André Pacheco de Assis

Design aspects of the underground structures of the Serra da Mesa Hydroelectric Power Plant

The Serra da Mesa Hydroelectric Power Plant, located in the Tocantins river, 210 km north of Brasilia, Brazil, is under construction and power generation will start in 1998. The installed power capacity is 1,200 MW. This project includes one of the largest underground power plants in Brazil, totaling 550,000 m 3 of underground excavations for the hydraulic circuit. The underground hydraulic circuit, composed by 3 intake tunnels, 3 pressure tunnels, a powerhouse cavern, 3 suction tunnels, a surge chamber, an tailrace tunnel and 8 shafts, was excavated in a very high quality granite. Laboratory and in-situ tests, including hydraulic fracturing, small flat jack and triaxial tests, were carried out to determine the in-situ stress tensor and the geomechanical properties of the rock. Test results were summarized to yield parameters for a 3D numerical simulation of the structures, using the Boundary Element Method. Results in terms of strength factors, using the Mohr-Coulomb and Drucker-Prager criteria, are presented and discussed. Copyright

Modelling the Shear Behaviour of Clean Rock Discontinuities Using Artificial Neural Networks

Since the mechanical behaviour of rock masses is influenced by the shear behaviour of their discontinuities, analytical models are being developed to describe the shear behaviour of rock discontinuities. The aim of this paper is to present a model to predict the shear behaviour of clean rock discontinuities developed by using artificial neural networks (ANN), as an alternative to the existing analytical models which sometimes require certain parameters obtained from large-scale laboratory tests which are not always available. Results from direct shear tests on different boundary conditions and types of discontinuities have been used to develop this ANN model, whose input parameters contain the boundary normal stiffness, the initial normal stress, the joint roughness coefficient, the compressive strength of the intact rock, the basic friction angle and the horizontal displacement of a joint. This proposed ANN model fits the experimental data better than some existing analytical models, and it can satisfactorily describe how governing parameters influence the shear behaviour of clean rock discontinuities. This paper also presents a practical application where the proposed ANN model is used to analyse the stability of a rock slope.

Analytical Slope Stability Analysis Based on Statistical Characterization of Soil Primary Properties

The role of statistical tools in the modeling of slope stability has been increasingly studied in the last decades. Mainly, this growth can be related to the availability of fast computational routines which enable massive-repetition numerical algorithms such as Monte Carlo simulations. On the other hand, analytical approaches to this problem, in the majority of cases, rely on considering the random variables of interest being normally distributed. This latter assumption tends to provide incorrect results while dealing with skewed data since normal distribution is symmetric about its mean value. In order to address this issue, a complete statistical study of a set of porosities data is undertaken. A total of seven well-known statistical distributions are adjusted to such data, showing that normal distribution is the worst possible fit for the considered data set. By means of an empirical correlation between strength properties and porosity, the probability density function of the factor of safety of a hypothetical slope is analytically derived based on a Mohr-Coulomb failure criterion. This way, it is shown that the probability of failure can be explicitly obtained by means of solely the distribution of a primary property of the material in study, namely, its porosity. Also, the results obtained by considering the porosity data normally distributed are compared to the ones hereby developed showing considerable differences.

Experimental and Physical Modeling of Bed Load Heterogeneous Sediment Transport

AbstractThe hydraulic deposition process linked to tailings dams normally presents some technical deficiencies mainly because of the lack of control of hydraulic parameters. Problems of instability in tailings dams usually arise from poor management practices, such as poor decant pond control, in addition to the material being placed randomly. Hydraulic parameters, such as the flow rate and concentration of the slurry, play an important role in the formation of hydraulic fills. Grain size, distribution, segregation, and density, and slope inclination are the main characteristics of tailings dams, which are affected by the hydraulic parameters of the slurry. To investigate these factors, an experimental apparatus [hydraulic deposition simulation test (HDST)] was used to simulate the hydraulic deposition process in the laboratory, where all variables that could affect the phenomenon could be controlled carefully. The HDSTs using different flow rates and concentrations were carried out using the iron waste f...

Influence of the Canopy Filling on Shallow Tunnels

The construction of shallow tunnels using a pre-support system based on pipe canopies may require important decisions during the planning stage. Once the decision of using pipe canopies has been made, the engineer has to choose between steel cylinders or steel tubes for the canopy. If the later ones are chosen, it is necessary to decide whether to keep the soil inside, remove the soil or remove the soil and fill the tube with concrete. In order to clarify the benefits and harms that guide this interactive solution, a study about the influence of the filling material in the bending behavior of the canopy was conducted. The results were obtained for steel behaving as an elastic-perfectly plastic material and concrete being represented by the parabola-rectangle diagram, however the methodology may be applied for different stress-strain curves. A case study, based on the site conditions proposed in Domingues et al. (Society for Mining, Metallurgy and Exploration, The World Tunnel Congress 2016, San Francisco, California, United States, 2016) exemplify the usefulness of the methodology proposed. As a result, it was shown that it may be favorable to keep the soil inside the beam instead of replacing it by concrete or use a massive structural element. In other words, as any other structural system, there is an optimal solution that leads to an economic and safe construction. Hence, this paper provides important tools for decision-making as it expands the possibilities of using the pre-supported pipe canopy technology in tunnel construction.