The Australian Standard AS2870 ‘Residential Slabs and Footings’ sets out the requirements for the classification of a site and the design and construction of footing systems for houses.
There are a few basic foundation types which can be considered during the foundation design for a building.
Stiffened Raft Slab: A reinforced concrete slab which has integral beams along its perimeter and a grid of beams internally. The concrete for both the beams and the slab are all poured at the same time. Raft slabs are generally suited to flat sites.
Waffle Raft: A waffle raft is similar to a stiffened raft slab except that the beams are not cut into the ground but formed using polystyrene void formers. The polystyrene void formers (pods) are placed on the ground with spacers between them to form gaps which later become the reinforced concrete beams. Perimeter beams require formwork to create the outer edge. Beams are typically much thinner and more closely spaced than in a raft slab. Waffle raft systems were developed in the 1970’s in South Australia. Early void formers were made from cardboard.These were supplied to the market by James Hardie. The cardboard boxes though lost their strength when they got wet in the rain and partial boxes made by cutting did not have sufficient rigidity. Later (mid 90’s ) waffle void formers or pods were made from 225mm thick solid polystyrene. Current waffle pods are made from polystyrene and have a ‘waffle within a waffle’. That is the polystyrene blocks have ribs and voids underneath. A typical size of a waffle pod is 1090mm x 1090mm and typical depths are 150mm, 225mm, 300mm and 375mm. The beams formed by the gaps between the pods are typically 110mm and spacing of the pods and the positioning of the reinforcement is achieved by plastic spacers purpose built to achieve this. Generally a waffle raft slab is more cost efficient than an equivalent raft slab. This cost saving is generally due to less concrete as the main portion of the slab can be made 85mm thick. A waffle raft is suited to flat sites and site excavation costs can be reduced by not needing to excavate for beams as is the case in stiffened raft slabs.
Strip Footings: Strip footings are reinforced concrete beams which generally follow the profile of the natural ground and the height difference between the natural ground and the floor level is accounted for by masonry walls. Strip footings can be used to support reinforced concrete slabs or timber floors. Strip footings are generally suited to sloping sites. Steps can be incorporated in strip footings allowing them to follow the natural shape of the ground. Step configurations should be constructed to details contained in AS2870 or your structural engineering drawings.
Both raft slabs and strip footings can be made to standard designs contained within AS2870 (Residential slabs and footings)
Pier and Beam: Generally pier and beam or pier and slab footing systems are designed in accordance with engineering principles. That is there will be no standard design in AS2870 and all reinforced concrete should be designed to AS3600 (Concrete Structures).
Isolated piers and pads: Isolated piers and pads are used to take localised concentrated loadings from constructions including timber pole frame houses, steel posts, masonry piers and reinforced concrete columns.
Screw Piles: Screw piles are as the name suggests large screws which are installed by applying a torque. The ultimate compression capacity of a screw pier is related directly to the installation torque (Ultimate capacity in kN is approximately 10 x the installation torque in kNm).