Southbank is a new development by Ganellen at Wolli Creek of 1,2,and 3 bedroom apartments next to the Mercure Hotel. The development is in soft marine clays and silts and the groundwater is tidal. Grosvenor are the subcontractor and McDonald sheetpiling are responsible for the excavation retaining wall. David Lees was requested to assist with a new sewer connection which is required to the manhole on the corner of Levy Street and Gertrude Street.
The new sewer consists of a 225mm diameter poly-ethylene pipe encased in a 450mm diameter steel pipe. The encased pipe is then sealed in grout.
The 450mm diameter steel pipe was inserted through a hole cut at the base of the sheet pile retaining wall to provide a fall of 1:40 to intersect the base of the manhole at a distance of 5.2m from the edge of the sheetpiles. Line and level were controlled by stringline and plumb-bob. The steel pipe was inserted in two 3m length which were then welded together with band after the first section had been installed. The pipe was pushed in to within 100mm of the manhole using the 30 tonne excavator.
On completion of the insertion of the 450mm steel pipe, the pipe was cleaned out with a sucker truck to expose timber form work from the construction of the manhole. The timber was removed with a circular saw prior to coring the concrete shell of the manhole with a 250mm diameter diamond corer. The breakthrough occurred with half above and half below the base of the sewer. A channel was then cut through the base slab to allow for the 225mm diameter sewer pipe to enter the manhole and direct the flow into the main channel.
The 225mm diameter sewer pipe was installed in the steel encasement with centralisers to maintain its position in the encasement pipe. The level of the sewer pipe was then checked by survey.
10m long 300mm diameter timber piles were then installed in front of the sewer connection to provide a stable base for the sewer pipe.
The penetration of the 225 mm diameter sewer pipe was sealed with polyurethane foam where it entered the manhole, with quickset concrete used to seal the wet zone in the invert. On the up side of the sewer pipe, a drainage pipe was installed in quickset concrete in the invert of the encasement pipe. The rest of the annulus between the encasement pipe and the sewer pipe was sealed with polyurethane foam and plaster with a 5m long grout injection hose pushed in at half way and a short section of hose placed at the crown to indicate when the pipe was full. A bentonite/cement grout mix of 1:1 water:cement ratio – i.e. 15 litres of water per 20kg bag of cement. 5kg of bentonite was added per 200 litres of grout. 650 litres of bentonite/cement grout was injected until grout was discharged at the pipe in the crown. 6 No samples were taken to determine the grout strength at 7 and 28 days.