Basement Slab with High water table

[slickdeals]

I am looking at a project that has a significantly large basement (40,000 sq.m). There are (8) six story buildings that will rise above the ground level.

The building columns and basement columns will be supported by piles. I was thinking of the following as a scheme to avoid doing a ridiculous raft and to mitigate hydrostatic loads.

The basement is 3.5 below the water table. I propose the columns being supported by individual pile caps. Once the piling and pile caps are done (with dewatering the formed caps) --> continue with constructing the superstructure.

Once the superstructure has risen enough to have sufficient dead weight + tension capacity of piles to resist the hydrostatic force, construct the basement slab as a suspended slab spanning between columns. This slab could be 400mm thick to span the 11m x 8m column spacing. Punching shear should not be an issue considering the big pile caps.

Is the above an adequate method to construct the basement without having to do a big raft?

If so, is it adequate to tie the slabs to the pile cap with dowels from the cap hooked into the slab?

Any other ideas/thoughts will be appreciated.

 

[rowingengineer]

slickdeals,
You are confusing me. why would the dead weight of the above help with the raft/slab design for hydrostatic force? The only thing I see it helping with is the design of the piles.

If the basement is deep into the ground a bottom down construction might be the answer, but generally these are done without pile caps.


ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[slickdeals]

The dead weight is only to help the piles. When the basement slab is constructed, the additional dead weight from the superstructure will help to reduce the tension in the piles.

 

[rowingengineer]

Here is a good guide as to your "normal" basement construction. what you suggesting is possible, but I would recommend selecting a more common method. you approach is fine you could just get a little more savings by using these approaches.

http://www.gnpgeo.com.my/download/publication/SL_04.pdf

ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[hokie66]

As your basement is 3.5 metres below the water table, the base slab has to resist 35 kPa uplift. It may be more economical to provide additional tension.s piles between the columns.

Punching shear is on the area of the uplift restraint, which will be less than the area of the pile cap. A problem with tying down the slab to the pile caps is achieving continuity of waterproofing.

Regardless of the quality of tanking/waterproofing system used, I would design the slab as a water resisting structure, i.e. about .6%Ag reinforcement.

The perimeter walls are perhaps the most difficult to waterproof. What is your proposed system and how do you see the slab to wall connection to be made?

 

[slickdeals]

@hokie:
I don't quite understand how the punching shear area would be less than the pile cap? Are you assuming that the slab is not connected to the pile cap , but only to the column that starts on the pile cap?

Good call regarding the waterproofing. I hadn't thought that far ahead. But if I don't tie the slab to the pile cap, then the waterproofing could be continuous over the pile cap?

The perimeter walls will be tied to the basement slab with epoxied bars. I am not sure yet if sheet piles will be used or it will be a wall. In the past, I have welded shear studs to sheet piles to tie the slab. That project went on hold, so not sure how the waterproofing would have been there.

Background:
The dynamic of projects in India is very different. It is all D&C (design and construct) and every contractor is trying to save money all the while keeping the structure good.

Our company (and me as a structural engineer) is providing value engineering services to Leighton Contractors (from Down Under) as they try to win large projects in India.

 

[dik]

It's extremely difficult to waterproof this... like an inverted swimming pool with more joints. You might consider 'popouts'... These are small 2'x2' square inserts in the slab with slightly tapered edges and no rebar. In the event of flooding (often used for parkades), the water pressure lifts the popouts and floods the basement without damaging the slab. If you really need it waterproof, design it like a swimming pool and use bentonite rod in all joints and be diligent about cracks.

Dik

 

[paddingtongreen]

I like your procedure, it makes sense.

One suggestion is that you build the basement after the piles are in place, and then flood it. If the basement has separate compartments, you might be able to use the dewatering water to flood the sections, so as to maintain balanced pressure inside and out.

I suggest this because I worked with people who used similar procedures with coffer dams.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[slickdeals]

@paddington:

I like your procedure, it makes sense

Are you referring to my OP or dik's post?

@dik: I don't think the owner would like this basement to get flooded. I want to design the slab to resist hydrostatic loads. Yes, any and all joints will be waterproofed.

 

[dik]

It's often done, and always with the 'blessing' of the owner... it's easier than repairing uplift damage and cheaper than providing a structural slab... and, noted that it's generally for parkades. I would not use this for a finished basement, with furniture <G>.

Dik

 

[dik]

should have added that it only works for intermittent flooding... not continuous high water table.

Dik

 

[paddingtongreen]

@slick, I like your procedure.

We successfully used a protected elastomeric membrane system in a large basement, part under a building, part under a road. We placed a mud slab. then fiber board then the membrane, placed the slab and the walls, and ran the membrane up the walls. We ran the membrane over the roof. As we backfilled, we put fiber board in place, up the walls and over the roof. It was still dry five years later ~ of course, we had a quake a few says ago and a storm coming is so who knows.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

Slick,

I assume it as paddington said...the waterproofing is under the structural slab and over the pile caps, so the slab is tied down to the pile caps within the perimeter of the caps.

I assumed this was the water table, not intermittent flooding, so dik's relief plugs don't apply.

Talk to Leighton. They know how to waterproof basements. However, they have lost a lot of money lately, so be wary.