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

First Record of Cheiloneurus clαviger (Thomson) (Hymenoptera: Encyrtidae) on Corfu Island

Αναφέρεται για πρώτη φορά στην Κέρκυρα η παρουσία του Cheiloneurus claviger (Thomson) (Hymenoptera: Encytridae) υπερπαράσιτου του Saissetia oleae (Oliv.) (Homoptera: Coccidae). Ακμαία του C. claviger εξήλθαν την περίοδο Ιουνίου-Ιουλίου από θηλυκά του S. oleae που είχαν παρασιτιστεί από το Metaphycus lounsburyi (Hymenoptera: Encytridae). Προς το παρόν, το υπερπαράσιτο αυτό δεν φαίνεται να αποτελεί σημαντικό περιοριστικό παράγοντα για τον πληθυσμό του Μ. lounsburyi που είναι ένα από τα κυριώτερα παράσιτα του S. oleae. Δίνονταιμορφολογικά στοιχεία για την αναγνώριση του και βιβλιογραφικά δεδομένα για την υπερπαρασιτική του δράση σε άλλα κοκκοειδή.During an ongoing research programme on Saissetia oleae (Oliv.) (Homoptera: Coccidae) and its parasites on the island of Corfu, the presence of the hyperparasite Cheiloneurus clαviger (Thomson) (Hymenoptera: Encyrtidae) was recorded, for the first time, in samples of parasitized Saissetia oleae in 1991. The identification of C. clagiver was made by the junior author. The adults of Cheiloneurus clαviger emerged from S. oleae females parasitized by Metaphycus lounsburyi (How.) (Hymenoptera: Encytridae), after they were placed in plastic capsules. Tables 1 and 2 give the number of adults of S. oleae, C. claviger and M. lounsburyi that emerged from S. oleae at two experimental fields. M. lounsburyi is one of the main parasitoids of S. oleae acting as an internal parasite of the fourth larval instar. The numbers of C. claviger emerged were very low compared with the numbers of M. lounsburyi, indicating that at present C. claviger is not a serious limiting factor for the population of M. lounsburyi. The presence of a group of coarse bristles at the apex of the scutellum and the long marginal vein of fore wings are the peculiarities of the genus Cheiloneurus. The incomplete infuscation of the fore wings and the colour of the antennae with only the 5th funicular segment blackish can easily allow the discrimination of C. claviger females from those of congeneric species. Cheiloneurus clαviger is reported as a hyperparasitoid of several coccids of economic impor tance mainly via other encyrtid parasitoids. In Italy, C. claviger has been found to act as a hyperparasite of Kermesidi (genus Kermes) which infest Pinoideae. Previous reports on the entomophagous fauna of S. oleae on Corfu do not include C. claviger and it is assumed that the hyperparasitoid has been introduced rather recently. The actual role oil the parasitic complex of S. oleae and the distribution of this hyperparasite on the island should be further investigated

Timelapse ultrasonic tomography for measuring damage localization in geomechanics laboratory tests.

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., micro-cracking 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

A two-rigid block model for sliding gravity retaining walls

This paper presents a new two rigid block model for sliding gravity retaining walls. Some conceptual limitations of a direct application of Newmark's sliding block method to the case of retaining walls are discussed with reference to a simple scheme of two interacting rigid blocks on an inclined plane. In particular, it is shown that both the internal force between the blocks and their absolute acceleration are not constant during sliding, and must be computed by direct consideration of the dynamic equilibrium and kinematic constraints for the whole system. The same concepts are extended to the analysis of the active soil wedge-wall system, leading to an extremely simple procedure to compute the relative displacements of the wall when subjected to base accelerations exceeding the critical value. A comparison with the results of numerical analyses demonstrates that the proposed method is capable of describing fully the kinematics of the soil wedge-wall system under dynamic loading. On the contrary, direct application of Newmark's method may lead to inaccurate predictions of the final displacements, in excess or in defect depending on a coefficient, which emerges from direct consideration of the dynamic equilibrium of the whole system. This coefficient can be viewed as a corrective factor for the horizontal relative acceleration of the wall, related to the mechanical and geometrical properties of the soil-wall system

Soil-structure interaction for the seismic design of the Messina Strait Bridge

This paper illustrates an approach to the study of the seismic soil–structure interaction that was developed at the verification stage of the design of the Messina Strait Bridge in order to validate its seismic behaviour. It consisted of a series of two-dimensional, plane strain numerical analyses on models that included, in addition to the embedded foundation elements, a simplified structural description of the bridge towers: simplified structural models were specifically designed to reproduce the first vibrations modes of the towers, that were deemed to have the most significant influence on the system's dynamic response. Non-linear dynamic analyses were carried out in the time domain, studying the effects of two different natural records, each characterised by three orthogonal components of the soil motion. In the first part of the paper, essential information is provided about the foundations layout, the main properties of the foundation soil resulting from the in situ and laboratory investigation, and the assessment of the liquefaction potential. Then, the numerical models are discussed in some detail, with an emphasis on the modelling of the soil and of the structural elements. For sake of conciseness, details are provided only for one of the two shores. The results obtained with the present approach shed some light on the complex coupling between the soil's and the structure's behaviour, evidencing the significant role that the embedded, massive foundations of the bridge play in the dynamic response of the system. The computed time-histories of the displacements of the foundation elements are used to assess the seismic performance of the bridge

