The Timbertop Outfall Sewer Project involves the installation of approximately 3.5 km of new sewer main for the Timbertop Estate housing development, located in Officer, Melbourne. Due to a number of constraints including rail, major roads, and overhead power, it was necessary to bore approximately 1.5 km of sewer along the alignment.
The project proponent is South East Water, who awarded the contract for the project to Delplant in May 2012. Trenchless Civil was contracted by Delplant to complete all boring works for DN300-550 mm sewer on the project. Trenchless Civil is utilising the Vermeer AXIS laser-guided boring unit to install the 16 associated bores on the project. Pipe diameters for the project range from DN300-900 mm, with a HOBAS GRP pipe preferred by the client South East Water.
Project considerations
The Timbertop Outfall Sewer alignment had a number of constraints preventing open trench installation, including the presence of overhead power lines alongside a major highway. Wetlands are also present along the highway, which would have required significant diversion for an open cut operation. A trenchless alternative also enabled the extensive tree line to be preserved along large lengths of the alignment. Lastly, the alignment crossed and ran parallel to a Melbourne train line, which couldn’t be disturbed by the project.
The sewer depth along the trenchless section of the alignment varied from 5-7 m. Given the highly saturated ground conditions throughout, open trenching would have provided its own issues. Geological conditions were variable, including highly saturated sands, clays and hard mudstone.
Trenchless Civil chose to complete the bores required with the Vermeer AXIS system. A relatively new technology, the system offered benefits including:
- The vacuum system was considered to provide the fastest laser-guided solution, as production rates achieved with this unit far outweighed slurry or auger technology
- The AXIS system was seen to be competent in a wide variety of ground conditions, from hard rock to saturated clays
- Vermeer provided strong technical and mechanical support to Trenchless Civil.
Technology overview
The installation of small diameter gravity sewers precisely on-grade without employing open cut excavation is not new to the wastewater industry. Proven methods have existed for some time, allowing precise trenchless pipe installation to be completed in conditions where digging a trench would be difficult.
In the past, two factors prevented Trenchless Technology from replacing the age-old method of open-cut installation. The first was the productivity of on-grade trenchless installations, which historically fell short of a rate which would make trenchless the preferred method on any gravity sewer project. The second factor was related to ground conditions. If the below-ground characteristics did not allow for the displacement of surrounding soil, then the trenchless methods available were ineffective, inaccurate, extremely expensive or required the boring of a tunnel much larger than necessary for the smaller pipe to be installed.
This Vermeer AXIS guided boring system is a technology which was introduced to address many of the challenges that face boring contractors today. It is a pit-launched trenchless installation method designed to achieve pinpoint, on-grade accuracy, while eliminating some of the difficult steps associated with other installation techniques. The system is comprised of several modular components: the vacuum power unit with a liquid ring vacuum pump, chiller system, 7,500 litre vacuum tank, rack power unit, drill rack, drill head and casings.
The process begins with the digging of a launch pit with a laser set to the specified line and grade required for the pipe installation. The drill rack is then lowered into the pit, and set in position relative to the laser beam. Next, the drill head is set in the rack, and drilling commences, boring a pilot hole to 343 mm in diameter. As the drill head advances, additional drill casings are added to extend the length of the bore. A rotating cutter bit cores the hole, while the vacuum system removes the cuttings through the centre of the drill casings.
At all times, the laser beam is monitored by a camera viewing the target inside the drill head, and minor steering corrections are made to maintain the head on course. Upon reaching the exit pit, the drill head is removed and replaced with a puller head, and the product pipe is pulled into the hole.
The system was created to combine accuracy with productivity. It was also designed to offer an alternative to some of the limitations of current trenchless installation methods, as well as open-cut.
Benefits of the Vermeer AXIS
One of the primary benefits is the flexibility to install a wide range of product pipes. Though the system can also be used to push pipe, the ability to pull pipe does not restrict this technology to only vitrified clay pipe (VCP),
reinforced concrete pipe (RCP) or steel. In fact, the preferred method of installation with the system is to pull in the pipe, allowing the use of high density polyethylene (HDPE), polyvinyl chloride (PVC), or ductile iron pipe (DIP). As would be expected, a fused or restrained joint is always the best option when pulling.
Requiring only ground resistance to steer, the cutting technology of the system allows a wide range of ground conditions to be addressed, from soft soils to hard rock. If conditions change from soft to hard ground during a bore, the drill head can be backed out of the hole. Cutters can be changed from clay cutters to rock cutters or vice versa to better suit the ground conditions.
