Challenging soil conditions necessitated contractors to adapt on site and opt for a concrete raft foundation rather than follow the initial piled foundation design.
During the construction of the Hazeldean Office Park development in Pretoria, the contractors, JC van der Linde & Venter Projects, and piling sub-contractor, VNA Piling, were confronted by very complex ground conditions right from the early stages.
A large portion of the site for the 800m² three-storey structure was overladen with large boulders and extremely loose soil.
“The boulders restricted drilling to a depth of just one to one-and-a-half metres to sink the piles that were initially specified for the foundations of the buildings. The removal of the boulders required extensive use of excavators. And, in some places, when the derrick was removed, the drill hole would rapidly fill up with sand and rocks, which then had to be removed by hand,” says Thomas Joubert, director of JC van der Linde & Venter Projects.
This problem significantly delayed the start of construction on the larger building, while the adjacent three-storey structure was founded on ground conditions that allowed for more routine piling and could go ahead.
Constructing the concrete mat
To avoid further delays, Joubert, in consultation with SiVest, suggested that a large raft foundation should be erected on the challenging terrain. While it cost slightly more than the initial piled foundation design, a week of valuable construction time was saved.
Designed by structural engineering consultants, SiVest, the raft foundation was an 800m² free-standing 600mm thick concrete slab. “A raft foundation is basically a concrete mat – or continuous footing – that can support an entire structure. On sites with low soil-bearing conditions, the raft distributes the building’s pressure over a large area so the soil can bear the stress,” Joubert explains.
“The raft’s construction called for about 420m³ of concrete. We had to mobilise the entire resources of the Eastern Readymix batching plant nearby to supply our material needs, and had 13 concrete-mixing trucks driving to and from the batching plant to feed two concrete pumps to finish the 12-hour continuous concrete pour for the raft foundation,” he added.
Once the raft foundation was completed, the construction on the building continued rapidly and was completed in June 2016.
A piece of the pie
Situated near the Silver Lakes Golf Estate, this office park is an important component of an 800-hectare nodal development by local property developer, Abland, which also comprises residential and retirement villages, schools, churches, clinics, colleges, retail centres and showrooms.
The buildings have collectively brought an additional 5 600m² of floor space to the Hazeldean Office Park.
JC van der Linde & Venter Projects
Tel: 012 803 0392
Caption: JC van der Linde & Venter Projects recently completed new additions to the Hazeldean Office Park development in Pretoria East.