Alternative to underpinning

[Heldbaum]

Hello everyone,

I have a question regarding an underpinning. What is an alternative to the typical underpinning method ? An architect works on the project where they are adding an extra floor to the exist. residential building plus they want to deepen the exist. cellar of about 10 feet. There is no ground water expected at this level. There are adjacent buildings at both sides. The thing is that the neighbors do not want to agree on any underpinning. I am supposed to prepare the proposal for SOE design for this project. But before I do that, I'd like to find a solution. I was thinking about titan micropiles..But I have never done this before so I Am not sure how this works for cellar deepening with adjacent buildings around..? I know it works well for foundation remedies.. Does anyone have any solution/good didactic materials? I would appreciate every advice. Thanks!

 

[oldestguy]

If the job is in tghe States, I'd also ask Hayward - Baker.

http://www.haywardbaker.com/

 

[SLTA]

What about a cantilever retaining wall inside the existing exterior walls? It's not really underpinning, and it's all on the one property, so the neighbors can't fuss.

Please remember: we're not all guys!

 

[Heldbaum]

Oldestguy - Yes thanks, I have visited the site already, I am going to call them tomorrow.

SLTA - how would you excavate 10' below the existing foundation wall/slab in order to build a retaining wall? You mean narrow excavation and build the retaining wall in sections..? Thanks !

 

[SLTA]

I guess I forget sometimes that not everyone has our soil. We've got red clay dirt here - you can EASILY do a 10' cut and it will stand on its own. We did that for our own house. Sorry for the poor advice.

Please remember: we're not all guys!

 

[Heldbaum]

We've got sands there...Maybe your suggestion is doable but I Think it would be a real pain in the ass for contractor..

 

[canwesteng]

Could use sheet piling to build the retaining wall. Your best bet is probably going to involve piles though

 

[structSU10]

how deep is the existing basement? Is there a headroom issue with respect to installation?

 

[Heldbaum]

Canwesteng - Sheet piling is not an option because we would loose a lot of space inside and there is no optin to push sheets in just next to adjacent buildings..

structSU10 - the existing cellar is around 10' below the sidewalk level. They want to go another 10' lower and add Subcellar at elevetation (bottom of excavation at around -20') There shouldn't be any headroom issue since as far as I know the exist. cellar slab is going to be knock down and rebuild.
I have a headroom issue with a different SOE project, very similar to this one, but the ground water is high and it is a landmark building, all exterior brick walls have to stay untouched so we cannot bring in any heavy machinery.. We are trying to "design" our hydraulic press (steel frame with ballast) that would be able to press in the sheet piles next to adjacent buildings without any vibrations..It's a long shot but we have an idea, we will see if feasible.. But still for this project with no ground water I would like to go with something "simple", I was thinking about those Titan micropiles but I've never done this before..

 

[Heldbaum]

Folks, I am wondering if continuous dewatering in order to lower the ground water table within 6 foot (in sands) is really going to affect adjacent buildings? The buildings are 3 story with basement, brick walls and wood joists. Second, considering the fact that water table level changes frequently throughout the year so isn't it that the existing buildings kind of got used to that? Should I be worried about an excessive settlements of adjacent buildings while dewatering considering two assumptions mentioned above..? Thank you!

 

[ATSE]

Lowering the existing ground water level below the elevation of seasonal variation will usually negatively affect adjacent bldgs via differential settlement (unless you have dense sand or very stiff, very over-consolidated clay). Note that when you dewater, by definition it is not a uniform lowering but a not well defined 3-D gradient.

 

[oldestguy]

If it is send and somewhat porous that brings up the possibility of injecting into the voids something that will in effect glue it together. Imagine then everything is on something akin to bedrock. Hayward-Baker will have the answer to that as to chemical and how to do it. Might even do that beyond the PL and the neighbor won't even know what is going on..

 

[PEinc]

This is more complicated than you might realize. This is a bigger lateral support problem than it is an underpinning problem. The existing basements are about 10 feet deep but one basement will be deepened to 20 feet. Therefore, you will have a 10' high unbalanced earth pressure with existing walls stacked on top of the underpinning.

