Shallow foundations – residential
A shallow foundation distributes loads from the building into the upper layers of the ground. A shallow foundation (as defined by B1/VM4) is one in which the depth from the ground surface to the underside of the foundation is less than five times the width of the foundation. All other foundations are considered deep foundations.
Shallow foundations perform very well on sites with strong soils, sufficiently thick natural gravel rafts overlying weaker soils or where robust, engineered ground improvement is carried out. For sites with weaker soils, it may be more economic to improve the ground and use shallow foundations than to use deep pile foundations.
Shallow foundations are susceptible to any seismic effect that changes the ground contour, such as settlement or lateral movement, or alters the bearing capacity of the upper soil, such as liquefaction.
Primarily, shallow foundations must be designed for the load combinations given in AS/NZS 1170 Structural design actions, as amended by Verification Method B1/VM1.
There are several types of shallow foundation that can provide resistance to seismic loads.
Spread footing foundations
Spread footing foundations are a class of foundation that distributes loads from the superstructure, including seismic forces, horizontally across sufficient surface area that the bearing strength of the soil is not exceeded. On some sites, this may mean the foundation covers a larger area than the footprint of the building.
Spread footing foundations may be designed as one uniform block, as with slab-on-ground foundations. They can also be designed as a series of smaller continuous or non-continuous footings that support walls, columns and other parts of the structure. This is the case with perimeter foundations.
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A perimeter foundation is a type of spread footing foundation that consists of a concrete perimeter footing that extends around the whole perimeter of a dwelling. The width of the footing distributes gravity loads from the wall and roof of the structure across sufficient surface area that the bearing strength of the soil is not exceeded. Seismic loads are also transferred efficiently through the perimeter foundation to the soil beneath.
Perimeter foundations may use irregular-shaped footings and be combined with other types of foundations. Non-continuous perimeter foundations (for example, perimeter foundation elements at only the corners of the building) are generally not suitable for sites that are likely to experience differential settlement or lateral movement.
Slab-on-ground foundations, sometimes called traditional slab foundations, are a type of spread footing foundation where the slab ties the spread footings together as well as providing a solid flat floor surface. They are typically formed by excavating the ground to a shallow depth, building edge formwork and pouring concrete into the cavity.
Different types of steel-reinforced slab-on-ground foundations are widely used in residential construction because they provide a high degree of durability and seismic resilience. They are also relatively inexpensive to construct.
Changes to slab-on-ground reinforcing requirements
In response to the poor performance of unreinforced or non-ductile reinforced concrete slab-on-ground foundations on marginal or poor ground in the Christchurch earthquakes, the New Zealand Building Code (Acceptable Solution B1/AS1) was amended in May 2011. It requires all slab-on-ground foundations constructed according to NZS 3604:2011 Timber-framed buildings and NZS 4229:2013 Concrete masonry buildings not requiring specific engineering design to be reinforced with ductile reinforcing. The amendment also requires all perimeter foundations to be tied to the concrete slab with reinforcing steel.
Mat foundations, also known as raft foundations, are a type of slab-on-ground foundation used in both residential and commercial construction. The residential variety consists of a thick concrete slab, several times thicker than a traditional slab, reinforced with a larger amount of ductile reinforcing.
A mat foundation is often suitable for sites with marginal soil that does not require deep foundations but could undergo substantial differential settlement. They can also be used for buildings that are likely to experience large lateral forces during a major seismic event. In this situation, the stiff raft can bridge over soft areas and slide on the ground if lateral spreading occurs beneath it.
A waffle foundation, also known as a ribbed foundation, is a type of slab-on-ground foundation that consists of a series of evenly spaced reinforced ribs surrounded by a wider perimeter footing, topped with a reinforced slab. Voids are created in the foundation, usually using polystyrene pods, separated by the ribs.
Waffle foundations are typically lighter, stiffer and several times stronger than traditional slab-on-ground foundations.