Invercargill is New Zealand’s southern-most city, located at the bottom of the South Island. Founded in 1856 by Scottish immigrants, it is now home to some 50,000 residents and supports a thriving commercial and rural business sector. The DN150 sanitary sewer earthenware pipe to be replaced was located in Gala St, a minor arterial road with approximately 7,000 vehicle movements per day.

A low impact, fast and safe construction solution was preferred to accommodate the traffic density and proximity to residential homes, the city Museum and Southern Cross Hospital. The tender called for either open cut or trenchless construction methods using PVC or polyethylene pipe, plus re-connection of the existing domestic lateral drains and replacement of associated concrete manholes.

Gala St, Invercargill - trenchless installation by pipe reaming

Southern Civil chose to use a pipe reaming method on the Gala St project. A Ditch Witch JT 1720 HDD machine, rated at 20,000 lb pullback and 17,000 lb rotational thrust was used, with a water lubricated reaming head.

Pipe renovation and installation

Access pits were excavated at nominated house lateral connections and the earthenware laterals neatly cut from the main. The section of the main was then isolated from the network and cleaned in preparation for the reaming/renovation process.

A conventional HDD drill head was used for the initial drill rod insertion, with the HDD rig located in a small park adjacent to the Gala St sewer line. This enabled Southern Civil to drill under the adjacent road intersection, and through to the correct invert level, prior to entering the earthenware main pipe. At this point the drill head was advanced up the bore of the earthenware main pipe until the first termination point was reached. Typical renovation lengths were 80 to 100 m.

The reception pit was sized to accommodate 6 m lengths of RESTRAIN pipe although both 3 m and 6 m pipe was supplied to the project. Once the drill rod was advanced through to the receiving pit, the reaming head and starter pipe were fitted in preparation for pipe installation. The first full length of RESTRAIN pipe was then screwed into the starter pipe, the water feed to the reamer was turned on and reaming commenced.

With pipe reaming underway the average rate of advance was 1.2 – 1.5 m per minute. The earthenware pipe disintegrated relatively easily with the broken chards displaced laterally by the bulk and shape of the reaming head. The swivel attachment to the PVC pipe ensured that no rotational movement was transferred to the RESTRAIN pipe during reaming.

Depending on the length of main being renovated a combination of 3 m and 6 m lengths of RESTRAIN PVC pipe were used. RESTRAIN pipes were added to the pipe string at drill rod changes by a simple screwed assembly at the joint.

A length of RESTRAIN pipe is selected, the threaded spigot and seal ring groove cleaned if necessary and the rubber seal ring fitted. The seal ring lubricant, Gensil 601 aerosol silicon lubricant or similar, is applied to the seal ring and sealing face inside the preceding pipe socket and then the threaded spigot engaged with the threaded socket.

The joint uses a conventional right hand thread and the pipes can be turned by hand until the seal ring engages. At this point strap wrenches are used to tighten the RESTRAIN pipe until the joint is fully engaged. It was generally found that the time to fit a new length of RESTRAIN PVC pipe was about the same time taken for a drill rod change.

The reaming process then continues until the next RESTRAIN pipe length is required. In this way a long ‘pre-welded’ pipe string is not required so the overall work space is relatively small.

Upon completion of a section of renovated pipe, connections to the existing laterals were made using conventional ‘off the shelf’ rubber ring joint u-PVC sewer fittings. The newly inserted RESTRAIN DN150 main was cut and a DN150 X DN110 junction fitted in place using conventional moulded rubber ring joint slip couplers. The DN110 branch was then connected to an inspection bend followed by a PVC/ceramic adaptor completing the connection to the existing earthenware lateral.

Contractors review

Chris Peters of Southern Civil reported a successful outcome to the installation and advised that the project had run smoothly and on time. The RESTRAIN pipe provided a more cost effective solution than traditional open cut excavation and significantly reduced site risk and safety issues. With smaller access pits site safety and traffic management was improved and the ability to work in a small footprint area was a great advantage.

Unlike polyethylene pipe, no site welding was necessary for the sewer main or connection of the laterals. As a result there were no long pipe strings to block driveways and disrupt the movements of traffic and residents in the area. As the project was undertaken in mid winter, on a ‘live’ sewer line, in wet conditions, and with temperatures as low as –10oC, the difficulties of site welding polyethylene pipe were completely eliminated.

A further advantage of the RESTRAIN system was that pipe could be undone and removed from the excavation if drilling difficulties occurred. The pipe could then be re-used once the operation was back to normal.

Conclusions

Trenchless renovation of the earthenware sewer provided a cost effective solution to Invercargill City Council that met with their desired performance criteria. In turn, the RESTRAIN threaded joint PVC pipe was a cost effective solution that the contractor found easy to use. The pipe met with councils specified performance requirements and contractors expectations for functionality and ease of use.

It was found that a PVC pipe system simplified connections into existing lateral connections by using conventional rubber ring joint PVC fittings. The pipe was lightweight and easy to handle and the jointing mechanism meant that long welded pipe strings were not necessary. In addition, by using trenchless methods, disruption to local residents was decreased and site safety and traffic management improved primarily due to the small footprint working area required.

This article is a summary of the paper by Iain McNaught and Frank O’Callaghan entitled Pipe reaming replacement of earthenware sanitary sewer pipelines with PVC-U Pipe specifically designed for trenchless installation. The paper was presented at the Trenchless Australasia conference in March this year.

Acknowledgments

Peters, C. Southern Civil Ltd, Invercargill

O’Callaghan, F. Iplex Pipelines (NZ) Ltd

References

Iplex Pipelines in New Zealand initiated development of the RESTRAIN pipe system. Hydrostatic testing of the elastomeric seal ring was conducted at Iplex’s independently accredited laboratory, with tensile and compression testing at the Massey University Institute of Technology and Engineering.

Elastomeric seal ring compliance with AS / NZS 1260

The RESTRAIN seal ring complies with Section 3.4 ‘Tests on Elastomeric Seal Rings’ of AS/NZS 1260. The hydrostatic and liquid infiltration tests examine resistance to internal and external pressure. The contact width and pressure test confirms a minimum interface pressure and minimum continuous contact width between the elastomeric seal and the spigot of the pipe. This is to effectively eliminate tree root intrusion and liquid infiltration/exfiltration at joints in correctly installed u-PVC pipelines.

Tensile and compressive performance

Tensile and compression testing of RESTRAINT joint assemblies established the ultimate performance of the joint. Appropriate factors of safety were determined, giving maximum tensile and compressive loads for installation.

Laboratory testing on PVC-U material samples established a baseline for the pipe material in use.

Initial field-testing by guided auger boring, followed by conventional horizontal directional drilling (HDD) technology, tested the functionality of the RESTRAIN system during construction. The first successful HDD installation of DN 150 RESTRAIN was 105 m between manholes at an invert of 4 m.