Issue
This job came to us as the result of a landslide. An old access route in Seatoun, Wellington, comes off Tio Tio Road and climbs above it to access 4 properties and a complex which was once a secondary school campus. In 2022, part of this access route collapsed onto Tio Tio road in a major landslide that blocked the road and carried away almost the full width of the access way above, isolating one property and the old campus.
The repair was complex due to the terrain, and a civil engineering company – Griffiths Construction – had formed an impressive concrete cantilever on which the reformed access would sit. The sewer was in the middle of the access way, and a temporary line had been laid to service the 2 properties above it and 1 adjacent. Work was at a stage where the sewer could be re-laid in preparation for the road surface to be reinstated.
Civil Complications
As this is a main sewer, different standards apply and not just anyone is permitted to undertake this type of work. Drainage Control is, however, qualified. All works must comply with Wellington Water’s drainage requirements.
Solution
Drainage control had to emplace a new sewer in the road compliant with current standards, replacing the old one which was swept away by the slip, whilst leaving the temporary replacement in service until the new one was ready to be connected.
The requirement for a replacement in the road is very different from a domestic system. The pipe is different – we use thick-walled HDPE (High Density Polyethylene) pip which is welded together in sections instead of glued PVC pipe, although this is used in its thick-walled version (1.6mm as opposed to 0.6mm PVC) to connect individual houses to the main.
Because the old sewer ran toward the outside, or slip side of the road, we could not follow the prior path of the sewer. Instead, we excavated the sewer further up the access road and intercepted the main where it was further back from the edge with room to fit the required chamber which was modified to fit over the sewer main.
We chose the site of this top chamber to give us a straight shot through the new retaining works to the other side where the existing sewer was. The new main was not in line with the old one, so at the downstream end, 2 new chambers and manholes had to be emplaced to kick the main over far enough to connect to the older pipe.
We then formed a connection to the new main for the house adjacent to the slip, connected the new section to the old sewer at either end, and decommissioned the temporary main which had served its purpose for nearly 2 years.
We back filled the trench with drainage chip to a height of 300mm above the new pipe. To reinforce the stormwater capacity of the repair, a perforated Novacoil pipe was laid the full length of the repair under the kerb and channel as an interceptor trap for any groundwater that seeped under the road, to lead it via a new pipe installed down the face of the site to the kerb and channel below on Tio Tio road.
The remainder of the excavation was topped off by Griffiths Construction with basecourse and compacted in several stages to enable final site levelling and laying of the new road surface.
Observations
This was a big job, and Drainage Control acknowledges the cooperation of Griffiths Construction and Craven Landscaping.
Outcome
Replacement of the sewer and enhancement of the stormwater drains at this site was a key element of returning the access road to service.