Laura Caldeira

FMEA of a tailings dam

The concepts, principles, assumptions and fundamental rules of Failure Modes and Effects Analysis (FMEA) are introduced. An application to Cerro do Lobo tailings dam is presented, with the description of the system considered, the functionalities, the potential failure modes of each component, their corresponding root causes and the sequence of effects. Finally, the available measures in place for the detection and control of the sequence of effects are also identified. Although FMEA application in complex dam systems may constitute a time-consuming process, its outcome can be extremely useful as illustrated in this article. It makes it possible to assess and manage the major risks of dams so that mitigation actions, taken at an early stage, can be optimised from an efficiency standpoint.

Laboratory test for evaluating limitation of flows during internal erosion in zoned dams

A new laboratory test, named the flow limitation erosion test (FLET), has been designed and developed to study upstream flow limitation during the phase of progression of internal erosion in the embankment of zoned dams. Upstream flow limitation may occur because of the presence of the shell or a transition zone upstream of a cracked core. In the FLET, the soil specimen is composed of core and upstream shell materials. These are compacted inside a test apparatus made up of several pieces assembled in steps. Then the soil specimen is subjected to water flow through a predrilled hole in the core material to simulate a concentrated leak, initiated, for example, by large differential settlement or hydraulic fracture. In most tests, the hole is also drilled in the upstream material to simulate the scenario in which the mechanism causing the flaw in the core is also likely to affect the upstream zone. Combinations of the same core material and several types of coarse-grained upstream materials (broadly graded and gap-graded soils) are tested for a range of compaction conditions and hydraulic loads. It is shown that FLET is capable of assessing whether there is upstream flow limitation and whether the internal erosion process stops, shows a trend of slowing down, or progresses. Upstream flow limitation is shown to be dependent on the fines and gravel content, fines plasticity, and compaction water content of the upstream material. In the particular case of the tested gap-graded soils, it is shown to be also dependent on the initial gradient along the upstream material.

Laboratory Test for Evaluating Crack Filling During Internal Erosion in Zoned Dams

A new laboratory test, the crack-filling erosion test (CFET), was developed to study crack filling during the progression of internal erosion in the embankment of zoned dams. Crack filling involves the transport of eroded material from an upstream zone, through a flaw in the core, which is then retained by a downstream granular layer. In the CFET, the specimen comprises a core, an upstream shell material, and a downstream filter layer. These are compacted inside a test apparatus made up of several pieces. The specimen is subjected to water flow through a predrilled hole in the core to simulate a concentrated leak. Seven granular upstream materials, two core soils, and two granular filters are examined. Following an extensive testing program, experimental observations and physical descriptions are presented. Three main types of pattern behaviors are identified: rapid crack filling with almost “no erosion” of the core; filtering after “some erosion” or “excessive erosion” of the core and/or upstream material; and “continuing erosion” of the core and upstream material. When the core has moderate-to-high resistance to erosion, crack filling is mainly governed by grading properties of the upstream zone and of the filter. Crack filling is more likely to occur the finer the filter layer, the higher the fine-sand content of the upstream soil, and the lower the fines content of the upstream soil. Test results are checked against the subjective guidelines on crack-filling action available in the literature.

Hydromechanical Characterization of Cement-Bentonite Slurries in the Context of Cutoff Wall Applications

AbstractIn the one-phase construction of cutoff walls, the slurry plays two contradictory roles: it should be fluid while excavating the trench but behave like a solid of low permeability afterward. For hydraulic containment applications, a mixture of water, cement, and bentonite fulfils these requirements, but the range of mix proportions is very limited. In the past, this discouraged systematic research on the subject, and, in consequence, a certain empiricism still prevails in the definition of the slurry composition. This paper presents the results of an extensive experimental program, which is part of a broader investigation regarding the technology of self-hardening slurry walls. Referring to triaxial tests, the stress-strain behavior and permeability of cement-bentonite slurries fabricated and cured in the laboratory is characterized. A conceptual microstructure model is proposed to explain the slurry’s behavior in light of the chemical reactions between its constituents. The final objective is to ...

Limit state design approach for the safety evaluation of the foundations of concrete gravity dams

The application of the limit state design (LSD) in the geotechnical area has increased over the last two decades, but this approach is not yet widely used in dam safety evaluation. This study aims to widen the use of the LSD application for large dams, in particular concrete gravity dam foundations. This paper starts with a brief reference to the LSD approach in recently published guidelines for dam design, followed by a detailed description of the LSD formulation when applied to the foundation of concrete dams. The relevance of the joint application of the concepts of ultimate limit states and of numerical methods is highlighted. Comments are made regarding the criteria adopted in order to determine the characteristic values of the material mechanical properties, with an emphasis on discontinuities, taking into account the spatial variability. The sliding safety assessment of the foundation of a concrete gravity dam using the LSD and a discrete element model, both in persistent and in an accidental design situation, is presented. Results led to the conclusion that the LSD methodology may be followed for dam foundation design with the partial factor values prescribed in Eurocode 7.

