Behavior of piled-raft foundation (PRF) in compressible ground: A case study delineating the importance of adopting bored piles over driven piles

Piled raft foundations are a combined system of deep foundation and shallow foundation. In this type of foundation systems, the pile footings are provided beneath a raft or mat footing. Thus, the piled raft foundations can be constructed either by providing raft foundation connected to the pile footings or by providing a cushion layer between raft and pile footings. The first type of piled raft foundation is generally termed as connected piled raft foundations, and the latter piled raft foundations. Piled raft foundations is termed as disconnected can be subjected to different types of loadings, like vertical and inclined loading depending on the type of superstructure, lateral loading induced by earth pressure as well as seismic loads and moments induced by the eccentricity in loadings. Numerous studies have been conducted on the behavior of piled raft foundations under independent and combined effects of these loadings. This article aims to summarize these literature works to review the behavior of piled raft foundations under static and dynamic loadings acting in different directions.KeywordsPiled raft foundationsStatic loadingDynamic loading

Piled raft system proves to be more effective on such problematic soft soil conditions. It takes the high vertical load and used to bring the settlement, differential settlement and tilting of structure within the permissible limit. Piled raft system proves to be cost effective than the conventional pile foundation system. Piled raft foundation accounts for complex soil-structure interaction, which needs interaction between structural engineer and geotechnical engineer. Geo-foam application is introduced below the footing to enhance floating resistance. The performance behavior of piled raft foundation in clayey soil and layered soil is carried out using laboratory experimental and then verified by 2D PLAXIS. There is a significant relationship between the decreases in settlement of the piled raft due to hydrostatic uplift.

Previous research often relied on simplified models that assumed a rigid connection between the superstructure and raft foundation. These models disregarded the influence of superstructure stiffness on soil-foundation interaction, leading to inaccurate predictions of raft bending moments, pile loads, and settlement. This paper investigated the impact of superstructure stiffness for three different structural systems (framing, coring and shear wall systems) on the behavior of piled raft foundation as well as the presence impact of a 2-meter sand replacement beneath the raft under various loads. A 3D nonlinear finite element analysis using PLAXIS 3D and ETABS was conducted in soft to medium clay overlying a dense sand layer. Incorporating the superstructure stiffness reduced the maximum raft bending moments by (6.6% to 9.1%), (39.0% to 47.9%), and (18.2% to 22.3%) as well as the maximum raft shear forces by (39.2% to 46.9%), (25.6% to 49.2%), and (34.7% to 44.9%) for framing, coring and shear wall systems ,respectively, under gravity, wind, and seismic loads. The presence of a 2m replacement layer beneath the raft had a negligible effect on the system's behavior(Less than 1.6%). In conclusion, the variation of superstructure stiffness affected the raft's internal forces; therefore, it can be economically advantageous to consider this while designing piled raft foundation.

The conducts of pile raft basis depend on piles, raft, as well as soil. When designing a pile group, it is expected that only the piles carry out the applied loading. No contribution is given to soils underneath the raft because of the complicated interacting amid the piles and their cap with soils. When the soil under the raft foundation shares carrying the applied loads, this interaction will increment the piled raft system’s bearing capability as well as decrease the settlements. To comprehend the piled raft system’s complicated conduct, testing for twenty-seven experimental models was done within loose sands with a load applied to the piled raft foundation via a compressing jack then the measurement was done using a loading cell. Wide parametrical examinations had been executed with the variables number of piles, pile length, raft dimension ratio. The soil under the raft contributed a certain percentage in carrying the applied weights in the piled raft foundation. Similarly, there is an increase in the loading taken via piles to the overall load executed on pile raft with the growth of the number of piles in the series, and there is a decrease in the share of the soil under the raft for the applied loads when there is increasing within the piles’ amounts. The increment within the piles’ lengths causes an increase in the soil’s share underneath the raft, and the maximum settlement of the piled raft system does not exceed 5% of the raft width in all cases.

