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The work at hand proposes a method for assessing, under reasonable hypotheses from an engineering perspective, the failure envelope of a pile group subjected to generalized loading conditions involving a vertical and a lateral force along with a moment. Following different assumptions of increasing complexity, a simple closed-form expression, which is however capable of considering also the strong dependence of sectional yielding moment on the axial force, is derived. The use of such formula, which allows a practical hand calculation of the interaction diagrams at failure, returns conservative yet very accurate results. As a follow up, with reference to reinforced concrete piles, design considerations involving both structural and geotechnical failure under lateral load are reported. It is found that for most cases, if steel reinforcement is established to resist the design bending moment, the geotechnical Ultimate Limit State checks are automatically satisfied.

This work is a numerical investigation of the effect of thermally induced volumetric collapse of normally consolidated clays on the performance of energy piles. A series of coupled thermo‐hydro‐mechanical Finite Element simulations were carried out using the commercial software ABAQUS. These examined a single free‐head energy pile embedded in a normally consolidated clay layer subjected to a constant mechanical load and to a number of heating/cooling cycles, to reproduce operating conditions. The soil behaviour was described with two advanced hypoplastic constitutive models for clays, one of which incorporates the thermally induced volumetric collapse using an ad‐hoc algorithm developed by the authors. Both models predict a cyclic accumulation of settlement and excess pore water pressure, especially when the thermal collapse effect is considered. While the excess pore pressure distribution stabilises within a few cycles, the rate of settlement of the pile head does not show any tendency to decrease from one cycle to another. These results are in agreement with data from small scale tests on an isolated energy pile in normally consolidated clay, indicating that the numerical model developed in this study can be used to investigate the complex soil/pile/raft interaction processes occurring in real piled foundations incorporating energy piles.

This research provides an insight into the previously unexplored aspects of kinematic pile bending, especially for large-intensity earthquakes where the soil behaviour is highly non-linear. In this study, a series of dynamic centrifuge experiments was conducted on pile foundations embedded in a two-layered soil profile to investigate the kinematic effects on pile foundations during model earthquakes. A single pile and a closely spaced 3 × 1 row pile group were used as model pile foundations, and the soil model consisted of a soft clay underlain by dense sand. It was observed that the peak kinematic pile bending moment occurs slightly beneath the interface of the soil layers and this depth is larger for the pile group compared to a single pile. Also, the piles in a group attract lower bending moments but carry larger residual kinematic pile bending moments compared to a single pile. Furthermore, the elastic solutions available in the literature for estimating the kinematic pile bending moments are shown to yield satisfactory results only for small-intensity earthquakes, but vastly underestimate for large-intensity earthquakes, if methods are applied injudiciously. The importance of considering soil non-linearity effects and accurate determination of shear strain at the interface of layered soils during large-intensity earthquakes for a reliable assessment of kinematic pile bending moment from methods in the literature is demonstrated using dynamic centrifuge test data.

The paper investigates the problem of Soil-Foundation-Structure Interaction (SFSI) for buildings supported on piles through the comparative analysis between the fixed base and the compliant base assumptions. The structure, a nine-storey residential building (with or without infills), is modelled in non-linear regime while the piled foundation is idealized by means of independent lumped parameters models, either linear or non-linear. In this last case, the soil-foundation system is replaced by an assembly of viscous-dampers, fictitious masses and non-linear springs modelled according to the classical Bouc-Wen formulation, so as to account for the hysteretic behaviour of the foundation. A detailed calibration procedure for both linear and non-linear foundation models is also presented and discussed. Two different natural soil deposits are considered, a pyroclastic deposit and a deep layer of lacustrine clay. The results undertaken in the context of a probabilistic analysis show that SFSI may lead to a significant reduction of the seismic demand in infilled buildings at low and intermediate earthquake intensity levels. Conversely, at higher intensity earthquakes the seismic demand is not affected by the non-linear springs. It is shown that a proper modelling of radiation mechanism at foundation level is crucial for a reliable and sustainable prediction of SFSI effects.

Annular shaped pile groups are a very common foundation layout for onshore wind turbines and other slender structures. In this study, their performance under vertical loads of moderate to high eccentricity, including moment rotation response and bearing capacity, was investigated by centrifuge testing on small scale physical models embedded in kaolin clay. To identify experimentally the capacity of the examined pile groups under different load paths, the model foundations were loaded monotonically until a clear collapse mechanism was achieved. The testing procedure and the proposed interpretation methodology can be easily adapted to load paths or pile layouts other than those considered in the current study. The experimental data can be adopted as a useful benchmark for mathematical models aimed at predicting the response of pile groups to complex load paths. The results of this testing program can also be used to assess the degree of conservatism of current methods adopted by industry for the design of piled foundations subjected to eccentric loads.

