Waffle pod vs monolithic slab

TheStig · Jun 09, 2007 9:14 am

after being in the construction industry for the past 25 years. and being invovled inthe construction of an estimated 250-300 homes, i feel im qualified enough. with monolithic slabs we had numerous instances of cracking in the brickwork due to movement in the soil. but since the introduction of waffle pod's, this is a very rare fault these days. the more reactive the soil, the more reason to use waffle. my confidence in the waffle pod system can be back by any engineer

Casa2 · Jun 09, 2007 12:55 pm

TheStig,

How does waffle pod and monolithic slabs compare to suspended concrete slab on piers?

I know the suspended concrete slab on piers would cost more, but the way I look at it, it looks like the best way to go for peace of mind. Put the piers below the clay line (and preferably further down to something hard) and leave a gap beneath the slab. This way as the clay expands and contract the top soil just moves up and down but never presses on the slab. I can't imagine such an arrangement ever moving the slab - no cracked brickword, tiles, ceiling... Must be worth spending a few thousand more on.

Failing this, what is the Rolls Royce of bottom level slabs?

Cheers,
Casa

grime · Jun 09, 2007 2:01 pm

I seem to recall that the guy who invented the waffle slab was in South Australia and came up with it due to the nature of SA soils.Does anybody know if that is correct,or can add to it?
Also I don't think they have been around for all that many years,so I guess that could be a reason some builders prefer other types.

ahuang · Jun 09, 2007 11:41 pm

Casa2

How does waffle pod and monolithic slabs compare to suspended concrete slab on piers?

I think we got waffle pod + piers, is that another different option?
I am confused, so which is better, and which is more expensive?

Cheers
Allan
http://building-our-first-house.blogspot.com/

TheStig · Jun 10, 2007 9:29 am

piers are usually used in unstable land, and are drilled to a depth that penetrates the land fill and 300mm into natural ground. they are poured seperatly so as the waffle slab is independant from them.

Casa2 · Jun 10, 2007 11:24 am

TheStig,

Thanks for the info. I guess the piers have to be independant of the waffle slab to allow the slab to "slide", so to speak.

On another post (or this one?) someone mentioned that after their piers went in, they were not visible. Should this be the case !?!!

Cheers,
Casa

Hennig · Jun 10, 2007 10:15 pm

The Stig, knows his stuff.
I believe that if you have a M or S class site due to your soil type, then waffle or raft slab will be fine as they are both founded basically at the surface.
It is when you have site issues like fall of land or uncontrolled fill, that a waffle sticks its nose in front - if not only for the cost savings.
The easiest way for me to explain it is:
- the piers act as the foundation support as they are not directly attached to the slab,
- the slab sits on them and is therefore able to slip/move with any seasonal changes.
- whereas a raft slab will usually have bulk concrete in the deepened beams, which can be poured with the slab (not always as is only needs to be 15mpa)
Don't think that by any means a waffle on piers will never give you grief. You are still subject to movement, due to weather, garden beds up against the house etc...
At the end of the day, it is whichever you are most comfortable with - or cost.

Vlad76 · Jun 11, 2007 2:54 pm

Henning, since you seem to be an expert in this, could you please shed some more light on the piers design for me. I understand that they are not directly attached to the slab, but what is the distance between them and a slab? Is it right on them, or is there some small gap between them?

And if there is any movement to happen (waffle pod with piers option), when is it likely to occur? First year or two or more likely within 10 years?

I guess I am just trying to understand when to expect potential trouble, and what likely hood if it does move a bit, of it doing serious damage to brickwork and interior cracks appearing?

Appreciate your help,

Vlad

Hennig · Jun 11, 2007 9:11 pm

If done right, the piers should abutt the underside of the slab.
In most cases you'll probably find that between 0 and 50mm of underslab fill (sand or fines) is placed over the slab area, mainly for levelling purposes.
You can get undermining of the slab (a gap forming between pier and slab) if things aren't done right. If you see this, raise a stink because it shouldn't happen.
Regarding movement, you'll find that your house will settle within the first 12 to 18 months - but as far as foundations, never say never. If you get a few wet years and then quite a few dry years (refer regions north of the divide for Vic people) movement will occur.
It helps to follow the recommendations stated on the soil report and the slab design regarding site drainage to minimise chances of movement.

Vlad76 · Jun 12, 2007 12:58 am

Thanks Henning, I will try to watch out for the gap between piers that are in place and the slab which is going in this week.

It won't be easy though, as the piers are at or just below ground level and can't really be seen. I can just see bits of concrete on the ground to indicate where some of the piers have gone in.

Casa2 · Jun 12, 2007 9:45 am

I'd want to see all my piers !!!!

Vlad76 · Jun 12, 2007 12:25 pm

Casa2

I'd want to see all my piers !!!!

Have you had piers put in on your site?

It is physically impossible to see them as they are at the ground level, not above, so for me the only option is to rely on the idenpendent report verifying they've been put in correctly.