Poor Ground Bearing result..less than 100kpa

Andrea GL · Jul 16, 2024 1:58 pm

Our builder has just advised our homesite, unlike any others in the estate, has very poor ground bearing results below 100kph and that piers will be needed under the slab. Does anyone have any experience with this situation or advice to offer. much appreciated

stefcep · Jul 16, 2024 2:20 pm

type "slab piers" in search and a few topics come up

Andrea GL · Jul 16, 2024 2:30 pm

stefcep

type "slab piers" in search and a few topics come up

Thank you 🙏

Eng. B A · Jul 16, 2024 2:44 pm

Can you attach the site classification and soil log pages from the soil report? Also are you building single or two storey?

Andrea GL · Jul 16, 2024 3:27 pm

Eng. B A

Can you attach the site classification and soil log pages from the soil report? Also are you building single or two storey?

Hi,
This has all unfolded this morning and is still ongoing. I do not have access to any documents as yet. Single story.

r_13 · Jul 16, 2024 3:57 pm

Our site was on a hill and therefore built over half cut and half filled soil.

There are about 30 bored concrete piers under the filled part of our slab, and toward the back of the cut part also.

This added to the site costs about $1000 per pier. You can see a few of them in this picture, which is taken from the back of the house in the cut side, prior to the slab pouring.

Most of the piers are toward the front of the house on the right side, where the slope goes downhill and the build area had to be built up with fill.

viewtopic.php?f=31&t=91923&p=1868360#pImg1335308

stefcep · Jul 16, 2024 5:04 pm

So $30k in piering?

Andrea GL · Jul 16, 2024 5:39 pm

r_13

Our site was on a hill and therefore built over half cut and half filled soil.

There are about 30 bored concrete piers under the filled part of our slab, and toward the back of the cut part also.

This added to the site costs about $1000 per pier. You can see a few of them in this picture, which is taken from the back of the house in the cut side, prior to the slab pouring.

Most of the piers are toward the front of the house on the right side, where the slope goes downhill and the build area had to be built up with fill.

viewtopic.php?f=31&t=91923&p=1868360#pImg1335308

so, on a hill I can understand but this is flat and level ground and worst= case scenario is 42, yes 42 piers containing steel to a depth of 1.5 -2m. Quoted approx cost was $150 per square metre- about 8-10 grand. Mind blown.

r_13 · Jul 16, 2024 8:18 pm

stefcep

So $30k in piering?

Yep. But, aside from a little settling movement in the first year, which caused some tiny cracks in paint around architraves and cornices, which were fixed by the building company under warranty, the house has been rock solid. So I guess the piering was effective.

The building company added 80k in site costs allowance as soon as they saw the geotechnical report, so it was expected and budgeted in. All turned out great in the end I think.

stefcep · Jul 16, 2024 8:30 pm

I think Metricon got sued for their waffle slabs some years ago, so they may now be over-engineering

20 years ago the houses built on H class reactive soils used raft slabs. I've never seen piering with a raft slab-

Today all the volume builders use waffle slabs which are not suitable for reactive soils, unless they are "engineered" which means piering or screw piling, and with a waffle slab on top and just let the customer wear the exhorbitant site costs.

Ashington Homes · Jul 17, 2024 4:51 am

Andrea GL

Our builder has just advised our homesite, unlike any others in the estate, has very poor ground bearing results below 100kph and that piers will be needed under the slab. Does anyone have any experience with this situation or advice to offer. much appreciated

Hi Andrea

I am assuming you are with a project home builder?

There are two points I would like to make:

1.Every slab should be piered

Basically every quality builder whom I know, including ourselves pier every single slab no matter what the ground conditions are.

It is good practice to pier slabs to a solid foundation.

Piering is actually cheap, we pay around $110/lineal meter, with an average depth of around 1.5-2m each.

On a 200 sqm slab you might have 50-60, worst ones have 80.

But, I would be looking at piering as a positive and not something you want to convince them to do.

2. Your home site may be an old dam or an area of poorly controlled fill

Often in new estates which were old farms, there are dams or other holes that have been filled, and sometimes the civil contractors don't compact these enough.

I saw this regularly in my time at Metricon in the new estates, you could have awesome ground for most of the estate then you would get to a section of 3 or 4 homes where the fill was 4m or more deep and just rubbish.

So you may have gotten unlucky with your lot.

All the best

Simeon

Eng. B A · Jul 17, 2024 6:15 am

Use piers under every slab is a waste of money and can sometimes cause harm to footings if the footings are not adequately reinforced for piers.
If you have achieved the 100kPa at shallow depth, then why you use piers (deep footings). 100kPa which is generally means you are most likely in natural ground with adequate compaction for single and two storey buildings (limitations to size as per AS2870 apply).
If you are in uncontrolled fill/low soil bearing to a depth that it’s not economical to drop the strip footings and suspend the slab (use top and bottom mesh) you will have to suspend the ground beams too over piers and use heavy reinforcements in beams. This is for low bearing soil.
Using piers not only adding the cost of excavation, reinforcing and concrete of piers but also the cost of upsizing and extra reinforcing to the ground beams to suit the piers system.

Andrea GL · Jul 17, 2024 6:20 am

Hi Simeon,
Thank you so much for your comments. We have a small builder, who is acutely particular & careful over every aspect of his builds, something I am grateful for. I would much prefer a proactive approach to issues and practical solutions. As a novice, I was interested to hear of others' experiences to help me understand the situation. It does seem we are just unlucky in this case. I just didn't want an adequate solution, but a solid long lasting one. Thanks again for your input.
Cheers
Andrea

Ashington Homes · Jul 17, 2024 6:26 am

Andrea GL

Hi Simeon,
Thank you so much for your comments. We have a small builder, who is acutely particular & careful over every aspect of his builds, something I am grateful for. I would much prefer a proactive approach to issues and practical solutions. As a novice, I was interested to hear of others' experiences to help me understand the situation. It does seem we are just unlucky in this case. I just didn't want an adequate solution, but a solid long lasting one. Thanks again for your input.
Cheers
Andrea

Andrea

You are welcome.

I know we over engineer our slabs, as we have to give a 6 year structural warranty so we think its prudent to spend a little extra up front, for instance if our engineers specify 25 Mpa concrete we will use 32.

We explain to our clients that we are going the extra step and generally they are also happy to know that they have a string foundation rather then one built to minimum standards.

Good luck with your build

Simeon