Plankenbrug Main Outfall Sewer and Associated Works - Phases 2 & 3 by AECOM for Stellenbosch Municipality, Western Cape

The existing Stellenbosch bulk sewer network along the Plankenbrug River was under strain. AECOM was appointed for full professional services to investigate, design, and manage the construction of urgent remedial actions required. This appointment was to address the current capacity and pollution problem, as well as upgrading the system for a future estimated sewage flow of over 900 ℓ/s, according to the Sewer Master Plan.

AECOM’s design proposal recommended the replacement of the existing problematic sewers with the following: • 1 245 m HDPE-lined concrete bulk sewers (DN 900 mm to DN 1 000 mm) • 3 094 m GRP bulk sewers (DN 350 mm to

DN 600 mm) • 1 486 m uPVC collector sewers (DN 110 mm to DN 355 mm). The project also included: • 94 concrete manholes • two above-ground gravity flow sewer pipe bridges (DN 350 mm and DN 1 000 mm) across the Plankenbrug River, including associated river hydraulic modelling • two temporary sewage pump stations (300 ℓ/s each) and one temporary screening station for sewage overpumping around the construction works • reconstruction of a 150 m long x 4 m high river retaining wall to protect the new sewer • stormwater intercept structures to divert polluted stormwater and low flow run-off from the Kayamandi township into the sewer network and not to the downstream

Plankenbrug River • installation of bulk sewers by means of no-dig methods underneath live provincial roadways and railway lines (90 m microtunnelling DN 1 170 mm, 40 m

pipe jacking DN 1 050 mm, 50 m horizontal directional drilling DN 250 mm) • reconstruction of 900 m municipal roadways following sewer installation underneath • two open-cut railway line crossings. This project was considered challenging beyond the norm. The sewer had to be installed alongside a river located in a highly built-up area, which offered less than 7.5 m working width in some locations. This required careful and intrepid decisionmaking, as well as careful execution. In addition, seasonal river flooding needed to be managed, particularly during the new river retaining wall construction. Several existing services were accommodated to allow the gravity sewer to be built. With a robust safety culture on-site, the project team completed the project in time and within budget.

The new bulk sewer will unlock upstream housing development opportunities, for which there is a high demand in this area.

PROJECT TEAM

Client: Stellenbosch Municipality Consulting Engineer: AECOM Contractor: CSV Construction