Underpinning Waterproofing

[AOW]

We will be underpinning an existing elevator shaft and the groundwater is almost certain to come into play. What is the best way of waterproofing foundation underpinning?

I was considering applying the waterproofing on the inside of the shaft with protection board. However, I am reconsidering this option after speaking to a manufacturer's rep who told me they don't have negative side waterproofing products. I am thinking blind side waterproofing might be difficult to install considering the underpinning will be done in segments.

Thanks in advance.

 

[Ron]

Practically impossible to waterproof the underpinning. Consider chemical injection in soil against shaft and using neg side waterproofing as well. Assuming shaft is cast-in-place concrete, seal all cracks and floor interface with epoxy injection then waterproof

 

[hokie66]

Whatever you do, water will get in, so your sump pump must be reliable, and needs to have a source of emergency power.

 

[AOW]

Thanks Ron and Hokie for your responses. I was having a hard time seeing how this could be done. Seems like a proper sump pump is the way to go even if we use methods Ron suggested.

 

[PEinc]

In the past, I have installed bentonite sheets against the back side of the shored underpinning pits, prior to placing the underpinning concrete. I also installed vertical bentonite chamfer strips along the two rear corners of the excavated pits. I dthis a couple of times long ago and never heard of any problems. (Maybe there really wasn't any water?)

http://www.PeirceEngineering.com

 

[dcarr82775]

I have done things similar to PEinc. We would provide drainage board and bentonite sheets with a foundation drain to a sump. There was definitely water. It leaks a little, but not enough to be a problem.

 

[AOW]

This is what I was thinking of when I said blind side waterproofing.

PEinc, are the chamfer strips used to form the joint of the bentonite sheets? I can see extending the width of the sheets about 4 inches or so on each side and cover the extra 4 inches with 4 x 4 so that the concrete isn't placed directly against the extra width. Once the adjacent pier is excavated then the 4 x 4 can be stripped off revealing that 4 inches of the sheet which can now overlap with the new sheet.

Or am I overthinking it and this is means and methods that should be left up to the contractor? I just think that if am going to put something in the plans I should have an idea how it can be done.

 

[PEinc]

Your description above confuses me. We my not be doing the underpinning by the same method.
An underpinning pit should be shored on all 4 sides. The bentonite sheet (cardboard impregnated with bentonite) is nailed to the wood shoring boards at the rear face of the excavated, shored pit. The bentonite "chamfer strips" are nailed vertically in the two rear corners of the shored pit. These vertical strips provide additional bentonite along the vertical, cold joints between the poured concrete piers. The bentonite chamfer strips are cardboard tubes filed with bentonite. When building an underpinning pier between two previously built piers, the shoring boards on the common side only get removed. The rear shoring boards stay in place. The front shoring boards will be removed when excavating to subgrade in front of the completed underpinning.

http://www.PeirceEngineering.com

 

[AOW]

What I described is similar to what described except that I would be using 4x4 instead of chamfer strips at the rear corners. Now that I understand what the chamfer strips do, I see how your method works.

Thank you all for your advice.

 

[PEinc]

The system won't be perfect, but it's a very reasonable effort. You need to make sure the benonite does not get damaged when stripping the shoring boards between underpinning piers.

http://www.PeirceEngineering.com