Raft Thickness Research Articles

Actual Temperature Evolution of Thick Raft Concrete Foundations and Cracking Risk Analysis

Heat of the hydration-induced temperature evolution of a 3.30 m thick raft concrete foundation for wind turbines at the early ages was monitored in situ through a temperature sensor testing system. The temperature variation patterns and risk of cracking were studied. Finite element analysis (FEA) conducted on the temperature fields determined the lower thickness threshold requiring temperature control. A comprehensive temperature control approach suitable for thick raft foundations was proposed based on a practical engineering project. Temperature monitoring and analysis results showed that the early temperature field evolution featured two characteristic phases: heat accumulation and heat release. A remarkable temperature gradient was observed along the vertical direction of the foundation. The maximum temperature difference between the concrete core and the top surface was approximately 35°C, indicating a risk of cracking. The accuracy of the FEA was ensured by adopting the concrete heat generation rate obtained from the adiabatic temperature rise test. A further FEA performed on foundations with various thicknesses demonstrated that a thicker foundation corresponded to a higher vertical temperature gradient. Moreover, a raft thickness larger than 2.50 m corresponded to a maximum temperature difference between the concrete core and the surface higher than 25°C, above which cracking prevention measures should be taken. Field test results proved the applicability of a suite of temperature feedback regulation measures proposed herein, including layered pouring, thermal insulation, and in situ real-time temperature monitoring, to thick raft mass concrete structures with relatively small volumes. Good control of temperature difference was achieved using this approach.

Seismic response of non-connected piled raft foundations

ABSTRACT Connected piled raft foundations (CPRFs) have proven to support heavy structures under vertical load. However, when subjected to seismic loading, significant load transfer from the raft beneath to the pile head may cause huge shear stress and bending moments in the piles. Thus, this paper investigates the behaviour of non-connected piled raft foundations (NCPRFs) subjected to earthquake loading. For this purpose, a three-dimensional finite element model was presented and validated against centrifuge test results. This numerical model was then employed to perform a series of numerical analysis. According to the results, pile slenderness ratio and raft thickness can be considered as essential factors in evaluating the dynamic response of NCPRF. The comparison of CPRF and NCPRF has also indicated that using NCPR system leads to significantly less shear force and bending moments along the piles, which results in less required reinforcement and more economical design in comparison with CPR systems.

Finite Element Analysis of Raft Foundation under Coupled Moment

Due to wind wave actions, ships impacts, high-speed vehicles and others resources of loading, structures such as high buildings rise bridge and electric transmission towers undergo significant coupled moment loads. In this study, the effect of increasing the value of coupled moment and increasing the rigidity of raft footing on the horizontal deflection by using 3-D finite element using ABAQUS program. The results showed that the increasing the coupled moment value leads to an increase in lateral deflection and increase in the rotational angle (α◦). The rotational angle increases from (0.014, 0.15 to 0.19) at coupled moment (120 kN.m), (0.29, 0.31 and 0.49) at coupled moment (240 kN.m) and (0.57, 0.63 and 1.03) at coupled moment (480 kN.m) with decreasing the raft thickness from (1.5, 1.0 to 0.5m), respectively. The computed maximum lateral deflection decreases with increasing the rigidity of raft. The maximum deflection decreases from (40 to 3mm) at coupled moment 120 kN.m, (150 to 60mm) at coupled moment 240 kN.m and (210 to 118mm) at coupled moment 480 kN.m with increase raft thickness from (t = 0.5 to 1.5m) and the maximum reduction in maximum stress value and lateral deflection mobilized due to applied coupled moment is noticed when width to thickness of footing ratio is less than (w/t<12). The failure of the footing is noticed when the rotational angle is more than 4◦ (α > 4◦)

Performance Analysis of a Piled Raft Foundation System of Varying Pile Lengths in Controlling Angular Distortion

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.

Performance of micropiled rafts in clay: Numerical investigation

The micropiled raft (MPR) offers an efficient foundation system that combines the advantages of micropiles and piled rafts that can be used as primary foundation system or to enhance an existing raft foundation. In this paper, a finite element model (FEM) calibrated and verified with centrifuge tests was used to carry out a numerical investigation on the performance of MPR in clay. A total of 26 different cases were analyzed in this study to assess the behaviour of MPR in clay taking into account a number of factors that may influence its behaviour such as: the number of micropiles (MPs), the spacing to micropile diameter (S/Dmp), the raft thickness, and type of loading. The outcomes of this investigation should help in understanding the effect of these factors on the MPR axial stiffness, including; differential settlement; load sharing between the MPs and the raft; the raft bending moment and micropiles skin friction. Moreover, the ability of the Poulos-Davis-Randolph (PDR) method to evaluate the axial stiffness of a MPR for the preliminary design stage is examined. It was found that the MPR system can increase the tolerable bearing pressure by 100% compared to an isolated raft system. In addition, an adjustment factor (ωPR) for PDR method was introduced to account for the raft flexibility. Equations were proposed in order to design MPR systems in terms of load sharing between micropiles and the raft.

