One Big Dam ChallengeThe R212 million Imvutshane Dam in rural Maphumulo, KwaZulu-Natal (30 km north-west of KwaDukuza) has been completed. This will form a key link in creating a dependable source of safe drinking water and ending water scarcity during winter seasons and droughts in the region.
According to MBB Consulting Engineers Pietermaritzburg, appointed by Umgeni Water to design and supervise the construction of the 3.1 million m³ Imvutshane Dam, the project offered some fascinating challenges.
“The geology of the site of the dam, which has a 28 m high, 305 m long, earth embankment, posed some challenges to the design and construction of the dam. Highly weathered and fractured rock was encountered in the area designated for the spillway. To overcome this, a 40 m wide concrete lined spillway was built which is anchored into the surrounding ground using soil nails, as lateral support, for additional strength to ensure that the spillway can withstand extreme flood events”, commented Tyler Bain, MBB’s Project Engineer.
The foundation material of the dam in the existing river bed comprised sandstone boulders in a matrix of sand and quartz, sandstone and gravel alluvium up to 6 m deep in places and was not suitable to support the dam’s foundation. Excavations normally required to remove this unsuitable material and replace it with better material were not practical or cost effective.
To overcome this, jet grouting was used to form a strong, watertight foundation for the dam’s embankment. Jet grouting involves the formation of columns of soil and cement created by drilling into the alluvial material and injecting cement grout at high pressure into the ground. The overlapping, underground columns form an impervious “perimeter wall” up to 12 m high.
The alluvial material was also unsuitable to support the outlet and scour pipes, and, therefore, these pipes were moved to a better location on the right flank of the dam embankment.
The pipes are housed within a 120 m long, 2.4 m diameter wide, concrete sleeve pipe which has been pushed through the weathered rock using hydraulic jacks. This took place simultaneously with the excavations for the 2.4 m diameter concrete sleeve pipe. Cement grout was pumped at
high pressure around the concrete sleeve pipe, for the full length of the pipe, to ensure that the voids between the concrete sleeve pipe and the rock surrounding the pipe were water tight.
The construction of the Imvutshane Dam forms part of the extensive R402 million, Maphumulo Bulk Water Supply Scheme being implemented by Umgeni Water, as bulk water supplier to the iLembe District Municipality. In addition to the Imvutshane dam, this water supply scheme includes the construction of the Maphumulo Water Treatment Plant, an extensive network of pump stations, bulk water supply pipelines and reservoirs for treated water distribution.