Some Slab Protection Questions Please

Lisa06 · Sep 08, 2020 10:50 am

Could someone please help me understand a few things?

1. I’ve read that temporary downpipes should be installed as soon as the roof is up. Is there some law that states this or is it just good practice/common sense (that is clearly uncommon!) to protect the slab?

2. How is the slab protected up to the point that the temporary downpipes are supposed to be installed? Is it purely via the grading of the land away from the slab?

3. Lets assume it took a volume builder 8 months to build a house and they didn’t use temporary downpipes. Any idea roughly how long it would have taken between the time the roof went up and the official downpipes were installed? ie. roughly how long would the slab have been exposed to rain if no temporary downpipes were used?

4. Lastly, how much rain fall per month is an issue when it comes to building a house that has poor slope away from it and no temporary downpipes? The house is on very highly reactive clay.

KameGame · Sep 08, 2020 12:04 pm

As far as I know, temporary downpipes are within Australian Building standards but not enforceable or practices that a builder must do. It's a quality thing which the builder must do, if they've not fixed the temporary downpipes they must at least backfill the site around the slab to prevent pooling of water.
Personally , I wasn't able to reason myself as to why the temp downpipes are so important if the site has been backfilled.If it's pouring down anyways , would the downpipes really matter. In my opinion unless we have the concrete apron around, we're not protecting the base as much.

Lisa06 · Sep 08, 2020 12:18 pm

KameGame

As far as I know, temporary downpipes are within Australian Building standards but not enforceable or practices that a builder must do. It's a quality thing which the builder must do, if they've not fixed the temporary downpipes they must at least backfill the site around the slab to prevent pooling of water.
Personally , I wasn't able to reason myself as to why the temp downpipes are so important if the site has been backfilled.If it's pouring down anyways , would the downpipes really matter. In my opinion unless we have the concrete apron around, we're not protecting the base as much.

I've heard that the perimeter around the house should not be backfilled, it must be graded away from the house. Backfilling to create a slope away from the house is supposed to be pointless as the water will seep straight through it and affect the slab.

The temporary downpipes are designed to move all the water from the roof away from the slab footings. That can be a heck of a lot of water in a heavy downpour. Imagine all that water concentrated in specific areas around the slab (ie. where the downpipes are supposed to be).

It doesn't have to be a concrete apron for protection. As long as the grading is correct, I've read that plastic (eg. with stones on top) is just as effective.

StructuralBIMGuy · Sep 08, 2020 12:27 pm

Hey lisa06
This maybe a bit too late to help you but it will definitely help others
1.On highly reactive soils they should be installed and the rainwater directed away from the footings
BTW, I include this in the Engineering Notes that are certified, I'm guessing they left it off your plans?
" I've also been told by builders to remove my notes from the building documentation..my response is PO"
2.We put a road base down and grade it away... no ponding adjacent to footings
3&4 Have you properly measure the slab movement with a bosch Laser Level over time and collected all the data, This will directly relate to engineering geotech Multiphysics simulations. The Data and proofs are required for a court of law to prove builders negligence.
OT, This would be a great final year project for a civil engineering student, they would access to the Software and while harbouring no industry bias... contact one of the universities engineering faculties to discuss
HTH
Chris

alexp79 · Sep 08, 2020 12:39 pm

You would use non porous fill such as compacted clay when grading away (i.e. clay lock).
A good design will be to also install a layer of builder's film under the clay lock.

And another important aspect is to decide where do you grade away to. Ideally, it should be grading away to a drainage.

Lisa06 · Sep 08, 2020 12:46 pm

StructuralBIMGuy

Hey lisa06
This maybe a bit too late to help you but it will definitely help others
1.On highly reactive soils they should be installed and the rainwater directed away from the footings
BTW, I include this in the Engineering Notes that are certified, I'm guessing they left it off your plans?
" I've also been told by builders to remove my notes from the building documentation..my response is PO"
2.We put a road base down and grade it away... no ponding adjacent to footings
3&4 Have you properly measure the slab movement with a bosch Laser Level over time and collected all the data, This will directly relate to engineering geotech Multiphysics simulations. The Data and proofs are required for a court of law to prove builders negligence.
OT, This would be a great final year project for a civil engineering student, they would access to the Software and while harbouring no industry bias... contact one of the universities engineering faculties to discuss
HTH
Chris

Hi Chris,

Does any sort of guideline state that temporary downpipes are required on reactive soils?

Our engineer plans don't mention temporary downpipes.

Given that we are in lockdown for likely the rest of 2020, I don't think any students are going to be able to assist.

Thanks.

EDIT: Our engineer plans actually DO state that temporary downpipes must be used during construction!!! Interesting.

Lisa06 · Sep 08, 2020 12:49 pm

alexp79

You would use non porous fill such as compacted clay when grading away (i.e. clay lock).
A good design will be to also install a layer of builder's film under the clay lock.

And another important aspect is to decide where do you grade away to. Ideally, it should be grading away to a drainage.

Hi Alex,

Thanks for that info.

Yes, I agree with the drainage.

Is builder's film some sort of plastic?

Harts · Sep 08, 2020 9:37 pm

Hi Lisa06

Australian Standands 2870-2011 5.6.3 Drainage Requirements – Buildings on moderately, highly or extremely reactive sites shall be provided with drainage systems designed in accordance with the following:
(a) surface drainage shall be considered in the design of the footing system & necessary modifications shall be included in the design documentation. Surface drainage of the site should be controlled from the start of site preparation & construction. The drainage system shall be completed by the finish of construction of the building.

