From the magazine, HDD

Rodney Council pipe installation

The installation of this pipeline in Tindalls Bay eliminated the need to upgrade an existing pump station, providing the Rodney District Council with a $NZ2 million saving.

The pipe installation was part of a much larger project that included the installation of a pipeline the full length of the Whangaparaoa Peninsula, 25 km north of Auckland. The pipeline was largely installed by open cut but included seven directional drills.

The proposed pipeline route began under the watchful eyes of local residents at the Tindalls Bay pump station, situated adjacent to the Tindalls Bay beach.

A Vermeer D80 drill rig was placed into a cut out bank and was set up on line and grade at the Tindalls Bay pump station. From here the proposed pipeline route passed through 30 m of unstable ground before entering into hard siltstone. The 30 m of soft ground was formed during a pre-historic landslide and high value homes have now been built over the area. It was imperative that the hydrostatic water pressure at the slip zone was not increased. As a precaution, four slotted dewatering drains were installed into the clay and up to the rock interface, removing excess water from the ground and offsetting any hydraulic loading caused by the drilling operation.

Due to the depth of this drill a magnetic north steering tool located in a non-magnetic steering head utilising true magnetic north was used to obtain direction. A wire inside every drill rod transports the electronic information back to the drill operator and is displayed on a laptop. Special computer software interprets the transmitter information and plots a long section and plan of the pilot drills progression. It also displays the calculated drill head position.

Once into the siltstone the pipeline route continues to climb at 4.8 per cent grade and quickly reached a depth of 40 m as it passed under the ridge. After a further 150 m it exited the ground in a swampy gully. As a result of the unstable, saturated ground in the gully it was necessary to install a 1 m diameter steel casing 30 m along the pipeline route down to the rock interface. A 200 mm diameter PVC sleave was also installed into the clay at the drill end to support the drill rods and to hold the pilot hole open.

The siltstone was hard. Traditional ripper type reamers require huge toque and produce large particle cuttings that are not easily removed from the tunnel. A steel tooth tri-cone reamer was built in-house with a diameter of 760 mm. By grinding the stone the particle size was reduced and then better suspended in the drilling fluid.

Plenty of water was required, and council water on the Peninsula is unavailable in summer. The options were to truck water in and truck the drilling fluid with cuttings to the tip site (20 km away), obtain resource consent to remove water from the sea/truck drill fluid with cuttings to tip site, or recycle drilling fluid and take cuttings only to the tip site. Smythe decided to use the drill fluid recycle option.

The drilling fluid was pumped up to the cutting face and then travelled down the pilot hole (around the drill rod) and was collected in a pit at the drill. The silt laden fluid was then pumped into the fluid recycler. After removal of the solids the fluid was fed back to the drill where it was pumped back up to the face and undertook another cycle. This method saved pumping the cleaned fluid from one end of the project to the other.

When the siltstone was fully reamed, the reamer was backed out of the hole. It was decided not to ream the final 30 m as the ground was unstable and we suspected that it would collapse causing damage to the driveways above and possibly buildings near by.

When the reaming was complete the pre-welded and tested 710 mm diameter, SDR10, polyethylene pipe string was placed on round posts to aid in the movement of the pipe up Hobbs Road. The pipe string was 400 m long and weighed a total of 48 tonnes. The pipe was connected to the drill string and pulled into the tunnel. The pipe insertion required 14 men, three excavators and one tow truck to maynover the pipe as it was pulled up Hobbs Road. The pipe installation was completed in 6 hours. The final 30m of clay was reamed during the pipe insertion and the pipe/tunnel anulus was then pressure grouted at each end with a cement based grout and the connection to the pump station and trenched section was undertaken.

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