The Christchurch City Council is responsible for managing the city’s wastewater via the Christchurch Wastewater Treatment Plant. The plant processes sewage from its raw state before safe, clean water is discharged three kilometres oﬀ New Brighton beach.
While excavating down one side of a treatment plant culvert to seal a leaking silicone joint, a civil contracting company discovered large voids under the main eﬄuent route. Left untreated, this would potentially result in the culvert sinking under its own weight. Because this was Christchurch’s main wastewater culvert, the issue needed to be remediated quickly as a collapse could cause the entire wastewater network to shut down.
The voids were likely caused by smaller surface water drainage running adjacent to the culvert and needed to be filled to keep the culvert supported and the sewage pipeline operational.
Christchurch Wastewater Treatment Plant engaged Mainmark to fill the voids beneath the main eﬄuent lines and pipe gallery, remediate the ground, and improve its bearing capacity to limit further settlement. The structure also required monitoring to ensure it was adequately supported and safe.
The successful void-filling project has provided Christchurch Wastewater Treatment Plant with peace of mind, knowing that further voids are less likely to occur, thereby protecting one of its critical infrastructure points.
The Christchurch Wastewater Treatment Plant had a requirement to fill significant voids beneath its main culvert, which measured approximately 50m3.
Mainmark needed to remediate the ground and improve its bearing capacity. As lifting the culvert was not possible, it was essential to stabilise it in situ.
As the asset was critical to the Christchurch Wastewater Treatment Plant’s operations, monitoring for the structure was required to confirm it was adequately supported throughout the project.
Mainmark was required to complete the project quickly and with minimal disruption to the treatment plant without shutting down any machinery or equipment.
Mainmark also needed to limit water ingress while working below the water table in the pipe gallery.
Mainmark used its proprietary engineered resin injection solution, Teretek®, to fill the voids and re- support the culvert. Teretek®’s unique two-in-one capability made it ideal for the project because it fills the voids and strengthens weak ground, reducing the risk of similar voids forming in future. Teretek® is applied via small, precise injection points, using a fast and clean process that creates minimal disruption.
Mainmark’s engineered structural resins are a closed-cell membrane, meaning they are completely impervious to water ingress.
To minimise disruption to the plant’s operations, all the work was carried out through 16mm penetrations in the floor slab from a pipe gallery that ran parallel to the culvert. The holes were then leak-sealed using resin.
Mainmark was mobilised to the site within four days of being notified of the issue, and completed the project in just five days.
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Mainmark Ground Engineering (UK) Ltd.