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The retention system originally proposed in
the contract documents was replaced by a McMahon Services retention
system which saved the client $500,000.
The alternative system proposed using two MPa
sprayed stabilised sand in a batter and a large vertical block to
retain the existing basement walls. Two MPa sprayed stabilised sand
had never been used before so McMahon Services, with the supplier
and subcontractor, trialled the method until a suitable mix and
design was established that met the job requirements.
Large pads were excavated in the basement and
poured so that the new columns for the next structure could be erected
up through the basement floors before the stabilised sand was sprayed.
The large pad footing incorporated some of the old footings which
had large 250mm holes cored through the concrete up to 2m long to
enable the new pads to be constructed.
The works were undertaken in a basement 7m below
ground level with no ramp access to it. The area was extremely tight
for small excavators and plant to operate in, as well as the requirement
to maintain the atmosphere at a safe level so work could continue
without breathing apparatus.
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