Class A prediction of mechanised tunnelling in Rome

Contract T3 of Line C of Rome underground, currently under construction, crosses the archaeological area of the historical centre, with significant interferences with the existing monumental built environment. A fully instrumented green field control section was established at the beginning of this contract, in representative ground conditions. This paper presents a thorough Class A prediction of the passage of the tunnels through the control section, obtained using a recently developed advanced numerical procedure. The ground was modelled with a non-linear constitutive law, calibrated with all the available data from the geotechnical investigation. The main physical processes occurring around the shield, including cutter-head overcut, shield tapering and tail void grouting were modelled in detail. The numerical results agree qualitatively with the findings from well documented case histories and results from physical models. The installed instrumentation will provide an opportunity to test the ability of the adopted procedure to reproduce quantitatively the measured performance, once the tunnels will cross the control sections and the field data will become available

Filtering effect induced by rigid massless embedded foundations

It is well recognised that the dynamic interaction between structure, foundation and supporting soil can affect significantly the seismic behaviour of buildings. Among other effects, embedded and deep foundations can filter the seismic excitation, causing the foundation input motion (FIM) to differ substantially from the free-field motion. This paper presents a theoretical and numerical investigation on the filtering effect induced by rigid massless embedded foundations. Based on the results of dimensional analysis and numerical simulations, it is shown that the problem can be reasonably described by two sole dimensionless groups, namely: (1) ωH/VS, relating the wave length of the signal to the embedment depth of the foundation, and (2) the aspect ratio of the foundation, B/H, where B is the foundation width in the polarization plane. New simplified and physically sound expressions are derived for the kinematic interaction factors, Iu= uFIM/ uff 0 and Iθ= θFIMH/ uff 0, which are frequency-dependent transfer functions relating the harmonic steady-state motion experienced by the foundation to the amplitude of the corresponding free-field surface motion. Standard methods for using these functions in the evaluation of the FIM are critically reviewed, with reference to both static and dynamic procedures for the seismic design of structures

Breakage mechanisms of highly porous particles in 1D compression revealed by X-ray tomography

Grain breakage affects a number of geotechnical engineering problems. In this research study, the breakage of an artificial, porous granular material (light-expanded clay aggregate (LECA)) has been studied in one-dimensional compression with both standard laboratory techniques and in situ X-ray tomography during loading. X-ray tomography has revealed that there is a wide distribution of internal porosity among LECA particles, and particle tracking has been used, for the first time, to give an objective measurement of each particle's life expectancy. Links between micro- and macro-scale quantities are discussed. </jats:p

Numerical modelling of centrifuge dynamic tests of circular tunnels in dry sand

This paper describes the numerical simulation of two dynamic centrifuge tests on reduced scale models of shallow tunnels in dry sand, carried out using both an advanced bounding surface plasticity constitutive soil model and a simple Mohr–Coulomb elastic-perfectly plastic model with embedded nonlinear and hysteretic behaviour. The predictive capabilities of the two constitutive models are assessed by comparing numerical predictions and experimental data in terms of accelerations at several positions in the model, and bending moment and hoop forces in the lining. Computed and recorded accelerations match well, and a quite good agreement is achieved also in terms of dynamic bending moments in the lining, while numerical and experimental values of the hoop force differ significantly with one another. The influence of the contact assumption between the tunnel and the soil is investigated by comparing the experimental data and the numerical results obtained with different interface conditions with the analytical solutions. The overall performance of the two models is very similar indicating that at least for dry sand, where shear-volumetric coupling is less relevant, even a simple model can provide an adequate representation of soil behaviour under dynamic condition

Grading evolution of an artificial granular material from medium to high stress under one-dimensional compression

