Dewatering method

[Heldbaum]

Hello,

My company has been engaged in this project in NYC, we are going to prepare SOE for exist. building vertical and horizontal extension. They are deepening the existing cellar - new foundation will be 4-5' lower than exist. The ground water table is around 11' below the sidewalk elevation which means we will be around 5' under ground water table..There are 2 adjacent masonry buildings..all buildings around 100 years old. I've done recently SOE for the same scenario at nearby property where I did sheet piles and now my boss is trying to "Design" a custom made hydraulic press to press sheet piles in - but as you can imagine, I am not sure if that's gonna work so I don't want to rely on this. That's why I would like to look for an alternative. Can I dewater the site without causing any damages to the adjacent properties and then do a typical underpinning ? Or is it too risky? I know secant piles is the option but it might be too expensive.Any reference materials would be very helpful.Thank you for any advice.

Best,

Heldbaum

 

[SlideRuleEra]

Sheet pile hydraulic press has already been invented, here is a typical brochure. Might be what is needed for this project.

Unconfined dewatering (say, with well points) is too risky, IMHO. The dewatering zone of influence will potentially (probably) affect the soil under the adjacent masonry buildings (on shallow foundations, I assume). Suggest getting a geotechnical engineer involved, if you pursue this type approach.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Heldbaum]

SlideRuleEra thanks! I know it has been invented already but we can't use any 'heavy" machinery over there since it's a landmark building and front facade has to stay as it is..so the whole building is to be demolished beside the facade which has to be supported first. So we will be able to bring in "something small" but capable of pressing in the sheet pile next to the adjacent building. That's why I said custom made press..Thanks for you advice regarding dewatering.

 

[PEinc]

Sheet piling is flexible. Using SSP could and probably will settle the building. There is also the possibility that bracing for the façade support will interfere with installing the SSP. This sounds more like a jet grouting/underpinning project. The jet grouting could support the building and cut off the groundwater.

http://www.PeirceEngineering.com

 

[Heldbaum]

Hello,

So I visited the site today. Attached are two pictures from the cellar that is going to be deepen. The existing cellar is no more than 7' high. They want to make it deeper to have around 12' height. So we would have to go around 6-7' under existing slab level to build the footings. According to the geotechnical report, water table is expected to be around 1' below existing slab which means we are going to be around 5-6' in the water.
All exterior walls, including front facade, stay as they are. It's a landmark building. One extra floor ( penthouse) will be added. What would you do for SOE over there? There are adjacent buildings from both sides. We can't neither use any heavy machinery nor cause any vibration. I wold appreciate any thoughts on that subject. Thank you!

Heldbaum

 

[Heldbaum]

One more picture

 

[PEinc]

You better be careful and know what you are doing. You are changing a 7' high foundation wall into a minimum 13' to 14' high wall. You will have an original masonry foundation wall stacked on top of tall underpinning. You need to check the underpinned wall(s) for overturning and sliding. (Sliding may be your bigger problem.) Once you have more than about 10' from OG outside of a building to subgrade inside a building, you usually need lateral support for the wall and/or underpinning. Nobody will want permanent internal bracing or tieback anchors for this new minimum 12' high wall. Larger, more massive, underpinning piers may help but nobody ever wants to give up any area inside the basement.

http://www.PeirceEngineering.com

 

[Heldbaum]

Peinc thanks for your comment. I am aware of the problem and I will check the sliding and overturning etc. But for now I am more curious how to perform underpinning there since we have a high water table and adjacent buildings (party walls from both sides), so I guess dewatering is not an option..?
Thanks!

 

[EireChch]

Dewatering will increase effective stress and induce settlement, no two ways about it. It's a very tricky situation you have.

This idea is probably a bit far fetched but you could look to freeze the existing ground, then excavate and try install an L shaped retaining wall. It is potentially feasible but too expensive maybe!!

http://www.moretrench.com/wp-content/uploads/2014/11/New-Ground-Freezing-Brochure.pdf

 

[Heldbaum]

EireChch too expensive 100%..

 

[PEinc]

You cannot underpin below the water table unless you are jet grouting or using some type of deep foundation, like helical piers or maybe bracket piles. Even for jacked pile underpinning, you would need to dewater to below the bottom of the excavated pit that will then be concreted and dry packed after installing the jacked piles. The other option for high water table is a secant pile wall or a diaphragm wall - probably neither of which you will be using. Of the methods I just mentioned, only jet grouting, secant pile walls, and diaphragm walls will control the groundwater enough for you to excavate and deepen the basement. Unless you can dewater to below the planned level of excavation (without settling adjacent structures), somebody has a problem.

http://www.PeirceEngineering.com

 

[Heldbaum]

What would you say to the solution like that: first we would use drilled titan piles at each underpinning section to support the adjacent buildings. After piles are installed, before excavation for underpinning begins, 24-hours continuous dewatering will be required to maintain safe ground water level...I understand the risk associated with dewatering with adjacent buildings, my question is if those micropiles would change anything? Would they prevent the adjacent buildings from excessive settlement due to dewatering..? Thank you in advance.

 

[PEinc]

Dewatering could affect (settle?) more of the adjacent building than just the wall to be underpinned. The Titan bar micropiles would need to be properly positioned to concentrically pick up the wall or column loads. Easier said than done.

http://www.PeirceEngineering.com

 

[Heldbaum]

PEinc there are party walls at both sides..I was thinking to install those micropiles at around 3' to pick up the load and then make an underpinning in sections, formwork behind piles would secure the ground ( in sections)? I am looking for some solution..