This week we were visited by a group of three engineers from Wallace Stone's Glasgow office to discuss the issues surrounding conceptual design selection in a project involving public sector funding, the potential for large scale disruption to affected communities, and designing in the marine environment.
The project we were presented with was the extension of a harbour pier, allowing for the berthing of larger ferries, which had already been ordered from the supplier. Prior to the extension project, the pier was used for berthing of a smaller ferry which ran up to 4 times a day. Throughout the construction phase of the project, it was the desire of the client, an area council, to prioritise the avoidance of disruption to the ferry service. This was for several reasons, the most important of which was that this was a lifeline route thatincluded transport to and from the island communities the ferry route served for not only tourism, but also food supply, and import and export transportation.
There are two methods of berthing for both the existing and newer ferries at the pier. During calm weather the ferry pulls alongside the pier and uses lateral thrusters to manoeuvre itself against the pier before being moored to bollards. During poor weather conditions, defined by winds of over 25 knots (relatively common for the Scottish West coast!), the ferry would rest its port side against a round-head fendering at the end of the pier, with a mooring line securing it, while thrusters would pivot the stern around until the full port side was rested against the length of the pier and moored (as seen below).
Further complicating the design of the new pier extension and critically its construction process, was the positioning of the ice plant (seen in the photos above) which produces ice for the fishing industry in the harbour. It was necessary to ensure that any design should allow for access to the ice plant by delivery and collection trucks during working hours.
With these limitations in mind, our group of predominantly 1st and 2nd year students began by looking at the options for each section of the required structure. It would need to have foundations, an elevating section and a level surface from which harbour staff can work to berth the ferry.
We identified from the geological survey provided that apart from a slightly more stable gravel layer, below the sandy top soil, there was no evidence of a stable bedrock onto which a piled solution could rest. This drove us towards a raft solution, which could essentially float on the normally consolidated soil of the sea bed. To prevent scour we would have installed a rock bund around the foundation, much like those seen around concrete walkways at the seaside, ensuring that the sandy soil under our new foundation wasn't washed away by wave action. In order to create a level base for the foundation to rest on, a dredge of the ground off of the point of the existing pier would need to take place. This could take place at any time between ferry arrivals and departures in the harbour as this would not affect the ferry berthing in any way.
Alternatives were discussed in which a sheet pile wall would be employed to produce a coffer dam into which fill material could be placed to form the level work platform, however with no stable bedrock evident in the borehole log, and the fact that such a structure could not be put in place while maintaining the round-head berthing option makes it infeasible.
Elevating the pier surface is essentially a choice between four options, a solid structure of earth or concrete, concrete columns, steel columns, or wooden columns. In this case, given the marine environment which is going to be extremely harmful to any steel, and the long design life, both the steel and wooden options were removed. Leaving either concrete columns sitting on our raft foundation, or a solid structure, again resting upon the foundation.
I'm going to admit to some prior knowledge of the project at this point! During research I undertook in preparation for a Summer job application, I had read up on this project as it had been nominated for awards in recognition of the design and construction process which managed to avoid disrupting a single ferry service. So, from this point onwards, I am fairly accurately describing the actual solution as I couldn't resist but start directing my group towards the eventually selected solution.
The major sticking point in this project is in the realisation that the client really does not want to disrupt the ferry service, even if it means accepting a more expensive solution. With this in mind, it was clear that a solution had to be found that could be put in place within the time between ferry departures from the harbour. More specifically, what appears to be the major problem is maintaining the function of the roundhead for berthing in the case of windy weather, however in light weather this is not required, therefore as long as light weather can be confidently predicted, the works can take a lot longer than the time between ferry services, as long as they don't extend into the area required for calm weather berthing.
With this in mind the amount of time for installation of the eventual solution had been extended from a few hours to potentially a couple of days. This was still too long for installation of a sheet pile solution, so effort was made by the group to look at how the raft solution and elevation of the work platform could be achieved in shorter amounts off time.
Eventually we came to the idea of building off-site and delivering and installing the solution in short bursts. Dredging could take place from a stabilised barge between sailings. The raft foundation could be cast in a single elongated pour between sailings also, or even overnight. Columns could be put in place, attaching to fixing points cast into the foundation. Similarly the top surface could be put in place as a combination of pre-cast beams and a final in-situ pour of the top surface. The problem with this disjointed approach was, again, that it would likely take too long and the risk of disrupting an arriving ferry was too high.
The idea of cutting the construction time in each stage eventually led to the realisation that if you could make the entire extension off-site, and then deliver and install it on the dredged sea bed in one single movement, with all of the new fendering already installed, what you would essentially have is a ferry leaving one pier, then returning several hours later to a complete, extended pier.
The problem here would be weight and space. There was no way to lift that much mass (the foundations elevating columns and surface material) in one go from a barge, or even from the existing pier head which was likely to be too small for a large enough crane to get access, let alone adequate support. The solution was to float the entire thing in.
I had seen something similar during my Summer work experience with Babcock International Plc. last year. A dry dock gate, which could be sunk into place at the mouth of the dock while water was drained out, or floated up and away. This was called a caisson. Essentially a floating bulk of structural steel in this case, much like a barge but more tailored in its form, with no engines of its own, which could be moved as required en mass. Steel wouldn't work in this case as the permanent placement at the end of the pier would require far too much in the way of ongoing maintenance, but a concrete caisson would float and sink just as well.
This is what was done. A floating concrete box was constructed off-site, with large internal spaces where ballast could later be placed to sink it into place. It was made to be precisely as tall as required. The required services on the top surface could be put in place long before it was at the end of a pier where access would become an issue. And the placement and sinking of it, might only take a few hours, a time frame easily covered by the extended work period granted by choosing to perform the manoeuvre in calm weather.
This was a solution that, even given the increase and enduring allure of pre cast concrete elements in structural buildings, would not normally occur to us as structural engineers. This seems to be because so little of our time at university is given over to the idea of working in a marine environment, where floating things make up the majority of important infrastructure!
This project, when I first saw it, confirmed my growing desire to be working in a more dynamic environment when it comes to design. It's a project that so neatly brings together some of the greatest challenges in civil engineering: working with water, with public sector clients, with critical infrastructure. It is a solution to a problem which instantly fits into the background and fascinates me as the lay person would never think of the complexity of delivering such a simple thing as an extra 35m of pier for a bigger ferry to use.
After reading about this project for the first time, I got in touch with the designers Wallace Stone to apply for a Summer placement, and enjoyed quite a few conversations about what they do. I'm happy to say they accepted my application and this Summer I will be joining their team, something I'm very much looking forward to!