Catch-up Blog 13: Piles and Engineering (Oct 2018)

We had a “small” detour during preparation for the foundations, so adding a little more detail here for the record. The journey is not always a clearly marked path on a sunny day… in this case at times it felt like we were about to fall into a black hole, faith was required…

With the plans already submitted to council for the building consent, we locked in the final siting of the house on the building platform with compass, pegs and string lines.  It was clearly encroaching on the 10-metre setback from the edge of the building platform that was noted in the subdivision Geotech report for the building platform.  This meant updated Geotech would be required and, dependant on what they found and recommended, updated structural engineering for the foundations and so an update to our building consent submission already with the council.

Step 1 was approaching Civil Services to update their original Geotech report, they in turn engaged Initia as the consulting Geotech Engineers. We quickly discovered that, as a result of the major earthquakes in Christchurch and Kaikoura in recent years, Geotech had developed into a major industry in NZ and our engineers had consulting engineers...  While it added additional complication and costs to the design and build, it was still accepted on the principle that the testing and engineering would strengthen our house - an important investment…

With Civil Services initial site tests completed, the core samples showed that the North West corner of the building would be over some fill from the platform cut and Initia confirmed that the foundation design would require revision.  This lead us back to GK Consulting our Structural engineers for the house foundations, and unfortunately their principal engineer was overseas visiting family.

We are very grateful to Geoff as he picked up the design revision while still overseas and came back with a revised design incorporating 21 piles along the North face of the foundations with a minimum depth of 1.2 metres, the tips a minimum of 300mm into natural ground, augured and concreted in place.  These piles would lock the raft slab into the hill to eliminate subsidence risk.

So, it was back to council with the updated Geotech report and revised foundation design while we were still within their RFI period for our building consent.  The change was incorporated into the overall building consent approval we received on 1st October.

We would need to get the piles put in place as part of the earthworks, before beginning work on the concrete raft slab foundations, so to minimise handoffs we gave the job to our concrete contractors MPT so they could place the piles based on their own profiles as they need to lock them into their concrete slab reinforcing.

We had Civil Services come back to inspect the pile holes and perform further tests before we cemented the piles in place.  This resulted in us putting in 22 piles averaging around 2 metres with some nearly 3 metres deep, we made these decisions on the principle that it was better to be safe than sorry - no short-cuts. 

So, our first concrete poured was for the piles, a big day for the build! and as a bonus we managed to get an 8 wheeler Bridgeman’s ready-mix concrete truck up to the building site and turn it around, it didn’t make a dent in the engineered fill we had laid for the driveway and foundations, a great start as there would be many more trucks…