Work Report: Thorndon


Drainage Control undertook an inspection of a residential property in Thorndon, at the request of the property owner. Sewage was surcharging through a gully trap on the uphill side of the house at the side. Inspection of the line revealed that there was a blockage and the suspect was an old Buchan trap to the rear of the property.

This is an older property, and as was the practice many years ago, a common line servicing 4 houses (2 above and 1 below this property) led sewage to the main situated under Tinakori road. This meant that the sewage from the 2 houses above was also being surcharged through the gully trap of this property.


This house and the 2 uphill from it are built on a slope. When constructed, the sewer was buried about 300mm below service as was, and still is, the standard, following the slope of the hill which is steep. Over time, property owners wishing to develop flat outdoor sections have built retaining walls. This means that the sewer on the uphill side was still accessible, but at the downhill side were now buried very deep. In this case over 2.2 metres.

Worse, the material used to back fill the retaining wall is very different to today, and it proved to be a poorly compacted midden heap of old bottles and other detritus overlaid with large concrete tiles. If this were not enough, an out-house structure had been built partly over the deepest part of the sewer.


Drainage control began excavating to reach the blockage. We quickly uncovered the Buchan trap and removed it, but it was still functional. This was not the cause. We removed it anyway, as it was an impediment to the maintainability of the sewer and are no longer required as they are ineffective at their main purpose which is to prevent rodent access.

Upon further inspection, the problem was found to be behind a concrete retaining wall separating this property from the one below and deep. Nearly 2.5 metres down.

Due to the nature of the ground, there was no integrity to the trench, and from a quite shallow depth, it became clear that shoring would be needed to prevent collapse. This was implemented using heavy grade ply and timber spacers horizontally.

As we got closer to the blockage, attempts were made to clear it with no effect. Drainage Control then excavated the sewer at the back of the property below, where it was quite shallow. We employed high pressure water, and in the end started to flush terracotta pipe shards and rocks from the pipe.

It was clear that the sewer had entirely collapsed and could not be returned to service by any means other than replacement. Neither could it be repaired from the neighbor’s property as the blockage was behind a concrete retaining wall.

Back to excavating. We were now starting to dig under the shed which partially covered the sewer. This required the forming of a temporary support structure to emplace 3 steel props.

Even then, the depth and all the props made excavation very arduous, and all the material had to be hand carried in buckets to a truck parked in front of the house via a narrow access and steps, as there was no way we were going to reuse this material as fill.

The excavation began to pose unacceptable risks due to depth and instability of the land around it and the building above it. Fortunately, the homeowner below was sympathetic to the situation, and agreed to having the sewer re-routed to a hole drilled through the concrete wall above the existing sewer behind a permanent screen which only required us to remove and then reinstate it after. Thus, we employed a concrete drilling contractor to cut a hole through the wall at a level accessible to the trench we had excavated behind the wall. We were then able to route the sewer through this and run it down below ground level and connect it to the existing sewer on her property. This was done successfully, and the sewer returned to service.

The old, shattered pipes remain interred below. Finally, reinstatement could begin, and the new PVC section was surrounded by drainage chip and then covered by base-course which was compacted in several stages due to the depth. All that remained was to restore the pavers, do a final tidy-up, and so ended one of the most complex and physically committing jobs of 2023.


Whenever a property owner modifies the contours of their property with retaining walls or for any other reason, it is only prudent to establish the location of underground utilities, and at the least have them inspected via CCTV to confirm that they are fit for long term service, and preferably replace them with new code-compliant PVC drains that will stand the test of time, and be maintainable if an issue develops.

Drainage Control sees this sort of problem frequently and it is rare that they are easy and low-cost to fix. It is far more cost effective and far less disruptive to do this task first, yet underground utilities are seldom considered before work is done.


The property now has an effective and maintainable sewer. Drainage Control inspected the pipe all the way to the sewer main and are confident that this house and the one below it will have no further issues for a very long time.