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Allowing the transitory attainment of bearing capacity of caisson foundations supporting bridge piers during strong seismic events can lead to substantial optimisation in their design and major cost savings. If the approach of capacity design is applied to geotechnical systems, the temporary triggering of plastic mechanisms may be permitted if the resulting permanent displacements are smaller than given threshold values. To validate this design approach, the seismic performance of caisson foundations was assessed through dynamic centrifuge testing on reduced-scale models. This paper presents the results of two tests in which a caisson–pier–deck system was embedded in a typical alluvial deposit and subjected to a series of earthquakes of different intensities. The caissons were founded on soft and very soft clay, to either avoid or induce the attainment of plastic soil behaviour under the same seismic inputs. It is shown that both yielding and failure of the layer of very soft clay limit inertial forces transmitted to the superstructure, validating the design approach and some useful empirical relations available in the literature. In contrast, inelastic soil behaviour implies accumulation of permanent rotation and settlement of the system, which must be carefully evaluated to check for fulfilment of performance requirements. [ABSTRACT FROM AUTHOR]

In recent decades, research on renewable energy has been boosted by the emerging awareness of energy security and climate change and their consequences, such as the global cost of adapting to the climate impacts. Both onshore and offshore wind turbine farms have been considered as one of the main alternatives to fossil fuels. Their development currently involves seismic-prone areas, such as the Californian coastline and East Asia, where the risk of soil liquefaction is significant. Onshore wind turbines (OWTs) typically are founded on shallow rafts. Their operation can be affected strongly by the simultaneous presence of intense earthquakes and wind thrust, which may cause remarkable permanent tilting and loss of serviceability. In these conditions, accurate evaluation of the seismic performance of these structures requires the development of well-validated numerical tools capable of capturing the cyclic soil behavior and the build-up and contextual dissipation of seismic-induced pore-water pressures. In this paper, a numerical model developed in OpenSees, calibrated against the results of dynamic centrifuge tests, was used to evaluate the influence of some ground motion intensity Measures of the seismic behavior of OWTs included the amplitude, frequency content, strong-motion duration, and Arias intensity (energy content) of the earthquake, together with the effect of a coseismal wind thrust, which is not well explored in the literature. The seismic performance of an OWT was assessed in terms of peak and permanent settlement and tilting, the latter of which was compared with the threshold of 0.5° typically adopted in practice. [ABSTRACT FROM AUTHOR]

A reliable assessment of earthquake-induced permanent displacements is a fundamental step for both displacement-based and force-based methods applied to the seismic design of embedded anchored walls. Acceleration reduction factors recommended by the codes for a force-based design of these structures derive from parametric studies of the performance of gravity walls or slopes. This may lead to either conservative or unconservative results and is certainly misleading from the point of view of the physics of the problem. Based on the results of an extensive numerical study, this work clarifies the mechanisms by which anchored walls accumulate permanent displacements during earthquakes, showing that full mobilization of soil passive strength requires displacements of the order ur/H≈1%–2% , making the assumption of rigid-perfectly plastic behavior of the system unsuitable. Moreover, both the critical acceleration and the pattern of final displacements depend on the plastic mechanism effectively activated within the soil-wall-anchor system. The issue of the proper choice of performance factors to be used in a force-based design of anchored walls is also addressed, and a new method is outlined for a preliminary calculation of the permanent displacements of the wall. Finally, the paper critically reviews two methods recently proposed in the literature to compute earthquake-induced wall displacements. All three methods account for the soil-wall system nonlinearity and hardening during the earthquake. The results discussed herein, together with the complementary work presented by the same authors in a companion paper, provide a thorough conceptual framework for the performance-based seismic design of anchored walls. [ABSTRACT FROM AUTHOR]

An innovative yet extremely simple approach to reduce the seismic vulnerability of existing buildings on shallow foundations is to remove the lateral contact between the embedded foundation and the surrounding soil. This paper compares the dynamic response of two identical one-degree-of-freedom reduced-scale model sway frames tested in a geotechnical centrifuge. One of the models was founded on fully embedded shallow strip footings, whereas the foundations of the other were disconnected from the soil laterally. Both models were constructed in dry medium density sand whose strength and small strain shear stiffness profiles were obtained by cone penetration and air hammer tests carried out in flight. Various tools were used to monitor the displacements and accelerations both in the structure and in the soil, including micro-electro-mechanical systems (MEMS) accelerometers, piezoelectric accelerometers, LVDTs, and a high-frequency camera. The experimental results demonstrate that the lateral disconnection of the foundation from the adjacent soil significantly reduces the foundation translational and rotational stiffness, which results in a larger predominant period of the structure and a reduction of the absolute floor accelerations obtained for different sinusoidal input signals. The lateral disconnection also mitigates floor drift and story shear forces. The trade-off is the lower radiative damping observed for the laterally disconnected foundation, which results in a larger number of post-earthquake oscillations of the structure. [ABSTRACT FROM AUTHOR]

