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Technical challenges and solutions in the field of mechanized tunnellingFocus on the Italian admixtures used in a primary EPB-TBM excavation project in Buenos Aires

The “Desarrollo Sustentable de la Cuenca Matanza-Riachuelo - Lote 3” is part of a huge hydraulic project that has been developed in Buenos Aires (Argentina) to treat wastewater and later to discharge it into the “Rio de la Plata” river, increasing the sewer capacity of a metropolitan area where more than 14 million people live. The “Lote 3” consists of three main parts: 50m deep upstream main shafts, 12km long sub-fluvial outfall tunnel and 34 risers. The 12km tunnel has been excavated using a 5.2m diameter EPB-TBM through very complex geotechnical and hydrogeological conditions. The paper presents an overview of the technical solutions adopted for the tunnel excavation and which MAPEI products have used for their implementation.

Photo Gallery: at work under Buenos Aires, Argentina

Technical Aspects of the Project
The design and construction phases of the outfall tunnel have been characterized by several technical challenges and environmental restrictions due to the peculiarities of the project and its geographical position. Geotechnical conditions. The geological region surrounding the tunnel alignment is characterized by sedimentary deposits mainly made by alternation of sand and clay. More in details, the geological formations encountered are: “Puelche” formation - almost mono-granular and very permeable sand; “Postpampeano” formatio -: soft to very soft silt and clay. The outfall tunnel has been excavated mainly through the “Puelche” formation whose characteristics represented a “tricky” scenario both for soil conditioning and back-filling grout injection.

Tunnel face support. The counter pressure at which the TBM had to operate during the construction of the whole outfall tunnel was higher than 4 bars, which is a value close to the limit of EPB technology. To properly satisfy this critical design request, the foaming agent for soil conditioning and its application parameters played a key role for the success of the TBM excavation.

Final lining requirements. The tunnel lining design had a lot of critical aspects: high net internal pressure, peak flow rate higher than 25 m3/s, service life longer than 100 years, etc. Consequently, the segments production and the concrete mix-design study were very important aspects of the final lining quality.

Environmental restrictions. One of the most valuable benefits of the project is to avoid the discharge of sewer drains into the “Riachuelo” river in order to reduce its pollution. In agreement with this objective, the use of chemical products able to minimize the environmental impact of the tunnel construction was preferable.

Products and Solutions
Since the preliminary stages of the project, Mapei UTT division sets up a strong cooperation both with Designer and Contractor for the selection of the most suitable chemical products for the project as well as to provide high level Technical Service needed to support the management of the tunnel construction.

Soil conditioning. The foaming agent Polyfoamer ECO/100 PLUS has been selected for the soil conditioning during the EPB-TBM excavation and it has been successfully used for the whole tunnel drive both through the very difficult sand of the “Puelche” formation and when some lenses of silt/clay soil have been encountered (tab. 1). Furthermore, Polyfoamer ECO/100 PLUS represents an innovation among the soil conditioning products thanks to its extremely low toxicity (fig. 3) and high biodegradability. Using the Polyfoamer ECO/100 PLUS and varying its conditioning parameters, it was possible to keep high TBM advancing performance through all geological conditions encountered. Thanks to the extremely low environmental impact of Polyfoamer ECO/100 PLUS, the excavated soil has been reused as filling material within an industrial district promoting the develop of vegetation and “green” area.

Backfilling grout. The TBM used for the excavation of the outfall tunnel was equipped to inject a traditional one-component grout to fill the annular void between segments and ground. As it is well known by technical literature and practical experience, this backfilling technology is very sensitive to raw material availability and quality as well as to the mix-design study. The continuous monitoring of ingredients’ characteristics (cement, sand, fly ash, etc.) and the consequent adjustment of Mapequick CBS System 1 dosage was part of the jobsite weekly activity. The Contractor’s Quality Department Technicians and Mapei UTT international team worked together to fully meet the project’ requests and to guarantee the proper injection of the grout in the TBM.

Segments production with Mapei admixture. The same technical cooperation has been implemented for the mix-design study of the concrete used for the segments production. The super-plasticizer admixtures Dynamon NRG 1014 and Dynamon NRG 1022 and the silica-fume based product Mapeplast SF have been selected thanks to their technical performances with the local raw material.

The lot 3 of the project “Desarrollo Sustentable de la Cuenca Matanza-Riachuelo” was one of the most critical tunnel projects in the world because of the technical challenges of its design and location. A profitable cooperation between the Contractor’s staff and the Mapei UTT international team allowed to profitably manage the various aspects of the projects, from the segments production to the soil conditioning and backfilling grout for the TBM excavation. Finally, the unique environmental features of Polyfoamer ECO/100 PLUS gave an additional value to the project in accordance with an innovative and conscious approach for the selection of soil conditioning products, minimizing the conditioned soil impact and promoting its reuse as by-product.

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