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Just In > Drains damaged Merriwa-Willow Tree Road

Drains damaged Merriwa-Willow Tree Road

Filed in Just In by April 22, 2021

LAST night, geotechnical engineers confirmed remedial works will not be a viable option during the Merriwa-Willow Tree Road reconstruction, leaving the road unusable for at least three years.

Inadequate drainage structure from the initial project has caused pipes to break apart, allowing water to damage materials and seep into the formation overtime.

Michael Gawn, Douglas Partners Principal Geotechnical engineer told Merriwa residents the road now poses an “unacceptable risk to life,” due to the increasing hazards caused by excessive water build up.

“We’ve had quite significant rainfall event and we’ve had increased movement since the general inspection to the last time we went to site in March,” Mr. Gawn said.

“As time progressed, we’ve had significant instability in the fill material which then regresses up the slope and we’re getting cracking through the middle of the road,” he said.

“This material, even though there is gross displacement at the moment, it has essentially failed . . . there is a 500 to 600 millimetres zone across one area where the road moves under your foot. That means the soil in that slip zone has no strength,” he said.

“If it moves under your foot, consider what it would do under a heavy vehicle,” said Mr. Gawn.

Two options for rehabilitation works have been completed however, GHD geotechnical engineers found multiple issues within the initial designs.

Further geo analysis is currently being undertaken and a third design option will be availably by May 12, 2021, with documents for construction tenders ready by August 2021.

Adam Sneddon, Project Director at GHD said the road will need to be widened, pushed further into the hill and drainage issues will need to be addressed. 

“Because the [design] options required us to identify the water regress into the pavement and also the formation, we’ve also allowed for concrete table drains and dish drains on the high side of the formation,” Mr. Sneddon said.

“We’re going to have to line the table drains because that is one of the major reasons for the failure, as water got into the formation,” he said.

“The previous drains were lined, there was some assessment done in relation to the flows and velocities of the flows of the drains. Whether they were constructed with the right Geofabric underneath to stop silt coming out or whether they were sized to stop stuff going through them is a different idea,” he said.

Travel lanes will also be widened however, 21 out of 34 road bends don’t allow for 80 kilometre travel, therefore regardless of the final design, additional speed advisory signage will be needed for corners.

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