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What lies beneath

Posted on Sep 2, 2013

CymonBy Cymon Allfrey

Across Christchurch most of our land falls into three technical categories – TC1, TC2 and TC3 – which drive the manner in which we are to approach foundation design when repairing and rebuilding homes. Outside of these categories are the hill zones which, in regards to foundation design, remain relatively unchanged since before the quakes.

As we know, our land in Christchurch is susceptible to liquefaction, so we need to ensure we are designing, building and laying foundations that are robust enough to stay intact in a moderate quake and can be re-leveled.

The foundation of our home should be able to support its structure across small unsupported distances. As in, if the ground beneath your house slipped or fell away in a small area, your foundation should be able to hold the house securely in place without breaking and causing the house to slump.

However, it is what lies beneath the foundation that we need to address to ensure we are constructing and rebuilding robust homes that have been designed to keep us as safe as possible.

In broad terms TC1 land is business as usual; we are to build as though nothing has changed. Prior to the quakes we were undertaking site specific soil tests and in TC1 we are still doing this. These site tests are looking for uniqueness in the land in question – areas on the site that have been filled, the presence of organic materials such as peat and so forth. This is something that has always been done and which has always had specific design principles associated with it. Building codes and standards like that of NZS3604 control how to design foundations in these sites ensuring the end result is defined as ‘good’ ground.

In TC2 and TC3 land however, the condition of the land may need to be lifted to meet this ‘good’ ground standard.

While conceivably foundation solutions for TC2 and TC3 land may be similar, it is how we treat the land underneath that is very different.

Before a foundation solution can be reached in TC3 and those TC2 sites at the higher end of the scale, the land quality needs to be addressed – ensuring that what is to be constructed above will not only be repairable, but work to bring the quality of the ground up to scratch.

While the damage to these sites is often extensive and costly to combat, unlike Red Zone areas, these sites can be brought to a level which permits repair or rebuilding. It is important to remember that each of these sites, like those in TC1, are unique and site specific geo-technical engineer assessments are looking to reveal uniqueness. In these sites, the assessments are looking at things like, soil type, the ability of the ground to suppress liquefaction, proximity to rivers for lateral spread and so forth.

The way in which we repair these sites can range from shallow excavation (say 1-2 metres) below a building footprint - making way for an engineered compact gravel fill which will create a base on which to suppress liquefaction and provide a firm base to level a home - to the more onerous use of deep piles.

In some instances the use of a gravel raft is suitably adequate, while in others it may need to be combined with piles to support the ground repair. The use of piles provides a resistance when the ground moves and they can vary greatly in their depth. And depending on the capacity required, size and nature of the home, these piles can vary in material from concrete to timber to steel.

In most TC3 sites, gravel rafts are emerging as popular and adequate solutions, as they ensure that the problems underneath – the inadequacies in the land – are being dealt with where combined with a robust foundation and flooring construction.

If your land is classed as TC3, or you are required by a geo-technical engineer to undergo major floor and foundation repairs, it is possible that you will be required to repair the ground also.

It is important to be aware that it is the cost of this repair that is seeing a large amount of contention in TC3 land areas. In many instances TC3 land homes are being demolished due to the cost of repair out-weighing the value of the house.

Generally speaking, it is possible to surmise that deeper concrete floors or an engineered pod system – which sees reinforced concrete beams spanning across the floor – together with suitable ground should result in the floor of a home being repairable, reducing the risk of having to deconstruct the house to repair its foundations.

With more than 20 years industry experience, Cymon Allfrey holds the Chair of the ADNZ National Board and has won the ADNZ National Supreme Award three times in the past five years.
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