RESER VOIRS
Deviation from standard designs create a durable reservoir
Special vibrating shutters were specified and tremie pipes were used to make sure the concrete did not segregate
Kendall Slater, lead: Pump Stations at Knight Piésold
A
unique structure that is watertight and aesthetically pleasing was produced by engineering a solution around limited available space, the concerns from the community, historical leakage concerns, and the size of the structure. The old existing reservoir was a 5 Mℓ circular, reinforced concrete reservoir that was supplied by a DN500 offtake from the Western Aqueduct – a major supply pipeline running from Umlaas Road to Durban. Due to increased water demand, the old reservoir was no longer able to meet the 48-hour storage requirement as laid out in the guidelines for human settlement planning. Additional storage was therefore required, and the new 25 Mℓ reservoir was proposed by the eThekwini Water and Sanitation (EWS) Department. Construction joints “Typically, water reservoirs have several movement joints to allow for the movement and shrinkage of the concrete, while still maintaining watertightness of the structure through the use of rubber water stops. This design had been used
32
SE P/O CT 2021
Global consulting firm Knight Piésold was appointed by eThekwini Municipality for the design and construction supervision of a new reservoir in the Hillcrest suburb of Durban.
by eThekwini Municipality for most of the existing reservoirs in and around the Durban area,” says Kendall Slater, lead: Pump Stations at Knight Piésold. Making provision for expansion joints inevitably results in discontinuity in the cast concrete. Traditionally, designers allow for this by specifying the installation of rubberised water stops across each joint, as well as the installation of HDPE membrane liners. However, in many cases, leaks still occur because water stops often become misaligned when casting the adjacent concrete panels. As the membrane liner deteriorates, and the seals around joints fail, water loss is inevitable. That requires ongoing maintenance and the loss of precious potable water. Knight Piésold therefore decided on a different approach and removed a number of movement joints from the structure. As this was a very large concrete structure, the removal of movement joints required careful modelling: • The previously utilised tapered walls were changed to uniform walls. The modelling of the structure showed that the uniform walls performed better as a propped cantilever with smaller crack width, as well as making the reinforcing designs at the
corners simpler and easier to construct. •T he 8 m high walls of the reservoir were poured in a single lift – reducing the number of construction joints and the construction duration. Normal concrete pours are not usually done higher than 4 m, as there is a risk of aggregate separation during the pour and it is difficult to achieve adequate vibration at the base of the pour. In order to mitigate this, the concrete was poured using a concrete pump with tremie pipes extending to the base of the wall. Long needle vibrators were utilised in combination with special shutter vibrators attached to the formwork at the lower levels. The resulting concrete was of a high quality with minimal spalling. • I n order to mitigate the shrinkage cracking resulting from the lack of movement joints, additional reinforcing was required. The higher reinforcing cost was offset against the savings in water bars, as well as the decrease in construction duration and the lower risk of leakage. •C onstruction joints were still required; however, the specification on these was given careful consideration and closely monitored during construction.
