| PROJECTS |
“All developments must comply with an approved JRA form of stormwater attenuation to prevent downstream flooding, erosion and pollution.” 11,5m in the south-western corner. B ot h t he pond a nd t he bu i ld i ng platform walls were built with the lower half of each in closed-face configuration, using MVA-7 blocks and sliders supplied by MVA Bricks. The top sections were constructed in open-face configuration. Moreover, the cut-and-fill walls and the backfill in the attenuation pond walls were cement-stabilised. Maccaferri’s WG-6 geofabric was used to reinforce the fill material behind each wall at every fifth block course.
for building attenuation ponds and to save space, we made the ponds relatively deep. We looked at several building materials to meet our design criteria, but in the end, the MVA-7 block proved the most economical option,” says Croswell. Balancing act Croswell explains that all the material for general f ill and the selected layer works was sourced on site. “However, we did find some unsuitable material (about 20 000m³) at a depth of 300-400mm which had to be removed. This meant we had to adjust the level of the platform so that the cut-and-fill and the fill behind the block walls balanced and we used a sophisticated software program to do it. Moreover, there wasn’t as much rock on site as we’d anticipated, although there was enough to stabilise the critical areas.” The retaining system walls and the pond walls were built at 70º. The northern boundary platform wall is a f ill wall 260m long, r ising from 0,5m in the north-western corner to a height of 9m in the north-eastern corner. The western wall is a cut structure 315m long, rising from 0,5m in the north-western corner to
Future developments All the walls were built with 45º wick drains every 1,5m. They discharge into bottom drains and, in walls higher than 4m, into mid-level drains as well. Croswell adds that the ponds were designed for attenuating a 25-year post-development run-off, discharging at the five-year predevelopment outflow. The ponds have a weir capacity which accommodates a 1:100-year storm event. All the water collected off the building and the concrete platform which surrounds it drains into both western ponds. Lined (Armorflex) channels have been installed
along the bottom of the ponds to cater for low flows and an overflow culvert was constructed at +1,5m for draining high outflows from the south-western pond. The discharge from the south-western pond is routed through the north-western pond before being discharged into a 10 000m³ sub-reg ional pond via the township’s stormwater system. The northeastern pond drains into an open channel which runs along the township’s eastern boundary. It also discharges into the subregional pond, which is used to distribute greywater to the entire estate, primarily for the flushing of toilets and for watering plants. “The greywater is partially purified through the removal of hydrocarbons by the vegetation which grows at the bottom of all the attenuation ponds on the estate. Each developed site is allocated a greywater quota at a ratio determined by its catchment area and the water is pumped through a set of solenoid valves. Smaller sites use header tanks for storing the water, whereas two 10 000-litre JoJo tanks will be installed at the ‘One on 75’ site,” says Croswell. (Top left): The grass-covered bottom of one the attenuation ponds shortly after a severe thunderstorm. (Far left): A close-up of an attenuation pond closed-face wall which was built using MVA-7 retaining wall blocks and sliders. (Left): One of the attenuation ponds under construction. (Above right): A water channel runs adjacent to one of the attenuation walls which was constructed in a closed- and open-face configuration.
PRECAST | ISSUE ONE | 2021
29
