Civil Engineering July 2021 | Vol 29 No 6

42 July 2021 Civil Engineering In recent years, environmentalists have voiced concerns for the community settling in this area. Sewage overflowing into the works’ stormwater system and spilling onto nearby Cuttings Beach has resulted in the closure of the beach, limiting the community’s use, threatening the livelihood of local fishermen, and causing high levels of pollution. The long-term impact of these spillages on the marine environment, ecosystems and public health is still undetermined. TREATMENT WORKS UPGRADE eThekwini Municipality’s Water and Sanitation Unit decided to implement measures to reduce the number of spills onto the beach, enhance the community’s quality of life and reduce the impact on the beach environment. The selected solu- tions comprised three aspects: Q Q Constructing a 23 Mℓ tank to increase the capacity of effluent storage on site, ahead of the overflow to stormwater Q Q Rehabilitating the existing 23 Mℓ con- crete tank to extend the design life and for quicker implementation/resolution Q Q Replacing 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. NEW TANK CONSTRUCTION The new effluent storage tank is a mirror image of the existing tank, but the existing container had a base of 800 mm and is founded on a boulder raft. The new tank’s base is approximately 400 mm thick and is founded on piles. This optimised design was selected as a cost-saving measure in line with project goals. Considering that the tank excava- tion reached depths of 4 m below natural ground level, one of the key challenges of the project was the water table which lies approximately 1 m below natural ground level. De-watering was a critical require- ment of construction and a unique set of additional risks needed to be mitigated. The first of these risks was the configuration of the de-watering instal- lation. 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 compromise the tank’s water retaining capabilities. With this in mind, de-watering piping was routed to daylight through a single box- out which would then be repaired. De-watering also presented the pos- sibility of contaminants, such as heavy metals from neighbouring industrial sites, being leached into the excavated area. A robust water quality monitoring regime was developed to ensure the safety and wellbeing of workers. This regime comprised a comprehensive testing pro- gramme prior to commencing works and ongoing testing until de-watering was no longer required. Finally, de-watering adjacent to the existing tanks that share boundary walls with the new tank could result in the settlement of the existing structures. On this basis, low impact augured piles were selected to reduce the risk of rotation of the raft. As an additional precaution to limit the movement of tank walls, the team was careful to not cast against existing structures as the shrinkage of the new concrete could cause displacement of the adjacent walls. Monthly surveys of the shared walls were conducted to monitor potential movement. CONCRETE TANK REHABILITATION Inspections of the existing tank revealed substantial surface cement erosion to the extent that the aggregate was exposed to almost its full depth. Selected repairs were identified in specific locations following a detailed mapping exercise. However, a fundamental three-step approach was developed for the entire internal surface of the tank. Prior to backfilling over the 1200NB interconnecting pipe from the new 23 Mℓ tank to pump station sump Prior to backfilling over the 1200NB interconnecting pipe from the new 23 Mℓ tank to pump station sump Aerial view of the new tank (left) in relation to the existing tanks onsite