The problem of kinematic bending moments imposed at the head of a single pile during the passage of seismic waves is explored under large shear strains in the surrounding soil. To this end, non-linear soil response at free-field conditions is derived numerically by a freely-available 1D code and then utilized to calibrate the constitutive law of soil introduced in a rigorous 3D Finite-Difference (FD) model of the soil-pile system employed to obtain pile's head bending moments. The pile is considered embedded to a normally-consolidated clay and seven earthquake records with different amplitude and frequency content are imposed as input motions at the base of the soil layer, thus allowing the investigation of pile kinematic bending with increasing levels of shear strains in the soil, exceeding the limit of equivalent-linear soil behavior. The performance of a simple analytical expression for predicting the kinematic bending moment at the pile-head is compared to the rigorous FD solution. It is concluded that this simple solution is still applicable, with slight modifications, for high shear strains related to non-linear soil behavior close to shear failure, provided that the proper mobilized soil properties from 1D soil response analysis are introduced.

The problem of failure envelopes of pile groups subjected to vertical and eccentric load is investigated both theoretically and experimentally. A critical review of literature works on failure envelopes for pile groups under combined axial-moment loading is first provided. Emphasis is placed on a recent, exact solution derived from theorems of limit analysis by idealizing piles as uniaxial rigid-perfectly plastic elements. The application of the relevant equations over a practical range of problems needs only the axial capacities in compression and uplift of the isolated piles. An intense program of centrifuge experiments carried out along with different load paths on annular shaped pile groups aimed at validating the equations pertinent to the above solution is presented and discussed. The endpoints of the load paths followed in the centrifuge lie approximately above the analytical failure envelope, giving confidence that the reference equations can be reliably adopted to assess the capacity of a pile group under combined axial-moment loading. Finally, the kinematics of the collapse mechanism observed experimentally is compared to that determined from the application of the reference theory.

A novel analytical method is derived for the ultimate capacity interaction diagram (i.e., axial compression, N - bending moment resistance, M) of reinforced concrete (RC) columns with circular cross section. To this aim, the longitudinal rebar arrangement is replaced with a thin steel ring equivalent to the total steel area; moreover, according to modern design approaches, simplified stress–strain relationships for concrete and reinforcing steel are used. Illustrative applications demonstrate that the ultimate capacity computed by the proposed analytical approach agrees well with the results obtained by rigorous methods based on consolidated numerical algorithms. The new solution allows for a rapid, accurate assessment of circular cross section capacity by means of hand calculations; this is especially useful at the conceptual design stage of various structural and geotechnical systems. The method can be easily extended to more general configurations, such as multiple steel rings and composite concrete-steel sections.

The paper deals with the design and analysis of slope-stabilising piles and adopts limit equilibrium concepts to derive pile contribution to stability. Analytical expressions for pile ultimate load are derived in terms of the force and the moment to be used in routine slope-stability analyses to take into account pile contribution. Free and head-restrained isolated piles, with infinite and finite section capacity, in drained and undrained conditions are considered.

Energy piles are foundation elements having the double scope of transferring structural loads from the structure to the ground and of exchanging heat with the surrounding soil. It follows that pile state of stress and settlement are altered by the time-dependent temperature change in both pile and soil. This work is aimed at investigating the effect of thermal cycles on the behaviour of a single energy pile. To this end, fully coupled thermo-hydro-mechanical analyses have been carried out using the Finite Element code ABAQUS. The single pile is installed in a normally consolidated clay behaving according to different constitutive models involving Mohr-Coulomb, Modified Cam Clay and Hypoplastic. The latter is employed with and without the thermal formulation capable of accounting for the thermal collapse of NC clays during heating. A single free-head pile is considered and the results are presented in terms of pile axial force and settlement developed cycle by cycle.

