Seán Mason addresses the technical aspects of the civil and marine development of the Greystones Harbour PPP scheme, from its initial concept stage through to the inherent technical challenges and, finally, construction completion
Civil

 

Author: Seán Mason, geotechnical & maritime director, Arup Ireland

The fourth-best leisure harbour in Ireland? I am not a sailor, so I am not qualified to judge. However, as a Greystones resident, my clearly biased view is that Greystones Harbour has been transformed from a dilapidated and non-functioning old harbour into a vibrant maritime facility with huge growth potential.

It has been a long time coming, however. In the early days, a small fishing community grew around the rocky headland, taking its name from the local grey rock and beach shingle. Back in Victorian times, rock was quarried to create a small harbour, with the excavated rock used to build a protective breakwater.

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Even from its earliest days, the harbour suffered. The exposed northern end of the breakwater was continually undermined and damaged, with little protection afforded to ships from north-easterly storms. In the late 1800s, a northern breakwater was constructed out from the beach to provide a more protected harbour. Unfortunately, this structure failed as it was being constructed. In addition, the original ‘inner’ harbour completely silted up.

Many attempts have been made over the years to retain the harbour, including the use of a section of the base of the Kish Lighthouse, which was damaged during casting/float-out, to extend the main breakwater.

Greystones Harbour before the works

The impact of the sea on the Greystones community has been even more dramatic on the adjoining coastline. Having been forced by the Earl of Meath to locate the railway line on the seaward side of Bray Head, Isambard Kindgdom Brunel was brought over to design the original complex sequence of bridges, tunnels and coastal protection works.

Progressive coastal erosion over the subsequent century has resulted in the realignment of the railway further inland, with the loss of many coastal structures and construction of a significant length of new tunnel. Erosion of the sea cliffs along the north beach between Greystones Harbour and Bray Head has been estimated at up to half a metre every year – that is 50 metres of movement inland in just 100 years.

In 1930/31, winter storms from the north-east devastated the small Greystones community, with the loss of some 15 houses to the sea along the ‘Beach Road’. All in all, the Greystones coastline has proven historically to be dynamic, aggressive and hostile.

INITIAL CONCEPTS

As the harbour fell into further disrepair, with increasing health and safety concerns and reducing public amenity value, something had to be done. After some initial concepts and public consultations, a proposed harbour/marina site was designated in the 1999 Local Area Plan for Greystones/Delgany.

Wicklow County Council then initiated a development plan to procure a major new harbour/marina and associated waterside development, through a public-private partnership (PPP) mechanism. The PPP model was based on a 30-year concession period, during which the appointed private development consortium builds and operates:

  • a 230-berth marina and public harbour;
  • a 16-acre public park and new focal public square;
  • new facilities for local marine-based clubs;
  • coastal protection works along the beach;
  • 360 residential units; and
  • 6,000m2 of commercial space.

In essence, through its compulsory purchase order (CPO) powers, Wicklow County Council acquired the foreshore area to be reclaimed for development by the concessionaire who, in return for the development space, was to construct and provide all the public amenities. On expiry of the 30-year concession period, all public amenities return to Wicklow County Council ownership.

Design plans (click to enlarge)

In 2006, Sispar (John Sisk & Son and Park Developments) was appointed preferred bidder, with Arup as design consultants. The winning bid design rationalised the harbour entrance and the marina shape to optimise cut/fill quantities and separate new buildings from the open sea. The bid design also included environmentally sustainable solutions for retention and protection of the old municipal dump and coastal protection by beach nourishment measures. Retaining the natural sea cliffs by using beach nourishment to manage rates of coastal erosion was more appropriate than building a ‘hard’ and ‘engineered’ revetment solution along the north beach.

After 18 months of preliminary design, environmental impact assessment and two oral hearings with significant public opposition, the scheme was approved by An Bord Planeala in August 2007. Planning permission included the scheme, land and seabed acquisition (CPO limits), dredging and reclamation licenses.

