Buenos Aires Metro, Argentina
The Buenos Aires metro, known locally as “el subte” is an extensive underground metro system consisting of six lines. Line B has been extended to include additional stations with the line now terminating in a parking garage and workshop for the metro trains. The new main (garage) cavern is 18m in diameter at its widest and 11m high. It is of NATM construction, with primary and secondary linings of sprayed concrete. The tunnel is 10 – 15m below the water table in permeable ground. The second new structure, the workshop, which utilised a similar construction method is approximately 12m wide and 6.4m high and lies around 20m below the water table. The requirement from the client, Sbase, was for a higher level of watertightness than had been achieved in previous projects.
- Buenos Aires Metro
The design of both structures incorporated lattice girders and temporary sprayed concrete as the ground support primary lining. Due to a short construction programme requiring a fast build speed, the design was for permanent unreinforced sprayed concrete as the secondary (final) lining from axis to crown and cast in place concrete in the invert. A waterproof membrane was also required as the ground conditions in Buenos Aires are permeable and the structures are completely below the water table. To facilitate the use of sprayed concrete as the secondary lining, a sprayed waterproofing membrane was specified as these form a strong bond to a sprayed secondary lining. Sheet systems have a record of poor bonding to sprayed secondary concrete lining and may have required the use of a cast secondary concrete lining.
In common with all NATM constructed tunnels the waterproofing membrane would not be accessible after the secondary lining was installed, so any future maintenance or remedial works to the membrane, if required, would prove very difficult. In addition, due to the permeable ground and the location of the structures below the water table, the ground was to be de-watered locally. This meant that if the waterproofing membrane did contain any defects, they would not become apparent until after the secondary lining had been installed and the de-watering pumps switched off.
Consequently Benito Roggio, the main contractor required a sprayed waterproofing membrane that could be tested in-situ, after it had been installed but before the secondary lining was applied, to prove that a continuous, defect free waterproofing installation had been achieved.
Designing for Dry
Whilst there is an expectation in tunnelling that tunnels will leak to some degree, Sbase were adamant that the workshop cavern in particular needed to be as dry as possible. The workshop cavern was also the deepest structure on the site and subject to the most water ingress during construction with the ground remaining wet despite dewatering.
The Chosen System
The Integritank HF system was selected for use following not only presentations to Sbase and Benito Roggio but also after trial panels had been sprayed where the membranes waterproofing quality and ability to be tested in-situ were amply demonstrated. The trial also showed the systems speed of application and cure, which would aid contract progression and facilitate the work of other trades.
Once awarded the waterproofing contract Stirling Lloyd worked closely with Benito Roggio to agree the waterproofing details prior to construction and a local contractor was trained in the application and use of Integritank HF by Stirling Lloyd technical staff.
The primary lining of the tunnel was a locally designed sprayed concrete mix containing 20mm aggregates. Consequently, this “as shot” sprayed concrete surface was very rough and not suitable for the application of a spray applied waterproofing system. Therefore a render layer was applied to the concrete primary lining which was trowel finished to close up surface voids and remove any projections.
Though the ground was being de-watered, local conditions meant that damp spots in the primary lining were common at the time of the waterproofing application, requiring the Integritank HF system to cope with these conditions.
The first part of the Integritank HF system was the HF Primer. This not only generated a high bond strength between the concrete and the Integritank HF membrane, in excess of 0.5MPa, as measured by the pull-off tests conducted which are part of the unique Quality Assurance Programme carried out on site by Stirling Lloyd, but also allowed the system to be applied to concrete that was damp.
The primer changes from white to clear as it cures, providing visual confirmation that membrane application could begin.
Once the primer had cured, which took approximately twenty minutes, the Integritank HF was applied in two, 1.5mm coats of highly contrasting colours – the first being yellow and the second white. The yellow colour was used because it contrasted starkly with the concrete and would show up any potential defects well under artificial lighting, enabling the sprayers to ensure that all areas were adequately covered. Once this coat had cured, typically in thirty minutes on this project, the white second coat was applied. Two coats help ensure complete coverage to the required specification. Thickness was measured constantly during application by “wet film dipping” by a Quality Assurance Technician who guided the sprayer, further ensuring the correct thickness was applied. The thickness of any waterproofing membrane is important as it is directly related to the waterproofing capability of the membrane. Therefore controlling thickness to ensure optimum performance is essential to successful sprayed waterproofing applications.
The build programme of both caverns required that the waterproofing was installed in phases to suit the build schedule. Traditional sheet waterproofing usually requires installation in one continuous process which would have been restrictive to other works. By utilising the Integritank HF sprayed liquid system the requirement to apply in phases could be met. This was because not only was the membrane fully bonded to the primary lining so that it could be left securely in place until the secondary lining was ready to be applied but also as the membrane bonds to itself, irrespective of the length of time between applications, application could be discontinuous with the untreated areas between the coated areas being sprayed at a time more convenient to other trades.
In fact waterproofing was carried out in multiple locations across both structures to fit the requirements of the construction sequence, with the different areas of application being joined seamlessly to form a continuous seam-free membrane. The rapid cure of the Integritank HF meant that there was no requirement for extended cure times before the secondary lining could be installed allowing the contact to progress rapidly. In wet ground conditions, such as those experienced on this project, it is preferable to install the secondary lining close behind the waterproofing.
To confirm high performance, effective waterproofing had been achieved the membrane was tested qualitatively after installation by electrical integrity testing that provides reliable results quickly and does not damage the membrane.
The membrane surface was divided up into panels 500mm wide using a marker. The panels were then numbered and tested using a "holiday tester" with a 600mm detecting head. This ensured that the whole surface was tested and if any defects were found they were marked and repaired, with the panels then being marked off as complete.
This essential part of the waterproofing application was carried out in conjunction with the client and the main contractor, with each area being signed off and handed back to the main contractor as waterproof.
In total 20,000m² of Integritank HF was successfully installed in the two caverns and connecting tunnels with both the contractors requirement for rapid installation to meet a short programme and the clients for a high degree of watertightness being met.