Central Interceptor Access Stair

Client: Watercare

Access stair redesign delivers cost, schedule and safety benefits

As part of Watercare’s $1.2bn Central Interceptor project to overhaul Auckland’s ageing waste water system, a dry well shaft at the Māngere Pump Station required an access stair. With the facility being of such high importance to the community, it was essential that the proposed access stair be robust enough to withstand a strong seismic event and still be serviceable.

The original design of the access stair required the installation of steel brackets to the reinforced concrete walls at every stair landing. The work would be high-risk, time-consuming and require significant scaffolding. With the stair needed as quickly as possible to provide access for other works within the shaft, early completion was a critical requirement of the project.

A smarter, safer solution

With this in mind, GRIT collaborated with Watercare and main contractor Culham Engineering to develop an alternative solution - one that would deliver the access stair in a much shorter timeframe, eliminate scaffolding, and still provide the durability required.

The original design was reengineered as a modular steel tower, with three modules being lowered into position by crane and fixed into place. BIM modeling was used to develop and test concepts for seismic resilience, while enabling project stakeholders to visualise and influence the design to ensure a fit-for purpose solution. Actively participating in safety in design meetings with Culham Engineering also ensured the solution was practical and safe to construct.

Sophisticated structural analysis

Essentially behaving as a high-rise structure, the 38m high modular steel tower needed sophisticated structural analysis and finite element analysis (FEA) to design the steel members and their connections, as well as staged-construction analysis.

Utilising Revit to model the steel stair structure and its connections, the project team was able to visualise and easily detect any clashes with other services before construction began. The result was a trouble-free installation despite inconsistencies in the shaft walls, with GRIT monitoring construction, fabrication and installation to facilitate the successful delivery of the project.

Significant savings

The alternative modular solution streamlined the installation of the access stair, providing access for workers to complete other works within the shaft 55 days earlier than the original timeline. Not only that but it provided a $500m saving compared to the previous design, and significantly improved safety by reducing working at height activity by 90%.