Will you be applying to build a new building?
Will it be located ON THE Hauraki Plains?
If you are going to build a new building and in particular a dwelling on the Hauraki Plains you need to be aware of the risk of settlement of the foundations. Typically the subsurface profile on the Plains consists of tens of metres of compressible estuarine sediments which settle (consolidate) when loaded. For a building on fill the amount of settlement is generally proportional to the load imposed by the combined weight of the building and the fill platform. Settlement is also generally proportional to the width of the fill platform.
Generally the floor level of a house on the Plains needs to be significantly higher than the existing ground level. If this is achieved by a fill platform and slab on ground then there is potential for significant total and differential settlements. The types of mitigation options that could be considered by your Chartered Professional Engineer are the following:
- Pre-loaded building platform
- Lightweight fill
- Construct subsurface permeable drainage path system
- Settlement reducing piles
- Structure stiffening
A report by Tonkin & Taylor Ltd – Environmental and Engineering Consultants commissioned by the Hauraki District Council in 2009 discusses these options. View the report on Hauraki Plains - house sites (2.35 MB) .
The mitigation options provided in the report generally offer the choice of deferred construction versus higher mitigation cost. It is possible to use Options 1 and 3 in combination to achieve savings in time at a cost less than Options 2, 4 or 5. Option 5 can be used in combination with any of the other options, although it is suggested that the cost weighed against the marginal benefit of doing so would not be attractive.
The use of its findings as a guideline for evaluating a foundation design by your engineer for the building consent application is strongly recommended.