According to Esor senior contracts manager Anton Naude the main challenges of the contract were the Dolomitic conditions, the necessity of blasting in a built up area close to busy offices and the presence of ground water as three of the six shafts are founded below the water table.
Dolomitic conditions are never easy to work in and demand expert geotechnical investigation and solutions that take into account the highly variable nature of the rock.
Esor’s solution was to sink the shafts using drill and blast methods whilst supporting the sidewalls using soil nails acting in conjunction with fibre reinforced gunite and mesh reinforcing.
“The challenge of shaft sinking in the Dolomites is that the rock formation is decidedly uneven as one goes down, with Dolomitic formations such as pinnacles, cavities and large boulders known as “floaters” of different sizes occurring fairly randomly until solid bedrock is found,” says Naude.
“This unevenness is caused by Dolomite’s susceptibility to dissolution resulting from the percolation of sub-surface water. During this weathering process a residuum known as Wad - a very soft material which is easily eroded or compressed and unsuitable for the founding of Gautrain pier foundations - is left behind,” he says.
Esor’s contracts manager Jonathan Day takes up the explanation. “Wad is very weak and tends to fall out during excavation resulting in overbreak of the shaft sidewalls. This overbreak must then be filled up behind welded mesh and covered with gunite,” says Day.
He adds that a good understanding of the underlying geotechnical structure is a prerequisite of a job of this nature. “We must know as accurately as possible how much blasting will be required, the position and extent of cavities within the underlying strata and the depth to suitable bedrock onto which the shaft can be founded,” he says.
Bombela CJV did this pre-construction work themselves drilling five percussion drilled probe holes per shaft, to a maximum depth of 50m, whilst measuring penetration rate, thrust pressure, torque, air pressure and air flow. This, coupled with borehole radar surveys provided an accurate rendition of the position of cavities, the presence of variable rock and the depth to suitable founding rock.
“So, before we began excavation we knew fairly accurately where the cavities were, where the protruding rock was, at what depth the bedrock was and the relevant penetration rates,” says Day.
A critical factor in the construction of shafts of this nature is to decide where the bottom of the shaft is. “The toe of the shaft has to be socketed a minimum of one metre into hard rock free of any Wad material and open fissures greater than 0,5mm,” says Day. “The designed depth of the shaft is pre-determined by the information gathered from the geotechnical investigation and then confirmed once reaching the specified depth,” he adds.
Once reaching the bedrock a series of five percussion-drilled probe holes are drilled 12m into the rock below the base of the shaft. ”We record penetration rates and take drill chip samples of the rock at one metre intervals to check the consistency of the rock,” says Day. The holes are then water-tested and pressure grouted.
Day says this is known as primary grouting. “Based on the grout volume takes of each of the primary holes, secondary holes may be drilled, water tested and pressure grouted in the vicinity of the primary holes that had high grout volume takes. In some cases even tertiary and quaternary grouting may be required. The grout fills any fissures or voids below the base of the shafts and seals off any ground water allowing us to work in dry conditions below the water table.”
The grouting results, including the percussion drilling penetration rates and drill chip samples, are then used to determine the suitability of the rock for founding the base of the shaft.
Once this is complete the shaft is handed over to Bombela CJV who cast a reinforced concrete base plug and the 0,5metre thick permanent shaft lining. The base plug steel cage is fixed on surface and then lowered onto the base of the shaft. This requires a minimum shaft diameter of no less than 7,0metres. Conversely, to prevent excessive wastage of concrete whilst casting the permanent shaft lining a maximum shaft diameter of 7,2metres is allowed.
On completion of casting the permanent shaft lining and backfilling the shaft with compacted soil a concrete shaft cap is then cast on top of the shaft. This forms the foundation onto which the viaduct pier is constructed.
Another significant challenge was working near the Momentum Life building which contains a significant number of computer servers and other sensitive equipment. Here Esor had to be extremely cautious of vibrations caused by blasting the rock just below ground level.
To achieve this Esor used a device called a boulder buster. This device uses a cartridge filled with explosives to create a pressure impulse, which is directed into the rock via the machine’s barrel which, in turn, is placed into a pre-drilled hole filled with water. When the device is fired the resultant impulse is transferred by the water into the rock causing the rock to break.
Regular blasting techniques using explosives were used in the shafts that were not close to sensitive buildings.
The six shafts were completed by the end of May 2009 and handed over to Bombela CJV for construction of the permanent shaft lining and backfilling. In total 154m of shaft were excavated and temporally supported.
Esor has for more than 30 years been a stalwart in the provision of geotechnical services to the South African construction industry.
Its claim of being the only specialist company in the country able to offer the full range of geotechnical services is vindicated by the wide range of skills it possesses which, in addition to shaft sinking, include piling, diaphragm walling, lateral support, pipejacking, grouting, percussion drilling, guniting, underpinning, dewatering, marine works, and pile integrity testing.
Issued on behalf of:
Esor Limited
Tel: (011) 822.3906
Fax: (011) 822.3112
E.Mail:
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Contact: Mr. Jonathan Day
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