Tunneling can compromise the safety and serviceability of existing buried infrastructure because of settlements, crack openings, dislocations, and material strains. A continuum-based soil-structure interaction model, implementing an equivalent Timoshenko beam, is used to evaluate the excavation-induced settlements and longitudinal deformations of existing tunnels and pipelines. Both single and twin tunneling scenarios are examined. The shear flexibility of shield and sprayed concrete lining tunnels may be significant. This increases tunneling-induced settlements while it decreases tilt, curvature, and the associated bending moment, with implications for optimal monitoring installations. In this work, a new dimensionless shear factor is introduced, governing the relative importance of shear and bending deformations and accounting for the settlement trough width. This can be used to evaluate if a pure-bending, shear-dominated, or mixed deformation mode should be expected. Design charts to assess existing infrastructure affected by either single or twin tunnels are proposed. [ABSTRACT FROM AUTHOR]

This paper describes a new pseudostatic approach for an efficient seismic design of anchored steel sheet pile (ASSP) walls supported by shallow passive anchorages. As for other retaining structures, energy dissipation during strong earthquakes leading to reduced inertia forces can be achieved by allowing the activation of ductile plastic mechanisms. To this end, a robust method is required to identify all the possible yielding mechanisms and to guarantee the desired strength hierarchy. It is shown that dissipative mechanisms for ASSP walls correspond either to the local attainment of the soil shear strength in the supporting soil and around the anchor, or in the activation of a log-spiral global failure surface. A new limit equilibrium method is proposed to compute the critical acceleration of the system, corresponding to the actual mobilization of its strength, and the maximum internal forces in the structural members. Theoretical findings are validated against both existing dynamic centrifuge data and the results of original pseudostatic and fully dynamic numerical analyses. [ABSTRACT FROM AUTHOR]

Computational modelling of the effect of underground excavations on adjacent structures has shown great potential to aid the assessment of tunnelling-induced damage to structures. However, the complexity of the mechanisms involved and the uncertainties connected to the use of sophisticated constitutive laws still limit the application of numerical modelling in civil engineering practice. This paper evaluates the effectiveness of soil models with different levels of complexity when predicting tunnelling-induced displacements of the soil surface, and consequently the assessment of building deformations. The performance of a non-linear elastic, a linear elastic–perfectly plastic and a critical-state-based kinematic hardening soil model were compared with the results of centrifuge testing of a tunnel excavation in sand. Results demonstrated that both the non-linear elastic and the kinematic hardening models are suitable to reproduce the effect of soil–structure interaction on the soil surface displacements and the building deformations, while also demonstrating the limitations of these methods in predicting local soil strains around the tunnel itself. [ABSTRACT FROM AUTHOR]

Variation of mechanical properties in materials can be detected non-destructively using ultrasonic measurements. In particular, changes in elastic wave velocity can occur due to damage, i.e., microcracking and particles debonding. Here the challenge of characterizing damage in geomaterials, i.e., rocks and soils, is addressed. Geomaterials are naturally heterogeneous media in which the deformation can localize, so that few measurements of acoustic velocity across the sample are not sufficient to capture the heterogeneities. Therefore, an ultrasonic tomography procedure has been implemented to map the spatial and temporal variations in propagation velocity, which provides information on the damage process. Moreover, double beamforming has been successfully applied to identify and isolate multiple arrivals that are caused by strong heterogeneities (natural or induced by the deformation process). The applicability of the developed experimental technique to laboratory geomechanics testing is illustrated using data acquired on a sample of natural rock before and after being deformed under triaxial compression. The approach is then validated and extended to time-lapse monitoring using data acquired during plane strain compression of a sample including a well defined layer with different mechanical properties than the matrix. [ABSTRACT FROM AUTHOR]

The writers would like to thank the discussers for their discussion, providing analytical insights and practical considerations on the response of existing pipelines and tunnels to tunneling-induced settlements, and reply with four main comments: • The shear factor HT ht was presented to facilitate the description of the mechanical response of pipelines and tunnels in practical applications. • Concerning the observation that the published article is limited to (vertical) contact soil resistance, the writers agree with the discussers that it strictly applies to perfectly smooth pipeline/tunnel-soil interface conditions, considering either coupled or uncoupled vertical springs only for the soil model. Further research is needed for more sophisticated two-stage interaction models capable of considering both contact and frictional resistance of the soil in the context of pipeline or tunnel response to vertical and horizontal ground movements. [Extracted from the article]

This paper describes a method to compute the mobilized shear modulus, G, and damping ratio, D, using the nonlinear fit of experimental transfer functions obtained at different depths in centrifuge models, with the analytical expression of the amplification function for a viscoelastic soil layer on a rigid base. The corresponding shear strain, ?, is computed as a function of the particle velocity and shear wave velocity. The sources of potential error in the determination of G, D, and ? embedded in the proposed method are identified and discussed in comparison with two other methods that have been proposed in the literature, based either on the determination of the time lag of accelerations between two accelerometers or on the evaluation of the shear stress-strain cycles from acceleration time histories recorded at different depths in the model. The performance of the three methods is evaluated using the experimental data obtained from nine centrifuge tests on dry sand. The values of G obtained by the proposed method compare well with the results of laboratory and literature data; D values are more dispersed and slightly higher than the literature data. [ABSTRACT FROM AUTHOR]