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Design codes vary in their recommendations for the end-bearing response for bored piles founded in sand, both for the ultimate design value and the response at small settlements. The ultimate end-bearing resistance may be expressed either in terms of bearing factors Nq relative to the in situ vertical effective stress, with the Nq value varying with the friction angle of the sand, or as a factor applied to in situ test data, such as the standard penetration test blow count or the tip resistance qc. Numerical studies have led to proposed ratios of design end-bearing pressure to qc at specific settlement ratios, such as 5% and 10% of the pile diameter. The work presented here used numerical analysis to evaluate the full pile base response from initial stiffness to ultimate end-bearing resistance at a settlement ratio of 10% of the pile diameter. The resulting base responses are suitable for implementation in beam column analyses and have been validated by comparison with published design guidelines and with data from a full-scale instrumented pile load test. [ABSTRACT FROM AUTHOR]

Recent research on the topic of pile base resistance has demonstrated that the guidelines provided by existing design manuals and codes are rather conservative in the case of clayey soils under undrained conditions. With the aim of investigating this topic more rigorously, a computationally intensive study was carried out to quantify the effect of the soil rigidity index on the response of pile base resistance. A precise assessment of the results led to the formulation of hyperbolic relationships that capture the development of the base resistance as a function of settlement. The relationships were implemented in a beam column analysis and validated against a full axisymmetric numerical analysis and also with respect to field data from a pile test performed using an Osterberg cell near the pile base. The validation process demonstrated the usefulness of the method and recommendations arising with respect to mobilization of the pile base resistance. This study should, therefore, assist in the development of more scientifically based guidelines for pile design that balance safety and economy. [ABSTRACT FROM AUTHOR]

Overlapping jet-grout columns are often used to form a grout slab at the bottom of deep excavations to support the retaining structures. Columnar geometrical imperfections, spatially varying strength and stiffness and the post-peak behaviour of the grout, render the simulation of the jet-grout column slab a very complex issue. In practice, these aspects are not taken into account and simplified models are commonly used, in conjunction with the application of a strength reduction factor. With the aim to improve the existing design process and extend its applicability to serviceability limit state analysis, a consistent design procedure is presented. [ABSTRACT FROM AUTHOR]

Although simplified design methods for piled raft foundations have been proposed to allow for the group effect and soil-pile-raft interaction, most of them are concentrated on one type of loading, rendering the applicability of these methods limited to cases under such loads. In the case of a combined pile raft foundation (CPRF), the structural loads are carried partly by the piles and partly by the raft as a function of the foundation settlement, rendering the CPRF a complex soil-structure interaction issue. Despite the recent development of computational resources and advances in numerical expertise, a detailed three-dimensional (3-D) numerical analysis, accounting for soil nonlinearities, nonlinear behavior of the interfaces between the soil, piles, and raft under various combinations of loadings remains impractical. The objective of this paper is to provide a rather simplified and straightforward design methodology for pile foundations under combined loadings. To achieve this goal, previous research works on the group effect under axial and lateral loading have been evaluated and the piles-raft interaction effect has been considered. The proposed procedure is fully compatible with structural software codes and can be straightforwardly applied to the design of the structural members, as it is able to effectively solve a CPRF under the numerous combinations of loadings required by most design codes. [ABSTRACT FROM AUTHOR]

A three-dimensional (3D) numerical model simulating a twin tunnel excavated by a tunnel-boring machine (TBM)-earth-pressure balance (EPB) is presented. All relevant components are taken into account, and the influence of the parameters adopted in the TBM-EPB method on the induced ground settlements is investigated. The sensitivity to variations of the face support, the pressure applied to the steering gap slurry, and the tail gap grouting were also evaluated within the framework of parametric analyses. The effects from and on a building located at or beyond the greenfield trough width were also investigated. The greenfield volume loss at the ground surface was found to be 0.85%, increasing to 0.97% at the building foundation level. These values were further increased by 6% at the cross section where the building is located. Furthermore, a full soil-structure interaction using 3D numerical analysis was carried out to evaluate the ability of numerical methods to provide accurate displacement prognosis for buildings adjacent to tunnels excavated by the TBM-EPB method. Conclusions regarding the effects of relevant parameters, which have been drawn based on the application of the method to the ongoing Thessaloniki subway, could be useful for the future design and construction of shallow tunnels by the TBM-EPB method. [ABSTRACT FROM AUTHOR]

