'Keyhole surgery' rejuvenates 90-year old stormwater chamber

Published 24/11/2020

Undertaking a seismic assessment of an ageing chamber in Aro Street, Wellington, before then upgrading it was a tricky procedure.

By Stephen Molineux, Senior Environmental Engineer and Deputy Team Lead, Connect Water

Six metres beneath Aro Street in Wellington there sits a 1930s brick stormwater chamber that collects gravel from the Polhill Reserve and Holloway Road streams. The gravel is collected to prevent it from entering the pipe network, and was historically cleared out of the chamber annually, in a risky and time-consuming operation.

Before 0
The 1930s Aro St stormwater chamber full of gravel prior to its annual clean out and the upgrade carried out by Connect Water.
Before 1
Temporary supports were installed to support the chamber roof during the upgrade.

 

Wellington Water engaged Connect Water to undertake a seismic assessment of the ageing chamber and then upgrade it. The site presented a number of challenges, including:
• hazardous confined spaces and a working at heights hazard within a potentially unsound structure
• several high-voltage cables, trunk water mains, gas and fibre optic cables located over the brick arched roof
• traffic management around the site on a busy road
• groundwater seepage
• limited access for concrete pouring
• installation of a weir and bypass pipes through the chamber to control the flow of incoming water during strengthening.

Final 0
The chamber after seismic strengthening and functional improvements. It will need to be cleared of gravel once every very two or three years now, rather than annually, and will be much safer for workers. 

 

Initially, it was expected that sheet piling would be used to allow the removal of the brick arched chamber roof for strengthening from the inside, but the number of service lines over the roof ruled this out. The solution was to cut a hole in the roof to allow the installation of internal braces. Once the roof was strengthened, the hole was enlarged to complete the works. The operation could be described as seismic strengthening via keyhole surgery.

Throughout the project, Connect Water developed an excellent understanding of the operational challenges Wellington Water faced cleaning the chamber, which required the construction of a temporary weir within the incoming pipe, from inside the chamber, along with a temporary flume through the chamber into the downstream pipe.

section2
Figure 1: A cross-section view of the ‘keyhole’ entry into the stormwater chamber.

 

This process was extremely difficult, slow, and exposed workers to significant confined space hazards, reflected in Wellington Water’s budget of $50,000 and five days’ work to clean the chamber annually.

cut in
The first ‘keyhole’ being cut into the brick arched chamber roof. The entire roof could not be removed because of service lines in the area.
 

Connect Water recommended three operational improvements to the chamber while strengthening it:
1. Enlarging the access chamber lids to improve access and safety.
2. Installing a penstock and bypass pipe to shut off water into the chamber, thereby avoiding the need for workers to enter it to set up a temporary weir and flume during cleaning.
3. Improvements to the intakes at Polhill Reservoir and Holloway Road to reduce the volume of gravel accumulated and reduce the regularity of cleaning.


Seismic strengthening of the brick chamber will greatly increase its resilience and the improvements carried out during strengthening will reduce the frequency and duration of cleanouts. Previously an annual task, it will now be carried out every two to three years, and the time spent on site will be reduced from five days to three, as the construction of a temporary weir and flume will no longer be required due to the new penstock. This will provide operational savings of approximately $30,000-$40,000 per annum. In addition, the upgrade will significantly reduce traffic disruption and, most importantly, make future cleanouts significantly safer, as entry into a difficult and dangerous confined space will no longer be required.

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The enlarged chamber lid and penstock in operation. Due to space restrictions above the chamber, the penstock has to close upwards rather than downwards as they typically do when shutting off water.