BUFFALO CITY
East London’s planned sewer diversion tunnel Addressing the demands of a growing city, the proposed East London sewer diversion tunnel entails the diversion of effluent that currently flows into the Central Wastewater Treatment Works (WWTW) to the new regional Reeston WWTW.
Isometric view of tunnel shaft and base intersection
T
he objective for Buffalo City’s administrative capital, East London, is to unlock the development potential within Central's drainage basin and ensure that the new infrastructure can accommodate the future splitting of the waste streams. In addition, to minimise future operational and maintenance costs, the existing flow is to be diverted via a hard rock tunnel, thereby eliminating pumping costs. The site is situated on the east bank of the Buffalo River, approximately 10 km due north-east of East London Central. The first phase of the project to divert the existing effluent from Central WWTW
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IMIESA April 2021
to Reeston WWTW has been completed. This initial 10-month contract included the construction of interceptor sewers for the Wilsonia area. The second phase, which is a 30-month contract, includes the balance of the works to divert the existing flows. The project is currently with Buffalo City’s Bid Adjudication Committee for award. The scope includes: • excavating, laying, backfilling and testing of the gravity sewer from Central to inlet of tunnel, consisting of approximately 1 200 m of pipeline varying in diameter from 315 mm to 630 mm • excavating, laying, backfilling and testing of the gravity sewer from the interceptor sewer to the central shaft of tunnel, consisting of approximately 300 m (x2) of pipeline with a diameter of 560 mm • excavating, laying, backfilling and testing of the gravity sewer from the tunnel outlet to the head of outlet siphon, consisting of approximately 100 m (x2) of pipeline with a diameter of 630 mm • laying, fixing and testing of approximately 5 200 m of 630 mm diameter continuously welded pipeline in the tunnel, including pipe chairs at approximately 3 m intervals and the valves for the crossovers at the bottom of the central shaft • excavation, mucking, supporting and lining of a 3 m nominal diameter tunnel excavated using a tunnel boring machine (TBM) at a varying depth from 10 m to 70 m below ground, to a length of approximately 3.5 km.
TBM tunnel The TBM-bored tunnel will be constructed between a point at the western end of the trenched pipeline from Central WWTW to the central shaft, adjacent to the primary school in Scenery Park, and on to the outlet a short distance from Reeston WWTW. The tunnel will be bored primarily through interbedded mudstone and sandstone, possibly interspersed with intrusive hypabyssal
igneous dolerite dykes. The excavation for the outlet will be through decomposed to highly weathered medium hard to hard rock mudstone, overlain by mixed talus conditions. The tunnel minimum and maximum overburden covers a range between approximately 15 m and 70 m. The central shaft, situated approximately midway through the tunnel, will have a 4 m internal diameter, approximately 35 m in depth to the tunnel invert. Investigations showed that rotary core borehole drilling, which was carried out along the centreline of the tunnel to a depth of 1 m below the tunnel invert, revealed that the sedimentary rock, although highly jointed in some areas, has an average unconfined compressive strength of 266 MPa.
Key features A unique feature on this project is the use of a geotechnical baseline report (GBR). The purpose of this is to describe and set baseline values for the anticipated geotechnical subsurface conditions and ground behaviour expected to be encountered during construction. The pipe works include a gravity pipeline from Central WWTW through to the inlet of the tunnel, a gravity pipeline to the central shaft, a gravity pipeline to the outlet, an outfall gravity pipeline, and a portion of an outfall siphon. A single pipe will be laid from the inlet of the tunnel to the central shaft. From there to the outlet of the tunnel, twin pipes are to be laid. Manholes, using precast manhole rings and reinforced concrete drop structures, will also be constructed. In addition, two reinforced concrete vortex structures and stilling basins are required, along with a siphon inlet chamber with their associated pipework and valves. Over the longer term, once all the effluent is diverted, the moratorium on development within Central’s drainage basin will be lifted and land with a developable area for approximately 20 000 houses will become available.
