Westview Drive earthworks - What's the story?

22/02/2019 9:00 a.m.

​​An update on a complex project

We know residents have been wondering about progress of earthworks on Westfield Drive in Weston.

Doing this job properly has taken longer than we anticipated. To give an idea of scale, the Westview Drive hole’s volume is equivalent to 216 truckloads of rock! That’s one big hole! We thought we’d let you know exactly what it’s taking to fill it in, as while there have been times when no one was on site, there has been A LOT of preparing and planning going on in the background.

If Waitaki District Council had simply repaired the pipe at point of failure, we would most likely have been done by Christmas. However, the discovery of a sewer line through the railway culvert pointed to the need to realign the sewer as well as concern about the remaining condition of the culvert under the road. This necessitated a larger excavation but made it possible to begin the much more sensible process of replacing all of the road culvert back to the railway culvert. The downside was now that a large riser manhole would be needed to collect the pipes and provide future access to them.  

With the concerns from the future owner of the flooded residence being voiced, and  the increasing number and intensity of storm events, now seemed the time to provide greater security to the main Weston sewer line. The Christmas and New Year break and the unavailability of the specialist contractor delayed progress until the end of January. Of course, adverse weather may yet create further delays.

So, we had gone from this:


To this:


OK, that does look a tad worse, bear with us:​

Background

The masonry box culvert beneath Westview Drive failed during a rain event on the evening of the 21 November 2018. The failure caused back-flooding that resulted in water entering the residence under construction at 19 Grove Avenue.  Pumps were set up on site that evening (21st) which reduced the water level.​

SouthRoads began excavation of the pipeline on the next day (22 November) and discovered a blockage well into the pipe. Unfortunately the depth of the excavation necessary began to slow progress as battering and benching was required.  Once the blockage, or collapse, was unearthed it was found to be close to a previous failure (6 years earlier).  The logical decision was then to remove the road culvert back to the railway culvert and replace it in full as it was clearly experiencing some distress.  Once this had occurred it was then discovered that the larger railway masonry box culvert had a sewer line attached by brackets throughout its length and this restricted flow plus created potential catch-points for debris. The railway culvert was inspected using a camera to check its condition, which showed no signs of deterioration.

The future owners of 19 Grove Avenue were concerned that Council could not give a 100% guarantee that the culvert would not fail in the future. To mitigate this concern, though the existing structures had had the capacity to cope with the exceptional rain of a few months previous, it was agreed to remove the sewer pipe from the railway culvert and install a by-pass 450mm Stormwater pipe as an overflow. 

This approach meant that other utilities like fibre optic needed to be worked around, which added to the time for reinstatement. As this work was complicated by also being within a railway corridor, it was necessary to survey the site to determine exactly the locations of all of the different infrastructure and determine where to realign the sewer line. During this period SouthRoads commissioned KB Contracting to thrust bore the new sewer line and the additional stormwater line through the railway embankment. Unfortunately, the contractor was not available until late January at the earliest. On the upside this gave us time to complete the designs and order the replacement concrete pipes from Christchurch for the road culvert.

Christmas and New Year arrived delaying work for two weeks. KiwiRail have insisted that the sewer line, being a hazardous material, must be sleeved so the thrustboring contractor needed to source larger drilling equipment to meet the new diameter. This took an extra week.  There is a need to cross and turn the sewer and stormwater lines and this is accomplished by a manhole at the junction of the rail and road culvert.  Considerable care is required to construct this manhole riser as it has three holes broken into the casing which may affect its longevity. Regardless of this, creating an access point will permit rapid response should it ever be needed.

What’s happening now?

  • This week the new sewer under the A2O track is being re-connected
  • Next week the new overflow stormwater pipe is being installed under the A2O track on Monday, new 1200mm concrete pipe and 2300mm concrete riser will start to be installed on Weds
  • Back Fill is programed to start on the 11th March along with gabion baskets 
  • Work on the inlet is programed to be finished by the 8th of April as is sealing and roadmaking, after which the Parks team will be planting 3 new trees to replace those removed.

As you can see, this is a complex project with a number of interconnecting parts to line up. Our teams are working hard to get it done right, while taking the opportunity to simplify what was in place and make sure that the infrastructure will be much less likely to fail again.


Page reviewed: 27 Feb 2019 8:04am