This work investigates the role of foundation type and layout on the seismic response of a bridge pier. Reference is made to an Italian case study of a bridge pier to be founded on a well-characterized subsoil. Both a caisson and a 3×3 pile foundation are considered as suitable design options. For each foundation type, three different geometrical layouts satisfying Ultimate Limit State (ULS) checks are analysed. Equivalent-linear ground response analyses are preliminary performed to derive the mobilized soil stiffness and damping ratio. FE analyses of the complete soil-foundation-bridge pier model are then carried out. Results indicate that consideration of Soil-Structure Interaction effects strongly reduces the pier acceleration, especially for pile foundations, which allow for a higher dissipation of energy due to radiation damping. Further, the role of foundation type and layout is discussed by separating the kinematic and inertial components of interaction, with reference to both frequency and time domain response. Some considerations about possible simplifying assumptions to account for these effects in routine engineering are finally reported.

Piles have been traditionally designed to ensure the transfer of the whole structural load to the soil in which they are embedded. It is now largely accepted that this way of doing is unduly conservative, especially in those cases in which an unpiled raft is sufficient to guarantee a satisfactory safety level. Nevertheless, it is still very common in the daily practice, sometimes for the restrictions imposed by local codes and regulations. Much research efforts have been dedicated to a more rational design of piled foundations, essentially based on a proper collaboration among raft and piles not only for increasing the overall stiffness of an unpiled raft but, if required, also for increasing its capacity, for controlling differential settlements, for reducing the state of stress into the raft. With no claim to cover any single aspects, some design options will be presented, providing some insight into a rational approach to piled raft design.

Kinematic pile-soil interaction is investigated analytically through a Beam-on-Dynamic-Winkler-Foundation model. A cylindrical vertical pile in a homogeneous stratum, excited by vertically-propagating harmonic shear waves, is examined in the realm of linear viscoelastic material behaviour. New closed-form solutions for bending, displacements and rotations atop the pile, are derived for different boundary conditions at the head (free, fixed) and tip (free, hinged, fixed). Contrary to classical elastodynamic theory where pile response is governed by six dimensionless ratios, in the realm of the proposed Winkler analysis three dimensionless parameters suffice for describing pile-soil interaction: (1) a mechanical slenderness accounting for geometry and pile-soil stiffness contrast, (2) a dimensionless frequency (which is different from the classical elastodynamic parameter a0=ω d/Vs), and (3) soil material damping. With reference to kinematic pile bending, insight into the physics of the problem is gained through a rigorous superposition scheme involving an infinitely-long pile excited kinematically, and a pile of finite length excited by a concentrated force and a moment at the tip. It is shown that for long piles kinematic response is governed by a single dimensionless frequency parameter, leading to a unique master curve pertaining to all pile lengths and pile-soil stiffness ratios. © 2012 Elsevier Ltd. ispartof: SOIL DYNAMICS AND EARTHQUAKE ENGINEERING vol:44 pages:183-195 status: published

Piles have been used to increase the stability of landslides in static regime for many decades, as they provide resistance against a laterally moving soil mass by transferring the loads into more stable underlying ground. Nevertheless, the presence of such a reinforcement has beneficial effects even under seismic shaking. The paper aims at proposing a simple procedure for the estimation of permanent displacements of slopes stabilized with piles. The method is made up of 3 different stages: (a) evaluation of stability conditions of the natural slope and the ultimate load offered by the piles; (b) combination of the two ingredients above, through limit equilibrium considerations, to assess the critical acceleration of the reinforced slope; (c) use of the classical Newmark method to calculate the permanent displacement under specified input motions. A parametric study is also performed to quantify the beneficial effect of piles under earthquake shaking.

Pile foundations in seismic areas should be designed against two simultaneous actions arising from kinematic and inertial soil-structure interaction, which develop as a result of soil deformations in the vicinity of the pile and inertial loads imposed at the pile head. Due to the distinct nature of these phenomena, variable resistance patterns develop along the pile, which are affected in a different manner and extent by structural, seismological and geotechnical characteristics. A theoretical study is presented in this article, which aims at exploring the importance of pile diameter in resisting these actions. It is demonstrated that (a) for large diameter piles in soft soils, kinematic interaction dominates over inertial interaction; (b) a minimum and a maximum admissible diameter can be defined, beyond which a pile under a restraining cap will inevitably yield at the head i.e., even when highest material quality and/or amount of reinforcement are employed; (c) an optimal diameter can be defined that maximizes safety against bending failure. The role of diameter in seismically-induced bending is investigated for both steel and concrete piles in homogenous soils as well as soils with stiffness increasing proportionally with depth. A number of closed-form solutions are presented, by means of which a number of design issues are discussed.