A new qualitative method for the condition assessment of earth and rockfill dams

This article presents a new qualitative method, based in efficient indexes, for the condition assessment of groups of dams in operation. After a brief overview of the method, the proposed indexes, their descriptors and aggregation rules are presented, as well as the steps for its application and general guidelines for the classification and sorting of the examined dams, aiming to achieve optimal scheduling of more elaborate condition assessment and/or rehabilitation works. The method developed was applied to 36 Portuguese dams in operation. The results obtained agree with the available information about the dams under investigation, their condition and the expectation of future performance.

Engineering Geological Studies for the New Drainage Tunnels of Lisbon

The municipality of Lisbon is currently planning the construction of two drainage tunnels in order to control the periodic flooding that occurs in the city during the winter. The first tunnel is 5 km long with DN5500 diameter, crossing the downtown in a NW-SE direction, and probably will be constructed by TBM. The second tunnel is located in the north part of Lisbon. This tunnel is 1 km long and is planned to be constructed by TBM or by NATM. The first tunnel crosses volcanic rocks and calcareous rocks from the Cretaceous and then detrital and calcareous rocks from Miocene. The second tunnel only intersects Miocene detrital and calcareous rocks constituted mostly by sand, silty sand, clay and biocalcarenite. For engineering geological characterization of alignments of tunnels, a site investigation program was planned and carried out that included boreholes, in situ (SPT, self boring pressumeters tests, packer tests, pumping tests and crosshole test) and laboratory tests (index, oedometer and triaxial compression tests). An integrated analysis of the data obtained from the site investigation works was performed in order to define the engineering geological conditions along the alignments of the tunnels.

Factors Limiting the Progression of Internal Erosion in Zoned Dams: Flow Limiting by an Upstream Material

AbstractIn zoned dams, some types of materials, when located upstream of a damaged core, may impose high hydraulic losses capable of limiting the flows converging toward the flaw in the core. In this paper, the factors influencing flow limiting by an upstream material are addressed experimentally, using the flow limitation erosion test (FLET). The results of eighteen FLETs, on five coarse, broadly graded soils as upstream material, are presented. The laboratory testing shows that the flow-limiting action is strongly dependent on the grading and the compaction characteristics of the upstream soil. The critical parameters are the fines and the gravel content, the nature of the fines, and the compaction water content. Practical rules for the preliminary estimation of the likelihood of flow limiting by an upstream coarse material being effective in stopping the progression of erosion in zoned dams are suggested, based on the results of the experimental study.

Processes Involved in the Formation and Performance of Self-Hardening Slurry Walls: Santa Clara-a-Velha Monastery Cutoff Wall

AbstractSeveral different phenomena are involved in the formation and performance of self-hardening slurry walls, including slurry contamination by soil, penetration of slurry into the ground, confining with the trench, filtration, sedimentation, consolidation, and chemical reactions between the slurry constituents. To evaluate the relative influence of these phenomena, a case study is analyzed—the cutoff wall of Santa Clara-a-Velha Monastery, built in 2001–2002. The performance of this wall is related to the in situ phenomena through data obtained during construction and from a site investigation conducted in 2011. An extensive laboratorial program was performed on samples cored from the wall and on laboratory-made samples. It was found that consolidation was the major contributor to the significant settlement of the wall that occurred after trench excavation, whereas filtration and sedimentation made only a secondary contribution. Contamination by sand only interfered with the physical properties and co...

Mechanical Characterization of Lightweight Expanded Clay Aggregate Materials for Modeling Their Geotechnical Behavior

AbstractBased on an extensive testing program, this study presents the mechanical behavior of a lightweight expanded clay aggregate (LWA). This material is currently being used in many civil engineering works and particularly in geotechnical applications due to its low weight, high shear strength, and favorable drainage characteristics. The present investigation is based on a particulate mechanics approach aimed at the quantification of appropriate model parameters in order to describe the material’s mechanical performance. The first part of the study concentrates on the determination of the physical and mechanical properties of the particles themselves. Relevant hydro-mechanical aspects of the particulate material are investigated. The second part focuses on the mechanical behavior of the aggregates. Using the compaction characteristics and stress ranges prescribed by the product manufacturer, a series of laboratory tests was performed on materials with two different grain-size distributions and water co...