Numerical modelling of the coastline and proposed marina and harbour layout was required as part of the preliminary design process. Modelling of coastal erosion was used to predict the beach nourishment required to manage erosion rates within ‘do nothing’ rates. Modelling of wave heights within the harbour and marina was required to demonstrate compliance with the strict limits on marina sea conditions. Working with the contractors at bid stage and planning stage also led to innovation in terms of breakwater design and use of high-quality sand and gravel from within the overall site for reclamation.

The inclusion of a concrete batching plant within the overall site for marine construction was conceived at an early stage, addressing many key constraints and facilitating design of a pre-cast concrete block breakwater structure.

DETAILED DESIGN

Numerical modelling of waves

Ground conditions comprise of strong Greywake bedrock, outcropping at the southern end of the site, generally increasing in depth northwards along the site and eastwards out to sea. Stiff dark brown/black glacially deposited boulder clays overlie the bedrock throughout the bulk of the site. Predominantly granular soils overlie the boulder clays, ranging from thin gravel deposits on the sea bed, through ‘mobile’ shingle deposits along the beach, to glacially deposited sands and gravels on landside areas.

As noted, the site contained an old municipal ‘dump’ underlying a future public park. Through application of sustainable principles, proper investigation, analysis and risk assessment, both An Bord Pleanála and the Environmental Protection Agency accepted the solution of keeping the material in situ, as it poses no risk to the environment or end users within the overall design.

Similar sustainability principles were applied to the coastal protection works. Apart from revetment to protect against erosion of old landfill material, the beach nourishment solution allowed preservation of the natural cliffs, the habitats, environment and safe recreational uses for the entire north beach area.

The key design element was the 400m+ of breakwaters. Many designs were considered, including rockfill embankments, sheet-piled cofferdams and large pre-cast caissons. Given the constraints on space, height, ground conditions and environment, a pre-cast concrete block wall and rockfill revetment ‘hybrid’ structure was chosen.

A design was conceived that could for the most part be constructed on ‘dry land’, minimising the use of floating plant and attendant weather dependency risks. The base of the breakwater was designed to be porous, allowing use of the ‘Goda’ design method. The pre-cast block stacks had to be free-standing to allow movement due to ground settlement to occur, before connecting the stacks together and casting the top slab with in-situ concrete.

Settlement calculations were made and trials undertaken to predict settlement amounts and time period required. Significant temporary works were therefore required to keep this ‘leading-edge’ of block stacks stable until the seaward face could be protected by construction of the revetment and the ‘grout sock’ connections.

Physical 3D scaled modelling of breakwaters

As typical for detailed design of significant maritime structures, scaled physical modelling was conducted (by DHI in Copenhagen). Flume testing of the breakwater section to confirm stability, revetment block (antifer) and rock-armour size and overtopping was undertaken. A 3D scaled model of the entire harbour and marina was also constructed and tested for performance under wave conditions ranging from a one-year return event up to the design 100-year return event.

Wave heights were thus measured to ensure compliance with design requirements. This modelling yielded design developments in breakwater and revetment design, most notably the need for ‘porous’ roundheads at the harbour entrance in order to reduce wave reflection impacts.

CONSTRUCTION

The civil and marine works contract was undertaken by Sisk-Lagan Joint Venture, commencing in March 2008. Early establishment of the concrete batching plant and casting yard within the overall site was a key component of the construction strategy, providing flexibility in off-site materials delivery times, maximising control and use of pre-cast (and in-situ) concrete, ensuring material supply kept ahead of programme.

Construction of the breakwaters was the next significant and time-critical activity. A temporary causeway allowed construction of the northern breakwater in two directions, plus south breakwater construction at the same time. Unfortunately, at the early stage of breakwater construction in September 2008, a storm with Force 9 gale gusting from the north-east hit. Without any permanent connections or protective revetment in place, the block stacks suffered extensive displacement under the storm conditions.

