Rubble trouble

A GEOTECHNICAL report has thrown a spanner in the works for a planned housing development adjacent to the Eric Singleton bird sanctuary.

The Galt Geotechnical report, released by Bayswater council this week, says the ground underneath a block owned by the D’Orazio family is riddled with debris from constructions and demolitions; all sitting on four metres of “weak” Swan alluvial soils that would compress and settle if built on, potentially damaging buildings.

Galt undertook test pitting, borehole drilling and soil penetration tests on behalf of the Dale Alcock-controlled ABN group, and says if the developer follows a raft of measures, the site could be brought up to scratch.

• Soft alluvial soils and building rubble have given developers of Skippers Row a headache. Photo by Trilokesh Chanmugam

• Soft alluvial soils and building rubble have given developers of Skippers Row a headache. Photo by Trilokesh Chanmugam

“In its current state, we do not consider that the site [is] suitable for supporting the proposed residential development,” the Galt report says.

The developers would also have to fill the site — mostly a wetland until being filled in the 1980s and 90s — by between 1.5m and 2m to get it above once-in-100-year flood levels.

Galt has recommended a slew of conditions to make the block ready, including layers of limestone, geogrid and gravel across the site, wick drains to draw water out and major filling and compaction.

Large vibrations

“We note that the proposed HIDC compaction technique … has been known to cause large vibrations … to structures in close proximity to the works,” Galt’s report says.

“It is strongly recommended that a dilapidation survey of the nearby structures is carried out prior to the works and that vibration monitoring close to any sensitive receptors be carried out during the works.”

Even after compaction, the site will continue to settle another 3 to 5cm, and Galt says there can be no building within four to six metres of the boundary as a result. The D’Orazios recently offered to increase the buffer between the development and the bird sanctuary by 4.5 metres as a “compromise” to win council and community support.

Independent geotechnical consultant Keith Clements read over the report and says a site this unstable and requiring this much work is “extremely rare”.

It’s currently categorised as “Class P” in the geotech game, and even if Galt’s remediation plan is followed, it’ll be considered “Class M,” two notches below top-grade sites.

That means housing will need extensive stability measures.

In a deputation to council Mr Clements said potential contamination had to be addressed: “This is a health issue for the whole community and has to be dealt with.”  Galt says it is “not clear” if there is any contaminants in the fill.

The report is at http://www.bayswater.wa.gov.au/cproot/7345/2/skippers-row-geotech-report.pdf

by DAVID BELL

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