GEOTECHNICAL contractor Austress Freyssinet has designed and built a retaining and cut-off wall for a basement excavation as part of the refurbishment of the historic Jones Bay wharf in Sydney Harbour.
The cut-off wall was needed on all four sides of the 4m deep excavation to seal the basement off down to the underlying sandstone bedrock. This allowed groundwater to be pumped out and meant the new basement did not have to be a tanked structure.
Multiplex Constructions was the client for the AUS$1.4M (£517,000) contract.
Ground conditions were complex. A variable thickness of sandstone rubble fill in a matrix of silt, clay, sand and gravel, in direct hydraulic continuity with water in Sydney Harbour, overlay the sandstone.
The fill was known to contain sandstone boulders, old timber piles and possibly old railway lines acting as tiebacks to the sea walls. High tide level is about 1.5m below the top of the wall. To further complicate matters, headroom across the site was just 7m.
Instead of a traditional diaphragm or secant pile wall, Austress proposed a jet-grouted retaining and cut-off wall to overcome the likely obstructions.
Further benefits were a reduced volume of spoil and a high quality seal into the bedrock, including grouting of discontinuities up to 3m into the rock.
Given the 100-year design requirement for the jet grout wall, a project-specific grout design was carried out by Austress's engineer GHD to ensure the grout was sufficiently durable in the aggressive saltwater environment. The final design involving the use of silica fume and PFA/cement blend.
Jet grouting was carried out by a pre-bore drilling rig with a down-the-hole-hammer and a jet grouting rig and pump. Austress worked closely with the main contractor to allow the ground floor deck to be laid across one side of the site for an access road and permanent service ducting.
Austress geotechnical engineer Oliver Batchelor said one of the difficulties was having to work right next to seawall where the line of the cut-off wall was only 3m from the open water.
'The 1.5m tidal fluctuation and the open voided material meant there were concerns about a grout wash out, ' he said. The solution was to pre-grout using low slump grouts prior to the jet grouting. The method was successful, he added, with 'no flowaway of grout into the harbour'.
Jet grouting also meant that the quality of the fill and the rock was almost identical at the interface. During excavation the rock broke at its discontinuities rather than at the interface.
An old seawall running at 90° to the new wall was also strengthened, by jet cleaning out the joints between the blocks and replacing them with grout.