Reliability of Elastomeric Waterproofing between Gunite and poured Concrete walls or floors

[fivetoedsloth]

Hi...
I'm looking for some insights on a concrete waterproofing technique I have in mind, but haven't seen used before.
A residential restoration project I am working on in France needs a small concrete cellar built, approx. 2 by 3 meters by 2 meters high, within
the existing building footprint, below the floor slab. There are very old stone walls on three of the four sides, laid on the decomposing schist bedrock, which starts about 40 centimeters down. There are occasional high water-table issues to contend with in the wet season.
In order to avoid digging too close to, and possibly destabilizing, the very old and shallow stone foundations, it would
be advantageous to avoid the conventional over-digging needed for form-work and back-fill. Since previous excavations
on the site created fairly clean and stable trenches, my idea is to shoot concrete on the walls of the excavation, to level and fill all voids, level
the bedrock base with a fiber reinforced concrete slab, apply a heavy-bodied elastomeric coating to all the stabilized surfaces when cured,
and then proceed with steel reinforced floor-slab and wall pours, forming only from the inside. The intended result would be a continuous thick envelope
of elastomeric waterproofing that is sandwiched between and adhering to both the leveling gunite/concrete base surfaces and
the reinforced poured concrete vault.

So here are my questions...if anyone has some knowledge or leads on this...it would be much appreciated....
1) Is this an existing method, is it reliable at this scale, and if so, where might I find some info/specs on the best technique/products?
2) If this waterproofing technique has been proven and effective, would it be necessary for the concrete for the vault itself to be fully waterproof?
3) Since the vault is relatively small, and there are theoretically few voids created for groundwater to accumulate, and no corrodable metal used in the prep-work on the excavation side of the membrane, would a french-drain and pump still be logical, trying to fend off the seasonal water-table?
4) Any hints on the lifespan of such an elastomeric coating...

 

[Ron]

No problem with your proposed method. I like it in fact.

As for the elastomeric material, I would preferably use a coal tar material with typical roofing felt. If this is not acceptable in your area, my second choice would be an asphaltic material applied at twice the recommended thickness....you only have one shot at getting this right!

If you must use a fluid applied membrane, go with something like a Tremco ExoAir 120 or similar that is capable of wet service conditions.

A good French drain system will be necessary in any case.

 

[fivetoedsloth]

Hi Ron, thanks for the reply.
Seen the small surface involved, I would apply the membrane with a roller or trowel, depending on viscosity. The felt idea would
be problematic for following any non-plane surfaces I could otherwise accept in the gunite, and likely create voids that could compromise the seal.
As you mentioned, I have only one chance to get this right! I see this as a kind of bedrock-encased boat, or submarine, that is designed to resist seasonal
water-table variations. As long as nothing can move, crack, or settle with time, seems like the seal would be durable. The usual french drain concept seems in
conflict with my approach of sandwiching the elastomeric compound between gunite coated bedrock and the concrete vault...there is no void in which to create a
drain. If I do create a drainage system, the question is how and where to do so while maintaining the continuity of the surfaces which sandwich the seal...

 

[hokie66]

I think your approach is sound. The only place for a drain would be behind the shotcrete, with one of those flat drainage systems.

It will be a boat, so you need to make sure it won't float.

 

[fivetoedsloth]

Hello Hokie,
As far as guaranteeing that my boat doesn't float, I'm counting on leaving some irregular contours to the gunite-covered bedrock, rather than
floating the walls to a flat plane... the poured-in-place walls of the vault will thus be keyed into the mass of the bedrock.

I'm also thinking I could place any flat wall drainage system (as you recommend) intermittently rather than continuously,
to reduce any chances of vault movement associated with water table changes...in the event of a drainage pump failure or prolonged
power outage during major storms.

Thanks for the suggestions.