Waterproofing membrane under portal frame foundations and sliding

[brotherson]

I'm working on a project where the independent verifier has called into question the sliding resistance of one of our structures as our footings are on a waterproofing membrane. It is a portal frame type concrete structure on engineered fill with a friction angle of about 35 degrees.

Our engineer's calculations use tan(2/3)phi which basically means we're ignoring the waterproofing layer. The independent verifier is using a friction coefficient of about 0.25 because of the waterproofing layer.

My first thought was the if it moves any amount the membrane would be shredded anyway, but that argument falls over if waterproofing is required. Also if it moves the soil pressure will be relieved to Ka, but that's probably not going to help us either.

Does anyone know of any literature on the subject or how this is dealt with? I've been looking online but found nothing. We could just remove the waterproofing layer and use something like Xypex in the mix but I'd be interested to hear other points of view on this.

thanks

 

[GeoPaveTraffic]

I've never seen a waterproofing membrane under a footing. Why would you need a membrane under the footing?

As for your question, if I were doing the calculations, I would use a very small coef. on the assumption that the membrane would last long enough for some movement to occur. So I guess I agree with the verifier.

Mike Lambert

 

[brotherson]

Thanks for you response. I'm told it is a mandatory requirement on this project but I'll look into that further.

The water table is 30m below existing ground, we're surrounded by sand and it rains once a year. However I've been here in the middle east for 1 month and I've already learned to just accept some things.

 

[JedClampett]

The reason that waterproofing is required under structures is that the flooring (carpet, tiles, etc.) manufacturers won't warrant their materials unless there is a membrane under finished floors. They've figured out that this is a weak link between the flooring specifiers (architects) and the structural floor designer where they can point blame.
I would worry that Xypex would encounter the same issue. Unless it is a plastic membrane, they won't honor their warranty. You could call them and see if they would consider Xypex, but I'm guessing they won't ever give you an answer. If you don't have a architectural floor, you can eliminate the membrane. If you do, I'd just design for the reduced coefficient of friction, instead of removing the membrane. Another alternative is to take the chance of just eliminating it at footings, but that gets messy to specify and construct.

 

[darthsoilsguy2]

use Kp and maybe look at Ko. it is conventional practice to ignore passive and at-rest, but you are past conventional anyway since friction is off the table.

i don't know how common it is but i have worked on a 200-bed psychiatric hospital designed by others that was fully encapsulated in poly like you describe. If you must use crystalline waterproofing to satisfy this requirement..... what you may want to do is have the foundation bearing soils approved, then cast a 4" mudmat, apply the xypex to that mudmat and cast concrete in forms. After stripping the forms you may come up the walls with almost any system (bituthene sheet, elastomeric, crystalline) if you're not really worried about it but need to address the requirement.... you could try mix design methods like adding Barrier-1.... you will always need to do the vapor barrier under the slab regardless.

 

[oldestguy]

Here is a story for what it may be worth. Some time ago I had a trailer upon which I carried clean sand for driveway ice control where I lived. Later, the municipality started mixing salt with the sand, so I placed a sheet of Visqueen on the trailer to keep the salt off the metal(its wooden bed very sandy) and loaded the salted sand on it. On the way home I slowly turned into a road on about a 10 percent uphill grade. I noticed nothing special there but getting home I was surprised to see the trailer was empty. Went back to that slope and there sat my sand on the Visqueen in the middle of the road. What's the friction angle? 10%

 

[brotherson]

Thanks for the replies all. Working from multiple angles.

Jed - there are no carpets. They are strip footings on a portal structure. They shouldn't need waterproofing but it's a project requirement - blanket statement for structural elements below ground level.

darthsoil - looking into a variety of things, thanks for the suggestions.

oldestguy - interesting, proof that this issue is worth looking into.