Sheet Piling

[Heldbaum]

Hello folks,

In a nutshell, owner of the townhouse in new york city wants to enlarge his house vertically and horizontally, meaning going down around 12' in the existing basement plus he wants to extend the basement all the way to the rear (where there is no basement now) what means he would have to excavate roughly 25'-30' down over there which is a lot of course. There are adjacent buildings, he wants to avoid underpinning, ground water table is below expected bottom of excavation. The soil is class 3a sand with silt and gravel. No way to bring any heavy equipment in.
I am thinking about sheet piles that are pushed by a hydraulic jack. We would install two steel beams going from wall to wall (or new steel column to new steel column)and one beam across and attach hydraulic jack to its flange. We would put enough ballast on the top of those beams plus we could use the self weight of the exist. building. Then we would do that in stages pushing 8'-10' long sheet piles next to each other and brace them from wall to wall..then we would excavate 5'-6' so the first pile's tip is still below the dredge line and push another pile next to it..Please see attached sketch of what I would like to do. I think that way we would reduce a friction and make those sheet piles driving easier.. then the question is how to connect those piles together (moment connection) so it can withstand soil pressure and provide watertightness etc.
We would also pre-drill the soil before jacking up operation. I think we would need at least 30-40 tons force to push it it..

It is just my imagination working so please share if you have any thoughts on that issue. It will be greatly appreciated. Thanks!

Jack

 

[Heldbaum]

Thank you very much for your input guys.

 

[PEinc]

avscorreia, very impressive. It looks like you need very good ground conditions to stay out of trouble.

http://www.PeirceEngineering.com

 

[Heldbaum]

avscorreia - how often do you usually drill the micropiles ?

 

[Guest262653]

PEinc
Some cohesion is indeed needed for temporary face stability. This is not applicable in loose sands or high water tables. The soils around here (CDG) mostly allow a 12h stability time frame to place the premanufactured rebar panels and in the situ concrete and formwork.

Heldbaum
It really depends on the panel dimension, which in turn will depend on soil conditions. The micropiles are designed for vertical load only, which arises from the panel weight and the vertical component of the soil anchor prestress (minus some friction of the wall panel with the soil). For example, if the primary and secondary panels are 3m wide, we can place the micropiles at 2.0m cc for primary panels. There won't be any micropiles for the secondary panels as these are supported laterally in the primary ones.

 

[Heldbaum]

avscorreia - thank you very much. Do you have any reference (book, article) about this method (means and methods etc.) ? Primary and secondary panels - is it similar to secant piles that one pile is reinforced and another one is not? How the reinforcement is placed there? If you know any reference, I'd appreciate it.

 

[Guest262653]

The main references I have are in Portuguese. We call it concrete Berlin, Lisbon or even Munich-type walls. Here's some information from the Technical University of Lisbon:
Link 1 (look in page 21 and forward)
Link 2

Both primary and secondary panels are fully reinforced. The concept is different from secant piles as it refers only to the construction sequence. Look in page 22 of the reference I provided.

Some design aspects:
- define a proper construction sequence;
- horizontal actions are the same as for a multi-propped or multi-anchored retaining wall, considering Terazghi and Peck envelope diagrams;
- code-compliant reiforcement lap lengths are required between panels and between cap beam and panels;
- the panels must be analyzed for both directions and in every operation during the construction sequence;
- punching shear reinforcement is normally required around the anchor (strut) connection;
- soldier beams (or micropiles) are designed for compression and buckling only in the upper stages;
- soil embedment length of soldier piles (or micropiles) defined considering sufficient geotechnical capacity for vertical loads;
- over thickness of concrete panel must be considered when calculating vertical loads;
- if long-term support is provided by slabs, a proper connection must be designed (post-installed rebar, rebar in wall and unbending for connection to slab, etc).

 

[Heldbaum]

avscorreia - thanks for that. Any chance I can contact you via email and discuss the situation/site conditions a little bit more thoroughly..?

 

[PEinc]

Before you get too far along in this process, don't forget to check into availability of easements for permanent or temporary tieback anchors.

http://www.PeirceEngineering.com

 

[Guest262653]

We could all benefit if the discussion was public. Can't you post your data here?

 

[Heldbaum]

Ok sure, I can post it here. I will do that after work.

 

[Heldbaum]

Hi guys,

so I am adding two pictures with floor plans and one boring log. It's a masonry building with pocketed wood joists. On the left side we have a party wall that is shared with 3-story building and on the right side there is a 5-story building with independent masonry wall. All buildings, most likely,have cellars at the same level. So the owner would like to deepen the existing cellar of about 10-12'. The basement floor, which is roughly 10 feet above the cellar level, and around 4' below sidewalk, is to be aligned with cellar level which would mean to excavate roughly 22feet (10'existing + 10-12' new). Furthermore, there is going to be 2-story addition to the existing condition. Independent steel structure from the basement level. But we are not EOR on this project, we were hired to design SOE...The ground water was found 30' below the sidewalk level..I guess can have a small bobcat on site or small drilling rig..What more information would you need in order to elaborate this problem more? Thank you.

 

[Heldbaum]

And picture of a boring log

 

[PEinc]

http://www.philly.com/philly/news/p...pe-mural-arts-west-philadelphia-20170726.html

Here's what can happen did happen yesterday when digging next to and below another house. After partial demolition of one or more row houses, the remaining party wall and its foundation collapsed due to the excavation and lack of lateral wall support.

http://www.PeirceEngineering.com

 

[Heldbaum]

PEinc - Thanks! I know what can happen that's why I am trying to find the best solution/answer not to let it happen..

 

[PEinc]

In my opinion, the safest way to do your job economically is to underpin the adjacent buildings and install sheeting where there are no buildings or where they are far enough away to be of little concern. If you are going deeper than about 12' or 13' below the adjacent grades or about 10' below the existing, adjacent basement or cellar floors, you probably will need either tieback anchors or internal bracing (either cross bracing or inclined raker braces). If the new foundation walls are not or cannot be designed to support the lateral earth and surcharge pressures, you will need permanent tieback anchors. For using either temporary or permanent tiebacks, you will need to obtain underground easements. This might be difficult, especially if using permanent anchors.

Some of the other, above-described, support methods might work fine under the right ground conditions and with qualified and available contractors. I personally do not design structures that might work. Usually, underpinning will have the least effect on reducing the usable area of the basement/cellar, which I often see as the biggest concern with owners and their architects and structural engineers. As long as the water table is below the bottom of your proposed foundations (as you indicated), you will probably find more contractors interested in bidding conventional, concrete pit underpinning and sheeting than the other methods. I would definitely not support the adjacent buildings with soil nailing or with steel sheet piling unless my client insisted and also gave me a strongly written indemnification agreement, and even then, I still might not do the design.

http://www.PeirceEngineering.com

 

[Guest262653]

Couldn't agree more, PEinc, especially with "support methods might work fine under the right ground conditions and with qualified and available contractors". I would only add "and with an experienced and qualified geotechnical designer".

The suggestion I made regarding Munich-type walls was to present a solution that is typical in this area and that performs well in the typical waterless CDG ground conditions, even close to buildings. However, if there isn't an experienced contractor available who dominates the technique and doesn't have enough accumulated experience, this solution should be avoided and underpinning would be the best option.