Performance of an existing raft foundation rested on granular soils due to TBM tunneling process

In urban areas, there is a necessity to evaluate the performance of existing structures during and after the construction of nearby tunnels. Therefore, a 3D numerical modeling technique is needed to consider the geometrical interactions between the tunnel and the existing structures. To validate the results of 3D numerical simulation for the tunneling process, the surface settlement trough resulted from the numerical model has been compared with field measurements of a selected case study (Second Heinenoord Tunnel in Netherlands). The 3D finite element model has also been used to evaluate the raft settlements and bending moments. The study is performed during and after the tunnel advancement process. In addition, the effect of related parameters, such as soil relative density (Dr), tunnel diameter (D), tunnel cover (Z), horizontal clearance between the raft centerline and the tunnel centerline (CL), raft inclination angle (i), and raft thickness (d), have been studied.

Effect of Change in Thickness of Raft & Grade of Concreteon Bearing Capacity of Piled Raft Foundation

Effect of Change in Thickness of Raft & Grade of Concreteon Bearing Capacity of Piled Raft Foundation

Improvement in creep life of a nickel-based single-crystal superalloy via composition homogeneity on the multiscales by magnetic-field-assisted directional solidification

The improvement of the creep properties of single-crystal superalloys is always strongly motivated by the vast growing demand from the aviation, aerospace, and gas engine. In this study, a static magnetic-field-assisted solidification process significantly improves the creep life of single-crystal superalloys. The mechanism originates from an increase in the composition homogeneity on the multiscales, which further decreases the lattice misfit of γ/γ′ phases and affects the phase precipitation. The phase-precipitation change is reflected as the decrease in the γ′ size and the contents of carbides and γ/γ′ eutectic, which can be further verified by the variation of the cracks number and raft thickness near the fracture surface. The variation of element partition decreases the dislocation quantity within the γ/γ′ phases of the samples during the crept deformation. Though the magnetic field in the study destroys the single-crystal integrity, it does not offset the benefits from the compositional homogeneity. The proposed means shows a great potential application in industry owing to its easy implement. The uncovered mechanism provides a guideline for controlling microstructures and mechanical properties of alloys with multiple components and multiple phases using a magnetic field.

Analysis of load sharing characteristics for a piled raft foundation

The load sharing ratio (apr) of piles is one of the most common problems in the preliminary design of piled raft foundations. A series of 3D numerical analysis are conducted so that special attentions are given to load sharing characteristics under varying conditions, such as pile configuration, pile diameter, pile length, raft thickness, and settlement level. Based on the 3D FE analysis, influencing factors on load sharing behavior of piled raft are investigated. As a result, it is shown that the load sharing ratio of piled raft decreases with increasing settlement level. The load sharing ratio is not only highly dependent on the system geometries of the foundation but also on the settlement level. Based on the results of parametric studies, the load sharing ratio is proposed as a function of the various influencing factors. In addition, the parametric analyses suggest that the load sharing ratios to minimize the differential settlement of piled raft are ranging from 15 to 48% for friction pile and from 15 to 54% for end-bearing pile. The recommendations can provide a basis for an optimum design that would be applicable to piled rafts taking into account the load sharing characteristics.

