ISSN / EISSN : 0016-8505 / 1751-7656
Published by: Thomas Telford Ltd. (10.1680)
Total articles ≅ 5,220
Latest articles in this journal
Géotechnique pp 1-44; https://doi.org/10.1680/jgeot.21.00017
Fort d'Issy-Vanves-Clamart metro station is a 32 m deep excavation part of the new subway line 15 of the Grand Paris Express project. The performance of the support system is assessed through a wide monitoring program covering wall displacements, wall bending moments, strut loads, ground surface settlements, and earth pressure. Classical instrumentation was set up in redundancy to consolidate field measurements (several inclinometers, strain gauges with temperature sensors). Advanced devices were used to provide accurate measurement data, in particular fiber optic was installed along the retaining wall with total pressure and pore-water pressure cells placed at the soil/wall interface at 4 in-depth locations. In addition, measurements data were provided for different excavation levels in order to assess the support system behavior as excavation progressed. The present paper aims at providing a full description of a deep excavation behavior through a complete monitoring system. Bending moments captured with fiber optic are more accurate than those derived from inclinometers. An analysis methodology is proposed to address the temperature effect on strut loads measurements in order to separate the thermal expansion contribution on the strut loading from the excavation process contribution. The stress redistribution behind the wall was observed with the lateral earth pressure increase of top cells while excavating deep levels. The comprehensive field measurements provided in this paper can supply further back analysis to improve numerical modelling prediction.
Géotechnique pp 1-40; https://doi.org/10.1680/jgeot.21.00145
Widespread liquefaction was frequently observed in silty sand deposits during recent large earthquakes. How to properly evaluate the liquefaction potential of silty sands has emerged as a matter of great concern. This paper presents an investigation into the problem through a comprehensive experimental program in conjunction with a newly developed framework of analysis. Focus is placed on the effect of particle size disparity and its interplay with the effect of fines content − two fundamental issues for which current understanding is limited. A novel finding of the study is that the liquefaction resistance of sand-fines mixtures, defined as cyclic resistance ratio (CRR) leading to failure at a specific number of loading cycles, is almost uniquely related to the state parameter (ψ) defined in the critical state theory, regardless of particle size disparity and fines content. To quantify the combined effects of fines content and particle size disparity, an empirical model is put forward along with a practical method for estimating the critical state parameters of silty sands. The model is evaluated using experimental data on a range of sand-fines mixtures in the literature, showing reasonably good performance and attractive advantages. A grain-scale mechanism is also suggested to explain the observed effects.
Géotechnique pp 1-58; https://doi.org/10.1680/jgeot.21.00083
The paper studies strike-slip fault rupture propagation through dense sand and its interaction with surface foundations, combining physical and numerical modelling. A series of centrifuge tests are conducted using a 3-section split-box, which allows modelling 2 strike-slip faults per test. A free-field test is initially conducted, followed by four interaction tests. Eight different foundation configurations are studied, varying the foundation location, surcharge load, aspect ratio, and rigidity. The experiments are numerically simulated employing 3D finite element (FE) modelling, combining periodic boundaries and a relatively simple yet efficient constitutive model, developed as part of this study. Based on a Mohr–Coulomb yield criterion, the model incorporates post-yield isotropic frictional hardening and softening (MC–HS). Carefully calibrated on the basis of triaxial tests, the model is validated against the centrifuge model tests, and exploited to derive further insights. The MC-HS model covers the entire range from elastic to fully-softened response, capturing the deviatoric and volumetric behaviour of dense sand, and especially its pre-softening volumetric response, which is proven crucial for the simulation of the complex mechanisms of strike-slip faulting. Both physical and numerical modelling reveal the formation of diagonal shear ruptures at the ground surface (Riedel shears). These are complex helicoidal structures, formed due to the spatial variation of shear stresses. Foundation response is mainly governed by the kinematic constraint offered by its presence. Fault rupture locations close to its sides typically lead to a translational mechanism, whereas locations close to its centreline to a rotational one. Foundation rigidity is proven to be a prerequisite for the development of both mechanisms, which rely on the ability of the foundation to resist the developing normal and shear stresses.
Géotechnique pp 1-40; https://doi.org/10.1680/jgeot.21.00150
The limit analysis approach is used in this paper to analyse the problem of a strip foundation on cohesive-frictional soil under seismic conditions, based on the pseudo-static approach, considering different inclination factors. In particular, for a weightless c – ϕ soil the Upper Bound (UB) and the Lower Bound (LB) solutions are found to be coincident and then the exact solutions for the bearing capacity factors Nqe and Nce under seismic conditions are obtained. The effect of soil weight is considered with the UB method. A discussion is made on seismic reduction factors and on their superposition.
Géotechnique pp 1-18; https://doi.org/10.1680/jgeot.20.p.432
Seepage-induced instabilities pose a challenge in many geotechnical applications. Particle-scale mechanisms govern the initiation of instability. However, current understanding is based on a macro-scale perspective that draws on continuum mechanics. Recent developments in imaging and numerical analysis can provide the particle-scale fundamental perspective needed to develop a comprehensive insight. This contribution demonstrates the value of combining particle-scale experimental and numerical studies. The experiments consider transparent soil samples created using refractive image matching and monitored by particle image velocimetry (PIV). Three-dimensional pore topology is extracted from a series of 2D images and imported into computational fluid dynamics (CFD) simulations. Permeability is estimated by three distinct approaches: using flow rate, PIV- and CFD-generated data. The flow fields obtained from PIV and CFD are in good agreement considering both flow rate contour plots and flow rate distributions; this demonstrates the successful reconstruction of three-dimensional pore structure and flow-field analysis. The comparison also reveals that the side boundary effects in CFD simulations are constrained within a limited region. The multi-plane results characterize the variance of flow velocity with the three-dimensional pore topology. Finally, the fluid-particle interactions obtained from CFD results show a larger variance in the angular particle packings.
