Civil Engineering June 2022 | Vol 30 No 5

10 June 2022 Civil Engineering An underground aquafer was discov- ered in an isolated portion of the site. Friction piles were the only solution to carry the building loads. From a contractor’s perspective, moving plant and equipment around the site when working on a steep gradient with dominant clay surfaces becomes very difficult. The expected annual rainfall for Rosetta is 1 300 mm with 147 rainy days p.a., posing an additional challenge. Careful planning was needed to carry out all bulk earthworks in the dry season and to get the G5 layers in place before the rainy season started. It was therefore more practical to use two tower cranes on rail as opposed to mobile cranes because of the slippery clay conditions. The benefits of tower cranes were felt when placing concrete during the rainy season. Water-tight concrete structures The main requirement for the reinforced concrete works was to construct water- tight concrete structures that did not leak. The general philosophy was to get the basics rights (mix design selection, joint preparation, placement and vibration) and to further reduce the number of joints by the setup of several larger pours in key structures. The mix design included an 80/20 blend with a water-cement ratio of 0.46 and a well graded combination of fine aggregates. A retarding paste was used during placement of horizontal joints, followed by pressure spraying the next day. The joints were then either reinforced with XYPEX Concentrate or a hydrophilic water bar, or both, depending on the thickness of the wall and the position of the joint. Formwork tie holes through walls were also substantially reduced by using the bulky NOE Formwork system which comes in 2.4 m × 2.4 m panels, requiring only four tie holes. The handling of the forms was made easy by the two tower cranes and the mobile crane setup on site. One of the more challenging structures was the 35 m high, 115 000 ℓ capacity elevated water tower that will supply processed water to the plant. The shaft was constructed in 3.5 m lifts using a hand-over-hand technique and custom steel shutters. To construct the conical section at the top, a series of prefabricated timber wedges were assembled on the floor using GT girders and plywood and reassembled at the top of the shaft. Through ties were used to resist concrete pressure while placing by catching a series of hoops made of radiused scaffolding tube and twin channels. Because of the angle of the cone, windows were installed on the inside shutter through which concrete was placed using a tremie pipe, and the windows were closed as the pour proceeded upwards. Domed roofs are always a challenge, and for this tower in particular stripping out the staging and formwork would provide significant restrictions in terms of both member sizes and the duration needed to strip. The roof was therefore precast on the floor next to the tower and placed on top of the RC conical water tank at completion of the conical section. The dome shaped roof was formed using com- pacted excavation material and covered in a steel floated blinding with a specialised release agent to provide a suitable finish to the roof soffit. COMPLETION The project was completed in March 2021, with a final water quality of 0.5 NTU being produced. This was a sat- isfactory achievement, with the civil, me- chanical and electrical contractors being able to fine-tune their skills to address the many interface challenges experienced on a daily basis. Sincere thanks must go to the Umgeni Water Capital Projects Team and its principals who had faith in the contractor to complete the project, despite the fact that Basil Read went into business rescue during the construction phase. Basil Read was in attendance during the one-year defects liability period which concluded in March 2022. The backup needed to attend to electrical and me- chanical issues was professionally handled by the specialised sub-contracting com- panies. On 18 March 2022 Umgeni Water signed off the Final Approval Certificate – a job well done. CHALLENGES UNIQUE TO THIS PLANT Rosetta is located in the KwaZulu-Natal Midlands which is popular because of Pulsators under construction Completed elevated water tower