3D numerical analysis of piled raft foundation in stone column improved soft soil

The concept of composite piled raft foundation incorporates piles of different lengths and stiffness, where short columns of flexible materials improve the shallow soft soil and longer columns of relatively rigid material support loads and reduce settlement. Present study utilised this concept by employing stone columns beneath the piled raft foundation and investigated its response through three-dimensional finite element analyses. Effects of area replacement ratio, slenderness ratio and material properties of stone column, slenderness ratio of pile and thickness of raft have been examined on the proportion of load shared by piles and stone columns and settlement response of piled raft under different levels of vertical load. Results showed that strengthening the shallow soft soil with stone columns was effective in improving the bearing interaction of raft, i.e. increasing the proportion of load shared by stone columns, and reducing the total settlement of the piled raft. Percentage load shared by piles was significantly influenced by the slenderness ratio of piles and level of vertical load on raft, but was almost independent of properties of stone columns and thickness of raft. Increment in diameter of stone columns (area replacement ratio) significantly improved the bearing interaction of raft, and increment in their lengths (slenderness ratio of stone columns) reduced the settlements of the foundation system. Increase in angle of friction of constituent stone column material improved both the bearing interaction of raft and settlement performance of the foundation system. Increment in raft thickness not only enhanced the proportion of load shared by stone columns, but also drastically reduced the total and differential settlements of the foundation.

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.

Numerical analysis of settlement and bearing behaviour of piled raft in Babol clay

Piled raft foundations are the composite construction of a spread foundation and a few number of piles that are usually used in soft soils to compensate the weakness of a raft in satisfying the design requirements. Due to the complex behaviour of piled raft, it is necessary to take into account a number of factors such as pile geometry and arrangement, raft thicknesses, soil properties and loading condition in the design. In this study, a parametric study has been conducted by a three-dimensional finite-element method considering the full interaction between the components of piled raft foundation. The underlying soil consists of Babol clay in drained condition with various stiffness and plasticity, determined from the results of a geotechnical investigation. The results of numerical analyses show that the bearing capacity of piled raft obviously increases with increasing pile length, pile spacing and raft thickness, especially in stiff clay. The effect of load type is more significant for the differential settlement and pile loads than other parameters. Finally, the advantages of using an optimum design method are examined in a special case of analyses and a case history.

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.

3D Numerical Model for Piled Raft Foundation

AbstractLoad sharing of piled raft foundations is known as an economical design for deep foundations. Nevertheless, research in this area has been lagging because of the complexity of the problem and lack of field data. Numerical modeling can be used to provide valuable data with a high level of success. A three-dimensional finite-element model of a piled raft foundation was developed to simulate the case of a piled raft foundation. The model accounts for pile-to-pile, raft-to-pile, pile-to-soil, and raft-to-soil interactions. The model was used to examine the effect of the key parameters governing the performance of this foundation during loading and, accordingly, the load shared by the piles and the raft. After validating the numerical model with available data in the literature, the model was used to develop data for a wide range of parameters and to examine the role of the foundation geometry, including pile spacing in the group, pile length, pile shape, pile diameter, and raft thickness. Furthermore,...

Analysis of piled raft coefficient and load-settlement on sandy soil

A piled raft foundation is a combined foundation, which is developed to utilize the load-carrying capabilities of both raft and piles. To obtain an optimum piled raft design, it is important to properly evaluate and consider the load-sharing behavior between the raft and piles, which changes according to the settlement level of the piled raft. In this study, 27 three-dimensional finite element models were analyzed to investigate the piled raft coefficient with linear and nonlinear load-settlement behaviors. The length of piles was varied between 10, 15, and 20 m. The spacing between pile centers was varied between 3D, 5D, and 7D, and the pile diameter was kept constant. The number of piles and the distance between the exterior piles and the edge of the raft were maintained at 9 and 1 m, respectively. The sand conditions varied between dense, medium, and loose. The results indicated that the piled raft coefficient increases when the load-settlement curve is linear and decreases when the load-settlement curve is nonlinear. The influence of the incremental increase in pile length on the piled raft coefficient is more pronounced in short piles than in longer piles. The raft thickness has a negligible effect on the piled raft coefficient.

Numerical investigation of the performance of micropiled rafts in sand

The micropiled raft (MPR) offers an efficient foundation system that combines the advantages of micropiles and piled rafts that can be used as primary foundation system or to enhance an existing raft foundation. In this paper, a calibrated and verified finite element model (FEM) with centrifuge tests was used to carry out a numerical investigation on the performance of MPR in sand. A total of 78 different cases were analyzed in this study to assess the behavior of MPR in sand taking into account a number of factors that may influence its behavior such as: the number of micropiles (MPs), the spacing to micropile diameter (S/Dmp), the raft thickness, type of loading and soil density. The outcomes of this investigation should help in understanding the effect of these factors on the MPR axial stiffness, including; differential settlement; load sharing between the MPs and the raft; and the raft bending moment. Moreover, the ability of the PDR method to evaluate the axial stiffness of a MPR for the preliminary design stage is examined. It was found that the MPR system has the ability to increase the tolerable bearing pressure by 190% compared to an isolated raft system. In addition, an adjustment factor (ωPR) for PDR method was introduced to account for the raft flexibility.

