Consultant identifies cause of collapse

A Roads and Traffic Authority geotechnical consultant has said he believes the cause of the Woy Woy Rd collapse was the result of ground materials not being removed and replaced and poor compaction during construction.

He stated that the failure itself was triggered by stormwater discharge or surface water infiltration into the cracks of the embankment.

"It is understood that many longitudinal cracks were presented in the road along the low side which had been covered by the pavement resurfacing in March," consultant Mr Bo Xiao said.

"The results from boreholes numbers 1 and 2 indicate that the subsurface at the low side of the road generally consists of a thickness of about 0.6m to 0.8m road pavement and base layer overlying very loose/soft to loose/firm materials down to about four metres depth.

"Bedrock/sandstone is encountered about 4.5m below the road level.

"Based on the available geotechnical information, we consider that the major cause of failure is likely due to poor ground condition (unsuitable materials were not removed and replaced, and poor compaction during construction) at the low side of the road embankment."

The details were provided to Gosford Council officer Mr Terence Gibbs in a memo on May 12.

Mr Xiao said the results from the preliminary assessment of remedial measures for the road embankment failure should be used for concept design only since the consultant had not yet carried out detailed analyses, and a design and quality assurance review was required.

Mr Xiao said that, based on available information, several remedial optioned had been assessed.

The options included a gabion wall, a soldier pile wall, a soil nail wall, a block retaining wall, a reinforced soil wall and cemented stabilised backfill.

"The objective of geotechnical concept development [is] to investigate and to provide geotechnical design remedial options and the recommended option for the road embankment," Mr Xiao said.

"Based on the results of assessment, it is considered that the gabion wall option is most suitable option for this project.

"Gabion walls are a popular wall type capable of supporting roads on steep slopes, and most contractors are capable of constructing them.

"However, the disadvantage of gabion wall is that the wall needs to be founded on a sound foundation material or rock and a large volume of material is required to be excavated in order to reach the foundation level and provide sufficient room for the wall."

Mr Xiao also said he had identified two other critical sections of road which needed remedial work to be carried out on.

These two critical sections of road were described as boreholes 3 and 4 in the preliminary assessment.

"Boreholes numbers 3 and 4 drilled at the other locations also encountered poor ground condition down to two to three metres depth," Mr Xiao said.

"Therefore, we recommend that remedial work should also be carried out at these sections where poor ground condition has been identified to prevent future road collapse."

Mr Xiao said several options could be adopted for remedial work at the other critical sections, in particular, sections with cracks on the road surface.

The options being considered by the RTA and Gosford Council included a gabion wall, a geotextile wall, a steel reomesh facing wall, soil nail wall and pile wall.

Mr Xiao said that further geotechnical borehole investigation was "current and on-going between Horsfield Rd and Woy Woy Creek" and would determine a recommended option.

"We would like to carry out detailed analyses and design once a preferred remedial option is selected by Gosford Council and the RTA."

Memo, 12 May 2008 Bo Xiao, RTA