Parametric Study on Unconnected Piled Raft Foundation Using Numerical Modelling

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.

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 performance of a vertically-loaded floating piled raft foundation in very soft and deep layers of compressible subsoil, in terms of its ability to control key factors such as bending moment and pile-raft coefficient, was presented and discussed in previous research. The other critical factor that needs to be evaluated is the angular distortion of the foundation. This paper presents the effectiveness of the piled raft foundation system in controlling the angular distortion of the foundation. A comprehensive analysis of this proposed foundation system for various pile lengths in soft compressible subsoil was carried out. Different pile groups with 9 × 9 arrangement of piles were loaded vertically. A total combination of 45 pile group cases with different pile lengths, pile spacing, and raft thicknesses was analyzed. Results showed lower angular distortion for the proposed foundation system, which indicated that a piled raft with varying pile lengths is most efficient in controlling angular distortion as compared to piled rafts with equal pile lengths.

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.

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.

The 3D nonlinear finite element model is developed to investigate thermo-mechanical response of a piled raft equipped with floating energy piles on sand subjected to combined thermo-mechanical loadings. Attention is focused on thermally induced group effects， the variations in temperature of the surrounding soil and the effect of the number and the layout of the energy piles on the thermally induced additional axial stress in piles and the differential settlements among the pile heads. The results show that when all the piles are heated， the additional axial compressive stress is much smaller for a pile in the energy pile group than that for single energy pile in the same group. In a pile-raft foundation with partial energy pile， the additional axial compressive stress is distributed in parabolic shape along the energy pile， and it decreases with the increasing number of energy piles. While the additional axial stress in a non-energy pile is distributed in S shape， and the additional axial tensile stresses in the upper parts of non-energy corner piles and edge piles increase with the increase of the number of energy piles. The layout of energy piles has great influence on the additional axial stresses in the energy piles， but has insignificant effect on the additional axial stresses in the non-energy piles. The maximum differential settlement among the pile heads is larger when the layout of energy piles is asymmetrical. The temperature of the surrounding soil depends on the number of energy piles around it. <fig fig-type="abstract-image" id="F1" orientation="portrait" position="float"><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="SP.J.1249-2022-39-1-67/9F0446D4-9E61-4374-AD12-215148B7C310-F001.jpg"/></fig>

The piled raft (PR) foundation system combines a shallow raft with deep piles, mobilising both components through complex raft–pile–soil interactions. Due to its effectiveness under vertical and lateral loads, it is widely used to support tall, heavily loaded structures. However, the complexity of the system often leads to reliance on simplified design methods. This study uses numerical simulations in FLAC3D to investigate the effect of pile battering on the behaviour of PR and pile group (PG) foundations in drained sandy soils. Unlike previous research combining vertical and battered piles, only battered piles are adopted here to isolate their influence under combined loading. A batter angle of 10° is selected to avoid excessive bending moments. Results indicate that battered piles improve lateral resistance in PR foundations by up to 66%, though vertical capacity is slightly reduced (by 14%) due to axial–bending interaction. Under combined loading, vertical piles outperform battered ones in overall resistance gain (55.7% vs. 38.8%), mainly due to improved soil confinement. Batter piles also play a key role in reducing tilt and enhancing foundation stability. This study offers practical insights for designing foundations exposed to strong lateral forces, such as in offshore structures and wind turbines.

Simplified method of estimating the dynamic impedance of a piled raft foundation subjected to inertial loading due to an earthquake

Piled rafts have been widely adopted as an effective foundation for designing high-rise buildings because of their efficiency in controlling the total and differential settlements and improving bearing capacity. In many cases the piled rafts settlements are likely to be large, which leads to an increase of the pile length and/or number of piles required to reduce the settlements. However, this increase does not satisfy the design requirements or an economical design. The majority of piled raft foundations has been designed with a uniform pile length and configuration. This paper describes the process of optimizing the design of a piled raft foundation for a high rise building in the Mazandaran province in Iran by considering an economical design methodology in which piles are placed more densely beneath the maximum load positions when the piled raft is subjected to non-uniform loads. By using the ELPLA software in the analysis process, the validity of the software is examined through the results of a report prepared on behalf of Technical Committee TC18 on piled foundations. The study shows that the pile arrangement method can help to considerably reduce the total and differential settlements with similar total pile length as well as the induced bending moments and shear forces of the raft. This study can help practicing engineers to choose pile and raft parameters with the pile arrangement method to produce an economical design.

