THE NEW SUMNER Surf Life Saving Club building was under the triple threats of sea erosion, liquefaction and lateral movement, and engineering consultancy Aurecon was tasked to come up with an innovative design for the foundations. It came up with a combination of a raft type foundation and a reinforced gravel mattress with sand filled geotextile containers to protect the foundations from erosion.
The site was particularly challenging as it was located on Crown land, within a Council managed reserve and was susceptible to coastal hazards and a harsh coastal environment. All these factors had to be accounted for in the design process.
Robert Kamuhangire, senior geotechnical engineer for Aurecon, says this is the first time that a ‘sleeping defence’ system utilising geotextile containers has been developed at design stage to form part of a composite shallow foundation.
“The best way to describe the containers is they are specialist geotextile sandbags that are relatively damage resistant on the exterior but have sufficient filtration on the inside that water can easily get in and out during a storm event without dislodging the sand inside the bag,” he says.
The bags are filled with locally available beach sand avoiding expensive imported fill, but are strong enough as a unit to resist damage from wave borne objects. They act together to prevent damaging effect of strong waves, storm surges, erosion or undercutting by wave action.
They incorporate a special ‘flexible self-healing toe’, a feature that reduced required excavation below erosion level and provides protection during extreme erosion events.
Unlike traditional sea defence structures that tend to be expensive and intrusive to the eye, sand bags can easily be employed as ‘sleeping defences’. They are not visible as they can be covered with natural dunes or dune plants.
“In terms of coastal erosion protection, we explored all the traditional solutions such as gabions, rockwalls, piled walls and found the sand filled geotextile containers to be more cost effective, easily adaptable in the future, and provided more flexibility to landscape designers.
“This was particularly important for Sumner SLSC since the final location of the Christchurch Coastal Pathway was not yet confirmed and we needed a system that could easily be adaptable to different landscape requirements.”
Aurecon explored a variety of solutions for this geotechnically challenging site and this combination was preferred because of its effectiveness and cost.
“In terms of building foundations, we assessed the option of deep piles to minimise the amount of fill required on site but considered this solution would be much more expensive to design and build due to varying depth to rock and liquefiable sand layers,” says Robert.
“Further still, and based on our experience around Christchurch, piled buildings are usually difficult to repair following a damaging earthquake compared with buildings on shallow foundations that are easily re-levellable if they have been designed for that purpose.
“The 800mm minimum thick gravel raft will span over areas where soil could liquefy during an earthquake, redistribute building loads and moderate differential settlement, and reduce building damage if lateral spreading occurs.”
The thickness of the gravel raft was also dictated by the finished floor levels that had to be higher than design flood levels.
The Sumner SLSC is a steel and timber frame building with precast panels in places. The main building occupies an approximate footprint of 550 square metre with an additional 200 square metre timber deck towards the beach side. It is single storey except the lifeguard tower that has two levels.
The project is due for completion in September 2015 before the busy summer season.