Settlement of the piled raft can be estimated even after years of completing the construction of any structure over the foundation. This study is devoted to carry out numerical analysis by the finite element method of the consolidation settlement of piled rafts over clayey soils and detecting the dissipation of excess pore water pressure and its effect on bearing capacity of piled raft foundations. The ABAQUS computer program is used as a finite element tool and the soil is represented by the modified Drucker-Prager/cap model. Five different configurations of pile groups are simulated in the finite element analysis. It was found that the settlement beneath the piled raft foundation resulted from the dissipation of excess pore water pressure considerably affects the final settlement of the foundation, and enough attention should be paid to settlement variation with time. The settlement behavior of unpiled raft shows bowl shaped settlement profile with maximum at the center. The degree of curvature of the raft under vertical load increases with the decrease of the raft thickness. For the same vertical load, the differential settlement of raft of (10x10 m) size decreases by more than 90% when the raft thickness increased from 0.75 m to 1.5 m. The average load carried by piles depends on the number of piles in the group. The groups of (2x1, 3x1, 2x2, 3x2, and 3x3) piles were found to carry about 24%, 32%, 42%, 58%, and 79% of the total vertical load. The distribution of load between piles becomes more uniform with the increase of raft thickness.

The micropile-raft system is used in the foundation system of buildings due to the combination of the advantages of raft foundations and micropiles. In this paper, the numerical model was calibrated and validated with a field test, and then several parametric studies were performed. The overarching aim of the study is to determine the efficiency of different arrangements of micropiles and the effects of these parameters on the behavior of piled raft foundations in sandy soil. Parameters including diameter, length, and distance of micropiles from each other were considered in the analyses. Prestressed rebars have been used to increase the load-bearing capacity and prevent premature buckling under severe compressive load. Therefore, to compensate for the weak bearing capacity of the raft foundation, a micropile system equipped with prestressed rebars was considered to increase the bearing capacity of the foundation. The results revealed that using a set of micropiles with a diameter of 30 cm and a length of 10 m had the maximum efficiency among other models. Furthermore, an innovative arrangement of micropiles was proposed in which a set of micropiles with a length of 30 m was placed at the middle region of the foundation, and in the peripheral area of the foundation, several short micropiles were used.

The objective of this research work is to study the behavior of piled-raft foundations subjected to vertical and lateral loads. Study for most of the previous research in this field shows that most of these studies analyzed piled-raft foundations in 2-D plane presenting the pile as a spring which may not give very accurate results. In this research, 3-D finite element method is suggested to represent piled-raft interaction. This leads to better representation for the elements which gives more reliable results such as (axial displacement, rotational displacement, vertical forces and moments acting on the pile and the raft). The results from this analysis will be compared with the results of a case study on a bridge in Basra, Iraq called (Fly Over Bridge) in order to verify the model. An extensive parametric study will be performed to assess the effect of various factors on the internal forces generated in the raft and piles. From analysis, there are important recommendations concerning the maximum inclination angle of the pile, and the diameter of the piles on the internal forces generated in the foundation and the piles and their equilibrium.

Tall buildings are normally supported on certain kind of piled foundation that is exposed to a mixture of vertical, horizontal, and overturning forces. Combination of the pile raft foundation may be a telling sort of foundation system for skyscraper as a result of the raft which is in a position to provide an inexpensive rate of each unwieldiness and load resistance. Now in practice, all the foundation of high-rise building are design as a fixed base, i.e., the relative behavior of soil and foundation are not to be considered. In this paper, soil–structure interaction effect is considered in pile raft foundation of a multi-story building. The G+21 multi-story building is analyzed in software ETABS 2016, and SSI is carried out in PLAXIS 3D. Here, vertical irregularities are taken in the modeling of G+21 story RC MRF building. The seismic analysis of the model is carried under the three different types of analysis methods, i.e., response spectrum (RS), equivalent static load (ESL), and time history (TH) analysis suit to Bhuj time history record which is used for analysis of the moment resistant frame (MRF) reinforced concrete (RC) building. The parameters being used for the comparative study of soil–structure interaction (SSI) are base shear, story drift, and story displacement. Numerical results acquired using SSI model situations are matched to those equivalent to fixed-base situations. The present study shows that the effects of soft soil deposit might be adverse to the earth vibration reaction of the building and avoiding SSI in the evaluation might be prime to unsafe design.

