Underground Parking 3

[JAE]

I've got a potential project where there will be two-stories of underground parking below a 5 story bulding.

I'm pretty sure I'd like to have cast-in-place concrete perimeter walls for this two-story basement. However, the walls will extend out to the property line, or perhaps out to the street curb line under the sidewalk and I was wondering:

What would be some feasible, economical ways to construct this in-place without digging a huge hole beyond the garage footprint? I know how to design the walls in-place - what I'm looking for is constructability means and methods. I've done concrete soldier piers before, but only for a single, 16' deep excavation. Sheet Piling?
Dig down and tie-back as you go?

And with those methods, what I'm curious about is waterproofing. For my soldier pier project, we added drainboard between the piers and cast an integral wall across the pier faces, draining the wall to a perimeter pipe under the wall. We then used a "negative" integral waterproofing on the inside face of the wall.

I'm just not as confident with a two-story condition, though.

 

[eric1037]

Have you considered using a diaphragm wall system? See the following link for some interesting construction techniques. These were utilized recently at the US Capital Building.

http://www.nicholsonconstruction.com/services/walls.html

 

[PEinc]

Diaphragm walls are very expensive and usually leak when holding back ground water. They are usually used when you need a very stiff wall due to adjacent building surcharges and when the diaphragm wall is to be the final foundation wall. If not, they usually are nowhere near being economical.

Tiedback or braced sheeting can be installed about 3 to 4 feet off the proposed foundation wall or can be installed on wall line as long as the proposed footings do not extend beyond the outside face of the proposed foundation wall. Sheeting 3 or 4 feet off the face of wall does not require "digging a huge hole beyond the garage footprint."

Sheet piling may be economical if you are holding back ground water and the sheets can be driven to the required depth. If there is not ground water or if you can temporarily dewater the site, then soldier beams and lagging, tiedback or braced as required, is a more economical way to go.

The subsurface ground conditions may influence the type of support system you need. Also, you must decide if the support system is temporary or permanent. There is a premium to pay for a permanent system.

 

[JAE]

Well, the site is most likely going to have water as its somewhat near a major river. Sheet piling was a first thought, but again, how do you ensure a water-tight bath-tub? I can see casting concrete walls onto the sheet piling (whether tie-back or cantilevering) ...but the water! I don't want the garage having water running into the floor areas.

The bottom slab would be an on-grade concrete and I would presume some sort of bentonitic waterproof panels would be required? Volclay?

 

[PEinc]

Somewhat near a major river? What does your geotech say about the water? Usually he will do an analysis of how much water to expect. This will affect structure uplift, pumping (temporary & permanent), water proofing, and the appropriate support system.

Steel sheeting holds out most of the water. There will be leakage through the interlocks and possibly up through the subgrade.

It's usually easier to waterproof soldier beams and lagging than it is to waterproof steel sheet piling. Soldier beams and lagging do not hold back water. Therefore, temporary dewatering would be needed.

Diaphragm walls are usually the last choice due to cost.

You should discuss this job with a local geotech who has experience with a similar project.

 

[JAE]

This is a project in concept phase only. In fact, we don't have the project under contract yet so there is no geotech report yet (there will be).

 

[PEinc]

Money and time can be saved now if an experienced geotech comes on board now. The report can come later.

 

[miecz]

Are you more concerned about water leaking through the concrete walls or at the joint with the base slab?

In either case, you could design the foundation according to ACI 350, to ensure water tightness. Essentially, this is a matter of adding more reinforcing to ensure that cracks are small enough so water won't seep through.

 

[PEinc]

I'm concerned about the correct foundation wall system and excavation support system being chosen.

If ground water is a big concern, the water proofing method may work better with one type of excavation support wall than with another.

Big bucks are at stake. Many buildings are built with conventional soldier beam and lagging sheeting 3 to 4 feet off the foundation wall and with waterproofing then applied to the outside face of the concrete foundation wall prior to backfilling. This is usually the first method considered.

If wall line sheeting or wall line underpinning is needed, the waterproofing is applied to the face of the sheeting or underpinning. Then the new foundation wall is poured against the sheeting or underpinning.

For a much smaller number of usually major buildings (usually in Boston, D.C., NYC), the design team decides to use diaphragm (slurry) walls or secant pile walls as both the excavation support (or substitute for underpinning) and the foundation wall. If this method were cheaper, it would be used more often, but it isn't.

Talk to the design team for their recommendations on waterproofing and to an experienced local geotech for thoughts on the waterproofing requirements and possibly the need for a permanent dewatering system. Find out what the adjacent, similar structures did or are doing and how successful they have been.