AS2870-2011 5.2 Drainage design requirements
- “Surface drainage shall be designed & constructed to avoid water ponding against or near the footing. The ground in the immediate vicinity of the perimeter footing shall be graded to a fall of 50mm minimum away from the footing over a distance of 1m & shaped to prevent ponding of water. Where filling is placed adjacent to the building, the filling shall be compacted & graded to ensure drainage of water away from the building.

Hope this helps

alexp79 · Sep 09, 2020 12:47 am

Lisa06

alexp79

You would use non porous fill such as compacted clay when grading away (i.e. clay lock).
A good design will be to also install a layer of builder's film under the clay lock.

And another important aspect is to decide where do you grade away to. Ideally, it should be grading away to a drainage.

Hi Alex,

Thanks for that info.

Yes, I agree with the drainage.

Is builder's film some sort of plastic?

Yes, it is a film made of waterproof plastic, sometimes can be also referred as geomembrane.

Lisa06 · Sep 09, 2020 8:32 pm

Harts

Hi Lisa06

Australian Standands 2870-2011 5.6.3 Drainage Requirements – Buildings on moderately, highly or extremely reactive sites shall be provided with drainage systems designed in accordance with the following:
(a) surface drainage shall be considered in the design of the footing system & necessary modifications shall be included in the design documentation. Surface drainage of the site should be controlled from the start of site preparation & construction. The drainage system shall be completed by the finish of construction of the building.

AS2870-2011 5.2 Drainage design requirements
- “Surface drainage shall be designed & constructed to avoid water ponding against or near the footing. The ground in the immediate vicinity of the perimeter footing shall be graded to a fall of 50mm minimum away from the footing over a distance of 1m & shaped to prevent ponding of water. Where filling is placed adjacent to the building, the filling shall be compacted & graded to ensure drainage of water away from the building.

Hope this helps

Thank you -it does.

Lisa06 · Sep 09, 2020 8:37 pm

alexp79

Lisa06

alexp79

You would use non porous fill such as compacted clay when grading away (i.e. clay lock).
A good design will be to also install a layer of builder's film under the clay lock.

And another important aspect is to decide where do you grade away to. Ideally, it should be grading away to a drainage.

Hi Alex,

Thanks for that info.

Yes, I agree with the drainage.

Is builder's film some sort of plastic?

Yes, it is a film made of waterproof plastic, sometimes can be also referred as geomembrane.

Thanks for that.

Is there anyway we can find out if the builder graded the land away from the slab correctly during construction? ie. right angle and fill-type etc? The land is certainly not graded away from our slab correctly but the builder will likely blame the first owner.

alexp79 · Sep 09, 2020 9:10 pm

Lisa06

Thanks for that.

Is there anyway we can find out if the builder graded the land away from the slab correctly during construction? ie. right angle and fill-type etc? The land is certainly not graded away from our slab correctly but the builder will likely blame the first owner.

"... fall of 50mm minimum away from the footing over a distance of 1m "
clay should be well compacted (meaning it looks like it is being smoothed with something and tough when you press it)

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StructuralBIMGuy · Sep 09, 2020 9:21 pm

Lisa06

Hi Chris,

Does any sort of guideline state that temporary downpipes are required on reactive soils?

Our engineer plans don't mention temporary downpipes.

Given that we are in lockdown for likely the rest of 2020, I don't think any students are going to be able to assist.

Thanks.

EDIT: Our engineer plans actually DO state that temporary downpipes must be used during construction!!! Interesting.

So you can see how easy it is to miss critical information... guys on site overlook that information all the time.. It's not a hanging offence
The saving grace is that others check the works as it proceeds and FUs gets rectified.
Unless the engineer pointed out the consequences directly to the builder and he then ignored the details/instructions.
Then the case of the Builder Failing in his "duty of care" is difficult, onerous and legally expensive to prosecute.

OT, Student research programs requires Supervisory Guidance by a professor, Access to test facilities and the use of expensive software.
Most private engineering practices don't offer that service for housing.

Cheers
Chris

Lisa06 · Sep 10, 2020 11:57 am

alexp79

Lisa06

Thanks for that.

Is there anyway we can find out if the builder graded the land away from the slab correctly during construction? ie. right angle and fill-type etc? The land is certainly not graded away from our slab correctly but the builder will likely blame the first owner.

"... fall of 50mm minimum away from the footing over a distance of 1m "
clay should be well compacted (meaning it looks like it is being smoothed with something and tough when you press it)

Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅

Ours looks nothing like that. There is just soil around the house and it looks like it's at a very slight slope. But it is certainly not compacted at all.

Varsity · Sep 12, 2020 7:38 am

Just a point on those temporary light weight plastic downpipe hoses. In our case the builder installed them as soon as the guttering went up, but they were totally useless. Our block is exposed and when it rains it is usually also windy so the plastics just disconnected from the drainage points and were of no benefit. They were always just flapping in the breeze until proper downpipes could be attached. It was also impossible to grade away from the slab on one side because we are built to the boundary there. I now note (after handover) as we are about to start the landscaping, the yard gully points are also full of dirt from from having loose soil all around during the build, perhaps these should have been capped off after rough in as well.