Micromechanisms of inelastic deformation in sandstones: An insight using x-ray micro-tomography

This study investigates the grain-scale mechanisms that lead to failure by strain localisation in specimens of Fontainebleau sandstone with different degrees of cementation. While the effects of inter-particle bonding on the mechanical behaviour of granular geomaterials, including soft rocks, have been largely studied, the physical micro-scale mechanisms governing the material deformation are still poorly understood. In this study, laboratory techniques have been developed to allow a non-invasive investigation of the internal deformation of sandstones during triaxial compression to failure. The material investigated was Fontainebleau sandstone, a quartzite formation from the Paris basin (France), which can be found as very hard, tightly cemented sandstone or more porous and less cemented material. Specimens with porosities of 6 and 21% were investigated. Triaxial compression tests at confining pressures of 2 and 7 MPa were conducted on dry cylindrical specimens of 11 mm diameter and 22 mm height. Three-dimensional (3D) images of the full specimens were obtained by carrying out x-ray micro-tomography scans at key points throughout the test. The high-resolution 3D tomographic images have a voxel size of 8.5 μm (0.033d50), allowing clear identification of the grains. This analysis suggests that dilatancy of the material, which depends on the degree of bonding between grains, plays a fundamental role in the failure mode of granular media. Insights into bonding rupture mechanisms and grain damage by inter- and intra-granular cracking are presented

An experimental study of micro-scale deformation in a soft sandstone

This study investigates the grain scale mechanisms that lead to failure by strain localisation in a slightly cemented sandstone. While the effects of interparticle bonding on the mechanical behaviour of granular geomaterials, including soft rocks, have been largely studied, the physical micro-scale mechanisms governing the material deformation are still poorly understood. In this study, laboratory techniques have been developed to allow a non-invasive investigation of the internal deformation of a soft sandstone during triaxial compression to failure. The material studied is Fontainebleau sandstone, a quartzite formation from the Paris Basin, with a porosity of 21 %. Fontainebleau sandstone is a uniformly graded material with a mean grain size of 260 m. Triaxial compression tests at confining pressures of 2 MPa and 7 MPa were conducted on dry cylindrical specimens of 11 mm diameter by 22 mm height. Three-dimensional images of the full specimen were obtained by carrying out x-ray micro-tomography scans at key points throughout the test. The high-resolution tomographic images have a voxel size of 8.5 m (0.033d50) allowing a clear identification of the grains. A preliminary analysis has suggested that the failure mechanisms can be different for the samples tested at different confining pressures, which has a direct implication on the permeability of the material

Predicting the seismic behaviour of the foundations of the Messina Strait Bridge

This paper presents some of the geotechnical studies carried out for the seismic design of the one-span suspension bridge across the Messina Strait, that is to connect Sicily with mainland Italy. These studies included advanced geotechnical characterisation, through in situ and laboratory tests, estimate of site stability involving both liquefaction analysis and submerged slope stability, evaluation of soil-foundation stiffness for spectral analysis of the superstructure, 3D FE static calculations, evaluation of anchor block performance under seismic conditions, and full dynamic analyses of the soil-structure interaction. The paper summarises the main results obtained from the geotechnical characterisation of the foundation soils, reports the approach adopted for evaluating the seismic performance of the anchor blocks through a modified Newmark-type calculation, and presents the study of the soil-structure interaction carried out through a series of two-dimensional, plane strain numerical analyses. In these analyses, in addition to the embedded foundation elements, the models included a simplified structural description of the bridge towers specifically designed to reproduce their first vibrations modes, that were deemed to have the most significant influence on the system's dynamic response. The illustration is limited to the foundation systems of the bridge located on the Sicily shore

Discussion: Foundation design for gravity retaining walls under earthquake

The discussers read the paper by Pender (2018) with great interest. The author addresses the important issue of computing the pseudostatic critical acceleration of gravity/cantilever retaining walls, corresponding to which a plastic mechanism is activated within the soil–structure system and the wall starts to move under the applied earthquake. In fact, the critical acceleration is the key ingredient for the seismic design of these structures, controlling both the maximum internal forces and the final displacement (Conti and Caputo, 2018; Conti et al., 2013)