Getting home built - Is H1 class waffle pod slab ok?

PeterCurtis · Dec 01, 2023 10:57 am

We are getting our double story home build on 300 sqm land in Schofields Akuna Vista NSW area.

There was GeoTech report and it came with site classification of class “Class H1” site
classification.
2m-3m --> CLAY, high plasticity, brown with fine gravel CH ≥PL --> ALLUVIUM

My builder has stated they will construct "H1 class waffle pod concrete slab" with Screw Piers.

Could you please help understand if it is appropriate piers and slab for the house?
Is there something we need to be mindful?

ponzutwo · Dec 01, 2023 12:07 pm

PeterCurtis

We are getting our double story home build on 300 sqm land in Schofields Akuna Vista NSW area.

There was GeoTech report and it came with site classification of class “Class H1” site
classification.
2m-3m --> CLAY, high plasticity, brown with fine gravel CH ≥PL --> ALLUVIUM

My builder has stated they will construct "H1 class waffle pod concrete slab" with Screw Piers.

Could you please help understand if it is appropriate piers and slab for the house?
Is there something we need to be mindful?

The appropriate slab is the one that the engineers design based on the report. In Short, yes.

Is there things you need to be mindful of? heck yeah.

Take a look for cornell Engineers posts and content about waffle pods slabs and reactive soils on this forum. There are HEAPS of conditions that need to be observed for the slab and the site to perform and not fk your house up. Those conditions will be covered in a Whole a3 page of engineers notes in 4pt font attached to your slab design.

Realistically, while i sound absurdly over the top, there are some issues you just need to manage for and for teh most part your home should be absolutely fine. Make sure you have a private inspector inspect the footings before the slab is poured.

BuildingandLegal · Dec 15, 2023 7:39 pm

The Soil classification has little to do with piers.
The purpose of the classing of the soil is to identify the clay content and the "average expected range of movement under normal moisture conditions"... How much water will the clays take up when it gets wet (expand) and how much will it shrink when it dries out. The soils yS. Based on this the engineer can then design a slab based on the expected average range of shrink/swell and provide the ridgity to the slab required to withstand that movement. Remember this is for the average over the block.
The soil test also confirms if any fill has been placed on the block and if it is controlled or uncontrolled fill. We'll come back to this. It also looks at the bearing capacity of the soils, this is represented in DCP's or kPa. For a single storey brick and tile home you need 100kPa.
Back to controlled and uncontrolled fill. If there is fill on site and it has just been dumped in a pile and pushed around to level it out, this is UNcontrolled fill. It will include loose pockets, air pockets etc and over time this fill will compact and settle and that settlement may not be uniform across all the fill. Controlled fill is where the same soil as uncontrolled fill is laid down in 150mm levels watered and rolled, then another 150mm, and another etc etc. Then the soil is confirmed by an engineer as placed and compacted and was tested.
The waffle slab is essenctially a raft, it sits on the soil and raises and falls subject to the clay moisture. However if the bearing capacity of the soil is <100 kPa or less than 4 DCP the soil is soft and the weight of the slab will sink as the soil can't take the load. Solution pier to firmer ground and the house weight transfers down the pier into the firmer ground.
You might have a sewer main or a retaining wall close to the edge of the slab, this means the load of the house pushes down on the soil BUT that weight pushes out to the side where the soft or hollow section is, so again you pier down to below the soft section to transfer the load to firm ground. This is the requirement for piers.