When the loading sustained by the foundation is large, employing piled raft foundation is one of the best solutions. In the elasto-plastic design of piled raft, piles could improve the differential settlement and reduce raft thickness. As the raft sustains high earth and water pressures, by reducing the span length of raft and excavation depth, one can get economic design. Using elasto-plastic property of the pile is a better method to avoid increasing the pile length and pile diameter and making full capacity of the piled raft foundation in design compared to normal piled raft. With adoption of few small diameter piles, the raft thickness could be reduced significantly. This makes it much better than raft foundation.

The 3D finite element simulations are carried out to study the impact of different pile head connection conditions, i.e., rigid and hinged connection on the response of piled rafts in sand subjected to pseudo-static horizontal loading. The time-acceleration histories of Assam (2014), Bhuj (2001), Sikkim (2011) and Uttarkashi (1991) earthquakes are considered to calculate the horizontal pseudo-static loads. Analyses results confirm that pile head connection rigidity greatly influences the behavior of piled raft in many aspects. The horizontal stiffness of piled raft is found to decrease as the rigid pile head connection changes to hinged one. Piles in a piled raft foundation is found to carry about 52–75% of the pseudo-static horizontal load for rigid connection and 32–48% for hinged connection. The horizontal deflection of pile is found maximum at the pile head and becomes negligible along the depth of pile for both cases. For rigidly connected piles, the maximum bending moment occurs at the pile head, whereas piles with hinged connection show zero bending moment at top and maximum value at a certain depth from pile head.

The potential damages of low-rise building foundations on liquefiable sites were studied using the Midas GTS-NX analysis. Both free-field and foundation responses for single raft and piled raft foundations were monitored on layered sands using the UBCSand model. It was found that a single raft can be easily uplifted by surrounding groundwater and soil liquefaction occurred. The piled raft foundation has better seismic resistance compared to the single raft. The foundation load will enlarge the ground settlements but reduce the horizontal ground displacements underneath the foundation. The 2D analysis can provide rational predictions of the piled raft responses despite the foundation geometry not being fully simulated. Using a thicker raft can eliminate the angle variable of the foundation. Considerable amounts of foundation settlements were found for single rafts and piled rafts according to the design specification. Longer piles will help to reduce the foundation settlement.

The paper describes the calculation and design steps of deep excavations and foundations of a high-rise building on a territory with a high groundwater level and existing buildings with a physical wear of their bearing structures and foundations located in the zone of mutual influence. The aim is to find a rational solution for the diaphragm wall, pile-raft foundation of the high-rise building in order to protect adjacent objects and consider various factors that determine the nonlinear behavior of subsoil layers and the elements of geotechnical protection. As an example, a project of a high-rise building with a multi-level underground parking located in the central part of Krasnodar is shown. It is designed and calculated considering the staged construction of the pile-raft foundation, non-linear behavior of the subsoil layers, the influence of a variable groundwater level, the risk of technological settlements and liquefaction of the underlying sandy soil layers. Due to the performed researches and by following the construction methods, it became possible to keep the existing buildings and structures safe during the construction period and further operations. The observed methods of construction of the pile-raft foundations and deep excavations in difficult subsoil conditions using the example of the central part of Krasnodar make it possible to recommend the obtained results as a rational solution applicable for similar cases in conditions of a dense urban infrastructure.

Behaviour of vertically loaded piled raft system on cohesionless soil

Ballastless tracks have been extensively used in newly built high-speed railroads in China. Most of their foundations are constructed without pile improvement; however, the pile-raft design concept has been increasingly used as a rapid yet effective construction technique to reinforce the foundations. Limited attention has been paid to the behavior of this foundation system under dynamic loads either in the laboratory trials or field applications. This study presented the results of displacement responses obtained from three-dimensional (3D) dynamic finite element simulations of a typical high-speed railroad ballastless track over the pile-raft foundation. Both the soft ground improved by the pile-raft technique and the unreinforced ground, as control, were modeled and compared to quantify the effectiveness of the ground improvement in mitigating the detrimental effects of dynamic impact loads induced by high-speed trains. Other features of the 3D finite element models included the simulation of the moving train at different speed levels, the use of nonlinear constitutive models for track foundation geomaterials, and the consideration of the pile-soil interaction. The results of this study could lead to further research in this area as guidelines to design piled raft foundations economically.