Civil Engineering December 2021 | Vol 29 No 11

32 December 2021 Civil Engineering L ocated in KwaZulu-Natal, the South Durban Basin is the industrial hub of Durban and is home to two large petrochemical refineries, a large paper mill, motor manu- facturers and at least 5 000 businesses. The area is also densely populated, housing 200 000 residents. In 1968, a deep-water outfall was constructed to discharge waste- water from the Southern Wastewater Treatment Works. 15 years later, the Department of Water Affairs permitted the discharge of raw sludge into the ocean without the prior removal of solids. During peak flows, and under normal operations, the effluent could bypass the low-level storage tanks and was discharged by gravity through diffusers along the sea outfall and pumped out to sea. However, peak wet weather conditions resulted in exponen- tially greater flows at the plant. Sewage overflowing into the works’ stormwater system and spilling onto nearby Cuttings Beach has resulted in the closure of the beach, limiting the com- munity’s use, as well as threatening the livelihood of the local fishermen and causing high levels of pollution. eThekwini Municipality’s Water and Sanitation Unit decided to implement the following measures to address the problem: Q Q Construction of a 23 Mℓ tank to increase the capacity of ef- fluent storage on site, ahead of the overflow to stormwater Q Q Rehabilitation of the existing 23 Mℓ concrete tank to extend the design life and for quicker implementation/resolution Q Q Replacement of a landside portion of the existing sea outfall with two 1 m diameter HDPE pipes to increase the discharge rate of effluent via the sea outfall, preventing overflow. DEWATERING With the tank excavation reaching depths of 4 m below natural ground level, one of the key challenges was the water table which lies approximately 1 m below natural ground level. Typically, several pumps would be connected to de-watering piping, which would surface at various points along the floor slab. However, many box-outs on the floor of the tank could compro- mise the tank’s water retaining capabilities. De-watering piping was instead routed to daylight through a single box-out which would be repaired. A robust water quality monitoring regime was developed to miti- gate the risks of possible contaminants, such as heavy metals from neighbouring industrial sites, being leached into the excavated area. Low impact augured piles were used to mitigate the risk of settlement of the existing structures. PIPELINE TIE-IN The tie-in of the new HDPE pipelines was the most critical part of the project works. As the sea outfall would have to be decom- missioned during the tie-in, the spillage of effluent onto the beach and ocean was unavoidable. The affected beaches were closed during the tie-in and the CSIR was appointed to confirm when they could be reopened safely. A 72-hour window was agreed on to undertake the tie-in activities. During the shutdown, the team tried to insert an inflat- able rubber bag to prohibit backflow from the sea outfall near the works, but the tide was higher than expected and made this impossible. An additional four hours were required to complete the works due to this challenge. SOCIAL SOLUTIONS The project’s construction logistics had several social impacts that needed to be considered. It was anticipated that Tanjore Drive – the main road to Cuttings Beach – would become unavailable to the public. A temporary gravel parking area and designated footpath were therefore constructed. In addition, the contractor’s office occupied the entrance to the beach itself, necessitating alternative access to the shore via a timber boardwalk through the beach dunes. This assured safe passage for everyone, including young children and wheelchair users.  SANRAL SA I CE 2021 Nat i onal Awards Category: Techn i cal Excellence FINALIST Technical Excellence category (Sponsored by Maccaferri SA) KEY PLAYERS Client: eThekwini Municipality Consultant: AECOM Contractor: Group Five Coastal Southern Wastewater Treatment Works: Upgrades to Sea Outfall Pumping Scheme The new tank (left) in relation to the existing site Work in progress at the northern chamber of the sea outfall pipeline installation