Utilities are making increasing use of trenchless technologies to install or upgrade their infrastructure, taking advantage of the opportunity to minimise disruptions such as closed roads, resulting in big savings in time and money for the local community. Wide Bay Water Corporation (WBWC) has extensive experience with directional drilling, but recently undertook their first microtunnelling project.
The project required a new 450mm gravity trunk sewer to be constructed along a busy section of roadway through the Pialba business district of Hervey Bay. Traffic on the road was as high as 7,000 vehicles per day. The route was also very close to a parallel 300 AC water main, 150mm below a 4 x 1200 RCP drainage culvert and for a short distance, 5.5 metres below the road surface level. There were also numerous other service crossings to be negotiated.
According to WBWC, “the extensive disruption to local business that would have occurred by open trenching the work, even as night work, justified consideration of using trenchless technologies. Needing to accurately maintain the design 1 in 714 grade of the sewer over reach lengths of 140 metres then became the defining requirement for the use of microtunnelling over other trenchless technologies.”
The project presented some significant challenges, not least the need to design around the many constraints of an older area of the business district. At one point of the sewer, in natural swale behind the beach dune, the sewer invert is above the existing ground level. This area of the work had to be filled and stormwater drainage provided above and below the sewer transecting a large stormwater chamber. The stormwater, earthworks and one of the drill launch sites were in close proximity to heritage listed fig trees and minimising impact on these trees was a major consideration. Maintaining day time traffic flow around the retrieval pit in the parking lane of a busy street required sheeting of the excavation with steel plates. A significant amount of night work was also required to minimise the impact on local business.
The contract for the works was awarded to Bothar Boring & Tunnelling, who started setting up site at the beginning of August, using a Herrenknecht AVN 400. Bothar was chosen through a public tender process and WBWC explained that the combination of micro tunnelling and civil works required for the project resulted in tender submissions from both civil and drilling contractors. Bothar was chosen after evaluation of both price and their extensive experience in microtunnelling that was the most critical performance requirement for the success of the project.
As with all projects various difficulties were encountered along the way. The loss of drilling mud into piped stormwater drains and from there into open drains was a concern. The loss was not discovered for some hours until it became evident in open drains some 400 metres from the drill site. While the spill was contained and environmental damage avoided, the clean-up was expensive and public perception of the drilling operation tainted.
The soft wet clay, through which much of the drilling operation was conducted, was not conducive to accurately achieving the design grades. A level non-compliance of 45mm occurred over the longest reach of 140 metres. While this was able to be countered in the next reach, as constructed grades varied between 1 in 500 and 1 in 1300 compared to the design 1 in 714. There was also a localised sag approaching 10mm deep in one reach of the completed pipe. Once the pipes are in place there is nothing that can be done to correct these non-compliances. The constructed sewer was capable of performing the design intent and was accepted. WBWC noted that any future micro tunnelling contracts they prepared will have clearly defined construction tolerances specifically applicable to drilled and jacked pipe and financial penalties for measurable non-compliance.
Now complete, the new sewer trunk has solved a number of problems for WBWC. Urban intensification in the catchment of the existing 300mm trunk sewer had started to over load the sewer and downstream pump station. The new 450 sewer short-circuited the pump station and was able to achieve gravity flow direct to the receiving 900mm gravity sewer approximately one kilometre downstream from the pump station. The success of the project avoided the need to upgrade of the existing 300 trunk sewer previously laid through commercial properties within the central business district and the augmentation of the pump station which would otherwise have been necessary to cater for the increased flows.
About Wide Bay Water Corporation
Wide Bay Water Corporation is based in Hervey Bay and Maryborough, about 250 km north of Brisbane.
Wide Bay Water Corporation is wholly owned by the Fraser Coast Regional Council. Their core business is to provide water and wastewater services to the Fraser Coast.
Their staff motto is “Whatever you do, do it well”. More than 200 people, across a broad skills base, work with this ethic in mind in an environment that encourages and rewards excellence.
WBWC has an onsite laboratory, a dedicated environmental services team, a strong commitment to workplace health and safety, an in-house engineering team and an award-winning reuse scheme that includes the irrigation of more than 500 hectares of hardwood tree plantations.