Abstract: A new approach for simulating the excavation and construction of subsequent panels is proposed to investigate the effects from the installation of diaphragm walls on the surrounding and adjacent buildings. The method has been combined with a 3-D nonlinear analysis and a constitutive law providing bulk and shear modulus variation, depending on the stress path (loading, unloading, reloading). From the application of the method in a normally to slightly over-consolidated clayey soil it was found that the panel length is the most affecting factor of ground movements and lateral stress reduction during panel installation. Moreover, from the evaluation of horizontal stress reduction and the variation of horizontal displacements arises that the effects from the construction of a panel are mainly limited to a zone within a distance of the order of the panel length. The effects on an adjacent building have also been investigated by applying a full soil–structure interaction including the whole building. Settlement profiles and settlements are given at specific points as increasing with subsequent installation of panels, providing the ability of specific monitoring guidelines for the upcoming construction of the diaphragm wall in front of the building. Contrary to lateral movements, which mostly take place at the panel under construction, it was found that the effect of settlements covers a larger area leading to a progressive settlement increase. The effect highly depends on the distance from the panel under construction. [Copyright &y& Elsevier]

Abstract: Although simplified numerical methods are reliable for evaluating the response of a single pile under horizontal load, their application is questionable for assessing the response of pile groups. The notion of “p–y” curves has been considered with the aim of establishing a transformation relationship able to provide the “p G–y G” curves of soil resistance around a pile in a group from the well-known curves of soil resistance around the single pile. This transformation extends the applicability of the “p–y” method to pile groups, without the need for time consuming numerical computations, rendering the proposed method efficient and attractive. Comparative examples demonstrated the applicability and the effectiveness of the proposed method. In addition, the method can be straightforwardly extended to account for varying soil resistance, according to the particular location of a pile in a group. It can therefore be used to estimate accurately force and bending moment distributions along the characteristic piles of a group, which are required for the efficient design of foundations. [Copyright &y& Elsevier]

Abstract: The aim of this paper is to investigate the behavior of laterally loaded pile groups in sands with a rigid head and correlate the response of a pile group it to that of a single pile. For this purpose, a computationally intensive study using 3-D nonlinear numerical analysis was carried out for different pile group arrangements in sandy soils. The responses of the pile groups were compared to that of the single pile and the variation of the displacement amplification factor R a was then quantified. The influence of the number of piles, the spacing, and the deflection level on the group response is discussed. A relationship for predicting the response of a pile group, based on its configuration and the response of a single pile, has been formulated allowing also for soil shear strength which was found to affect the group response. The relationship provides a reasonable prediction for various group configurations in sandy soils. [ABSTRACT FROM AUTHOR]

Capacity-based design of structures limits the soil-structure interaction mechanism to the determination of the bearing capacity of a pile group. However, in many cases the criterion for the design of piles to resist lateral loads is not the ultimate lateral capacity but the deflection of the piles. Many procedures exist for estimating the response of single piles and pile groups under lateral loading, ranging from application of empirical relationships and simple closed-form solutions to sophisticated nonlinear numerical procedures. With the aim of investigating the effect of cracking, disregarded by most of the above-mentioned methods, a three-dimensional (3D) nonlinear analysis that accounts for cracking is presented. Response prediction correlates well with the experimental data from a full-scale pile load test. Interesting conclusions have also been drawn regarding the discretization of the computational domain and the combination of 3D numerical nonlinear analysis and the structural beam theory. [ABSTRACT FROM AUTHOR]

Abstract: Capacity based design of pile foundations limits the soil-structure interaction mechanism to group bearing capacity estimation, neglecting, in most cases, the contribution of the raft. On the other hand, a straightforward, nonlinear, 3-D analysis, accounting for soil and structural nonlinearities and the effects arising from pile–soil–pile interaction, would be extremely high CPU-time demanding and will necessitate the use of exceptionally powerful numerical tools. With the aim of investigating the most efficient, precise, and economical design for a bridge foundation, a hybrid method, compatible with the notion of sub-structuring is proposed. It is based on both experimental data and nonlinear 3-D analysis. The first step to achieve these targets is a back-analysis of a static pile load test, fitting values for soil shear strength, deformation modulus, and shear strength mobilization at the soil–pile interface. Subsequently, the response of 2×2 and 3×3 pile group configurations is numerically established and the distribution of the applied load to the raft and the characteristic piles is discussed. Finally, a design strategy for an optimized design of pile raft foundations subjected to non-uniform vertical loading is proposed. [Copyright &y& Elsevier]

