underpinning 13 feet below water table any tips 1

[propell]

I am currently working on a project in which the next phase requires me to underpin a 7 story masonry building. the building is approx 70' long and i must underpin it 10' deep. the water table is 9' feet below grade the footing of the building is 12' below grade. I must bring it to 22' below grade. I understand that when i dewater below the buildings footings i will undermine the structure. I am considering helical piles or resistance pile to support the building and then dewatering and underpinning. Please if any one can give me some tips or ideas it will be greatful, thanks.

 

[morokanu]

Hi,
Could you be more specific? Why do you have to underpin the building, what sort of access is inside and around the building (including height clearance inside), is there any other building nearby, what are the recommendation(s) from the Geotech people.
Thanks.

 

[PEinc]

Did you consider jet grouting?

 

[cap4000]

Google Hayward Baker and Jet Grout as PEinc suggests. Do Not pump out the water as the adjoing structures may also settle espically if you have clayey soils to deal with.

 

[PEinc]

I agree.

 

[Prepakt1]

If you can excavate with the water in place it would be better.Any pumping will dry out the clay or suck out fines from under the existing structure causing voids or settlement.Being under an existing stucture you may need a to dewater quite a large area due to access constraints. A lower foundation and supporting wall can be built underwater. How this would be done depends on the site. There are a couple of methods you could use.
In any event excavation will leave some voids under the existing structure.
Grout tubes should be placed in the new wall and the voids behind pressure grouted.


Intrusion Prepakt

http://www.marineconcrete.com

 

[PEinc]

I would not do that. The hole would have to be dug under a building, in the wet, and unshored or with drilling mud or slurry. Sounds like big trouble to me. Any case histories of similar, successfully or unsuccessfully completed projects?

 

[DRC1]

Jet groutig woks well in sands, but clays and silts are a different story. Lowering the water table increases the effective stress on the underlying soil. depending on the nature of the soil, this may or may not signifcantly increase foundation settlement. generally you only underpin a few feet below the proposed excavation, so dewatering and its effects will need to be considered at some point. If you do not dewater before underpinning, traditional pit underpinning is not feasible. Jet grouting or minipiles are certinally options. However if you dewater to a few feet below the existing basement footing, you may be able to underpin with jacked pipe. The pipe would need to be lagged and tied back as the excvation proceeded.

 

[PEinc]

propell never described what the reason is for underpinning the building. Even if the building could be vertically supported (underpinned) by one of the above-mentioned methods, the site may still need to be dewatered in order to build a new adjacent structure. If so, how are the excavation support and dewatering going to be handled for the other 3 sides of the site? You can't look at just one side of a difficult excavation.

 

[propell]

The reason for underpinning is because the foundation for the new building is 10 below the exsisting adjasent building. we will have controlled dewatering allowing us to control the lift hights. i would like to use helical pile to support the structure and then embed the piles into the concrete of the underpinning. this will control soil erosion. the other three side will recive soilder beams and lagging with good drainage. Please if any one feels that this can work contact me i will be glad to work with some one on getting this designed. thanks to all with your helpful posts

 

[Prepakt1]

If you are going to do a lot of dewatering you might have to consider totally supporting the adjacent structure or structures before you dewater.There will be less damage to the existing structures foundations if the dewatering system extends outside of the old structure. Dewatering your excavation from just one side of the old building will increase the wash out considerably.
You also have to consider the permeability of the ground and the flow rate into you excavation. You can't just dig a hole and hope the pumps will keep up. You may have to seal of the area before you start pumping.
Have you considered freezing the area.

Intrusion Prepakt

http://www.marineconcrete.com

 

[ankerman]

Have you looked at micropiles? More specificly for your application, injection grouted micropiles using a hollow threaded bar with disposable drill bit. No dewatering or excavation required. Can be used in any soil type from sand to rock. The drill steel becomes your anchor for tension and compression loads. Especially useful in expansive clays for new as well as retrofit construction. They can be used as ground improvement under mat slabs or terminated in the footing/foundation with an embedment plate.

 

[PEinc]

If propell intends to use soldier beams and lagging on three sides, he will need to dewater to below subgrade. If he dewaters well enough to lag to subgrade and build the new building, he will have dewatered well enough to conventionally underpin the building or use any other type of underpinning. Underpinning plus helical piers or micropiles is probably overkill.

I recommend that propell speak to a engineer or design build contractor who is very experienced in underpinning and excavation support in that particular location.

Where is the job?