Titanic Belfast has recently been announced as the second-biggest tourist attraction on the Island of Ireland, attracting over 800,000 visitors in its first year of operation (807,340 between March 31, 2012 and March 31, 2013, just behind the annual figures for Dublin’s Guinness Storehouse). The iconic signature building itself, which has just celebrated its first anniversary, stands almost 40m tall and has an instantly recognisable, aluminium-clad external façade that replicates four hulls. The building is located a stone's throw from where the RMS Titanic was originally launched and its next-door neighbour is the Harland and Wolff drawing office, where the ill-fated ship was designed. Belfast native Mark Cassidy is an associate director in Byrne Looby Partner's Belfast office. He played an important part in the delivery of the project, albeit his contribution is the least visible aspect of the building. Cassidy and his team designed the foundations and substructure that hold the iconic building in place. The company worked with the specialist piling contractor Dawson Wam from tender stage on the foundation package. According to Cassidy, in Belfast, local knowledge means everything. “To be competitive in tenders like this, you have to know the soils, determine what sort of scheme design will be appropriate and be able to come up with an efficient solution,” he said. [login type="readmore"] TIE-BACK ANCHORS The project was particularly interesting for Cassidy as, despite designing almost a dozen deep basements in the city centre, this was the first time that he used tie-back anchors on such a project. “The particular set of circumstances allowed us to use anchors, which is very unusual in Belfast - the site location and land take facilitated this and the anchors provided significant efficiencies for our design,” he explained. "The basement perimeter is over 400m long and anchors were used along three of the four boundaries, as there were no buildings in close proximity. The remaining boundary is adjacent to the historic drawing office and, in order to reduce ground movements, the team used a much stiffer propping system here." [caption id="attachment_5336" align="alignright" width="1024"] The building's neighbour is the Harland and Wolff drawing office[/caption] There were many considerations to take into account on this particular site. Located adjacent to the Belfast Lough, it is tidal, which has big implications for basement construction. There were old key walls which needed to be considered and to add to this, Belfast is full of geotechnical challenges and the city has very variable ground conditions. Much of Belfast is underlain instead by a deposit of soft grey mud, silt and fine sand with numerous sea shells. This was deposited at a time of elevated sea level in the estuary of Belfast Lough and is graphically known as sleech. It is over 15m thick in the docks area of the city and 3-10m in the commercial and inner-city areas. This deposit presents engineering problems because of its very low-bearing strength and large buildings are generally founded on a forest of piles to transfer the weight down to stronger material. Cassidy advises caution when working with sleech. “You must be very careful; you need to understand its strength and also must be aware of flighting where installing augered piles,” he explained. Flighting is where the soft material is pulled into the auger from the adjacent strata and is a big problem for piling in built-up areas, as it can cause undesirable pile settlements. CHALLENGING CONDITIONS The challenges of Belfast ground conditions did not end there - a sizable portion of north Belfast is located on a 10,000-year-old landslip and the bedrock and overlying strata vary significantly. Belfast's sands vary from very wet to very dry and the distinction is important. “The type of basement wall you install is hugely dependant on the groundwater regime - if it’s a wet sand, you’ll need to use a secant wall; if not, a contiguous pile wall is often appropriate and this decision can have big cost implications for contractors,” said Cassidy. "On this site, the sands had a big implication for the construction of the thrust blocks that supported the raking propping system adjacent to the drawing office. The team designed temporary works comprising sheet piles to facilitate installation in the wet sands." [caption id="attachment_5343" align="alignright" width="1024"] The iconic building during construction in August 2011[/caption] The piling works were delivered successfully, meeting their programmed deadline. According to Cassidy, the design team’s main contribution to the delivery was setting out a clear construction sequence for the basement with its complex propping. “We worked closely with the piling contractor and main contractor to fully develop a construction sequence that balanced design requirements with practical construction requirements and produced detailed drawings to clearly communicate this the contractor’s site operatives. "On a project like this, clear and regular communication between the design and construction teams is essential to mitigate risk,” he said. Another factor that helped the programme was the delivery to standard details for the anchors. “We had numerous anchors along the capping beam, so when we designed the capping beam, we provided a standard detail that was the same at all locations and was easy to construct,” he added. A technically challenging project with a demanding deadline is never easy; however, Cassidy said that he and his team members were very pleased to have been part of the Titanic Belfast project. “It’s great to know that you had a part in such a significant historical and cultural development in your home town,” he concluded. Mark Cassidy is an associate director in Byrne Looby Partner's Belfast office and assists part time in the teaching of geotechnics at Queen’s University Belfast. Byrne Looby delivers civil-engineering design services and has particular experience in contractor design and temporary works. The company has offices in Belfast, Cork, Dublin and Galway and it also operates in the UK and across Europe and the Middle East.