What lies beneath: why screw piling matters for raft slabs
A raft slab might look like one large concrete foundation, but what happens underneath it can be just as important as the concrete itself.
Depending on the soil report and structural engineer's design, screw piles may be required beneath the slab to provide additional support and transfer the weight of the building into suitable ground deeper below the surface.
Not every raft slab needs screw piles. When they are specified, however, installing them correctly is an important part of getting the foundation right.
What are screw piles?
Screw piles are long steel shafts with large spiral-shaped plates near the bottom, similar to the thread on a screw.
Using specialised machinery, they're screwed into the ground until the conditions required by the engineering are achieved. Once installed, they become part of the foundation system supporting the raft slab and structure above.
Why would a raft slab need screw piles?
It starts with the soil report.
Ground conditions vary from site to site. Some sites may have reactive clay, fill or other conditions that need to be considered when designing the foundations.
The soil report identifies what's happening below the site, and the structural engineer uses this information to design an appropriate footing system for the building.
On some projects, a raft slab may be suitable without piles. On others, the engineer may specify screw piles to provide support deeper in the ground.
At Mancuso Constructions, we always work from the soil report and structural engineering rather than making assumptions about what a site requires.
How are screw piles installed?
Screw piles aren't simply screwed into the ground to a set depth.
Ground conditions can vary across the same site, so the resistance encountered by each pile can also vary.
During installation, we monitor torque, which simply means how much resistance the machine feels as it screws the pile into the ground.
Think of driving a screw into soft timber compared with hard timber. The harder material creates more resistance and requires more force to turn the screw.
It's a similar principle with screw piling. The depth, ground conditions and resistance encountered during installation are considered against the engineer's requirements for the project.
Why does getting it right matter?
The foundation is designed to support the house and manage movement in the ground below it.
If foundations experience excessive or uneven movement, the effects can eventually appear throughout the house, including:
Cracked walls or slabs
Uneven or sloping floors
Doors and windows that stick
Movement or distortion through the building
Screw piles don't prevent all building movement or cracking. But when they've been specified by the engineer, they're an important part of the foundation system designed to support the structure and manage the site's ground conditions.
That's why something you'll eventually never see needs to be installed properly from the beginning.
From screw piles to raft slab
Once the screw piles are installed, the raft slab construction can continue in accordance with the engineering.
Depending on the project, this can involve:
site preparation → set-out → screw piling → formwork → reinforcement → inspections → concrete pour → curing
The screw piles, reinforcement and concrete all need to work together as part of the engineered foundation system.
Planning a structural concrete project in Melbourne?
We complete screw piling as part of our structural concrete works where it has been specified, coordinating the piling with the raft slab and concrete works that follow.
If you're planning a raft slab or structural concrete project in Melbourne, send through your architectural plans, structural engineering and soil report and speak with Mancuso Constructions about your project.