Monica Barbero

Dynamic shear strength of rock joints subjected to impulse loading

Abstract The shear testing of rock joints under impulse loading is studied in the present paper. Following the description of novel shear equipment which allows the shear tests to be carried out under both static and dynamic loading conditions, the results of tests on saw-cut surfaces are given, with attention being paid to the mechanical behaviour of rock discontinuities prior to peak strength in a stage where no irreversible shearing has yet occurred. The objective is to describe the initial dynamic strength of a rock discontinuity versus the shearing stress rate applied during the test. It is shown that: • (i) under low normal stress and no initially mobilized strength, the dynamic shear strength is greater than the corresponding static value and is greater in relation to the higher shearing stress rate applied • (ii) the dynamic to static shear strength ratio has a tendency to decrease as the mobilized shear strength prior to loading increases • (iii) with the normal stress increasing, the dynamic influence on the shear strength reduces.

The failure of the Stava Valley tailings dams (Northern Italy): numerical analysis of the flow dynamics and rheological properties

BackgroundTailings dams are made up of mining residue deposits, and they represent a high risk, in terms of mechanical instability. In the event of collapse, the tailings in such dams may be released and flow over long distances, with the potential risk of extensive damage to property and life. The traditional geotechnical assessment of tailings facilities has mainly concentrated on the stability of tailings dams, while relatively few studies have investigated the flow of tailings released after dam failure. In this context, it is possible to state that, if the complex rheological behaviour of the tailings material is captured correctly during the flow, numerical modelling can be used to contribute to a better comprehension of the flow characteristics and for the assessment of the possible extension of the impact area.ResultsConsidering the wide range of possible rheological behaviour that tailings flows can assume (from laminar to turbulent), this paper presents the new version of a computer model, which was designed to simulate the motion of rapid flow movements across 3D terrain. This new version integrates the existing rheological kernel (Frictional, Voellmy) with two new rheological laws (Bingham and Turbulent), and adds the possibility of changing the rheological properties of the flowing mass during the propagation process. The code has been applied to the disastrous flow that was caused by the failure of a pair of tailings impoundments in the Stava Creek Valley (Italy) in 1985. Since different interpretations on this flow behaviour already exist in literature, and since a large number of changes in the rheological values along the run-out path have been proposed to recreate its dynamics, new simulations, carried out with different rheological combinations, are presented and discussed here in order to obtain a better understanding of the flow dynamics and to identify the rheology that reproduces the phenomenon that occurred with the fewest possible changes in the rheological values along the runout path. The latter aspect is particularly important when numerical analyses are used for prediction purposes.ConclusionsThe great rheological flexibility of the new code has allowed the Voellmy rheology and a combination of its parameters to be identified as the most suitable to describe the Stava flow, even where the run-out path presented critical characteristics.

An Apparatus for In Situ Direct Shear Tests on Snow

The article presents a prototype of a new device for measuring the shear strength of snow specimens in situ. The resistance to sliding of a snow slab on a mountain slope is a key parameter in snow mechanics. The proposed apparatus acts as a sampler, to obtain a nearly undisturbed specimen, and as a shear box, similar to those used in the laboratory, with complete control of the test procedure. The main components of the device are a pneumatic system, for the application of normal and shear forces to the specimen, a real-time controller for commanding and recording of the data, and a computer. The apparatus can be carried to the place of the experiments and operated by a team of two researchers. Calibration and preliminary tests are also described in the article.

Snow Pressure on a Semiflexible Retaining Structure

AbstractSnow avalanche hazard is one of the most critical aspects in the urbanization of mountainous areas. The risk from these natural phenomena can be reduced if proper countermeasures for reducing the danger or preventing avalanche triggering are taken. For instance, in the avalanche initiation zone, devices able to withstand snowpack gliding and prevent crack propagation can be installed. These can be classified as rigid (snow bridges) or flexible (snow net) structures, depending on the mechanisms involved under snow loading. In recent years, a new kind of structure called a snow umbrella, based on a net panel supported by rigid beams, has been designed and installed in the northwestern Italian Alps. As the real behavior of these structures is far from being simple to understand because of the complex interaction with snowpack movements, an umbrella was instrumented and monitored during the winter season of 2010–2011. The recorded data are used for testing two possible pressure distributions and evalu...