• The ULS analysis provides extremely high displacements to DW and adjacent buildings. • The SLS analysis accounting for concrete cracking provides the most realistic results. • Lower DW bending moments discrepancy was observed between ULS and SLS results. • Concrete cracking was found the most SSI affecting factor. • Satisfactory approach is provided by a constant DW flexural rigidity equal to 70% E c I c. Recent research works into the design and response evaluation of deep diaphragm walls and induced settlements to adjacent buildings demonstrated that the nonlinear behavior of surrounding soil, cracking effects affecting the structural members' flexural rigidity and spatial variability of soil parameters are a prerequisite for a rigorous analysis of serviceability performance. However, as a result of the complexity of the mechanisms developed in the case of deep multi-propped retaining structures, most of the proposed Soil Structure Interaction (SSI) design methodologies are biased towards geotechnical or structural behavior by adequately capturing the nonlinearities of the one field and considering a rather simplified behavior of the other. With the aim to bridge the gap between the above design methodologies, a fully coupled analysis of a multi-propped diaphragm wall of an underground station with a supporting grout slab at the excavation bottom has been carried out. Further to the nonlinear behavior of the soil, the spatial variability of the grout treated zone strength and stiffness together with the post-peak behavior have been considered. In addition, the concrete cracking effects, associated with substantial decrease of structural members' stiffness, have been taken into account. Comparative simplified empirical approaches disregarding cracking effects by adopting constant diaphragm wall flexural rigidity instead, have been carried out as well. Interesting conclusions were drawn relating the results of the above simplified approaches and the Serviceability Limit State (SLS) analyses with regard to the response of the diaphragm wall, the bending moment distribution along the wall and the displacement field in the surrounding soil and adjacent buildings. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this paper is to evaluate the influence of negative skin friction in pile foundations. Three dimensional nonlinear analyses for a single pile and pile groups were carried out for a specific case and some case studies as well. Contrary to simplified conventional analysis in which the predictions are usually overestimated and could be considered as an upper limit, it was found that the dragload of a pile in a group depends on the surface load, the pile configuration, the pile position in a group, the ultimate skin friction and the interface stiffness. It has been demonstrated that for fixed-head friction pile groups the dragload group effect is significantly greater than in the case of free-head end-bearing pile groups. Moreover, predictions for internal piles have shown considerably smaller dragloads for fixed-head piles, which is in accordance with experimental findings. It has also been demonstrated that when the construction of an embankment precedes the application of the foundation working load, the effect of negative skin friction is considerably smaller than in the reverse case. [Copyright &y& Elsevier]

Axial pile load tests are considered within the design procedure of most major construction projects that include pile foundations, aiming to determine both the ultimate bearing capacity and the pile stiffness at working load level. Commonly used pile spacings of 3–4 pile diameters between the test pile and the reaction piles cause significant interaction with drastic effect on to the load-settlement relationship. The objective of this paper is to evaluate the influence of this interaction on both bearing capacity and stiffness of single piles and pile groups. For this purpose, a back analysis of a pile load test was initially performed which facilitated the determination of the single pile response and the verification of the soil properties. Subsequently, a numerical analysis was carried out to establish load-displacement relationships for several different layouts of pile groups. Based on these non-linear analyses the effect of the interaction was quantified for both the pile load test and pile group layouts examined. [Copyright &y& Elsevier]

The settlement behaviour of vertically loaded pile groups has been the subject of an extensive body of research over the past two decades. In particular, this work has identified the over-conservatism associated with predictions of pile interaction derived from elastic theory and the corresponding amplification of group settlement relative to single pile values. Researchers have since redoubled efforts to refine settlement predictions for pile groups towards more economical design, largely through more rigorous treatment of soil stiffness non-linearity. Although foundation design engineers are increasingly employing three-dimensional continuum analyses to quantify pile interaction on a site-specific basis, simplified design approaches remain an integral part of preliminary foundation design. The purpose of this paper is to undertake a critical examination of these methods with a view to increasing their potential for take-up by foundation engineering practitioners. A database of simplified models has been collated for the prediction of non-linear pile interaction that exists within vertically loaded pile groups. These models are categorised as either analytical or empirical. The development, limitations and range of applicability of these models are explored in detail in the context of some published case histories. [ABSTRACT FROM AUTHOR]

The author comments on the article "A simplified nonlinear approach for single pile settlement analysis," from the volume 49 number 11, 2012 issue, noting an error in the equation of the load-transfer relationship of a pile base.