Reprofiling Dublin's north shoreline

Concrete, Apr 2007 by Cridge, Clive

The specialist Environmental division of concrete manufacturer and supplier RPC provides a range of erosion control systems in coastal protection. A recent project saw the reprofiling of the shoreline embankment at Dublin.

At the Port of Dublin, the northern shoreline - comprising a sloping embankment - had deteriorated due to erosion around the toe, causing the embankment to become steep and unstable. Various attempts had been made to stabilise the slope by dumping old concrete and builders' rubble, creating an unsightly appearance that attracted the deposition of waterborne debris.

The reasons for the project were two-fold: first, to stabilise the embankment and protect it from further erosion by wave action and second, to facilitate environmental and amenity improvements to the commercial premises along the northern shoreline. A further aim was to improve the visual impact from the Dublin residential area of Clontarf along the south shore. In addition to the reprofiling and protection of the shoreline embankment, substantial landscaping and tree planting was undertaken.

Design

Following the collection of all relevant wave, tidal and geotechnical data, a full design assessment was carried out by Dublin Port, assisted by RPC Environmental, with the help of its consultant, Hydraulic Engineer. The design was carried out in accordance with Guidelines for the design and construction of flexible revetments, incorporating geotextiles, in marine environments(1) and Revetment systems against wave attack(2). The boundary conditions for the new protection works were established and agreed, together with the dimensioning of the primary armour layer as well as the granular and geotextile filter underlayers, and the toe and crest details.

The final design included the use of Dycel 151 concrete revetment mattresses for the main revetment along the north shore, with Dycel 221 at the easterly end where the embankment returns through 90° and where the conditions are deemed to be more severe. Granular and geotextile filter underlayers were included to provide adequate drainage beneath the new revetment and to prevent the build-up of pore water pressure.

The design was completed and tender documents prepared over a five-month period with the contract awarded, and work commencing, some two months later.

Construction

Site preparation works began with the clearing of debris from the existing embankment, together with re-profiling works to establish a uniform slope of one in two (26°). At the same time as site works commenced, manufacture of the 11,500m^sup 2^ of Dycel revetment mattresses commenced at RPC's factory. The Dycel mats were assembled using steel cables in various mattress dimensions to suit the specific site requirements. A close working relationship was established between the contractor, Padhraic Moneley, and RPC Environmental to ensure that the mats were manufactured and delivered to suit the installation programme and available tidal windows. The assembled mats were loaded onto articulated lorries at RPC's factory and taken by ferry from the docks at Heysham and Liverpool directly to the Port of Dublin.

Installation of the revetment, together with other site works and landscaping proceeded in an orderly manner, being dictated in part by the limited access along the top of the embankment for the contractors' plant and the mattress delivery lorries. Good liaison and cooperation with the haulage contractor were essential for the smooth running of the contract. The Dycel revetment mattresses were unloaded and placed onto the embankment by crane using special lifting cradles provided by RPC.

Concluding remarks

Installation of the new revetment was completed over an 11-month period, which included a two-month break to allow part of the landscaping works to be completed to comply with local planning requirements.

RPC's Dycel is a fully interlocking precast concrete revetment system designed to provide exceptional protection against embankment erosion above and below water level whilst ensuring the flexibility essential to cope with variations in embankment profile.

In addition to the solid mattress system used at Dublin, Dycel is available as an open system with a full-depth cellular structure that enables the system's stability to be enhanced by filling and seeding voids above water level and by filling any voids below water level with graded aggregate. Seeding with prevailing natural vegetation also enables structures to harmonise perfectly with the local environment.

References:

1. PIANC. Guidelines for the design and construction of flexible revetments, incorporating geotextiles. In marine environments. K W Pilarczyk (1992).

2. WALLINGFORD, H., Revetment systems against wave attack - A design manual. Kirsty McConnell.

CLIVE CRIDGE, RPC ENVIRONMENTAL