96 publications from this institution
The combination of methane – air can cause potentially explosive mixtures, which in contact with an energy source can ignite, resulting not only in the destruction of infrastructure but also in the death of people. This paper studies the ignition of different methane–air mixtures as a function of the type of mixture (methane–air, methane–nitrogen–oxygen), the kind of atmosphere (homogeneous, stratified) and the environmental conditions (initial temperature, pressure, and humidity). For this purpose, the Rock Dynamics research group of the University of Oviedo has designed and manufactured an explosive chamber, quasi-spherical in shape, instrumented with dynamic pressure sensors and accelerometers. With the results obtained after a laboratory-scale experimental campaign, pressure-concentration graphs were plotted, obtaining the explosion limits of the different mixtures. Higher oxygen concentrations (O2) have therefore been found to increase the upper explosive limit of mixtures. However, increasing methane (CH4) concentrations lead to different behaviours, depending on the type of mixture. The kind of atmosphere does not seem to affect the recorded pressures, as they are similar for homogeneous and stratified atmospheres. Finally, initial pressures above atmospheric lead to higher detonation pressures, while a high initial humidity content decreases the likelihood of explosion.
Uniaxial compressive strength is an essential mechanical parameter to adequately characterize any given material. Numerous standards have been developed to guarantee reliable testing execution, as well as the repeatability of results. In this sense, not only the geometric dimensions and tolerances of both the platen and the specimen have been prescribed, but also the testing parameters, such as the load application speed. However, all these recommendations are based on the assumption that the stresses are uniformly distributed across the contact interface between the platen and the specimen. Nevertheless, this is major elastic simplification that allows for obtaining a handy and useful formula to determine the compressive strength, but this strongly deviates the theoretical foundations from the actual experimental reality. Experimental and numerical research to determine the influence of relative stiffness between the specimen and the platen on the stress distribution generated during the execution of the uniaxial compressive test is performed. The results prove that the stresses are not uniformly distributed across the contact when the platen material is significantly stiffer or softer (less stiff) than that of the tested specimen, and additionally, an undesired triaxial stress field is induced inside the specimen. For these reasons, the use of platens with a similar stiffness to that of the specimen is strongly recommended, as it allows for the uniform distribution of the compressive contact stresses and minimizes the influence of the triaxial stress field.
Instability and high permeability are two of the problems facing tunnelling excavations in soils with high fines content. Among the different techniques used to improve these soils, the injection of cement grouts stands out. In this work, a grouting treatment is designed to ensure the stability of the ground during the construction of two tunnels linking two municipalities in the north of Spain in Biscay, and to reduce the inflow of water from the aquifer located in the vicinity of these tunnels. First of all, the rock mass is analysed and the material to be injected is selected on the basis of the authors’ experience as well as setting time and compressive strength. Subsequently, with a test device designed by the DinRock research group of the University of Oviedo, two types of laboratory tests are carried out in order to analyse the effect of fines migration and washing on the water flows and the effect of re-injections of grouts with different densities on the permeability value. The results show that, in sandy materials, obtaining high degrees of waterproofing together with large stable zones can only be achieved by a combination of treatments and stages with different materials and densities. In addition, maximum values for both injection pressure and flow rate must be established depending on the type of grout and the permeability of the soil. Once the problem has been analysed, the injection treatment is designed and executed. The treatment consists of one pre-injection in four stages with 30 boreholes drilled in the top heading, 19–20 boreholes drilled in the bench, and one post-injection with boreholes drilled around the perimeter of the tunnel in those areas where the pre-injection does not achieve the desired degree of waterproofing.
The Cartagena-La Unión mountain range was the focus of an intense mining activity between early XIX and late XX centuries. Most of Spanish national production of lead and zinc was extracted from its mines. During the ore concentration process, contaminated wastes containing heavy metal minerals, cyanides and sulfates were produced and deposited in earth dams. The Spanish National Institute of Geology and Mining had catalogued 75 earth dams in the councils of Cartagena and La Unión. These deposits pose a potential risk for the environment and nearby populations. Without suitable and precautionary measures, contaminated particles can be transported far away due to the wind action and runoff water, and may be incorporated to the food chain. This risk is increase due to the fact that it is a seismically active area, and breakage of these dams can lead to the dumping of thousands of tons of contaminated wastes. The SOIL TAKE CARE Project is an international project co financed by the European Regional Development Fund (ERDF) through the Interreg Sudoe Cooperation Programme. It aims to improve the management and rehabilitation of contaminated soils in South-Western Europe that includes Spain, Portugal and south of France. The University of Oviedo takes part of that Project by the instrumentation and monitoring of two of those earth dams. Among the work realized so far highlights the perforation of two boreholes and the installation of several sensors. It aims a double objective: to analyze the erosion and infiltration capacity of rainfall into the dams and to detect possible symptoms of slope instability. Although the investigation is still in course, preliminary results shows fast rainfall infiltration into the superficial soil layers, being discharge curves much more extended. This water retention capacity, coupled with the existence of impermeable layers into the dams, could lead to a complete saturation of superficial soil layers and trigger slope instability processes.
Uniaxial compressive strength is a variable necessary for adequately characterizing a material’s mechanical properties. However, a specimen’s geometric deviations and elastic properties may lead to undesirable stress states, which cause strong discrepancies between the results of the uniaxial compression test and its theoretical foundations. While geometric deviations may cause non-uniform contact between the platen and the specimen, elastic properties can provoke severe end effects that disturb the local stress field near the points of contact. To address how the relative stiffness between the platen and the specimen influences the induced stress field, numerical simulations considering the stiffness ratios Ep/Es=3, Ep/Es=1 and Ep/Es=0.05 were performed. Subsequently, these results were employed to establish the relation between relative stiffness and specimen failure patterns in brittle materials, particularly in three different rocks. The results prove that the platen stiffness must be accurately selected to match that of the tested material, in order to avoid undesirable local stress fields near the point of contact and to induce homogeneous uniaxial compression that guarantees reliable uniaxial compressive strength characterization. Furthermore, the brittle failure patterns reported in previous studies were correlated with the induced stress fields inside the specimen depending on its platen stiffness, allowing the validity of the test results to be verified based on a simple visual inspection.
In the last decades, rigorous research has been carried out with the end of understanding the gas dynamic phenomenon and although different preventive techniques have been employed, even today there are numerous accidents even with the loss of life. This work analyses an alternative and innovative method of fracturing and degassing coal, by generating CO2 with a pyrotechnic device called PYROC (Pyrotechnic Break Cartridges). Medium-scale tests of generation of CO2 into coal samples are carried out and their effect is analysed comparing the initial and final permeabilities of the coal samples once the generation of CO2 has finished. These permeabilities are calculated by injecting methane. Besides, the influence of different parameters as the length of the boreholes, the pressure of the gas or the initial permeability of the coal have been analysed with a numerical simulation of one face of one of the sublevels of a mine. The results show that the method increases the safety in mining operations because it fractures and degasses the coal, increases the permeability of the coal in the borehole of injection from 9.5 mD to 31 mD, decreases the methane gas pressure below pre-detonation levels for 1 min, achieves decompressed lengths between 8 and 10 m ahead of the face with pressures of injection of 50 MPa, relaxes the total length of the borehole for initial coal permeability values equal to or greater than 0.002 mD, and allows to work with low permeable coals with high induced stresses and high methane concentrations.
