UEA Trenchless recently completed a sewer augmentation project for ACTEW Water which encompassed four separate locations throughout the ACT. UEA was originally contracted to upsize approximately 637 metres of 150mm concrete and vitreous clay pipe to DN250 PE using pipe bursting technology. However, with varied conditions across the four locations, UEA recommended using a number of different trenchless techniques including pipe bursting, horizontal directional drilling (HDD), auger boring and slip lining to ensure the best outcome was achieved.
Location 1: Minimal Ground Cover
With only 900mm of cover between the existing main and the road surface pipe bursting was deemed to be unsuitable for this 115 metre section of work. As the existing main required upsizing it meant that in all likelihood the upsize would occur in a vertical alignment which would most likely lift the road. To ensure this didn’t happen UEA opted to perform the works using Horizontal Directional Drilling (HDD). Rather than expanding the existing pipe into the surrounding ground, as is the case with pipe bursting technology, the HDD reaming process removes the existing pipe by grinding it down and – using drill mud – carries the cuttings to the exit or entry pit to be vacuum removed and disposed of.
The DitchWitch AT3020 directional drilling rig used to upsize this section of the pipe requires a launch/receipt pit approximately 2m L x 1.5m W x 1.5m D to be excavated. However it requires a 6m x 3m footprint above the ground, adequately setback to be able to achieve the desired grade and depth at a certain position. As this site had plenty of room to perform this type of work HDD was the preferred method of replacement.
Location 2: Residential Areas
Unlike the previous example ground conditions in this location were ideally suited to pipe bursting. Adequate cover meant the existing pipe could be ‘cracked’ and expanded into the surrounding ground without causing heave or cracking on the surface. Pipe bursting is ideally suited to residential areas where maintaining alignment or limited accesses are components that need to be considered.
As well as upsizing the existing main from 150mm to DN 250 PE, UEA was also responsible for connections into the manholes and reconnecting each house service. While the pipe bursting process in this suburb was relatively straight forward a number of properties affected by the works were up for sale. Effective communication with residents was required so the works could be completed within the week and asphalt driveways restored with hotmix before the open house on the weekend.
Location 3: Rocky Conditions
The rocky ground conditions in Campbell were both unexpected and challenging – 20m into the 148m bore the pipe bursting head could go no further. Further investigation showed that due to the rocky ground conditions minimal surround had been provided when installing the original sewer pipe, therefore preventing expansion during the cracking process. To overcome this issue UEA used an auger boring system to bore out the existing concrete pipe. Even after this process the bore hole was still very tight so UEA used an air hammer and winch to install the new 250mm PE. HDD could not be used in this circumstance as there was insufficient room to set up the directional drilling rig.
Location 4: Trenchless Technology overcomes complex conditions
Due to varying ground conditions, access issues, the close proximity of residential properties and high voltage electrical infrastructure a combination of HDD, slip lining and pipe bursting techniques were used to complete 295m of upsizing works from 150mm VC to 250mm PE.
HDD was used to complete one of the street runs which was deep in hard compacted soils and located directly under a row of large trees whose roots were wrapped around the existing pipe. These trees had also sucked the moisture out of the ground which contributed to the dry hard conditions. Given there was adequate space to set up the drill rig HDD was identified as the most effective trenchless solution.
Slip lining was used for the house services due to the restricted access to both ends and the close proximity to power poles that needed to be held up during the construction process. Therefore a quick excavation, installation, connection and backfill process was required – something the slip lining process easily offers. However, a reduction in the diameter of the new pipe was required which was acceptable to the client.
Pipe Bursting was utilised for the remainder of the works. These sections were located in residential backyards with difficult access. Pipe bursting is the most suitable replacement technique in these conditions.
This sewer replacement project clearly shows the benefit of operating and understanding multiple trenchless replacement techniques. The challenges faced were easily overcome with little impact on the program. Five different techniques were used to install the pipe over the whole of the project – horizontal directional drilling, auger boring, slip lining and both air and hydraulic pipe bursting methods. This clearly shows how important it can be to a client to understand the options available and that not one type of method is necessarily suitable for all situations.
UEA works closely with its clients to ensure the correct trenchless method is chosen and this sewer augmentation project has demonstrated the diversity of trenchless technology and the importance of selecting the right trenchless technique for the job.