Soil -structure interaction analysis of a building frame supported on piled raft

The study deals with physical modeling of a typical building frame resting on pile raft foundation and embedded in cohesive soil mass using finite element based software ETABS. Both- the elements of superstructure and substructure (i.e., foundation) including soil is assumed to remain in elastic state at all the time. The raft is modelled as a thin plate and the pile and soils are treated as interactive springs. Both- the resistance of the piles as well as that of raft base - are incorporated into the model. Interactions between raft-soil-pile are computed. The proposed method makes it possible to solve the problems of uniformly and large non-uniformly arranged piled rafts in a time saving way using finite element based software ETABS. The effect of the various parameters of the pile raft foundation such as thickness of raft and pile diameter is evaluated on the response of superstructure. The response included the displacement at the top of the frame and bending moment in columns. The soil-structure interaction effect is found to increase displacement and increase the absolute maximum positive and negative moments. The effect of the soil- structure interaction is observed to be significant for the type of foundation and soil considered in the present study.

Wind load analysis of tall chimneys with piled raft foundation considering the flexibility of soil

Soil–structure interaction (SSI) analysis was carried out for tall reinforced concrete chimneys with piled raft foundation subjected to wind loads. To understand the significance of SSI, four types of soil were considered based on different material properties. Chimneys of different elevations and different ratios of height to base diameter of chimney were selected for the parametric study. The thickness of raft of piled raft foundation was also varied based on different ratios of outer diameter to thickness of raft. The chimneys were assumed to be located in open terrain and subjected to a maximum wind speed of 50 m/s. The along-wind and across-wind loads were computed according to IS: 4998 (Part 1)-1992 and applied along the height of the chimney. The analysis was carried out using three-dimensional finite element technique based on the direct method of SSI. The linear elastic material behaviour was assumed for the integrated chimney–foundation–soil system. The radial and tangential moments, lateral deflection and base moment of chimney were evaluated through SSI analysis and compared with the response obtained from chimney with fixed base. The base moment of chimney considerably reduces due to the effect of SSI. It is found that the variation of different responses in chimney due to the effect of SSI depends significantly on the geometrical properties of chimney and foundations. The response variation at base for a distance of 1/40th of the height of chimney should be considered for a safe design.

Response in piled raft foundation of tall chimneys under along-wind load incorporating flexibility of soil

The present paper deals with the numerical analysis of tall reinforced concrete chimneys with piled raft foundation subjected to along-wind loads considering the flexibility of soil. The analysis was carried out using finite element method on the basis of direct method of soil-structure interaction (SSI). The linear elastic material behavior was assumed for chimney, piled raft and soil. Four different material properties of soil stratum were selected in order to study the effect of SSI. The chimney elevation and the thickness of raft of piled raft foundation were also varied for the parametric study. The chimneys were assumed to be located in terrain category 2 and subjected to a maximum wind speed of 50 m/s as per IS:875 (Part 3)-1987. The along-wind loads were computed according to IS:4998 (Part 1)-1992. The base moments of chimney evaluated from the SSI analysis were compared with those obtained as per IS:4998 (Part 1)-1992. The tangential and radial bending moments of raft of piled raft foundation were evaluated through SSI analysis and compared with those obtained from conventional analysis as per IS:11089-1984, assuming rigidity at the base of the raft foundation. The settlements of raft of piled raft foundation, deflection of pile and moments of the pile due to interaction with different soil stratum were also evaluated. From the analysis, considerable reduction in the base moment of chimney due to the effect of SSI is observed. Higher radial moments and lower tangential moments were obtained for lower elevation chimneys with piled raft resting on loose sand when compared with conventional analysis results. The effect of SSI in the response of the pile is more significant when the structure-foundation system interacts with loose sand.

Stepping between Membrane Microdomains

Lateral membrane microdomains are postulated to be involved in a number of important functions in mammalian cells, such as membrane trafficking, exocytosis, endocytosis, signal transduction, and protein activity (1,2). These putative cell microdomains have been the subject of extensive research using a variety of biophysical and biochemical techniques, including detergent extraction (2), fluorescence microscopy (3), electron microscopy (4), fluorescence resonance energy transfer microscopy (5), fluorescence correlation spectroscopy (6), fluorescence recovery after photobleaching (7), and single-molecule tracking (8).