Slab design - contract issues

Hi All,

Our builder has asked us to pay almost 12k extra for extra piercing for the waffle raft slab as there are 62 piers.

I asked the builder several times do we need to provision for this and he said our soil test is a M And he said it was okay.but other builders put provision for it. I think this was one of his tactics to get a lower price in preliminary

So in the contract, the slab design is allowed for the is for M class site. If we require extra piercing, we will need to choose to pay the variation or cancel the contract mutual agreement.

I don’t know what to do, we don’t want to go over budget. Trying to work it out with the builder and not pay extra.



Anyone else experience this or provide any advice?

Thank you in advance

Usually M class wouldn't require any piering. There should be something else why extra piering is needed (knockdown, trees, uncontrolled fill, etc.). What does your geotech report say?

What is your site classification?, what are geotechnical report recommendations?

Thanks for the reply. We demolish the house , and did the soil test straight away. So it’s a class P Otherwise Class M. The geotech and building designer said it’s because of the demolishing , there is movement that’s why they classify it as P site.

Class P
Otherwise Class M -Soft clay, dwelling removed. trees, fill
Iss =2.7
Ys =30
Bearing Capacity = <100 kPa to 600mm (see DCP logs)

Also I got a conventional slab design done from my engineered and it only had 10 piers.so I don’t know why the builders engineer required 62 piers.

Your builder has to warrant your home for 10 years in Victoria. M slab on P site does not add up for me. It seems your builder also is not comfortable with your design.

So this is P class side due to trees and low bearing capacity and this is where extra piering comes into play.

Yeah the builder didn’t want to go with my design as it was conventional and he wanted to use the waffle design for economical.

The trees we have is all in the back of the land and we have cut it all down.

doesnt matter that you gut them down. the soil conditions where ethe roots have been growing are varied and will remain so for a long time.

Rotting tree roots are underground conduits for water

So just a quick update. The builder has now deleted the piering cost. He said it was base on good will. I have asked multiple times if there was something wrong with engineering and asked for new report but he has been ignoring my question. What do u guys think?

It looks your builder was trying his luck