Buoyancy Mat Slab 1 Pour vs 2 Pours 1

[jdgengineer]

We are working on a high end residential project with a basement that due to the potential for a perched high water table the geotech has asked that we design the basement for a buoyant condition with water up to 3'-0" from the ground surface. To counteract the buoyancy we investigate retaining walls with heels, helical/micropiles and a really thick matslab. Due to constructability reasons and waterproofing the contractor prefers the thick mat slab option. We have determined that we will need a 4'-0" thick matslab to offset the potential buoyancy caused by the high ground water.

From a structural perspective, we do not need the matslab to be this thick. It's sole purpose is really just weight to offset the buoyancy. We were thinking of a couple of options and I wanted to see if anyone had opinions.

1) Pour a single 48" thick matslab. To meet the 0.0018 reinforcing we would utilize #8 bars @ 8" oc each way top & bottom.

2) Pour two 24" thick matslabs. In this situation we would design the lower 24" matslab as the structural foundation and detail the retaining walls to tie into this matslab. In this scenario we would likely reinforce the lower matslab with #6 @ 10" oc each way top & bottom. We would then pour another matslab ontop with similiar reinforcing (potentially we could omit the bottom layer and only have steel at the top?). All of the interior shearwalls, posts, etc. would land on the top matslab. The top matslab would be doweled into the retaining walls at the perimeter.

Currently, we have our drawing detailed for the first option. The contractor prefers the 2nd option so we are looking at revising the details for this situation. With the second option, I'm trying to decide if I want to even positively connect the two slabs together. My initial plan was to tie everything together, but thinking about it more, I'm not sure if it is really necessary and I think having hooked bars/single legged stirrups, epoxy dowels, etc. would be a fair amount of work and potentially a nuisance with the first pour.

My questions if we went with the two slab option:

1) Would you only include top steel in the top matslab? It's a pretty thick section so I like the idea of top and bottom steel, but I'm not sure what the bottom steel would be doing.

2) Would you connect the two slabs?

 

[jayrod12]

Why not pour the lower as unreinforced and only reinforce the upper? Then the lower would just be added weight and essentially ground improvement.

 

[BAretired]

1) Better to use top and bottom steel to satisfy minimum temperature and shrinkage requirements.

2) If the top slab is required only to provide dead load, no need to connect the two slabs.

Will you be providing an underdrainage network of drain tile connected to a sewer system?

Will you be providing PVC waterstops between the walls and the bottom slab?

BA

 

[BAretired]

Why not pour the lower as unreinforced and only reinforce the upper? Then the lower would just be added weight and essentially ground improvement.

Be careful with that idea! If water can enter the space between the two slabs, the lower slab cannot be considered part of the dead weight resisting uplift. The lower slab should be designed to resist net uplift pressure and should be sealed to the walls.


BA

 

[jayrod12]

Great point BA... wouldn't that apply regardless Of which one is reinforced?

Therefore they should be tying the two together, or making sure the perimeter is adequately water stopped in my mind.

 

[jdgengineer]

Be careful with that idea! If water can enter the space between the two slabs, the lower slab cannot be considered part of the dead weight resisting uplift. The lower slab should be designed to resist net uplift pressure and should be sealed to the walls.

This was our concern as well. By sealing in the lower slab I thought we could prevent the water intrusion.

Will you be providing an underdrainage network of drain tile connected to a sewer system?
Will you be providing PVC waterstops between the walls and the bottom slab?

We aren't designing the waterproofing but its my understanding there will be waterstops installed, likely xypex, and a robust waterproofing system. Some areas can't be drained due to proximity to a septic system, but the waterproofing will likely consist of grace preprufe and bentonite. Waterproofing is outside of our scope.

 

[jdgengineer]

making sure the perimeter is adequately water stopped in my mind.

This is our intent. The retaining wall will be continuous down to the lower slab with rigorous waterproofing applied to the outside of the wall and under the slab (designed by others).

 

[BAretired]

If only the top slab is reinforced, the bottom slab could crack and allow water to rise through those cracks causing pressure on the upper slab. Then the dead weight of the bottom slab would not be available to resist pressure.

Tying the slabs together is not a substitute for water stopping because water can still enter through joints between floor and wall. Water stopping between walls and lower slab is essential to resist upward pressure. Without it, the basement will be flooded.


BA

 

[structSU10]

Not sure if this falls into the same issues discussed above about a middle layer, but could you do the bottom slab, cast the walls, backfill with runner crush, then cast a standard SOG on top? Maybe it gets a little thicker overall, but when they backfill the walls they can just dump into the 'bath tub' and come back to make a standard floor. Might also make any underslab utilities that may be required easier to access if they are in the fill space so you can get to them if there is ever an issue.

 

[jdgengineer]

Not sure if this falls into the same issues discussed above about a middle layer, but could you do the bottom slab, cast the walls, backfill with runner crush, then cast a standard SOG on top? Maybe it gets a little thicker overall, but when they backfill the walls they can just dump into the 'bath tub' and come back to make a standard floor. Might also make any underslab utilities that may be required easier to access if they are in the fill space so you can get to them if there is ever an issue.

I like the idea, but we have a fair amount of interior bearing walls, steel columns, shearwalls, etc. which would require more of a footing than a standard SOG. I think the added forming etc around these items may make the thicker slab potentially easier.

 

[canwesteng]

How can waterproofin be out of your scope? If the foundation leaks is no one going ask who the engineer was?

Anyway, another vote for just a mass of concrete on top of the bottom slab.

 

[jdgengineer]

How can waterproofin be out of your scope? If the foundation leaks is no one going ask who the engineer was?

In our area waterproofing is always outside of the structural engineers scope. It is either detailed by the project architect or for more sophisticated projects by a waterproofing consultant. As a structural engineer my knowledge of waterproofing is quite limited and I have no business designing the waterproofing. I believe this is fairly standard practice.

 

[KootK]

X2 for SU10's SOG over raft proposal. It's all about those under-slab utilities in my opinion. Thicken & downturn locally where you gotta and put some weeping tile in the interstitial just in case if the stuff in there is not free draining etc. Of course, the name of the game is never letting the ground water get there to begin with.