Mt Crosby & Wilson Weirs

Project Scope

The project involved remediation works to two of Seqwater’s weirs, namely Mt Crosby and Wilson Weir.

Works at Wilson Weir near Gatton involved the following:

Construction of gabion and reno mattress structures

±5,000m³ of select backfill material to the above structures

±11,000m³ of impervious fill

Construction of concrete capping slabs to the weir abutments

Miscellaneous fencing, signage, landscaping etc.

A particularly challenging aspect of the contract was the construction of the bottom row of gabions below the water level and on the irregular river bed of the Lockyer Creek.

In order to facilitate this a coffer dam structure was constructed  which allowed a binding layer of concrete to be poured to ensure a level surface for the commencement of gabion works. The construction of the gabion structures and reno mattresses to tie in with existing structures and at varying levels also required special attention to detail.

Works at the Mt Crosby Weir involved:

±25,000m³ of cut to spoil

±3,000m³ of filter material

±6,000m³ of transition rockfill

±12,500m³ of rip rap (±900Ømm)

Each of the engineered layers mentioned above required compliance with stringent material quality specifications and in particular, a very narrow grading envelope.

In order to achieve compliance with these grading envelopes the blending of several different materials was required.

It was also vitally important that there was no difference in the grading of the material produced at the quarry to that delivered and tipped on site. Hence regular visits to the quarry to inspect stockpiles and loading operations were necessary.

Due to the steepness of the existing slope on which the engineered layers needed to be constructed, it was necessary to construct the layers horizontally and to repeatedly provide new access and departure ramps to allow for body truck movements. Measures to avoid the contamination of these layers also required attention.

The close proximity of both sites to waterways required the implementation of several erosion and sediment control measures including the use of floating silt curtains and the construction of geofabric lined sediment ponds and channels.

Water quality tests were conducted daily to ensure compliance with environmental requirements. Working close to water and on steep and unstable slopes also posed several safety risks, which through their early identification and associated implementation of risk management mechanisms ensured the safety of all employees on site.

Although plagued by an extended 2013 wet season, both contracts were completed on time and to the satisfaction of Seqwater.

Key Staff

Ben Dyson – Construction Manager

Anton Rheeder – Project Manager

Bryce Wilson – Project Manager

Brian Moore – Site Supervisor

Rob Forrest – Site Supervisor

Challenges

High water levels on Lockyer Creek during the construction period necessitated the construction of coffer dams for gabion installation on both banks

Steep existing slopes required the use of innovative construction techniques to ensure the safety of construction personnel

Strict material grading envelopes required the blending of material in order to achieve compliance. Regular inspection of quarry processes was needed to ensure consistency in materials

Extensive erosion and sediment control measures were required to avoid the contamination of Lockyer Creek and the Brisbane River

• Project:

  Mt Crosby & Wilson Weirs

• Location:

  Gatton & Mt Crosby, Qld

• Client:

  Seqwater

• Practical Completion:

  September 2013

• Contract Period:

  32 Weeks

• Contract Value:

  $6.4 Million