Leading geotechnical solutions provider Franki, part of the JSE-listed Esorfranki group, has employed David and Goliath on a building site in Mozambique. But, unlike in the biblical story, this David and Goliath are not adversaries.

On the contrary! ’Goliath’, a Bauer MGB24/48 – one of Franki’s biggest and most expensive rigs – and ‘David’, a Dynamic Cone Penetration (DPSH) test rig and one of its smallest and most inexpensive rigs, are working side by side on a challenging contract in Maputo.

The contract, for the piling for the ‘Tip-Top’ office block tower with two parking garages, is situated on Maputo’s 25th September Avenue - one of the most prestigious locations in Southern Africa – overlooking the Bay of Maputo and marina. Valued at more than R4.5 million, the contract was awarded to Franki by Insitec Constroi.  

‘Goliath’ – the Bauer MGB24/48 - is a unique piling rig used to install Full Displacement Screw (FDS) piles, which are advantageous in a host of differing conditions.

Franki technical director Gavin Byrne explains. “FDS piling’s well-established track record in other parts of the world shows it to be a fast and economical piling methodology with no vibrations and limited noise levels and our burgeoning experience with this technique in Southern Africa supports this completely,” says Byrne.

He adds that the FDS has overcome many of the negative features of the Continuous Flight Augur (CFA) piles.  “While (CFA) piles, also known as auger cast piles, have become increasingly popular since their introduction some fifty years ago in North America because of the high production rates they can achieve and their excellent cost-efficiency, there is still room for improvement.”

The positive features of the FDS method are:

High production levels attainable in suitable soil conditions.
The soil displacement technology delivers good pile performance at significantly shallower depths to CFA and bored piles.
The system is vibration free.
Noise levels are low and limited to the engine noise of the piling rigs.
The pile shaft can be reinforced over its full length using a variation to the normal installation methodology.
Pile load capacity can be ensured on each pile by monitoring the installation energy.
No soil removal is required with the soil displacement technology.

The ‘vibration free’ feature was the tipping point as far as the Tip-Top site was concerned. “The neighbouring site housed the old Televisao de Moçambique (TVM) building and prior to commencing work a study was done of the building and all the cracks in the structure noted,” explains Franki Mozambique branch administrator Ilonja Rabe.

“The client’s main concern was that the vibrations of a normal piling rig on the already cracked TVM building could further damage it. This was completely averted by using ‘Goliath’,” she says.

Rabe added that because of the large number of older – normally un-piled - buildings in Maputo, the FDS system is proving increasingly popular. “

In the last six months Franki has used ‘David and Goliath’ on four separate contracts – Campismo, where 280 piles were installed in four weeks; Horto, where 360 piles were installed in seven weeks (including two weeks delay); JAT where 246 piles were installed in five weeks and Tip-Top where 322 piles were installed in 5 weeks.

Apart from the advantages of vibration free and fast track installation, Rabe also points out that generally speaking the Maputo ground conditions are ideal for this pile type. “Currently Franki has out for tender six more contracts all based on the FDS pile.”

“Overall the FDS pile has generated considerable interest from the local engineering fraternity and clients and we have had excellent feedback from those for whom we have used it.” 

Turning to ‘David’, Rabe says it assists in determining the depth to which the FDS pile should be founded to give optimal load capacity for the specific pile size. “Generally the soils information supplied for tender purposes is minimal and, on these jobs, we use the DPSH rig to supplement this information and provide us with more detailed geotechnical information for the specific site where the pile is to be installed.”
 
A disposable cone is fitted onto an extendable rod and driven   down using a fixed-mass free-fall hammer with a fixed drop height. The number of blows required to drive the rods through a specified distance gives the DPSH value. These rods are driven to “refusal” which is generally where the blows exceed 100mm for 300mm in depth. These results give an indication of the resistance of the ground to penetration and thus give an excellent indication of where piles can be founded.                                                      

Rabe says that on the Tip-Top site varying soil conditions were found with ‘refusal’ depths ranging between 5 and 10 meters. “As the top layers were dense with high alpha values (measurement of penetration resistance), it was impossible to determine the depths without having very comprehensive soils investigation across the site and ‘David’ was really the hero in this regard.”

She added that it was almost comical to see the tiny ‘David’ moving around the site “with the monstrous ‘Goliath’ in obedient tow.” 

Rabe says they have had to acquire additional plant for these contracts especially concrete pumps and compressors. “Against common advice, we opted to purchase Chinese concrete pumps rather than their more popular but very expensive European counterparts.  We’ve had them some six months now and we’re most impressed with their performance,” she says.

She adds that ready-mix supply in Maputo for fast track work like FDP piling has been most unreliable. “Deliveries are erratic and can cause major delays. We found that mixing the concrete on site with Fiori mixer trucks by and large solved the problem. Using two trucks which continuously supply each Bauer rig, ensures good production with no delays,” she concluded.

Issued on behalf of:    
Franki Africa (SA) (Pty) Ltd   
Tel: (011) 531.2700    
E.Mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it    
Contact: Mr. Gavin Byrne