Piled raft foundation is used to reduce the settlement of soft clay by using the friction between the pile and soil. Recently, a numerical approach (FEM) was adopted in a study to clarify the mechanism of the bearing capacity of piled raft foundation. In addition, the finite deformation theory and the subloading model based on the unconventional plasticity theory are introduced. Under a small load, the distinct difference between the infinitesimal and finite deformation analyses cannot be observed, but when the deformation is large, the difference is remarkable. This is because the influence of geometric nonlinearity becomes large. The behavior of over-consolidated soil can be expressed appropriately using the subloading model. When the load is applied at the piled raft foundation, compression and shear behavior are observed around the element at the tip of the pile. Finally, the possibility of deformation prediction using FEM is demonstrated.

SUMMARYFinite element simulations of the behavior of a piled raft foundation have been carried out using a multiphase model conceived as an improved homogenization approach. According to this model, the ground reinforced by a group of piles is treated as a homogeneous continuous medium. In this approach, no specific interface elements are necessary to account for the mechanical interaction between the piles and the ground: this interaction is described by means of two scalar parameters, one stiffness parameter and one which can easily be derived from the maximum ground‐pile friction. The implementation of the model into a finite element code provides an efficient tool for the analysis of the influence of the pile number or length on the settlement and bearing capacity of a square piled raft foundation and of the way the total applied load is shared between the raft and the piles. Results are compared with a standard 3D finite element analysis. The comparison highlights the fact that the proposed approach remains to be improved to account for tip resistance. Copyright © 2012 John Wiley & Sons, Ltd.

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.

The structural behavior of instrumented cast-in-situ reinforced concrete bored piles when subjected to lateral movements due to deep excavation has been monitored and studied. The most important structural parameter required, from the design standpoint for bored piles, is the magnitude of bending moment induced in the pile in such situations. Two methods have been used to derive the bending moment induced in instrumented bored piles. In the first method, which is a direct method, strain readings measured via strain gauges installed along the pile depth were used to back-analyze the induced bending moment. In the second method the deflection profile measured via inclinometers installed in the bored piles were used to back-analyze the bending moment. In this paper the results obtained and the comparison of the bending moments obtained from the two different methods are discussed. From this discussion, an idea of the pile elastic modulus and moment of inertia that can be generally used to determine the bending moment from pile lateral deflection profiles is further explored and discussed.

The effect of lime piles on the behavior of piled-raft foundation constructed on such soil and subjected to static and cyclic vertical loading was also investigated. The soil samples were contaminated artificially by light fuel oil (LFO) mixed with water and kerosene to simulate the contaminants that are usually disposed directly to the soil from the electricity power plant as a byproduct. The lime columns are holes of 20 mm diameter and 500 mm deep were drilled in the soil and filled with lime mixture. The lime mixture was prepared by mixing a dry soil with 15% of lime and natural water content and then poured into the holes and mixed thoroughly. The lime mixture was left for four weeks to allow the completion of most chemical reactions necessary for improvement and remediation of oil contaminated soil. Aluminum piles of 20 mm in diameter and 300 mm long were installed in soil models and subjected to static and cyclic vertical loading. Load-settlement curves showed that the contaminated soils have more settlement values compared with the intact soil, but improvement and remediation of soil samples showless settlement than that of contaminated samples. Lime piles are considered an efficient technique for decreasing the settlement of piled-raft foundations.

Nonlinear analysis of flexible piled raft foundations subjected to vertical loads in layered soils

Behaviour of Disconnected Piled Raft Foundation on Sandy Soil: An Experimental Study