Strong Foundations For An Earthquake Resistant Home

Unfortunately earthquakes are a part of life in New Zealand. Geonet estimates that there are around 250 earthquakes felt each year. Most are small enough to cause very little damage, however there are unexpected large quakes which can be extremely damaging. 

For the health and safety of the population, lessons about house and building design need to be learnt from the earthquakes of the past, especially the big event in Christchurch, and more recently Kaikoura. These big event quakes can also show seismic engineers which technologies are effective in the event of earthquake.

When structural engineers and architects design buildings, either high rise or residential houses within a seismically active area, they design with two key goals. The number one priority, is to keep the building intact long enough to allow the occupants to escape to safety, in the event of a major earthquake. A secondary priority of the design, is to enable the structure to remain functional and habitable if a small earthquake strikes.

Designing from the ground down

As most buildings sit on foundations, which contact the earth, the design, preparation and construction of a building’s foundation is of prime importance within a seismic region. Experience has shown that the ground works for a building, can contribute to a building surviving a quake. If a building is constructed on loose or filled-in soil, there is more likelihood of it sliding off its foundation. When it is constructed on solid, firm ground or even better bedrock, a structure is more likely to remain intact through an earthquake.

Before constructing a building’s foundations, it is important to prepare the ground surface. Removing any loose soil, and digging down onto firm compacted soil is a basic level of preparation. If there is a risk of a moderate earthquake, sinking piles onto solid bedrock, will provide the structure with more stability.

Designing foundations to suit the risk

Depending on the level of seismic activity, within a region, there are a number of foundation styles which may be used. There are four basic types of foundation used in earthquake prone areas. Ranging from the basic floating slab, through to deep piled structures. The decision of which one to use, should be made in conjunction with an architect after studying the regional building code. As expected, the construction costs increase with the level of complication, and security.

Large steel reinforced floating slabs

This is often the standard type of flooring in brick, and often timber framed modern buildings. It consists of a footing under the load bearing walls, ideally the footings should extend in depth to exceed the frost depth for the area. Then a foundation wall, which is poured around the entire foot print of the building. Once this wall is set, a large steel reinforced slab of concrete, which covers the entire floor area of the structure is created. Each stage of the construction needs to set and cure, before the next section is poured, this curing time is usually about a week.

Steel reinforced grade beam slabs

In areas where more earthquake activity may occur it is possible to provide greater reinforcement to a slab floor, to provide more anchorage, and more stability to the structure. After the foundation of the floor is cured, a structural steel or steel reinforced concrete sill is created to provide additional support to the flooring of the building. These grade beam foundations work very well on houses with non-traditional footprints, like hexagonal structures. The size and design pattern of the graded beams will need to be calculated by an architect, based on the structure, and the level of the earth movement anticipated in the geographical region. Although more expensive than a floating slab, it is cheaper than the more expensive ground work options.

Pinned to stable rock formations

Where greater stability is required due to frequent or moderate sized earthquakes, it is essential to pin the foundation of the house to a stable rock formation. This is achieved by locating, a bedrock formation, and building the footings directly onto the rock. Where the depth is too great to clear the overriding soil, sink the reinforced concrete piles onto the rock base, then build the footings of the house or other structure on the footings, to achieve a stable and strong build.

Deep anchored pedestal structures

In areas with high seismic activity, structures require high levels of stability. This stability is achieved by developing a deep anchored foundation with a small foot print, and then building the house on a pedestal. Reinforced piles are driven or poured, to a great depth, to anchor the structure into the ground. Instead of creating a footing for the entire footprint of the building, a very strong, reinforced and compact pedestal is built above the surface of the ground. This pedestal, then supports the entire structure, through the central core. The floor, walls and the roof are all anchored to the central pedestal, producing a very stable structure.

Read more about the builders of sound foundations at digger.nz