Construction World December 2018

Consulting Engineers

CONSTRUCTION OF A NEW 12,5 M ℓ RESERVOIR, GEORGE OLD WATER TREATMENT WORKS

T he reservoir provides storage of potable water mainly to facilitate the indigent communities of Pacaltsdorp and Thembalethu. The reservoir is located on a unique, confined site between the main raw Malgas pumping main to the north and three of the main supply pipelines to the south. Many historically placed services had to be relocated to accommodate the reservoir construction. The reservoir had to be designed to accommodate the existing reservoir levels while achieving the required capacity. The existing pipe systems on the site were reconfigured to allow improved operation and functionality. The project is an example of excellent innovation and high-quality infrastructure, in an extremely challenging and restricted area to be utilised by the community for decades to come. It is a success story of achievement through teamwork between the client, engineer and the contractor, while the supply to the community could not be interrupted. The geotechnical investigation showed that the site is generally suitable for the proposed structure in terms of the geology and soil conditions. There are, however, the mildly problematic soils and precautionary measures were incorporated into the design to prevent excessive soil movements. These measures include an engineered fill layer below the floor slab, a basal reinforcement geogrid and subsoil drinage. During construction, further geotechnical investigations were done at foundation level of the reservoir. Further problematic soils had to be removed to a Royal HaskoningDHV was commissioned by George Municipality for the design, tender and construction stages of the 12,5 M ℓ capacity reservoir at the Old Water Treatment Works.

The excavation of the 6 m deep outlet chamber extended into one of these private erven. Access on site was limited to one road to the reservoir; a secondary temporary access road was built to the north of the site to allow circular heavy vehicle movement. Existing services had to be temporarily relocated to accommodate the reservoir excavation footprint. HDPE pipe material was used to allow the re-use of the pipe in the final position. The existing pipes on site date back to the original George WTW; no reliable as-built information was available, what sections of pipes are in use or what sections had been disconnected. 3D scanning was not considered feasible for the project. Pipe crossing one another at deep depths within confined areas. Exposing of the services had to be done and new special fittings designed to allow connection of new pipes to existing pipes. Commercial divers assisted with faulty valves inside the 9 Mℓ existing reservoir, which had prevented construction activities. The existing pipe network consisting of individual systems installed in separate historic projects was evaluated and improvements made to allow improved and easier operation, flexibility and the removal of bottle necks in the system. The main pipework had to be designed to accommodate future extensions, as there is no space for future additional services through/or in this corridor.  PROJECT INFORMATION

• Company entering: Royal HaskoningDHV • Client: George Municipality • Main Contractor: Urhwebo E-Transand

deeper depth, but the geogrid was omitted as a saving. The increased excavation depth made the drainage during construction, together with increased excavation slope stability challenges while dealing with the existing services, much more challenging. The top water level of the new 12,5 Mℓ reservoir has been kept to match the existing 8 Mℓ reservoirs to ensure compatible system hydraulics. The new 12,5 Mℓ reservoir has a flat floor, whereas the floors of the 8 Mℓ reservoirs are deeply graded towards a central outlet point. The site is restricted and within a small and confined space, a rectangular reservoir was designed within the site limitations. Additionally, a portion of the Transnet property had to be secured to accommodate relocation of existing pipework and new pipework. The southern site fence is positioned within very close proximity to the first row of houses.

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