This contribution presents the results of an experimental investigation of the mechanical behaviour of granular materials with crushable grains under one-dimensional compression at medium to high stress. The material used for the experimental work is a Light Expanded Clay Aggregate (LECA) whose grains break at relatively low stress. Reconstituted samples were prepared with different initial grain size distributions and their evolution observed under one-dimensional compression. The grain size distributions before and after testing were used to calibrate a bimodal model obtained from the superposition of two Weibull functions. The observed evolution of the micro and macro diameters on loading are linked to the characteristics of the one-dimensional compressibility curve obtained under displacement controlled conditions, such as its shape and two characteristic stress values, namely the pre-consolidation stress and the stress corresponding to the point of inflection

Improved Method for the Seismic Design of Anchored Steel Sheet Pile Walls

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

Artificial ground freezing of a volcanic ash: Laboratory tests and modelling

The use of artificial ground freezing (AGF) to form earth support systems has had applications worldwide. These cover a variety of construction problems, including the formation of frozen earth walls to support deep excavations, structural underpinning for foundation improvement and temporary control of ground water in construction processes. On one hand, the main advantage of AGF as a temporary support system in comparison to other support methods, such as those based on injections of chemical or cement grout into the soil, is the low impact on the surrounding environment as the refrigerating medium required to obtain AGF is circulated in pipes and exhausted in the atmosphere or re-circulated without contamination of the ground water. On the other hand, the available methods may vary significantly in their sustainability and complexity in terms of times and costs required for their installation and maintenance. The ability to predict the effects induced by AGF on granular materials is therefore crucial to assessing construction time and cost and to optimising the method. In this work, the thermo-hydro-mechanical processes induced by artificial freezing of a soil body are studied using a constitutive model that encompasses frozen and unfrozen behaviour within a unified effective-stress-based framework. It makes use of a combination of ice pressure, liquid water pressure and total stress as state variables. The model is validated and calibrated using the results of a series of laboratory tests on natural samples of a volcanic ash (Pozzolana) retrieved during construction of Napoli underground, where the technique of AGF was used extensively to stabilise temporarily the ground and control the ground water

Multi-scale morphological descriptors from the fractal analysis of particle contour

The increasing understanding of the connection between particle morphology and mechanical behaviour of granular materials has generated significant research on the quantitative characterisation of particle shape. This work proposes a simple and effective method, based on the fractal analysis of their contour, to characterise the morphology of soil particles over the range of experimentally accessible scales. In this paper, three new non-dimensional quantitative morphological descriptors are introduced to describe (1) overall particle shape at the macro-scale, (2) particle regularity at the meso-scale, and (3) particle texture at the micro-scale. The characteristic size separating structural features and textural features emerges directly from the results of the fractal analysis of the contour of the particle, and is a decreasing fraction of particle dimension. To explore the meaning of the descriptors, the method is applied first to a variety of Euclidean smooth and artificially roughened regular shapes and then to four natural and artificial sands with different levels of irregularity. Relationships are established between the new morphological descriptors and other quantities commonly adopted in the technical literature

Guidelines for comparing field or physical model observations with numerical simulations

This paper presents short report on the activities of TC204 Working Group on "Guidelines to compare field or physical model observations with numerical simulations". The main objective of the WG is that of proposing recommendations concerning the comparison between calculated and observed behaviour, specifically adapted for the type of works covered by TC204, i.e. tunnels and open excavations in soft ground. © 2012 Taylor & Francis Group

Geotechnical design and construction issues for Lines 1 and 6 of the Naples Underground

Since 1839, when the historical railway Naples - Portici was inaugurated, Naples has been characterized by a significant system of urban and suburban railways. The idea of a fully integrated urban rail network dates back to the 1950s, but construction only began in the 1990s. At present, the system includes 54 km of tracks and 69 stations in operation; ten lines with 93 km of track and a further 30 km of new light rail linking 114 stations with 21 interchanges are planned with the completion of the City Transport Plan. The present paper reports on some geotechnical aspects of the design and construction of Lines 1 and 6, which are presently underway, discussing how the experience gathered during construction of Line 1 influenced some of the design choices for Line 6