1,720,984 research outputs found
Osservazioni sul progetto di paratie secondo gli Eurocodici e le Norme Tecniche per le Costruzioni (NTC08)
No full textIn questo articolo si prende in esame la progettazione geotecnica di opere di sostegno flessibili – paratie a sbalzo e ancorate – secondo le prescrizioni degli Eurocodici EC7, EC8 e della recente normativa italiana (NTC08). Per tali opere, la normativa italiana prescrive al progettista di eseguire verifiche SLU, in condizioni statiche e sismiche, secondo l’Approccio 1 dell’Eurocodice (DA1). Considerando i diversi valori dei coefficienti di sicurezza parziali per le azioni e le loro combinazioni, il progettista deve generalmente svolgere le verifiche di sicurezza in tre distinte condizioni (DA1-C1 e DA1-C2 in condizioni statiche; DA1-C2 in condizioni sismiche), al fine di valutare quella più severa nei confronti del dimensionamento geotecnico della paratia.
Nell’articolo, le equazioni che soddisfano il requisito fondamentale Ed Rd sono sviluppate in forma adimensionale per le tre condizioni su citate; ciò consente di individuare i gruppi adimensionali delle grandezze che governano l’equilibrio alla rotazione del sistema. Nello studio sono esaminate due semplici tipologie di strutture di sostegno - paratia a sbalzo e paratia ancorata in testa - in terreni incoerenti; per queste, è condotto uno studio parametrico considerando un ampio intervallo di valori dell’angolo di attrito del terreno, del coefficiente sismico orizzontale e dei sovraccarichi permanenti e variabili. I risultati ottenuti dallo studio, sintetizzati in grafici adimensionali, mostrano che spesso non è necessario analizzare le tre differenti condizioni di verifica e consentono di individuare immediatamente la condizione più gravosa per molte situazioni reali, in condizioni sia statiche sia sismiche; ciò conduce a una notevole utilità e semplificazione nella prassi progettuale corrente
Advanced numerical modeling of seismic response of a propped r.c. diaphragm wall
No full textThe paper presents some results from a number of dynamic FE simulations carried out to investigate the seismic response of a propped flexible retaining wall in a dry coarse-grained soil, considering two bedrock acceleration time histories as seismic input. Two different soil plasticity models have been considered in this study: an anisotropic hardening, critical-state model for cyclic/dynamic loading of sands and the classical Mohr–Coulomb elastic-perfectly plastic model with nonassociative flow rule. The results obtained allow to highlight the main features of the seismic performance of such type of flexible retaining structures and to evaluate the effects of the constitutive assumptions made on soil behavior on the predicted wall displacements and structural loads
Advanced numerical modelling of seismic response of a propped r.c. diaphragm wall
No full textThe paper presents some results from a number of dynamic FE simulations carried out to investigate the seismic response of a propped flexible retaining wall in a dry coarse-grained soil, considering two bedrock acceleration time histories as seismic input. Two dierent soil plasticity models have been considered in this study: an anisotropic hardening, critical state model for cyclic/dynamic loading of sands and the classical Mohr-Coulomb elastic-perfectly plastic model with non-associative flow rule. The results obtained allow to highlight the main features of the seismic performance of such type of flexible retaining structures and to evaluate the effects of the constitutive assumptions made on soil behavior on the predicted wall displacements and structural loads
Critical Accelerations for Propped Diaphragm Walls in Sand by Finite Element Limit Analysis
No full textThe paper presents the results of a parametric study aimed at determining the pseudo-static critical acceleration of a propped retaining wall in sand under seismic conditions, as a function of problem geometry, soil strength and wall strength. The critical acceleration ratio has been computed by means of a finite element formulation of the limit analysis (LA) theorems. The results obtained, presented in dimensionless form, could be used in the safety assessment of similar structures under earthquake loading by means of the classical pseudo-static approach or in simplified Newmark-based methods, where the seismic performance is evaluated in terms of permanent displacements
Geotechnical properties of an unsaturated pyroclastic soil from Roma
No full textThe paper presents the results of an experimental investigation on the mechanical behaviour of an unsaturated pyroclastic soil typical of the subsoil of Roma (Italy). In its natural state, the material is a bonded coarsegrained weak rock. The deposits of pozzolana are unsaturated in situ; this has an important practical relevance in the evaluation of the stability conditions of natural slopes and cuts. The experimental investigation consisted mainly of pressure plate tests, oedometer tests and special isotropic and triaxial compression tests at increasing values of mean net pressures in the range 50–400 kPa and at different constant levels of suction (20–75 kPa). The compressibility and shear strength properties of the material are presented in some detail. Special attention is given to defining the failure envelope at relatively low confining stresses and assessing the influence of the degree of saturation on the failure conditions
Direct Displacement-Based Design of Embedded Retaining Structures
No full textThe paper presents the displacement seismic design method known as ???Direct Displacement ???
