Issue
At the request of the property owner, Drainage Control undertook an inspection of the stormwater services at a residential property on the flat land in the Kilbirnie/Lyall Bay area. The property is on a back section accessed via a long paved driveway. The stormwater from this driveway is collected by a sump located at the house end of the driveway, where it is led around to the North of the house collecting two downpipes from the house on the way, and on to a ‘soak pit’ located to the west (Rear) of the property inside the boundary. This soak pit also receives the flow of a third down pipe on that side.
The issue was that water was surcharging back through the sump in the driveway and pooling near the front entrance, indicating a restricted flow or a blockage.
Solution
An attempt was made to clear the obstruction using a water jetter. No progress could be made, and the obstruction remained, a drain CCTV showed water ponding the pipe. At this point, there was no alternative other than to manually excavate the stormwater pipe and trace it to the soak pit to find the cause of the blockage.
We began by excavating toward the point reached by the water jet where the pipe from the downpipe at the rear and north of the property connects to the pipe leading from the driveway sump. It was not long before we encountered the first serious fault. The line bends towards the centre of the backyard, and the fitting used to affect this turn had been laid incorrectly and at an invert – in other words, the drain was now running slightly uphill making this bend the low point of the system! This can be seen in the image below. The blue arrows show the direction of flow.
At the very least, this would require the pipe to be re-aligned to establish fall along its full length. We continued to trace the line. At this point, we attempted to inspect the downpipe at the rear of the house which clearly would join the soak pit from the opposite direction. It was entirely blocked with roots and sand. Now we turned our attention to finally uncover the cause of the problem and when the actual ‘soak pit’ was reached, the problem became clear at last.
The installers of this system had trenched and installed the pipes, and instead of forming a proper soak pit had converted part of the trench into a maintenance issue waiting to happen in the future. They have transitioned from PVC pipe to what looks like non-perforated nova coil, but instead of using a joiner, they just butted it up to the PVC pipe relying on the surrounding fill to keep it in place.
The pipe connecting to the other side is the same. Then they backfilled the trench with drainage chip to approximately 150mm below the surface where they covered it all with folded sheets of black plastic before covering it with sand for the pavers to be laid over the top. No attempt was made to protect this drainage chip from contamination from the side and inevitably roots and sand penetrated this material. The next images illustrate this.
Summary
- The pipe to the north was blocked with sand as far back as the downpipe at the side of the house.
- The pipe to the south (Rear) was blocked with roots along its entire length.
- There is a bend that has been installed incorrectly that has shifted the low point of the system from the attempted soak pit to this bend, which would explain the presence of sand blocking the pipe on the north side.
- The sump in the driveway was completely inadequate to accommodate the flow required of it.
- The result is, there was no effective stormwater drainage at all for this property.
- The entire stormwater system needed replacement with one designed and fit for purpose.
The Solution
Unfortunately, because of the degree of poor workmanship and design, most of the underground components of the stormwater system needed to be realigned and replaced.
The works included:
- Excavation of the current ‘soak pit’ to a depth of at least 1.2 metres, where the water table is generally found in Lyall Bay and the installation of a concrete chamber with a manhole and lid at ground level to enable inspection/maintenance.
- Installation of a channel and grate interceptor trap across the flattest part of the driveway slightly to the east of the current driveway sump and piped into a new driveway sump near the existing location giving at least 4 times the capacity of the existing sump alone.
- It hardly needs to be stated, but we made sure that the system has correct fall at all locations eliminating the current invert present due to the misaligned bend referred to above.
- Replacing the pipes from both downspouts in the back yard and connecting them to the new soak pit.
- At least we did not have to dig up the section of pipe from the driveway at the front of the house to the system at the rear, as it was in good condition, and so this section of pipe was retained in service. We used a drain camera CCTV to confirm this.
- Backfilling the excavations and removing all waste/debris to an approved disposal facility, and relaying about half the pavers to a point indicated by the homeowner so that a lawn area could be developed instead.
Just to reassure ourselves that this would finally fix the issue, we tested the length of the driveway to make sure the flow was all being dealt with. Despite it looking very flat, it in fact had good fall, and the entire driveway was drained by gravity alone. Good job to the people who laid this driveway and paved it.
Drainage Control NZ Conclusion
The stormwater system did not work at all, and a small lake was forming during even moderate rain at the front of the house where the front and garage doors were located.
We replaced the entire system with a code compliant higher capacity arrangement as explained above, which will keep the property from flooding under all conditions currently foreseeable.