Civil Engineering April 2021 | Vol 29 No 3

When client requirements change, we don’t have to go through cumbersome organisa- tional hierarchies as we have the authority to immediately adapt to that change,” adds Lize Louw, Director at SDG Legal. In addition to agility and cost-effec- tiveness, the consortium is 100% South African-owned and has a high degree of diversity and a large female-owned equity participation.  Keeping houses from a sinkhole A 6.5 m deep sinkhole – 25 m wide and 45 m long – was formed about five years ago in Irene, Centurion and filling it up posed a threat to nearby residences. Gauteng Piling was appointed by the sinkhole repair main contractor, AST Africa Trading 465CC, to provide lateral support for the unstable walls of the sinkhole to prevent the movement of the foundations of two adjacent houses in the upmarket Irene Farm Villages complex in central Irene. “The one residence was about 5 m from the sinkhole and the other just 2 m away. In general, excavations deeper than 1.5 m always need to be secured to prevent the collapse of the walls, and this sinkhole’s walls were more than four times deeper,” says Keoatlaretse Tema, contracts manager for Gauteng Piling. Tema says a Casagrande C6 Crawler drilling rig, suitable for drilling micro piles, soil nails and anchors, was brought to site by Gauteng Piling. The specialised and extremely versatile rig’s capacity to drill vertically, horizontally and at angles was vital for this project. “To provide the lateral support for the sinkhole walls, we had to install 40 micro piles, placed to a depth of 6 m. Micro piles are small diameter drilled and grouted friction steel piles that are bonded into the bearing soil or rock with gunite, a mixture of cement, sand, and water applied through a pressure hose to produce a dense hard layer of concrete,” she explained. “We also had to drill horizontally into the sinkhole walls to install 40 soil nails 6 m long, as well as 40 other soil nails 4.5 m long and 40 more, 3 m in length. The soil nails were inserted and gunited into place into the sinkhole walls. Mesh was attached to the bar ends to hold the slope face in position.” Among the challenges of the contract was the exceptional – and unexpected – hardness of the dolomite rock which delayed drilling progress. It also proved difficult to install the micro piles as the drilled holes were collapsing as soon as the drill bit was extracted. This problem was overcome by using self-drilling anchors. “After we had completed the required lateral support to the walls of the sinkhole, the main contractors filled it with dump rock and pumped concrete,” Tema adds. Nico Maas, MD of Gauteng Piling, says the successful completion of this unusual and formidable contract highlights the value of experience when it comes to pro- viding foundations and site stabilisation.  Drilling of soil nails into the sinkhole walls Civillain by Jonah Ptak: Lab experience For more engineering humour, please visit “Unreal Bridges” on Facebook and “@TheUnrealBridge” on Twitter. Civil Engineering April 2021 61