Hydro-Mechanical Behaviour of Tailings in Unsaturated Conditions

Many geotechnical problems are related with soils and structures in unsaturated conditions. Among them, a significant role is played by tailing dams that represent complex geotechnical systems. Because of their wide extension, tailing storage facilities are subjected to many external actions and interactions with the atmosphere, whose effects governing the position of phreatic surface. Hence, the low permeability deposited tailings are subjected to drying-wetting processes, leading variation in the extensions of the unsaturated zone above the phreatic surface with a significant role in stability of the embankment. The current research is aimed to study the hydro-mechanical behaviour and the water retention response of unsaturated silt and silt-sand tailings mixtures, respectively, in order to extend a previous characterization of the same soil performed in saturated conditions. This research is also aimed at investigating the occurrence of the critical state in unsaturated conditions, showing the range of state variables leading the static liquefaction. Finally, irradiation by means of gamma rays is proposed as an accelerated technique in order to simulate the natural ionizing radiations acting on stored tailings that, in many countries, are beginning to be re-used as backfill, landscaping material or feedstock for cement and concrete. Wet and dry silty samples were exposed to gamma rays and then characterized in order to study any possible physical and chemical modifications of the exposed matter. The starting point of this research is an available hydro-mechanical characterization of saturated tailings coming from Stava tailing dams collapsed in 1985 (Italy). The first series of experimental tests was carried out to study the hydraulic behaviour of the Stava tailings by means of water retention tests. To account for the heterogeneity of the tailing material within the basin, the dependency of the water retention relationship on grain size distribution and void ratio was investigated. In order to extend the limited range of matrix suction allowed by the axis translation technique, the water retention curves were also implemented with data obtained by indirect soil suction measurement and soil suction controlling methods, dew point technique and vapour equilibrium technique, respectively. Since the pre-shearing state of the soil has a relevant effect on the behaviour shown during shearing before reaching the critical state conditions, a second campaign of experimental tests, consisting in five mono-dimensional compression tests, was performed. Results were compared with those in saturated conditions obtained by Carrera (2008) in order to investigate the dependency of preconsolidation stress and stiffness of the soil on suction. Aimed to extend the hydraulic characterization and to approach the possible critical state in unsaturated conditions from drying and wetting sides, a third campaign of experimental tests was carried out by using a suction controlled triaxial cell and imposing different hydro-mechanical stress paths. Statically compacted silty samples were subjected at different suction levels, then isotropically consolidated at different net stresses, and finally sheared at constant water content or constant suction level by means of the axis translation technique or the vapour equilibrium technique. Finally, due to the importance of the static liquefaction phenomena in stability problems of tailing dams, a fourth series of experimental tests was performed by means of a triaxial cell. Silt specimens were tested in unsaturated or close to saturated conditions, in order to investigate some of the main factors (degree of saturation, initial density and preparation technique) that are supposed to influence the liquefaction response

Evaluating the capability of a critical state constitutive model to predict the collapse potential of loose sand

Many catastrophic flow failures in granular soil slopes are believed to be caused by a rise in pore water pressure associated with substantial loss of soil shear strength. This failure mechanism is known as prefailure instability or static liquefaction. Constant shear (CS) and consolidated undrained (CU) triaxial tests can reproduce stress paths, in which such instability may occur before reaching the failure. In the present study, a previously proposed critical state constitutive model was first used to simulate the behavior of loose saturated sand in CU tests. It was then employed to predict the instability of loose sand subjected to the CS loading. Under such loading, loose dry sand initially experience small volume increase, and then start to contract substantially. In saturated sand, such contractions can lead to the generation of pore water pressure and sudden decrease of shear strength. The capability of the model to predict the onset of the volume contraction and collapse potential of loose dry sand was examined by comparing the model predictions with experimental results of CS tests. The comparison showed that the effect of initial void ratio, consolidation and deviatoric stresses on behavior of loose dry sand can be well predicted by the model

Cardiac magnetic resonance characterization of atrial pseudo-mass in Erdheim-Chester disease

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Chemical-Physical Characterization of Stava Tailings Subjected to an Innovative Aging Technique

Tailing dams are realized to store the waste products resulting from the mining extraction processes. These complex geotechnical structures should be designed taking into account long-term stability and long-term properties of the deposited materials. Depending on the interactions between source mineralogy and local conditions, tailing wastes can undergo aging processes with chemical and physical modifications. Recently, in many countries tailing wastes are re-used as feedstock for cement and concrete, backfill or landscaping material, so if any, the long-term chemical and physical modifications could affect the hydro-mechanic response of tailings, resulting in relevant environmental and economic consequences. An increased interlocking of particles and oxidation, sometimes making previously safely held contaminants available and mobile, are recognized as common aging processes. Among the long-term aging processes, the natural ionizing radiation due to ultraviolet rays or cosmic rays can be considered. Moving from these reasons, this paper presents an innovative accelerated aging technique to simulate the natural ionizing radiation from the sun. Tailing fluorite ore samples collected form the collapsed Stava dams (Italy) were characterized in dry and wet conditions, before and after the gamma rays treatment. Stava silty tailings showed some physical modifications in terms of specific surface, size particle distribution and inner porosity of the particles, while they revealed a certain chemical stability

