The latest issue of Dam Engineering (Volume XXXI, Issue 2) has been published:

An Exposed Geomembrane System at Paradela CFRD

A Camelo, A Gomes, L Caldeira, G Vaschetti & A Scuero

Abstract: Paradela Dam, owned by EDP (Energias de Portugal SA) is a 110m high concrete face rockfill dam at El. 742m, completed in 1958 and used for hydropower. The upstream reinforced concrete slabs, placed at a 1V:1.3H slope, are 15m wide and 10m long below El. 709m, and 15m wide and 15m long above El. 709m. The joints between the slabs are waterproofed with an embedded copper waterstop covered by a bituminous product. The grout and drain curtain extending into the rock foundation was made from the gallery in the toe wall. Since first impounding the dam has experienced leakage due to large displacements whose main effect was the failure of several waterstops and cracking of the concrete slabs. Several repairs have been made over the years, up until 1980 when the upstream face was lined with an in situ liner consisting of a geotextile impregnated with a water-expansive ‘hydrogel’ glued onto the concrete slabs. In 1987 flow rates started to increase again, and by the middle of the 1990s leakage was up to about 1660l/sec (≈100,000l/min) at normal water level. In 2019 EDP launched a Tender to restore watertightness in the lower part of the dam, from the toe wall to El. 691m, i.e. on less than a third of the dam’s upstream face, with an exposed geomembrane system of the same type successfully performing since 1992 at EDP’s Pracana buttress dam. Carpi was awarded the waterproofing works. The waterproofing liner is a 3mm thick geomembrane formulated with a special compound of polyvinylchloride, plasticised with high molecular weight branched plasticisers, and heat-bonded during fabrication to a 500g/m2 non-woven needle punched polypropylene geotextile. The liner is anchored with a mechanical tensioning system, and sealed at the peripheries with a watertight mechanical seal. A drainage system divided into three separate compartments allows for monitoring of the performance of the geomembrane. This paper discusses the design details and installation, including the milestones set by EDP for completion. The dam is back in service, and with water level at El. 729m, total leakage has been reduced by 98% compared to the previous flow.

Back Analysis of Coupled Seepage and Heat Transfer Through Embankment Dams and Levees by Analytical Solutions and Numerical Simulations

Chiara Cesali, Francesco Federico & Vincenzo Federico

Abstract: Careful and quick detection of seepage variations due to internal erosion processes (i.e. permeability defects) within embankment dams and levees allows us to evaluate their current/effective safety conditions with respect to serviceability and ultimate limit states (e.g. local or global instabilities, structural collapses, etc). In addition to the conventional piezometric monitoring of the seepage, temperature sensors have recently been placed and employed to detect possible piping erosion mechanisms in earthen structures and their foundation soils due to their high accuracy and reliability, as well as their spatial distribution and resolution. The meaning of measured temperature distribution is not, however, immediately linkable to the effects of internal erosion (i.e. permeability defects), due to the non-linear coupling of seepage flow and heat transfer mechanisms through granular materials and foundation soils. Thus, their mutual dependence must be evaluated and interpreted by means of advanced analyses. To this purpose, analytical (closed form) solutions and numerical simulations (finite element analyses) describing the coupled transient seepage and heat transport processes through soils or granular materials are firstly proposed and discussed. Theoretical (analytical and numerical) results have then been compared with results obtained through small-scale laboratory tests. Finally, the effects of permeability defects due to internal erosion processes on the temperature distribution have also been numerically investigated.

Critical Success Factors of a Dam Erection or Rehabilitation Contract

Bettina Geisseler

Abstract: Dam erection or rehabilitation projects are complex and experience shows that (future) Owners/ Operators of a dam (“Employers” in the contractual relationship) and their Contractors (the enterprise executing the construction and erection works) are often faced with considerable delays respectively project hindrances or cost overruns/ additional costs. Thought-out contractual concepts and project-adapted contractual provisions might help to minimise project-related risks.

The key issues of a dam erection or rehabilitation contract between an Owner/ Employer and his Contractor:

Essential is a clear-cut description of the Scope of Works including the guaranteed technical parameters; this is the core clause in the contract and the benchmark for the evaluation of the quantity/ quality of the executed works at the moment of the (Provisional) Acceptance. Potential claims of the Contractor for additional money and an extension of time (“EOT” claims) asserting “Extra Work” ordered by the Employer will be examined in reference to the definition of the Scope of Works. The wording has to avoid any ambiguities.