philosophy behind DDBD is that structures ??? including geotechnical structures ??? should be designed to achieve a specified performance level under a specified seismic intensity. The calculation steps of the method are described in detail with considerable discussion on their applicability to cantilever and anchored retaining structures. As a final result, the DDBD procedure provides a rational design approach which relies on a convenient reduction of seismic thrusts based on the system capacity of dissipating energy (ductility capacity)
On the design of embedded retaining structures following Eurocodes and the italian national code
No full textThe paper focuses on the design process of cantilever and anchored embedded retaining structures, following the prescriptions of Eurocodes EC7, EC8 and their implementation in the recent italian Code for Constructions (NTC08). For such structures, the Italian Code prescribes to the designer to perform ULS analyses in static and seismic conditions following the Eurocode Design Approach 1 (DA1). Considering the distinct values of partial factors relevant to the different actions and combinations, the designer generally has to face and to verify the stability of the structure in three distinct cases (i.e.:DA1-C1 and DA1-C2 in static condition; DA1-C2 in seismic condition) in order to assess the most critical one. In the paper, the equations satisfying the fundamental requirement Ed <= Rd for the three cases above are developed in a nondimensional form; this allows to detect the non-dimensional groups (geometrical, mechanical, loading) affecting the rotational equilibrium. Two simple kinds of retaining structure are considered, namely, cantilever and singly-propped embedded walls in cohesionless soil. A parametric study is then performed by considering wide ranges of values for soil friction angle, seismic horizontal coefficient, and unfavourable permanent and variable surcharges. The results obtained from the study, which are summarised into non-dimensional plots, show that often it is not necessary to analyse the three different conditions and immediately provide the most critical case for a wide range of practical situations, in both static and seismic conditions. This leads to a significant usefulness and simplification in current design practice
La protezione superficiale dei pendii con tecniche di inerbimento
No full textI fenomeni erosivi sul territorio italiano sono molto diffusi. Nel corso degli anni sono state sviluppate diverse tecniche di protezione del suolo e di rinaturalizzazione. Fra queste emerge un’innovativa tecnologia naturale che impiega esclusivamente piante erbacee perenni a radicazione profonda e consente di operare anche in aree in cui le condizioni pedoclimatiche erano fino a pochi anni fa ritenute proibitive per lo sviluppo della vegetazione. Tale tecnologia appare promettente anche nei riguardi dei fenomeni di instabilità superficiale dei versanti: l’inerbimento con radici profonde può indurre effetti meccanici ed idraulici sulle condizioni di equilibrio del versante tali da incrementare tipicamente la resistenza a taglio del terreno. Gli effetti meccanici dell’impianto di radici sono attribuibili ai processi di interazione radice/terreno; gli effetti di natura idraulica derivano dal meccanismo secondo cui l’inerbimento può ridurre significativamente il contenuto d’acqua ed il grado di saturazione del terreno. La ricerca si pone come obiettivo finale la valutazione quantitativa di tali effetti, mediante la loro modellazione ed implementazione in un algoritmo di calcolo
An experimental study on a partially saturated pyroclastic soil: the Pozzolana Nera from Roma
No full textThe paper presents the results of an experimental investigation on the mechanical behaviour of partially saturated Pozzolana Nera (Roma, Italy). Such natural deposits, which have been extensively studied in saturated conditions, are characterised by a marked heterogeneity in terms of grading, nature of grains, and inter-particle bonds. In situ, deposits of pozzolana are generally found in unsaturated conditions; this has an important practical relevance in the evaluation of the stability conditions of natural slopes and cuts. The experimental investigation consisted mainly of isotropic and triaxial compression tests on reconstituted samples, at increasing values of mean net pressures in the range 50 – 400 kPa, at different constant levels of suction (20 – 75 kPa). Volume pressure plate extractor tests were also conducted to obtain the water retention curve. In addition some wetting tests were carried out by means of conventional oedometer cells. The testing programme made it possible to examine the hydraulic and mechanical properties of unsaturated Pozzolana Nera at different values of the degree of saturation and to compare the mechanical behaviour of the same material in saturated conditions. Special attention was focused on defining the failure envelope at relatively low confining stress and assessing the influence of the degree of saturation on the failure conditions of the material
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