Here it is confirmed by Mike Stannard from MBIE that surface structures do not work in the Eastern suburbs where there is 30-40 m down to gravel.
High groundwater and liquefaction down to 20m or more.
So far NO SOLUTIONS are available.
“There needs to be a decent thickness in the layer you’re founding on to.”
A single-storey house needs at least three metres of solid ground, and a two-storey house needs four metres.
Deep piling will not be an option on land with a major risk of liquefaction or lateral spread.”
“Site ground improvements This technique is new to residential housing but has long been used by civil engineers for road building. Land is strengthened by one of two methods: mixing cement with the ground to make it denser, or compacting up to two metres of earth to give a better crust for building on.
Once treated the land can be piled or have a concrete pad placed on top of it.
Again, it isn’t suitable on land with major liquefaction risk. Anywhere with liquefiable soil deeper than 10m is out, Stannard says.”