Like in all large-scale projects there are several stakeholders, whose obligations depend and whose interests have to be aligned to each other. The overall coordination will be the task of the Employer. Well determined responsibilities regarding the management of interfaces, might it be among several contractors in a multi-contracting set-up or – which is often forgotten – between the Employer and the Contractor, are important.

Balanced provisions regarding the risk allocation in case of unforeseeable events and risks, which suddenly materialise, are essential. The contractual parties (Employer on the one hand, Contractor on the other hand) are well advised to carefully evaluate potential risks such as the subsoil conditions being important for the stability of a dam. The decision about the question, who assumes the risk, if e.g. the underground conditions prove to be different than previously assumed with the consequence of time delays and additional costs, can be crucial for the party affected and the whole project progress. There is a whole range of contractual possibilities from overburden the risk to one party, e.g. the party which better might know or control the situation, up to totally or partially (above a certain threshold) sharing the risk (cf. Alliance Agreements).

And last, but not least: mutual obligations for a regular/ ad hoc and accurate, transparent reporting combined with an adapted Quality Assurance Program are crucial to enable the Employer to adequately monitor the whole project.

Application of Fault Tree Analysis in Understanding the Mechanism of Internal Erosion in Dams

Natalia P Souza, Teresa C Fusaro, Olivia C Marques, Terezinha Espósito & Daniel E Mendes

Abstract: This paper presents an application of the Fault Tree Analysis – FTA – as a support methodology in the process of understanding the relationship between the factors that can affect internal erosion processes. Its development is part of the ongoing Research and Development Program CEMIG-ANEEL GT0597 (PAEplay) and was used for modelling possible dam failure scenarios to be considered in the construction of a game developed for training emergency action plans among hydropower dams’ employees. Once internal erosion is a failure mode not assessed by numerical methods usually applied to dam stability analysis, understanding the mechanisms involved in this failure process is essential to allow it to be adequately addressed. In this context, Failure Trees were found to be extremely helpful to identify, in successive stages, possible causes that could lead to dam failure by internal erosion through the embankment, through the foundation or from the embankment into or at the foundation. At this point, the objective is not to conduct probabilistic risk analysis, nor to quantify risks, but to simulate the behaviour of dam structures and allow the construction of random emergency scenarios. The methodology proved to be an adequate tool for understanding the different initiation mechanisms of internal erosion (concentrated leak, backward erosion, contact erosion and suffusion), and for identifying events that could influence the continuation, progression and development of the final failure mode. Therefore, the FTA proved to be useful in the assessment of internal erosion of dams, as well as to assist decision-making and risk management, and has shown to be a potentially useful tool in the design and management of dam safety.

Numerical Assessment of Shear Strength of Cohesive and Rough Rock-Concrete Interface Based on a Limit Analysis Approach

Eleni Stavropoulou, Frédéric Dufour, Matthieu Briffaut, Romain Tajetti & Grégory Coubard

Abstract: Due to the re-evaluation of loadings (increase of hydrology), as well as the evolution of design criteria (increase of safety factors), the reassessment of the stability of existing concrete or masonry dams may be a difficult issue. In this context the characterisation of the mechanical properties of the dam-foundation interface (it governing the failure mode of gravity dams) is a main concern given their intrinsic hidden margins. In the actual design calculation, shear strength properties are often undervalued in a conservative approach. One of the initial conservative assumptions concerning this shear strength is to smoothen the real geometry of the contact (assumed horizontal or slightly inclined) which can, however, be very irregular in reality (roughness or indentations), and to use macroscopic mechanical parameters. Recently, EDF has launched two theses that deal with the estimation of shear strength. These experimental studies have contributed to highlighting the strong influence of the contact roughness on the shear strength parameters. In order to precisely estimate the influence of the contact geometry, a numerical tool is currently being developed. The overall objective is to evaluate the global shear strength of a dam-foundation interface knowing the contact geometry and the mechanical properties of the different materials involved (concrete-to-rock contact, concrete and rock). The main steps of the original approach in this contribution is to obtain the 3D geometry of the contact surface, and discretise it into 2D profiles along the shear direction. All possible failure paths are geometrically selected, passing along the contact and/or in the bulk of materials. Based on a limit analysis approach at the roughness scale, the strength of each possible segment is estimated according to its orientation, the material properties and the macroscopic load. The Dijkstra algorithm (GPS like) is then used to select the weakest path among all, which provides the global strength of the interface by summing up the strength along each 2D profile. The first results are very encouraging, and the final tool will be validated by means of experimental data collected in previous research programmes at different scales.

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