Miracles of St Marys Bay Water Quality Improvement Project

Abstract

In January 2020, McConnell Dowell Constructors (MCD) commenced the St Mary’s Bay Water Quality Improvement Project on behalf of Auckland Council Healthy Waters (ACHW). The project involved the construction of 1,125m of 1.8m ID storage tunnel, which was designed to intercept 3 existing Combined Sewer Outfalls. These outfalls discharged into the inner bays of Auckland’s Waitemata Harbour during rainfall events - up to 200 times per annum, causing pollution and hazardous water conditions for several days after. The project was multidisciplinary, not only involving the tunnelled pipeline, but also the construction of 3 large shafts, 500m of large diameter open cut pipework, a fully operational pump station and a 460m long 1.6m dia. HDPE marine outfall pipe. Adding complexity, the project works were located within one of Auckland’s oldest and most affluent suburbs – St Mary’s Bay. This required extensive construction planning to ensure Resource Consent compliance was met whilst also causing minimal disruption to residents. This was best demonstrated at London Street where the MCD team constructed a 25m deep, 4m dia. shaft used to retrieve the TBM at the completion of tunnelling works. The shaft was less than 10m from the nearest house and the works were completed without incident. This paper describes the construction of the scheme, that now completed has seen overflow events reduce to less than 20 times per annum, drastically improving water quality in the harbour. This has ensured local beaches are now safe for swimming more than 98% of year. Page 1 of 10 No-Dig Down Under 2022 Miracles of St Mary’s Bay Water Quality Improvement ProjectPaper title M Beswick and K Kumar 1. PROJECT PURPOSE AND SCOPE The St Marys Bay and Masefield Beach Water Quality Improvement Project was an Auckland Council Healthy Waters (ACHW) project undertaken to mitigate the very frequent combined wastewater/stormwater overflows (CSOs) that occured at the St Marys Bay and Masefield beaches in Auckland. The project was designed to intercept 3 overflow points and direct the overflow volumes into a new underground storage pipeline. Once the rainfall events ceased and the flow levels subsided, the combined overflow within the storage pipe will be pumped clear through a new pump station and rising main into the sewer network for treatment. Figure 1. Location and direction of the 3no. existing Overflow points The project scope included the following: • Construction of three vertical shafts • The installation of the new 1.125km x 1.8m (ID) storage pipeline by way of pipe jacking (constructed in 3 Drives) • The construction of a new Pump Station and associated discharge rising mian and air treatment devices • The construction of 500m x 900mm and 1050mm dia.open trenched gravity sewer main • The installation of 468m x 1.6m HDPE marine outfall pipeline. (MCD Design and Construct Element) 2. FUNCTIONAL DESIGN – HOW IT WORKS Essentially the scheme works by intercepting the 3no. existing overflow points and storing the flows within the new storage pipeline. Once sufficient capacity is available within the local sewer network, the new Pumps Station pumps the stored flows through a new rising main to the local sewer network for treatment. Page 2 of 10 No-Dig Down Under 2022 Miracles of St Mary’s Bay Water Quality Improvement ProjectPaper M Beswick and K Kumar