"An innovative bio-engineering retaining structure for supporting unstable soil"

The paper presents a new prefabricated bio-engineering structure for the support of unstable soil. This prefabricated structure is made of a steel frame which is completely filled with soil and a face made of tree trunks among which scions or autochthonous bushes are planted. Due to the difficulties in interpreting the complex interaction between soil and structure during the installation and lifetime, an in situ test was carried out in order to evaluate the state of stress in the steel frame and to understand the global behavior of the structure under service loads. On the basis of the obtained results, a procedure for checking the structure safety was proposed and discussed. An easy design method was developed during the research. Moreover, the use of this type of prefabricated structure shows several advantages, such as good performances in terms of stabilizing effects, and easy assembly and transport

Numerical models for the design and construction of new underground structures at CERN (HL-LHC), Point 5

The Large Hadron Collider (LHC) is the latest, most powerful, world’s largest underground particle accelerator realized on the CERN site. High-Luminosity LHC (HL-LHC) is a new project aimed to upgrade the LHC, at Point 1 (ATLAS in Switzerland) and Point 5 (CMS in France) in order to enhance scientific progress. This paper describes the design and construction issues developed at the Point 5 for the new underground structures, located near the existing LHC tunnel. The project requires new technical infrastructure: an additional shaft with a 12 m-diameter and 60 m-height, cavern with 270 m2 cross-section, approximately 500 meters of tunnels connected to the LHC tunnel, vertical linkage cores and additional technical buildings at the surface. The geological ground model of this site lies in an area covered by Quaternary moraine with two independent aquifers. The bedrock of Molasse comprises sub-horizontal lenses of heterogeneous sedimentary rock, that is known to locally retain hydrocarbons and to have a swelling behaviour. In order to investigate the heterogeneous behaviour of the rock mass composed of several layers with different strengths, numerical calculations have been performed, under a 2D plane strain condition with RS2 9.0 FEM-software.. The purpose of using the software was to design both the rock-supports and the concrete inner lining for the tunnels and the shaft. Data from a comprehensive monitoring system with pre-defined threshold values was compared to the 2D FEM results, confirming the importance of the observational method to verify the assumptions used in the numerical modelling. The execution of the underground works started in April 2018. The excavation of the main un-derground works has been successfully completed without any critical impact on the nearby ex-isting underground structures. The completion of the works is scheduled for September 2022

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

Effect of season, late embryonic mortality and progesterone production on pregnancy rates in pluriparous buffaloes (Bubalus bubalis) after artificial insemination with sexed semen

The use of sexed semen technology in buffaloes is nowadays becoming more and more accepted by farmers, to overcome the burden of unwanted male calves with related costs and to more efficiently improve production and genetic gain. The aim of this study was to verify the coupling of some variables on the efficiency of pregnancy outcome after deposition of sexed semen through AI. Pluriparous buffaloes from two different farms (N = 152) were screened, selected, and subjected to Ovsynch protocol for AI using nonsexed and sexed semen from four tested bulls. AI was performed in two distinct periods of the year: September to October and January to February. Neither farms nor bulls had a significant effect on pregnancy rates pooled from the two periods. The process for sexing sperm cells did not affect pregnancy rates at 28 days after AI, for nonsexed and sexed semen, respectively 44/73 (60.2%) and 50/79 (63.2%), P = 0.70, and at 45 days after AI, for nonsexed and sexed semen, respectively 33/73 (45.2%) and 33/79 (49.3%), P = 0.60. Pregnancy rate at 28 days after AI during the transitional period of January to February was higher when compared with September to October, respectively 47/67 (70.1%) versus 47/85 (55.2%), P = 0.06. When the same pregnant animals were checked at Day 45 after AI, the difference disappeared between the two periods, because of a higher embryonic mortality, respectively 32/67 (47.7%) versus 40/85 (47.0%), P = 0.93. Hematic progesterone concentration at Day 10 after AI did not distinguish animals pregnant at Day 28 that would or would not maintain pregnancy until Day 45 (P = 0.21). On the contrary, when blood samples were taken at Day 20 after AI, the difference in progesterone concentration between pregnant animals that would maintain their pregnancy until Day 45 was significant for both pooled (P = 0.00) and nonsexed (P = 0.00) and sexed semen (P = 0.09). A similar trend was reported when blood samples were taken at Day 25, being highly significant for pooled, nonsexed, and sexed semen (P = 0.00). Hematic progesterone concentration between the two periods of the year was highly significant for pregnant animals at 28 days from AI when blood samples were taken at Day 20 after AI for pooled, nonsexed, and sexed semen, respectively P = 0.00, 0.00, and 0.06, and for pregnant animals at Day 45 for pooled, nonsexed, and sexed semen, respectively P = 0.00, 0.00, and 0.01. From these results, it can be stated that hematic progesterone concentration measurement since Day 20 after AI can be predictive of possible pregnancy maintenance until Day 45. Furthermore, the transitional period of January to February, although characterized by a higher pregnancy outcome when compared with September to October, suffers from a higher late embryonic mortality as evidenced by a significant different hematic progesterone concentration between the two periods at Day 20 after AI