In September 2015, Unitywater’s sewer rising main crossing Weyba Creek at the mouth of the Noosa River in Queensland failed. The 30-year-old pipeline, installed around 2 m below the creek bed, separated at the failure location, meaning that repair by relining was not possible.
“The broken main was located in an environmentally sensitive area and close to holiday accommodation,” says Unitywater Sustainable Infrastructure Solutions Executive Manager Simon Taylor.
“The high-peak tourist season was approaching and although we had carried out a temporary solution by re-directing sewage to an alternative network, if we had experienced a high rainfall event it may have overloaded the system.”
The high-peak Christmas holiday and tourist season was approaching – an essential part of Noosa’s economy – so finding a fast and permanent solution was imperative.
Proactive response
After evaluating the challenges of the situation, Unitywater decided that the project required urgent reactive work using innovative thinking to overcome the obstacles and complete the project within the limited time available. The utility mobilised to undertake an emergency horizontal directional drilling (HDD) installation of DN315 high density polyethylene (HDPE) pipe.
Despite being confident in this choice, the project faced a complex situation. Generally, HDD is employed on projects with more time available for project planning and approvals; within the limited time, Unitywater needed to account for a number of situational factors.
The HDD locations needed to avoid impact to all surrounding areas of environmental significance and choices were limited. After assessment, the entry pit and rig were set up at a Noosa Council park on
the north side of Weyba Creek, while the exit drill was set up within a nature refuge on the south side.
At the same time, Unitywater had to run several processes in parallel, including engaging drilling expert Bamser to determine the capacity of drilling companies to start on site within three weeks. It also liaised with Noosa Council to scope out access for vehicles and geotechnical surveys, and presented site assessments and an Environmental Management Plan to the Noosa Council and state government departments for the required approvals.
Work management plans took into consideration:
- The creek’s designation as a fish habitat
- Protected vegetation in the nature refuge
- Protected trees and vegetation in the council park
- Protected animal species
- Pedestrian access requirements
- Noise considerations in proximity to nearby residences
To navigate these requirements, Unitywater confined activity to the 650 m narrow concrete pathway, limited machinery accessing the site to 5 t, completed a pre-construction survey and used fauna spotters, and operated the vacuum excavator on low pressure. Throughout the project, Unitywater engaged stakeholders to keep them informed and address any concerns.
“The creek is a designated fish habitat area, some of the vegetation is protected and the nature refuge is a protected habitat for several species of animal,” says Mr Taylor.
“We were also very conscious of the noise impact on local residents. We managed this by placing sound curtains around machinery and on sections of temporary fencing.”
Unitywater cast a wide net for suitable machinery with rubber tracks to use on the concrete path of the nature refuge. A geotech drill rig was brought in from interstate, and the HDD contractor sourced lightweight machinery and developed construction methodologies to meet the project requirements.
Racing the clock
On 26 October 2015, Unitywater’s contractor, Pipeline Drillers, mobilised in the restricted space available in Mossman Park. Tough ground conditions under the creek made for relatively slow work; however, the pilot hole was completed by 18 November.
To manage the risks, project managers planned a conservative triple reaming process. Two 25,000 L sucker trucks were employed fulltime to transport the mud from one side of the creek to the other.
On 21 November, the first 180 mm pass was completed, followed by a 350 mm pass, with the 500 mm pass completed on 26 November. The final hole-conditioning drum was passed through during the next two days and, on 29 November, the DN315 HDPE product pipe was successfully installed.
The project was completed to specifications and delivered in record time, meeting the Christmas holiday deadline and achieving recognition as the ASTT Project of the Year – New Installation Award in 2017.
“This was an innovative project and we are thrilled to be recognised for the work we carried out in such a highly sensitive environment,” says Mr Taylor.
This article was featured in the December edition of Trenchless Australasia. To view the magazine on your PC, Mac, tablet, or mobile device, click here.
For more information visit the Unitywater website.
If you have a project you would like featured in Trenchless Australasia contact Journalist Chloe Jenkins at cjenkins@gs-press.com.au