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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.

This paper presents experimental data from two series of centrifuge tests on pile foundations embedded in a kaolin clay under earthquake excitation. A single pile and two pile-group configurations were tested in the centrifuge at 50g. In the first test series (flight 1) the seismic response of the soil-pile system was monitored, allowing experimentally-driven insights on the kinematic interaction between soil and pile. In the second series of tests (flight 2), a superstructure mass was attached to each foundation model, allowing investigation of the seismic behaviour of the coupled soil-foundation-structure (SFS) system. Two earthquake excitations were analysed during each flight having similar frequency and amplitude characteristics. Results are presented in terms of soil response along a vertical array, pile-caps spectral acceleration and superstructure top-to-ground surface response ratio. Experimental data suggests that for soft soils such as those employed in this experimental campaign, piles may amplify the motion with respect to the one at ground surface, while the natural period of the SFS system may be largely increased compared to its fixed-base counterpart, indicating a strong effect of soil compliance.

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.

The paper examines the problem of the soil-structure interaction for buildings founded on piles throughout the comparative analysis between the seismic demand in compliant base and fixed base models. The aim of the work is to introduce a simple approach for evaluating the effects of soil-foundation-structure interaction (SFSI) for buildings founded on piles and, hence, to quantify the relevance of SFSI effects for this kind of structure. Inertial interaction analyses are carried out by idealizing the complete system as a linear SDOF on a deformable base represented by frequency dependent springs and dashpots. An application of the proposed methodology to a case study of a nine-story building resting on two well-studied subsoils, a deep clay layer from Piana del Fucino (Italy) and a pyroclastic deposit from Napoli, is presented and discussed. Reference is made to a complete seismic risk analysis in which the input signals are grouped into strips according to the conditional spectrum method. As a further objective, the effect of wall infills on the seismic demand in the reference buildings is investigated. The results show that for the pyroclastic deposit the seismic demand in the compliant model is comparable with that in the fixed base. By contrast, SFSI leads to a relevant reduction of the seismic demand for buildings on the clay layer. It is concluded that, contrary to the common belief, SFSI may also be relevant for tall buildings if they rest on soft soils.

We examine the kinematic response of fixed-head vertical floating piles embedded in continuously nonhomogeneous soils and subjected to upward propagating seismic waves. The problem is explored numerically by means of a rigorous finite element (FE) model of the soil-pile system to quantify the kinematically induced reduction of the horizontal free-field spectral acceleration. Soil stiffness varies continuously with depth according to a generalized power law function. We show that kinematic pile response in the harmonic regime is controlled by a unique dimensionless frequency parameter involving the active pile length in a generalized nonhomogeneous soil. A new, simplified expression for the horizontal kinematic interaction factor Iuis proposed for practical time-domain applications while a novel physical interpretation of the filtering action of piles is reported by introducing the role of pile stiffness in averaging soil motion over an effective pile length. Following a parametric study under transient motion, we propose a set of novel, ready-to-use formulae for a rapid assessment of the pile-induced filtering action. An application of the proposed formulae to clayey soils is finally presented, leading to useful indications for the selection of the pile diameter associated with the maximum filtering potential.

The potential of piles to reduce the seismic motion transmitted to the superstructure with respect to the free field motion, due to the kinematic interplay between soil and pile, is explored by considering a single pile embedded in the soft marine clay of Maliakos Gulf subsoil in central Greece. The soil profile consists of a very soft normally consolidated clay layer with nearly-zero stiffness at ground surface, increasing proportionally with depth, underlain by a stiffer layer of sandy clay. The problem is tackled numerically by means of Equivalent linear (EL) and strength-controlled non-linear (NL) constitutive laws to model the behavior of the soil under seismic excitation. The latter is chosen on a magnitude-epicentral distance (M-R) basis from the European Strong-motion Database (ESD), taking into account the seismotectonic regime of the broader area under study. Seven (7) motions recorded at soil type A, according to EC8, were chosen to define an average code-compatible spectrum. The average mobilized stiffness of the free field soil obtained from EL and NL analyses is then introduced into a finite-element model of the soil-pile system to investigate the kinematic-induced filtering action of the pile as affected by pile cap embedment depth and pile diameter. The above is quantified in terms of ratios of spectral accelerations at the pile head and free field at ground surface. Results highlight the importance of considering NL response for such type of soils and conclude on the beneficial role of piles as reflected in a large reduction of the free field spectral acceleration, even for piles with relatively small diameter.

