Driven Ductile Iron Piling solution proven in boulders and cobbles
Driven Ductile Iron Piling (DIP) is a simpler, faster and safer method of installing piles compared to other conventional techniques.
The DIP approach is especially well-suited for challenging ground conditions with limited geotechnical information. The penetration rate of each pile is recorded per metre. In the case of end-bearing piles they are driven to practical refusal, with the final penetration “set” dictating when the pile is complete.
“Continuous flight auger piles and driven cast in-situ concrete piles tend to refuse where boulder/cobble obstructions occur,” explains Warwick Nel, New Business and Project Manager, Geopile Africa. “In other words, they often fail to penetrate through to reach suitable founding material.”
“The only traditional method that can compete with DIP in this geology is percussion drilling. However, the latter option is very expensive, requiring a specialised drilling head and the insertion of a casing to cast the pile. DIP is therefore the most competitive approach,” he continues.
The point is well illustrated by a recent Geopile Africa project in Stellenbosch, Western Cape. Here the geology included the presence of a 2 to 3m boulder layer interspersed in a loose sandy clay matrix. Individual oblong-shaped boulders were up to 800mm in width and length.
Faced with these challenges, the client’s professional team had recommended a deep raft foundation. However, shortly before the commencement date for construction, Geopile Africa was invited to carry out a trial installation to argue the case for a DIP design.
“This provided the perfect opportunity to prove that we could drive our DIPs through the boulder layer to reach bedrock,” Nel explains. The trial was carried out free-of-charge and Geopile was subsequently awarded the project. The DIP solution cost around R1 million less than the raft foundation option for the Stellenbosch project and was faster by approximately two months.
Before test piling commenced, two rotary cored boreholes were drilled by Geomechanics, a GeoGroup sister company, to verify the depth to bedrock. Geopile Africa then proceeded with the installation of three DIP piles, which all comfortably penetrated the boulder layer. These piles were installed using a 30 tonne hydraulic excavator fitted with a purpose-designed demolition hammer specified by the DIP manufacturer. This hammer achieves an output of approximately 280 tonnes per blow at up to 400 blows per minute.
“We reached refusal in medium hard sandstone bedrock at between 6 and 11 metres from platform level,” says Nel.
For the actual project, a combination of 118mm and 170mm diameter DIPs were used, installed either open-ended and/or with driving shoes. The latter keeps the pipe free of debris, ensuring a 100% concrete fill.
“Open-ended piles were used in lift and stair shafts where resistance to shear meant that the piles needed to carry some tension,” he expands.
The open-ended piles, socketed into the medium hard sandstone bedrock, provided tensile resistance from skin-friction on both the inside and outside of the pile. These piles were reinforced with a centrally located y32 bar with 500mm “L” protruding 500mm (or to the underside of the pile cap reinforcing cage).
Piles with driving shoes were installed for compressive column loads. They were filled with 30 MPa concrete, thus providing 5% more load carrying capacity. Compression piles were fitted with a Y16 rebar to centrally place the load plate: 200*200*30mm for 118/7,5mm piles (550kN SLS) and 250*250*40mm for 170/9mm piles (990kN SLS).
Low-cost housing development
Other Geopile Africa DIP projects currently under way include a low-cost housing development in Cape Town. The development entails the construction of a three-storey apartment building that will tie into an existing one.
The current building rests on a raft foundation, while the new one will be supported by a DIP installation. As with the Stellenbosch project, DIP proved to be the most cost-effective approach.