 

[JAE]

Thanks for the ideas all.

 

[eric1037]

I have been involved with several projects that used tangential auger-cast piles for earth retention. You would probably need to use additional internal support for the walls at that depth. Drain board is placed on the inside of the wall and a concrete wall was cast against it. It would be similar to using sheet piling, but with less vibration. As PEinc eluded to, it depends on the project location and the available contractors if these types of systems are cost-effective.

As far as the groundwater, you may need to provide drainage below the slab, design the slab to resist uplift, and/or install other permanent dewatering measures.

 

[JAE]

If you use a drain gravel below the bottom slab, and perforated drain pipes in a grid, all flowing to a sump pit, would that work?

Would you still need to have Volclay panels under the slab? This is a slab for a parking garage so a purely "dry" slab isn't essential.

 

[SlideRuleEra]

I agree with eric1037 about designing the foundation to resist uplift... but suggest doing that even if a permanent dewatering system is installed. Don't want to loose the parking garage because someone (in the future) ignored mechanical/electrical failure of the equipment.

Doubt if you need waterproofing panels under the slab. Volclay wall panels plus good water-resistant concrete placement practices for both foundation & walls should be enough. Gets back to the upcoming soils report though... If the water table is high enough there is no point in placing drains under the slab. Let the subgrade stay wet and put the water collection system "inside" the garage.

http://www.SlideRuleEra.net

 

[Ron]

JAE...I would go with sheet piling, leave in place and cast your concrete walls against it. You can waterproof the inside face of the sheet piling and if you place a work slab, you can continue the waterproofing up the inside face of the sheetpile, this making the outside face of the concrete waterproof. Don't forget the waterstops as this will be like a tank but dry (hopefully!) on the inside.

Yes, this approach will have a bit more expense (sheet pile loss versus forming the concrete on both sides), and some additional concrete (in the pile flutes), but it is easy to implement. Water won't even be much of a problem (laterally) until the steel corrodes in 15 or 20 years.

Consider a mastic waterproofing and cast the concrete against it while it is still soft. Coal tar is best, but asphalt will suffice. Use same material to waterproof the work slab before placing the final slab.

 

[PEinc]

I don't think enough information has been given to actually recommend the most appropriate support system. We have no information on adjacent structures, soil types, bedrock, noise restrictions, or building footprint shape (to name a few considerations). Any one of these considerations may be enough to preclude the use of steel sheet piling or another system.

With sheet piling, the cost of extra concrete would be significant, as would the extra waterproofing area needed to follow the surface of the sheet piling. Also, from my experience, a one-sided wall form can easily cost as much as a simpler two-sided form. Therefore, there may not be a savings in formwork or there may be a cost premium.

If the wall-line sheet pile wall needs tieback anchors, the sheeting wall may also need tieback wales which will interfere with the foundation wall.

Again, there are many things to be considered and little information has been provided.

 

[PEinc]

Also, the price of steel sheet piling has increased greatly in recent months. Sheet piling = more steel = more $$$$

 

[eric1037]

PEinc makes a good point with regard to the price of steel. It has recently become more cost-effective in many cases to install tangential auger-cast piles than steel sheeting.

It is important for design engineers to keep things like material prices in mind when doing their design. Don't just specify the same-old same-old. It may not be cost-effective even if it saves money on the engineering side for simplicity's sake.

 

[PEinc]

Soldier beams & lagging or soil nailing are usually cheaper than sheet piling, tangential auger-cast piling, or diaphragm walls. However, it also depends on the contractors who are bidding the work.

 

[dik]

I would seriously get a geotekkie involved and drill some holes and set in pipes to monitor the ground water. Is it possible to raise the building?

I've used 'pop-outs' where there is a fluctuating water table that permit the lower levels to flood with the pop-outs preventing hydrostatic uplift for the lower floor SOG.

Dik

 

[Azz90]

Quite often all you would need is a steel sheet pile wall. The wall can act as both temporary shoring and a permanent wall. A non-structural seal weld is applied over the interlocks to make the wall completely water tight. This will allow you to stay within the property line.

I have seen several different methods for the base slab and drainage issues. If the you can cut off the water with the sheet pile, you can than use an underdrain system below the base slab. Other wise a "raft" mat foundation may be required with tension anchors.

You would need temporary anchoring until the slabs are constructed. The slabs would than act as the bracing in the permanent condition.

Here is a website with more info

http://www.skylinesteel.com/applications/construction/parking_garages.aspx