If the building wall load is pretty high, you might be able to install underpinning without needing tieback anchors, soil nails, or internal cross bracing. However, massive underpinning piers (3 to 4 feet thick) will significantly decrease the useable floor space in the basement. Most owners don't want to do this. Soil nails and tiebacks need permanent easements from the adjacent property owners. Titan micropiles or helical piers can vertically support the walls but you still have to address the lateral earth and surcharge loads.

Your best choice is probably to underpin the walls with concrete underpinning piers, brace the top of the underpinning just above or below the new floor slab (near the original bottom of wall or basement slab), then build the new elevated floor slab which can accept the lateral load that the bracing was carrying. Then, remove the bracing. It may be cheaper for the owner to just buy or build another house with a deeper basement. I recently designed underpinning so an owner could increase basement height to 30 feet so he could have an indoor basketball/tennis court in the basement. It was very expensive!

http://www.PeirceEngineering.com

 

[Heldbaum]

PEinc thank you for your response. I might have confused you by talking about two different projects in the same topic. I want to use Titan micropiles in a project where we are lowering the exist. basement around 5' and water table is high (around 1-2' below the existing slab according to geotechnical report) I want to use those piles to support the existing building ( and adjacent buildings - there are party walls at both sides and buildings are kind of old) and then lower the water table and underpin...

Yes another project includes 10' lowering the exist. slab..I realize it is a complicated issue..but there is no neighbors agreement for underpinning anyways so we have to come up with an alternative..
My boss's got an idea for a self-made hydraulic press to push the sheet piles into the ground just next to the adjacent buildings with very little vibration..ballasted steel frame with hydraulic cylinders and custom-made guide..we have a contractor who likes the idea and is willing to invest and fabricate it..so we will see, maybe it will work out

 

[PEinc]

Job 1 - If you have to dewater to build the deeper basement, dewater first and then do conventional underpinning and then deepen the basement. However, check for settlement that might be caused by the dewatering.

Job 2 - If you need to underpin party walls, I don't believe the adjacent co-owner of the wall can stop you. He may be able to stop you from using tieback anchors under his building. Also, the underpinning may not be allowed to extend beyond the inside face of his basement foundation wall or wall footing. The hydraulic SSP driving system sounds like pie-in-the-sky. It will take up a lot of room when against the party wall. The SSP will be significantly offset from the wall, making the basement much narrower. Also, SSP is usually not stiff enough to prevent movement and settling of the adjacent structure.
See attached PDF.


www.PeirceEngineering.com

 

[Heldbaum]

Peinc so you wouldn't use Titan mircopiles ? That's what geotechnical engineer suggests in his report: "... because of the high groundwater table, age of the structure and sensitive nature of the party wall constrcution, we recommend that the underpinning be accomplished using drilled Titan piles.The titan piles should be installed at each of the underpinning pit locations before the pits are excavated(...)"

 

[PEinc]

Titans for which building? In an existing basement? With overhead clearance problems? Is the wall thick enough to drill into and directly bond the threaded rods or do you need underpinning brackets? Can you handle eccentricity from a thick wall or wide footing if necessary? Pre-installing micropiles and then trying to dig underpinning piers around the micropiles could be tricky. Has this geotech engineer ever done this before? Titan micropiles may be able to vertically support the building but you probably still have lateral and dewatering concerns. Sketches would help this discussion.

http://www.PeirceEngineering.com

 

[Heldbaum]

Peinc Titans for the project where we are lowering the basement of 5'..attached is architectural building section. There are party walls at both sides, rubble stone walls with no or very small footings.

 

[Heldbaum]

And plot plan I drew

 

[PEinc]

I don't know of any good way to attach micropiles to a rubble stone foundation wall unless you first underpin to at least some partial depth and then install the micropiles through the concrete underpinning. Bolting micropile underpinning brackets on a rubble stone wall probably will not work very well and the stone wall may not span between brackets. If you need to install concrete underpinning in order to attach the micropiles, you probably still need to dewater and you might as well eliminate the micropiles and just underpin the building.

http://www.PeirceEngineering.com