The components are modular, allowing for multiple truck configurations and job site set-ups. The drill rack, which is the ground engaging portion of the machine, can be set-up in a pit as small as 3.6 m
long by 2.4 m wide. This limits the disruption to only one lane of traffic if an entire road cannot be closed. In addition to the favorable pit dimensions, the system does not require a structural or engineered shaft. In other words, no thrust blocks or expensive shaft construction methods are required, since the machine does not generate massive thrust tonnages to force the pipe through the ground. The unique vacuum boring technology simply cores the hole and removes all the spoil as soon as it is cut in the hole. This also results in a clean pit that can then be used for manhole construction if called for in the project plan.
In most ground conditions, if the maximum outside diameter (OD) of the product pipe is smaller than the 343 mm pilot hole, a pullback head replaces the drill head at the exit pit, and the pipe installation is performed while the drill casing is removed, section by section, from the launch pit. If the OD is larger than the pilot hole, a cutter and back plate are attached to the pullback head, and the hole is cut to the desired dimension. Similar to the pilot bore, all cuttings are removed through the vacuum tube, so the hole remains clean and with a relatively low friction coefficient. Though fluids are used, they are also removed immediately by the vacuum, which lowers the probability of ground reaction.
Though designed primarily for the average 90 m manhole-to-manhole distance, bores of up to 150 m have been successfully completed. And all these features are available in a machine that is capable of achieving pinpoint accuracy due to the laser guidance. The system can also be used on large diameter, shallow bores without the risk for ground heave and inadvertent returns. This is because the bore hole is cored, with little or no ground compaction, since all the cut material and fluids are removed from the bore hole. Also, bores that may not require strict grade, but are classified as difficult access for other trenchless methods, are candidates for this technology.
Timbertop project site
Tunneling conditions have ranged from saturated sands to highly saturated clay and hard mudstone rock at the Timbertop project site. The project requirements were as follows:
- A total of 16 microtunnelled bores with individual bore lengths ranging from 50-130 m
- Shallow grades to 1:500 along the alignment requiring exacting tolerances
- Installation of approximately 1,500 m of HOBAS sewer pipe of size DN300-550 mm.
The Timbertop Outfall Sewer Project required a total of 16 bores varying in length and geology. The maximum length bore for the project is 130 m, from man hole (MH) 22 to MH 23. The pipe type being installed was a DN550 HOBAS GRP (SN 40,000) with a clay back reamer cut size of 575 mm. The grade for this bore is a shallow 1:500.
Geology for MH 32 to MH 34 indicated saturated clay and saturated sand. Moving further along the length of the pipeline, from MH 34 to MH 39 was highly saturated clay, while from MH 39 to MH 40 pockets of hard mudstone began to appear. MH 40 to MH 41 was hard mudstone for the full 125 m bore.
Delplant contracted Trenchless Civil to undertake works for a road crossing, MH 37 to MH 37-1. The road crossing is 53 m in length and required a cut size of 575 mm at a grade of 1:300. It consisted of a 550 mm diameter steel sleeve pipe which encased a 300 PVC product pipe. The steel sleeve sections are two metres in length and welded together at the joints with 100 mm stitch welds. Five HD-100 centralisers are used in one spacer around the perimeter of the PVC where there are two spacers per 3 m length of PVC. Once the centralisers have been attached, the PVC is jacked into the steel sleeve casing and grouted in position.
Trenchless Civil had numerous challenges to overcome while boring and installing the product pipe at Timbertop. The project required some bore lengths to be more than 120 m with the maximum length being 130 m. In doing so, Trenchless Civil had to contend with very minimal grades (1:500) and non-supporting ground including saturated sand and high saturated clay. Results to date have shown great success, achieving precision alignment and level control.
The installation of non-restrained joint pipe in non-supporting ground conditions proved a unique challenge for the project. An additional pipe jacking frame was fabricated specifically for the Timbertop Outfall Sewer, enabling the non-restrained joint HOBAS pipes to be pushed from the retrieval shaft as the drill casings were removed from the launch shaft. This was essential to maintain the integrity of the pipe string, given the possibility of ground collapse throughout the alignment.
The pipe jacking process involves a HOBAS pipe being placed onto the jacking frame and jacked or pushed into the tunnel bore immediately behind the pullback head. This enables full ground support throughout the length of the bore. With the aid of the power pack and hydraulic extensions running to the jacking frame, the HOBAS pipe can be easily jacked at the required speed and pressure that is needed to ensure there is no separation between the pullback cutter head and the HOBAS pipe. A steel can was fabricated ranging in sizes suited to the bore that allowed approximately 500 mm tolerance before separation. Close communication between launch and retrieval shafts was essential during this process.
Conclusion
Challenging ground conditions and jobsite circumstances required an innovative approach to the execution of the Timbertop Outfall Sewer Project in Officer. Trenchless Civil counted on years of experience and new microtunnelling technology to overcome those challenges, and has achieved success throughout the course of the project.
This is a prime example of how a technology can be adapted to varying ground conditions or pipe specifications when other construction methods or alternatives cannot offer the desired outcome or solution.