-static. That is, the displacements fields are essential the static fields under the load simply moving with it. For the soft ground, the displacement on the ballast surface is large at all observed train speeds. For the model case where the ground is improved by piled raft foundation, the peak displacement is reduced at all considered train speeds compared with the case without ground improvement. Based on the effect of energy-dissipating of ballast-embankment-ground system with damping, the train-induced vibration waves moving in ballast and embankment are trapped and dissipated, and thus the vibration amplitudes of dynamic displacement outside the embankment are significantly reduced. But for the vibration amplitude of dynamic velocity, the vibration waves in embankment are absorbed or reflected back, and the velocity amplitudes at the ballast and embankment surface are enhanced. For the change of the vibration character of embankment and ballast, the bearing capacity and dynamic character are improved. Therefore, both of the static and dynamic displacements are reduced by ground improvement; the dynamic velocity of ballast and embankment increases with the increase of train speed and its vibration noise is another issue of concern that should be carefully evaluated because it is associated with the running safety and comfort of high-speed trains.

Piled raft with hollow auger piles founded in a Brazilian granular deposit

Geotechnical projects typically achieve load transfer to the ground using shallow or deep foundations. The conventional design approach does not provide for the combination of these two types of foundation. The piled raft philosophy allows the association of the soil elements, raft, and piles to obtain technical and economic advantages over conventional design. The city of João Pessoa, in northeastern Brazil, has developed foundation practices with hollow auger piles in piled raft design. The coastal area of the city has topsoil layers with favorable conditions for using such a technique. This paper addresses the results of a research project with instrumented load tests on foundation systems of hollow auger piles and a piled raft. The analysis is based on the load–settlement curve through extrapolation criteria. The Poulos–Davis–Randolph (PDR) method is applied according to a trilinear and hyperbolic approach to simulate the load–settlement curve of piled rafts. The results indicate that the raft absorbs most of the load, and the raft–soil contact significantly increases the load capacity of the foundation. The PDR hyperbolic method could apply to practical use in the foundations of the region, as it allows a more detailed assessment of the behavior of the foundation and can forecast the behavior of the (locally nontraditional) piled raft foundation system.

An Experimental Study on Behaviour of Piled Raft Foundation

In order to study the behaviour of piled raft foundation system in sandy soil under vertical load, an experimental program in the laboratory has been conducted. The experimental program includes the model test on unpiled raft, raft supported by single pile, (2 × 2) and (3 × 3) pile groups. The piles used in this test are non-displacement piles. In the laboratory test, model mild steel piles of diameter 20 mm and a length of 200 mm were used, to represent the slenderness ratio, L/D of 10. The raft was made of mild steel plate with plan dimensions of 200 mm × 200 mm with different thicknesses of 5, 10 and 15 mm. The values of raft-soil stiffness of model rafts ranging from 0.37 to 10.11 covers flexible to very stiff raft. The refinement in the bearing capacity is represented by load improvement ratio and the reduction in average settlement is represented by settlement reduction ratio. The influence of raft-soil stiffness and the number of piles underneath the raft on the load improvement ratio and settlement reduction ratio are presented and discussed. The results of the tests show that as the number of piles underneath the raft increases from 1 to 9 piles, the load improvement ratio increases by 30–40 %, settlement reduction ratio increases by 18–90 % and percentage of load carried by the raft decreases by 22–25 %. Also load improvement ratio is slightly increases by 1.5–5 % and the settlement reduction ratio decreases by 1.5–30 % as the raft-soil stiffness ratio increases for the given number of piles, while the load carried by the raft slightly decreases by 1.5–4.5 % as the raft-soil stiffness increases.

Analysis of Piled Raft Foundation on Soft Soil Using PLAXIS 2D

Recent years, there are many construction projects constructed on soft soil. Due to the characteristics of soft soil, the structures built on it are subject to differential settlements. Raft foundation is one of the methods for reducing the differential settlement. Although it has an adequate bearing capacity, it may cause excessive settlement. Piles can be used with a raft foundation as a piled raft foundation system. The addition of piles is to reduce the settlements to an acceptable amount. The aim of this study is to analyze the settlements of the raft foundation and by adding piles, as the pile raft foundation, under the same loading. The numerical analysis has been done by finite element method using PLAXIS 2D with considering the various number of piles. As the results, the addition of piles could reduce the settlement, but after reach a certain number of piles, increasing the number of piles showed the settlement tends to be constant. For an economic design, it is necessary to consider the optimum number of piles in piled raft foundation system based on the allowable settlements.

3D-Analysis of Soil-Foundation-Structure Interaction in Layered Soil

The analysis of building structure in contact with soil involves an interactive process of stresses and strains developed within the structure and the soil field. The response of Piled-Raft Foundation system to the structure is very challenging because there is an important interplay between the component of building structure and the soil field. Herein, soil-foundation-structure interaction of buildings founded on Piled-Raft Foundation is evaluated through 3D-Nonlinear Finite Element Analyses using PLAXIS3D FOUNDATION code. The soil settlements and forces demand of the high-rise building structures and foundation is computed. The parametric study affecting the soil-foundation-structure response has been carried out. The parameters such as construction phasing, sequential loading, building aspect ratios, soil failure models and thickness proportion of soil field stiff layer, are considered. It is concluded that the interaction of building foundation-soil field and super-structure has remarkable effect on the structure.

Gravel Mat로 보강된 말뚝지지 전면기초의 실내모형실험

A piled raft foundation is one of the systems used to reduce the settlement of structures. However, the general design method for a piled raft foundation system assumes that the piles only support external loads, which exclude the bearing capacity of the raft itself. In this study, an experimental model test was performed to evaluate the raft capacity for the external load on the sand. Additionally, a part of the sandy ground under the raft was replaced with a gravel mat to reinforce the piled raft foundation system and increase the bearing capacity. Then, parametric studies of the reinforced ground were performed to determine the displacement and load-sharing ratio of the piled raft foundation system.

A piled raft interaction model

Based on the considerations of pile interaction as published by Randolph and Wroth (1978) and Van Impe (1991), a modified interaction model for a piled raft foundation system anda layered soil profile is proposed. Attention is payed particularly to the pile soil interaction radius and the relative stiffness of the pile group and raft. The paper shows the application of the model for a case study in France.