Deviation from standard designs creates a durable reservoir

Deviation from standard designs create a durable reservoir

Kendall Slater, lead: Pump Stations at Knight Piésold Special vibrating shutters were specified and tremie pipes were used to make sure the concrete did not segregate

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.

Aunique 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 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. • The 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. • In 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. • Construction joints were still required; however, the specification on these was given careful consideration and closely monitored during construction.

These design changes resulted in a structure that was commissioned with no leaks and a high quality of concrete.

“As the Emoyeni Reservoir is located in a residential suburb (Hillcrest), noise pollution, restricted working space, dust pollution and traffic congestion were taken into account during construction. Furthermore, the existing water storage system had to be kept in operation throughout construction,” explains Slater.

After several consultations between Knight Piésold and the community, eThekwini Municipality and the contractor committed to fixing portions of the road that were damaged by heavy construction vehicles and avoided using big trucks during school pick-up times. A 20 m high shade cloth fence was erected around the entire site to limit noise and dust pollution.

Space constraints and aesthetics The residents living adjacent to the construction site generally disapproved of the size and proximity of the structure to the site boundary. “Our scope of works was extended to re-analyse the future demands and to reassess the size requirements of the reservoir. We conducted hydraulic modelling of the combined Knelsby and Emoyeni reticulation systems and found that the reservoir size could be reduced from 30 Mℓ to 25 Mℓ, excluding the old 5 Mℓ reservoir (which would be demolished). This resulted in a smaller footprint with more space between the structure and the surrounding houses. The space between the structure and the site boundary increased from 1 m to 3 m,” adds Slater.

Knight Piésold’s reconfiguration of the network also optimised the flow and balance of the pressures in the system. The demand projections input into the models were all based on 2042 estimated growth. The sizing of the reservoir and pipework were all done for this scenario.

The overall size of the reservoir is 98 m (length) x 34 m (width) x 8.2 m (height) – 4 m of the reservoir is below ground level and 4 m above ground level. Residents were concerned that a large concrete structure would have a negative visual impact that would affect property prices in the area.

“Designers added face brick cladding to the outside of the structure in conjunction with an embankment berm around the structure. This lessened the visual impact that this large structure would have on the homes adjacent to the reservoir site. A grassed embankment was added around the structure as much as the site would allow. This reduced the structure’s visual height from 4 m to 3 m. The site was landscaped and grassed after completion, and additional trees were added. Inlet pipework to the reservoir and inlet control valve chambers were all positioned to the rear of the structure so that they were not visible,” says Slater.

“While Knight Piésold will always focus on client specifications and the technical aspects of any project, we hold a holistic vision. We will always consider the environment and social parts of any project. With the Emoyeni Reservoir project, Knight Piésold took ownership of community engagement, as a lack of communication with the surrounding community could have created significant delays.”

Operational water storage system “In order to keep the system operational during construction, the reservoir had to be

The 8 m high walls of the reservoir were poured in a single lift – reducing the number of construction joints and construction duration

Designers added face brick cladding to the outside of the structure in conjunction with an embankment berm around the structure

constructed into two compartments so that the old 5 Mℓ reservoir could continue to operate while Compartment 1 was built. Only once the first compartment was complete and commissioned could the old reservoir be demolished and Compartment 2 be constructed,” he says.

While water demand is low, only a single compartment can be used. Separate inlets are provided, as are dual outlets – affording the operators the flexibility of using either compartment or both, should the need occur. The dual compartments also give operators the opportunity to undertake inspections and maintenance while keeping the system operational.

Hillcrest has a relatively flat topography and this was taken into account during the study. “Even though there wasn’t a lot of elevation to work with, one of our goals was to minimise pumping costs. We achieved this by eradicating low-pressure zones and by altering the supply network,” Slater explains. “The main line feeding the new reservoir is now the Western Aqueduct pipeline from Umlaas Road. It also feeds designated water towers within the high-level zones.”

Excavation Excavation exposed weathered sandstone with local lenses of friable siltstone. Bearing capacity was not a concern, but

The site was landscaped and grassed after completion and additional trees were added

the variable nature of the material could have resulted in differential movement. Therefore, the unsuitable material was removed to a depth of 1 m and replaced with mass concrete. A soil raft base of G6 and G5 was constructed above this and underground drainage comprised a uniform, 100 mm thick, no-fines blanket with DN75 subsoil pipes embedded.

With a 5 m deep excavation required, and only 2 m to 3 m of working space around the structure, a normal battered excavation was not possible. In order to achieve adequate working space around the base of the structure, the excavation required the slopes to be vertical. Therefore, an earth-reinforcing system – in the form of earth anchors, wire mesh and gunnite – was designed in conjunction with geotechnical engineers.

Conclusion “While Knight Piésold will always focus on client specifications and the technical aspects of any project, we hold a holistic vision. We will always consider the environment and social parts of any project. With the Emoyeni Reservoir project, Knight Piésold took ownership of community engagement, as a lack of communication with the surrounding community could have created significant delays,” adds Slater.

Knight Piésold held several meetings with the community and kept in constant communication, while doing its best to address all concerns. Contract participation goals – requiring that 20% of the contract value be directed towards local labour and companies – were also successfully met.

EMOYENI RESERVOIR

• Client: eThekwini Municipality • Consulting engineer: Knight

Piésold Consulting • Contractor: Afrostructures