Géotechnique pp 1-39; https://doi.org/10.1680/jgeot.21.00068
Optimised design is essential to reduce the cost of monopiles for offshore wind turbines. For this purpose, an in-depth understanding of the behaviour of monopile-soil interaction is required. As more wind farms are planned in seismically active areas, the undrained behaviour of sandy soils (and possibility of soil liquefaction) and their effects on monopile cyclic response need a critical evaluation. Considering the lack of well-established test programs, implicit 3D Finite Element (FE) methods stand outas a robust tool to identify and highlight the governing geo-mechanisms in monopile design. In this work, an implicit 3D FE implementation of SANISAND-MS for undrained soil behaviour, termed SANISAND-MSu, is deployed in OpenSees to serve these objectives. The role of pore water pressure on monopile performance is comprehensively investigated by comparisons between drained and undrained soil behaviour. Local soil responses are studied in detail in relation to parameters in laboratory soil testing and application to monopile geotechnical design. The results of simulations are also used to evaluate numerical p – y curves as function of number of load cycles on the pile. The conclusions in this work contribute to the ongoing research on monopile-soil interaction and support the development of lifetime analysis for monopile-soil systems.
Géotechnique pp 1-31; https://doi.org/10.1680/jgeot.20.p.228
The state parameter ψ is widely used for soil characterization and as a controlling parameter in modern constitutive understanding of soil, but there remains a perception that the control of soil strength by ψ is merely that of a correlation. This perception possibly stems from ψ having been introduced from ‘principles’ of critical state theory rather than derived, which is now rectified. It is shown that the control of limiting dilatancy by the state parameter (and thus soil strength through stress-dilatancy) is a formal mathematical consequence of Casagrande's canonical characterization linking void ratio to soil constitutive behaviour. This formal consequence is independent of soil type, being applicable across the spectrum from clays to sands. Three dimensionless and familiar soil properties are involved in addition to those characterizing the critical state locus: Mtc, N, and X. The framework is kinematic, with no constitutive model: it is a constraint on models. Example data are shown for sands, silts, and clays to illustrate the independence of the theory from geological descriptors.
Géotechnique pp 1-31; https://doi.org/10.1680/jgeot.20.p.312
Pile foundations for masts supporting Overhead Line Equipment (OLE) on railways in Western Europe have traditionally been designed using empirical formulae derived from tests carried out in the 1950s under the auspices of the Union Internationale des Chemins de fer (UIC). Recent application in the UK of ostensibly more analytical approaches led to significantly increased pile lengths, which contributed to the high-profile cost over-run on the Great Western Electrification Programme. Further, the loads associated with some newer designs of mast are greater than those covered by the original field tests, hence are outside the evidence base for the empirical approach. To address this, full scale field tests were carried out on three, 610 mm diameter circular hollow section steel piles installed in a railway embankment at the High Marnham test track (Nottinghamshire, UK). Each pile was designed according to the empirical Overhead Line Equipment Master Index (OLEMI) method for a different form of modern OLE mast, and instrumented using Shape accelerometer arrays to determine pile deformations. The results provide valuable insights into the mechanisms of deformation of piles of different lengths, both at and beyond the expected in-service loads. p-y curves derived from the Shape accelerometer array measurements compare reasonably well with curves constructed using the American Petroleum Institute (API) method for undrained clay soils. The tests also demonstrate the suitability of the OLEMI approach for large structures and loads; and that the EC7 partial factors on load and undrained shear strength applied to an undrained (total stress) limit equilibrium calculation would likely be sufficient to meet serviceability requirements for standard single and twin track cantilever structures.
Géotechnique, Volume 71, pp 925-936; https://doi.org/10.1680/jgeot.19.p.260
The acoustic emission (AE) technique can be used to locate failure-induced AE sources and provides an alternative to continuously visualise the development of failures inside stressed materials; however, a literature review reveals almost no studies regarding its application for source location in granular materials. Yet the visualisation of ubiquitously observed strain localisation and shear banding is critical to an in-depth understanding of the progressive failure mechanism of granular materials. In this paper, an original AE source three-dimensional (3D) tracing code based on the idea of ‘time difference of arrival’ (TDOA) was developed using Matlab programming, for the first time, to continuously visualise particle-scale interactions involved in saturated granular soils. After validating its feasibility through a series of pencil lead break (PLB) tests, the developed AE source 3D tracing code was applied to saturated granular soils subjected to drained triaxial shearing. Both the results of PLB tests and drained triaxial shearing tests demonstrate a good consistency between the traced AE source and actual PLB/particle interaction-induced sources. This suggests that the developed AE source 3D tracing code could be used to visualise the initiation and evolution of strain localisation and shear banding in saturated granular soils subjected to drained triaxial shearing.
Géotechnique pp 1-6; https://doi.org/10.1680/jgeot.20.d.014