STIFFENING EFFECT OF BOLT-ON TRANSDUCERS ON STRAIN MEASUREMENTS

The strain of a structural element is often measured indirectly by determining the strain of a transducer that is supposed to follow the deformation of the element perfectly, and with negligible interaction, around the mounting point. However, this assumption is not always true, and depends on the stiffness of the transducer relative to the local stiffness of the piece to which it is attached. This paper deals with the problem of evaluating the influence that a relatively rigid transducer (the HBM SLB-700A was chosen as an example) has on the strain field, studied from both the theoretical and the experimental points of view. The findings of the research have shown that, in order to interpret the measurements obtained from this kind of instruments correctly, the perturbation induced on the strain field cannot be disregarded. A corrective factor, obtained numerically by means of the FEM and discussed in the paper, can be used as a first approximation to evaluate the true strain, stress and forces in common structural steel elements.

Experimental tests for the assessment of the shear strength of marble waste dumps

The large dumps of waste material called ravaneti from marble quarrying activities in Alpi Apuane basin are distinguished features of the local landscape. They are strongly heterogeneous, never compacted and frequently subject to local instabilities. The access roads to the marble quarries cross the dumps; therefore, it is concern of the local authorities to hinder such phenomena. In this respect it is of paramount importance to evaluate the shear strength of the material comprising the dumps at the proper scale. In this note, the results of in-situ direct shear and tilt tests on samples of metric size are illustrated, together with the interpretation according to a strength criterion relative to rockfills. The results of a series of lab tests on the finer fraction of the waste material are also described and interpreted. Finally, qualitative considerations on the local and global stability of the waste dumps, related to their shear strength, are reported.

Quarry- Induced Slope Instability at a Broadcasting Transmission Plant near Valcava, Lombardia, Italy

Since 2006, a broadcasting tower and ancillary buildings on the top of a hill adjacent to a large, active limestone quarry, in northern Italy near the village of Valcava, has been heavily damaged by ground deformation due to slope instability. In 2005, the quarry began to work the slope toward the broadcasting plant, releasing bedding planes that affected the tower site: a number of fractures occurred in the warehouse and in the ground around it, that grew particularly during significant rainfalls. In October 2006 a landslide occurred in the quarry area, close to the broadcasting site. In order to understand what induced the fracture process in the buildings and define a support system to stabilize the broadcasting site, a detailed geotechnical survey and a number of numerical slope stability analyses were carried out. One of the main goals of the paper is to underline the effectiveness of the numerical analyses to understand the instability phenomena when they are particularly complex. Furthermore, by using a reliable numerical model with a good understanding of site geology and of the main geotechnical parameters, an evaluation of the slope behavior under different scenarios was made and stabilization measures can be designed.

Aging Simulation of the Tailings from Stava Fluorite Extraction by Exposure to Gamma Rays

Abstract Tailings storage facilities are disposal systems for storing the waste products of the mining industry consisting of a slurry mixture made of soil, rock and water that remain after the mineral values have been removed from the patent ore. Tailings dams are supposed to last forever, so after their deposition, tailings can experience aging processes with physical and chemical changes depending on the interactions between local conditions and source mineralogy. The consequences of these aging processes are increased interlocking of particles and oxidation processes, sometimes making previously safely held contaminants available and mobile. Among the long-term aging processes, the natural ionizing radiation (from radioactive isotopes of the soils, cosmic rays, and also ultraviolet rays from the sun) can be considered, as proposed in the current research. Furthermore, in many countries, tailings are beginning to be re-used as backfill, landscaping material or feedstock for cement and concrete. So if any, the long-term physical and chemical modifications could affect the hydraulic and mechanical behaviour of tailings with relevant economic consequences. For these reasons, wet and dry silty samples of tailings spilled out after the failure of the Stava tailings dam (Trentino Alto Adige, Italy) were exposed to gamma rays, as an accelerated aging technique to simulate the natural ionizing radiation, and then characterized. The modifications on physical and chemical properties were observed and, despite certain chemical stability, some physical changes were observed, particularly in terms of size particle distribution, inner porosity of the particles and specific surface.

A quantitative approach for the evaluation of rockfall risk on buildings

A quantitative rockfall risk analysis at local scale is a complex and difficult task because it should consider both the randomness in the natural phenomenon and the variability of the response of the elements at risk. In engineering systems, such difficulties can be tackled with logical trees. In particular, event trees allow the determination of a set of outcomes of a given event in a probabilistic manner. Following recent publications on the estimation of rockfall risk on infrastructures (roads) by means of the event tree approach, a novel framework for the quantitative evaluation of the effects of the impact of a falling rock block on a building is presented. The method considers the occurrence of a given rockfall event, the kinetic energy of the falling block, the structural response of the impacted elements and the possibility of damage propagation within the building. An example is proposed to show the capabilities of the proposed approach.