CONCRETE SUPPORT A total of 32 concrete piles were needed on the Mbombela Fresh Produce Market project
Concrete foundation specialist Gauteng Piling’s most recent projects have showcased the company’s diverse capabilities in the concrete piling industry.
Gauteng Piling contracts manager Keoatlaretse Tema tells Engineering News that two of the projects include the installation of concrete piling and foundations for a new pedestrian bridge at a produce market in Mbombela, Mpumalanga, as well as the stabilisation of a sinkhole threatening a residential area in Irene, Tshwane, in Gauteng.
The project in Mpumalanga was completed in May while the one in Gauteng was completed early in the year.
Tema says Enza Construction, the main contractors for the construction of the Mpumalanga Economic Growth Agency International Fresh Produce Market at Mbombela, awarded the footbridge piling project to Gauteng Piling.
The 1 260 m2 concrete pedestrian bridge will link the market with a large taxi rank adjacent to the market, she adds.
The foundations for the pedestrian bridge required 32 piles, all 45 cm in diameter, to be sunk to depths varying between 8 m and 15 m, while 30-cm-diameter steel cages were used as reinforcement for the concrete piles.
“We brought one of our Williams straight-shafted auger diggers from our yard in Diepsloot, Gauteng, to handle this project,” she says.
During the piling process, soil and rock were loosened by the digger and removed on a continual basis to gradually create the cavity in preparation for concrete piling.
A major benefit of this type of drilling is that it can be conducted with comparatively low noise and vibration levels. Assembly of the piles is relatively quick and does not require any temporary casing.
Sinking Feeling
For the Irene sinkhole project, Gauteng Piling was contracted to provide lateral concrete support for the unstable walls of the sinkhole to prevent the movement of the foundations of two adjacent houses in the Irene Farm Villages complex.
Tema explains that the project required a bespoke solution, owing to the close proximity of the sinkhole to houses.
The 6.5-m-deep sinkhole – about 25 m wide and 45 m long – was within 2 m and 5 m of the houses.
“In general, excavations deeper than 1.5 m need to be secured to prevent the collapse of the walls and this sinkhole’s walls were four times as deep,” 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,” she enthuses.
To provide the lateral support for the sinkhole walls, Gauteng Piling had to install 40 micro piles, placed at a depth of 6 m.
Micro pilesare small diameter drilled and grouted friction steel piles that are installed by drilling to the required depth, with the drilling rig then grouted up while installing a very high tensile strength bar.
Thereafter, soil nails were inserted and gunited into the sinkhole walls after mesh was attached to the bar-ends to hold the slope face in position.
The guniting process involved a mixture of cement, sand and water applied through a pressure hose to produce a dense, hard layer of concrete.
Among the challenges of the contract was the exceptional – and unexpected – hardness of the dolomite and chert rock, which delayed drilling progress, says Tema.
It also proved difficult to install the micro piles, as the drilled holes collapsed as soon as the drill bit was extracted; self-drilling anchors were used to overcome this problem.
“Self-drilling anchor rods have sacrificial drill bits, with hollow steel bars of the appropriate outer and inner diameter and coupling nuts. The rods’ design provides consistent grout cover around the hollow steel bar and ensures that the bar remains in the centre of the drilled hole,” explains Tema.
After the required lateral support to the walls of the sinkhole were completed, the main contractors on the project – earthmoving contractors AST Africa Trading –filled it with dump rock and pumped concrete.