In addition to the delays caused by the storm, the scale of temporary works to protect the stacks and underlying granular formation needed to be substantially increased. The levelling frame used to guide placement and screeding of the granular formation and placement of the block stacks was also significantly enhanced.

Precast block breakwater construction

While significant improvements in productivity were achieved, the use of floating plant for dredging activities and divers for block placement remained under the influence of sea conditions. The use of GPS units on the excavator buckets helped dredging productivity.

The bulk of dredged material was stiff boulder clays, intended for re-use as reclamation fill above high-tide level, but softened somewhat by the dredging and barge transport used. The use of the sand and gravel borrow-pit within the overall site greatly reduced the need to import select granular fill for reclamation below high tide level. It also provided storage space for the dredged soils, allowing some ‘drying out’ of the boulder clays for reclamation re-use.

The reclamation of up to five metres depth was specified to provide suitable support for hard and soft landscaped areas. Future buildings will be founded on piles in reclaimed areas. Some lime stabilisation was used in the boulder clays to achieve target compaction levels. Driven steel tubular piles were used for a temporary load-out quay and for support of the future boardwalk around the marina.

Individual steel tubular piles were required as tension piles within the centre of the porous roundheads to resist uplift under wave loading. These piles were also used to help support the substantial temporary steelwork required for porous roundhead construction.

A system of monitoring and managing erosion along the north beach has been in place as required since before start of construction and will continue over the 30-year concession period and beyond. The movement of material under the actions of the sea is unpredictable and astounding. Monitoring indicates movement of large volumes of material up and down the beach over a period of a few months, with changes in beach levels of over one metre and complete shingle removal for a period over part of the beach. Overall, the rate of beach erosion to date has been within prescribed limits, with beach nourishment and redistribution provided as required.

PRESENT AND FUTURE

The completed project

The civil and marine works contract value was in the order of €50 million, within 2% of tender price. The contract was completed in February 2011 and involved:

  • 125,000m3 of concrete (more than Dundrum Shopping Centre);
  • 5,100 breakwater blocks;
  • 3,900 antifers;
  • 90,000 tonnes of 3-6 tonne rock armour;
  • 115,000m3 of dredging/excavation; and
  • 203,000m3 of reclamation

Since completion of construction, the new harbour has been in use and this year, a marina facility was opened with an interim capacity for 100 vessels. The various marine clubs are all using their allocated space and are active in the new harbour. The new public slipway and beach areas are being extensively used, as is the new public space provided as part of the development.

A new coastguard building and boatyard are due for 2014, with landside buildings, including new club facilities and the public park to follow. While a long time has passed since a new harbour and marina was first mooted, the maritime facilities now provided are a very welcome addition to the excellent public amenities available in Greystones.

Even in the ongoing challenging economic times, the extensive use of the facilities and related activities confirm the value of the facility and is a welcome boost to the local economy. Further development of the landside elements of the overall Greystones Harbour and Marina scheme is eagerly awaited.

Seán Mason presented this paper at Engineers Ireland in May 2013 and at the National Yacht Club, Dún Laoghaire in August. Most gratefully acknowledged is the collaboration with Wicklow County Council, developers Sispar and main contractors Sisk-Lagan JV. The involvement of many others is also acknowledged, including employers representative Jacobs, subcontractors, design team, subconsultants, Arup staff, Greystones public representatives and the people of Greystones. Historical photos from excellent books by Derek Paine are also acknowledged.

http://www.engineersjournal.ie/wp-content/uploads/2013/10/Greystones-1.pnghttp://www.engineersjournal.ie/wp-content/uploads/2013/10/Greystones-1-300x209.pngDavid O'RiordanCivilconstruction,local authorities,marine,public private partnership,Wicklow
  Author: Seán Mason, geotechnical & maritime director, Arup Ireland The fourth-best leisure harbour in Ireland? I am not a sailor, so I am not qualified to judge. However, as a Greystones resident, my clearly biased view is that Greystones Harbour has been transformed from a dilapidated and non-functioning old harbour into...