Pier depth for cut and fill site

Hi fellow builders!

After any advice/feedback/thoughts on my slab engineering specifications.

The site has been classed H1 and will require a cut/fill of around 450-500mm due to the downslope. The land is 10,000m2 so we avoid any zones of influence or having to do retaining walls etc...

Geotech report stated no fill material identified along with no rock being present. They determined the maximum estimated depth of 450mm concrete piers to be 1800mm.

Now my builders engineer has estimated piers to be at a depth of 1400mm but have placed a note stating the Geotech report has advised 1800mm pier depth to the entire slab.

Current provision is for 210 lineal metres (more than double the original estimate before engineering).

Would the piering depth usually be the same depth across both the cut and fill areas? I would of thought the cut side would require a lesser depth?

Should I get a 2nd Geotech report done to compare?

Any advice would be appreciated.

Thanks.

What do you plan on doing if a second engineering report is different?
Ultimately you have signed a contract and agreed to use the builders engineering.
Stop stressing.
Cheers

acro Welcome to the forum
You should have firmed up the site work costs with your own engineers report before you handed over money to the salesman
That's a big slab be prepared for Extra Over upgrade costs..What have they quoted?
Here Breaking Down and Checking slab & formwork costs
BTW sharing photos and data also helps others thanks
Chris

Thanks Chris,

I haven't signed a building contract just yet, but intend to in a few weeks time.

Only received the final tender, slab engineering and DA drawings on the last day before xmas break so just gathering some information before I speak to them again.

I trust my builder (not a volume builder) and expected to pay top dollar for quite a large slab to be engineered properly (peace of mind knowing structural issues would be less likely down the track). More just trying to get my head around why the engineer has stated 1400mm estimated pier depth, but insisting the $500 soil test guys 'maximum recommended pier depth required 1800mm' to over rule his calculation. Thats $5000 extra in costs for 400mm difference. It could well be a misinterpretation of the soil report!

I'll speak to my builder when they're back to clarify if they will actually go to maximum depth for every pier or adjust to suit on the day with how hard the ground is below. I would also like to be on site to verify measurements to know I'm getting what I'm paying for.

Cheers and happy new year!

Good idea to check the depth, compaction/bearing & take photos
generally anything buried & covered is often forgotten
BTW The pile depths will vary onsite
Cheers

acro

Can you post the recommendations of the Geotech report?

You have clay which turns into a mud when it gets wet.

Piers means point loading so your slab is more or less semi-suspended as there won't be much of the compaction achieved on a clay material anyway.

I would be considering alternative approach, if I were you, ie excavate and get rid of the clay soil up to approximately 700-800 mm level under your footings, put a well compactable clean fill such as a road base or crushed concrete or sandstone and compact it to high levels with compaction certificates being provided.

This will create a reliable building platform for your footings and turn your H1 site into almost an A-class site.

You will be able to save a lot of money on:

1) the cost of your footings as this approach will work fine with lighter footings;
2) thinner slab with less expensive reinforcement, e.g. 100 mm slab with SL82 reo, shallower edge beams.
3) you won't be requiring piering at all.

But you would have to pay for more excavation, soil removal, clean fill and compaction.

Also, if you consider 3D concrete re-inforcement fibers such like HelixSteel, you will be able to save many thousands on your steel and steel-fixing labour costs.


So the costs have to be evaluated. But point loading on uncompacted reactive clay soil... Don't know if it is ever a good idea.

Discuss with your engineer and see if it is viable option.

When it gets to piers (if you really want to stick to them), each pier depth won't be determined by engineer or your soil testing guys but by the level of hard soil (shale) to which you can drill, so some piers may be 1400 mm or less, others 1800 mm or more. You only have to pay extra for the cost of additional concrete.

Ah, just noticed it is actually a waffle pod.

Well, waffle pods are not recommended for highly reactive clay sites at all, you won't be able to achieve a good drainage on such sites (I personally wouldn't be recommending them even for moderately reactive clay sites too).

Especially, when there is cut and fill too.

In my opinion, this design is just asking for trouble.

Drainage can be achieved, you just need it sorted from day 1 until you take possession. Lots of yard drains, spoon drains etc and you should be apples. The builder and engineer will sort that out for you.

I wouldn't risk as builders generally have no idea of what good drainage on clay means.
Especially I would never go for waffles in cut and fill scenarios on reactive clay soils.

Cornell Engineers strongly believe that waffle pads are "red light" for H1/H2s.


https://www.cornellengineers.com.au/bew ... fle-slabs/

But considering that average Australian house has life expectancy of 25 years, this will probably work, anyway.

Just my 2 cents.