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

The behaviour of a kinematically stressed pile in layered soil under the passage of vertically propagating. seismic S waves is investigated by means of rigorous three-dimensional Finite-Element. (FE) analyses. Both pile and soil are idealized as linearly viscoelastic materials, modelled by solid. elements and pertinent interpolation functions in the realm of classical elastodynamic theory. The. system is analyzed by a time-Fourier approach in conjunction with a modal expansion in space.. Constant viscous damping is considered for each natural mode, and a FFT algorithm is employed to. switch from frequency to time domain and vice versa in natural or generalized coordinates. The. scope of the paper is to: (a) provide some rigorous elastodynamic results in both frequency and time. domains that can be used as reference cases; (b) elucidate the role of a number of key phenomena. and salient dimensionless parameters for the amplitude of pile bending at an interface separating. two soil layers of different stiffness; (c) propose a simplified semi-analytical formula for evaluating. such moments; (d) provide some remarks about the role of kinematic bending in seismic design of. pile foundations, with emphasis on the long-standing issue of establishing an optimal pile diameter. to resist such bending. The results of the study offer a new interpretation of kinematic pile bending. in terms of the interplay between pile and soil, expressed through dimensionless layer thickness,. pile-to-soil stiffness ratio and impedance contrast at the layer interface. A case study from Japan is. presented.

A tall public building in Naples (Italy) has recently undergone a seismic vulnerability assessment, following the new Italian code requirements. The building is about 100 m high and is founded on a piled raft floating in a thick layer of soft pyroclastic and alluvial soils. On the basis of a conventional subsoil classification, the inertial seismic actions on the building would lead to expensive measures for seismic retrofitting. By contrast, if site effects and soil–structure interaction are adequately addressed the picture is completely different. First, free-field seismic response analyses highlighted the beneficial effects of a peat layer, acting as a natural damper on the propagation of shear waves. Finite-element analyses of pile–soil kinematic interaction were then carried out to define the foundation input motion, which was found not to be significantly affected. The effects of inertial interaction were evaluated accounting for soil–foundation compliance; they resulted in an increase of the structural period of vibration, while the overall damping did not change compared to that of the fixed-base structure. The increased structural period led to further reduction of spectral acceleration. The results could lead to significant impacts on the seismic assessment of slender buildings founded on piles embedded in deformable soils.

A tall public building in Naples (Italy) has recently undergone a seismic vulnerability assessment, following the new Italian code requirements. The building is about 100 m high and is founded on a piled raft floating in a thick layer of soft pyroclastic and alluvial soils. On the basis of a conventional subsoil classification, the inertial seismic actions on the building would lead to expensive measures for seismic retrofitting. By contrast, if site effects and soil-structure interaction are adequately addressed the picture is completely different. First, free-field seismic response analyses highlighted the beneficial effects of a peat layer, acting as a natural damper on the propagation of shear waves. Finiteelement analyses of pile-soil kinematic interaction were then carried out to define the foundation input motion, which was found not to be significantly affected. The effects of inertial interaction were evaluated accounting for soil-foundation compliance; they resulted in an increase of the structural period of vibration, while the overall damping did not change compared to that of the fixed-base structure. The increased structural period led to further reduction of spectral acceleration. The results could lead to significant impacts on the seismic assessment of slender buildings founded on piles embedded in deformable soils.

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.

The design of piles under seismic loads is conventionally carried out with reference to the inertial forces arising from structural vibrations and, hence, neglecting kinematic interaction between pile and soil. Even if in the last years several research efforts gave particular attention to kinematic effects, they focused on bending moments at the interface between two soil layers with different stiffness, while few contributions investigated kinematic effects at pile head. The paper presents the results (both in the frequency and time domain) of extensive parametric studies, leading to a simplified formula for predicting kinematic bending moments at pile head. The paper gives some indications about their relative importance with respect to the inertial ones and provides simply rules to combine inertial and kinematic maximum effects taking into account their phase lag.