Keeping water out

Elsewhere in this issue, the consequences of having too much water on track or within embankments are discussed. However, water is also a problem for structures engineers. Water getting into bridge decks, or leaking down into tunnels, can cause major difficulties. Waterproofing therefore becomes an important consideration.

With over 30 years experience in the rail industry, Stirling Lloyd’s high performance waterproofing systems continue to play an important role in the protection of challenging rail projects in the UK and around the world. As some of its most recent projects confirm, tunnel and bridge deck waterproofing remains a critical consideration on any rail engineering programme.

Buenos Aires Metro

The growing popularity of sprayed waterproofing membranes for use in tunnels has again been highlighted on the Buenos Aires Metro, the mass transit system that serves the Argentinean capital.

The Buenos Aires metro, known locally as ‘el subte’, is an extensive underground system consisting of six lines. One of them, Line B has recently been extended to include additional stations and at the end of the line is a cavernous, four track wide parking garage for the metro trains. Excavated using the New Austrian Tunnelling Method (NATM), this huge space is 18 metres in diameter and 11 metres high and its tunnel lining comprises a cast in-situ invert section and an arch and crown upper half lined with sprayed concrete for both the primary and secondary linings. The tunnel sits 10-15 metres below the water table.

A second new structure, the workshop, which utilised a similar construction method, is approximately 12 metres wide and 6.4 metres high and lies around 20 metres below the water Blackfriars Bridge - View 2 IMG_0189 [online]table. With both spaces therefore significantly below the water table and the ground conditions in Buenos Aires very permeable, the requirement from the client, Sbase, was for a higher level of watertightness than had been achieved in previous projects.

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. To facilitate the use of sprayed concrete as the secondary lining, Integritank HF seamless sprayed waterproofing was specified as this forms a strong bond to both the primary and secondary linings. Successfully spraying on to a sheet system is very difficult and so they are not commonly used where sprayed concrete secondary linings are used.

In common with all NATM constructed tunnels, the waterproofing membrane would not be accessible after the secondary lining was installed. 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 a system was installed that contained 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.

The Integritank HF system has been specifically designed for incorporation between the primary and secondary concrete linings in NATM tunnels, as well as SEM (Sequential Excavation Method) and SCL (Spray Concrete Lined) types. To meet the challenge of the rapid build programme for both Metro caverns, the waterproofing was scheduled to be installed in phases. Traditional sheet waterproofing usually requires installation in one continuous process which can be restrictive to other works, particularly in a linear tunnel application as they wait for sections to cure.

Here, however, application could be discontinuous with the untreated areas between the coated areas being sprayed at a time more convenient to other trades.

In total 20,000m² of Integritank HF was successfully installed in the two caverns of Buenos Aires Metro Line B. Both the contractor’s requirement for rapid installation to meet a short programme and the client’s for a high degree of water tightness were met.

Blackfriars Bridge

Back in the UK, Stirling Lloyd’s bridge deck waterproofing systems have been used by VolkerLaser on the second phase of a £3 million refurbishment contract now nearing completion at London’s Blackfriars Bridge. The 125 year old landmark wrought iron bridge was refurbished as part of the Thameslink programme to give greater rail capacity through the centre of London.

The works, carried out in two phases, consisted of applying some 10,000m² of Eliminator spray waterproofing to the largely steel, with concrete elements, substrate and 1,000m² of Hytec sheet waterproofing to the end abutments.

VolkerLaser overlayed 4,000m² of the waterproofing with 900 tonnes of Laserphalt high modulus mastic asphalt, with the remaining area being covered in ballast. The company also installed 50 metres of bespoke expansion joints and applied 25,000 metres of Stirling Lloyd’s Metaset Flex Sealant to joint gaps in the deck plate.

Due to adverse weather conditions, VolkerLaser designed and erected bespoke tenting to cover the works and operated around the clock to ensure that each stage of the programme was completed on schedule and achieved the client’s key performance indicators. The scheme also provided another excellent rail bridge showcase for Stirling Lloyds’s waterproofing capability and, as Andrew Welsh, VolkerLaser’s associate director said: “Blackfriars Bridge is one of London’s most congested sites where rigid timescales and health, safety and Photo 3 - Spraying Membrane [online]environmental standards are of paramount importance.”

Londonderry to Coleraine rail track

In Northern Ireland, Londonderry’s status as the UK’s 2013 City of Culture has led to improvements in transport infrastructure to accommodate increased visitor numbers. This has included major engineering works on the main line between Londonderry and Coleraine with the track closed until April this year.

The work is the first phase of a £75 million upgrade to the track and represents a significant investment by Northern Ireland Railways. It includes a full relay of the track system between Coleraine and Castlerock and between Eglinton and Londonderry and it is on a number of bridges close to the Coleraine section that Stirling Lloyd’s approved bridge deck waterproofing systems have been used.

Four bridge structures have been treated in total. Two road-over-rail structures have seen the installation of Stirling Lloyd’s spray applied ‘tightly-bonded’ Eliminator system, with captive blasting of the concrete substrate used to prepare the deck ahead of the application of a coat of PAR1 Primer, membrane and Tack Coat No.2, the latter being used to enhance the strong bond between the membrane and the subsequent 120mm of surfacing. Two separate rail bridges have been waterproofed using Stirling Lloyd’s ‘loose-laid’ Hytec system, a flexible polypropylene sheet membrane offering exceptional mechanical properties. Hytec, designed especially for fast track rail possession where deck and climate conditions are uncertain, was the ideal choice for these Coleraine rail bridges.

These three examples show the variation of waterproofing challenges faced by structures engineers, and the lengths that they have to go to to keep water where it belongs – not on the railway.

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