Excavation support 4

[hoshang]

Hi all,
Please find the attached image.
The building has 2 basement floors+ 2 floors (used as shows)+ 4 typical floors (used as flats). The mat foundation is 800mm thick. The basement floors are 3.6m high floor-to-floor each. My first thought is using piles for excavation support. My worry is the piles will interfere with boundary columns. What is the best practice to avoid such a condition?

 

[hoshang]

If I get enough geotechnical soil properties, I'm thinking of using soldier pile beams with walers connecting all perimeter soldier beams @ elevations -2.40 and -6.00 (so it's now above the splice length of the columns since the columns have 16mm reinforcing bars in it), and struts @ corners. Another level of walers may be required between the two other waler levels. Your opinions on this would highly appreciated.

 

[PEinc]

If you are going to brace the 8 meter deep excavation, the upper braced waler should be about the same elevation as the bottom of the existing building foundations. Preferably, each braced waler would be just below a proposed, basement, floor slab to minimize brace interference with new building framing. Bracing at the corners of the excavation can be tricky with raker braces. You probably need to use horizontal corner braces at the corners instead of raker braces. The locations of your soldier beams and your raker braces need to be coordinated with the proposed foundation mat and column locations. Walers with inclined raker braces will need roll chocks at each brace to prevent the waler from rolling upward from the vertical component of the raker brace load. Use as few levels of braced walers as possible. Make sure you check the intermediate design cases: Cantilevered soldier beams before installing the upper level of bracing, One level of bracing before installing the second level of bracing, etc.

Additionally, despite having a temporary sheeting wall, the new building still needs to be designed for all lateral pressures and surcharge pressures from the existing buildings. As the building is being constructed, you cannot remove any temporary braces until the new building is backfilled, if needed, to the brace levels that need to be removed and until the foundation wall is properly and permanently braced by the new floor slabs.

Again, sheeting walls are generally flexible systems. Consider underpinning the existing buildings.

http://www.PeirceEngineering.com

 

[hoshang]

Preferably, each braced waler would be just below a proposed, basement, floor slab to minimize brace interference with new building framing.

The building framing is RC columns, beams, slabs, and walls. If each braced waler would be just below a proposed, basement, floor slab, then it would interfere with new building framing since bar splicing for columns from the basement floor would interfere with braced waler. My thought is each braced waler should be above bar splicing for columns. So my proposed elevations in the previous post. Am I wrong?

 

[EireChch]

hoshang, I am sorry to say that you are way out of your depth. If you do not know how to estimate phi from an SPT then you shouldnt be designing an 8m high retaining wall, or any for that matter.

Its great that you are coming hear and trying to get advice on what to do but the consequence's of an 8m high wall failing right beside a building are very serious. Someone could die.

If you are coming hear just to learn and ask questions then that is fine. But if you are coming hear to get advice on how to design the wall with you being principal designer then this thread should not go any further IMO.

 

[PEinc]

I agree with EireChch. Wall-line soldier beam sheeting with walers and braces will cause many construction problems. You could brace every soldier beam without walers but your concrete contractor and waterproofer will not be happy.

http://www.PeirceEngineering.com

 

[hoshang]

Hi all,
I made a preliminary soldier pile layout as in the attached file (not designed yet) at the elevations in my earlier post. Some struts are longer than 13m. The right and left boundary would be soldier piles braced by rakers. Your thought would be highly appreciated.

 

[PEinc]

It would be nice to know the soldier beam spacings. My first impression is that, for holding up buildings, the W8X31 soldier beams are very small and the 8-inch struts are very small and very long. The struts need to be designed for axial load and bending from self weight and any surcharges. In addition, where will your lagging be installed - behind the front flanges or behind the back flanges? Are your struts inclined raker braces or horizontal cross braces? How are you providing lateral support for the sheeting walls along Col. Lines A and E? Is there sheeting at the bottom, angled side of the site from Col. Line A to E? How will the site be excavated with all of those long, closely spaced struts? You need help from someone with extensive, successful, sheeting and underpinning design experience.

http://www.PeirceEngineering.com

 

[hoshang]

In addition, where will your lagging be installed - behind the front flanges or behind the back flanges?

behind the back flanges, at the property line.

Are your struts inclined raker braces or horizontal cross braces?

I'm thinking of using this configuration for struts longer than 13m.

and horizontal cross struts for struts less than 13m long.

 

[PEinc]

You need help from someone with extensive, successful, sheeting and underpinning design experience.

http://www.PeirceEngineering.com

 

[hoshang]

You need help from someone with extensive, successful, sheeting and underpinning design experience.

Thanks. I'm trying to understand whether my thought is correct. My thought from my post on 5 Jan 24 08:55 came to my mind as I read other threads here recommending struts not longer than 13m. Isn't my framing model for the struts bracing the soldier piles applicable? If not, I wonder why?

 

[PEinc]

I do not understand your strut sketch. Nothing is labelled. Don't you need soldier beams on all 4 sides of the excavation? Are you trying to brace the soldier beams with a truss? Will you have more than one level of that type of bracing?

http://www.PeirceEngineering.com

 

[hoshang]

I do not understand your strut sketch. Nothing is labelled.

Sorry for that. Please find the attached image.

Are you trying to brace the soldier beams with a truss?

Yes, I'm trying to brace the soldier beams with a bent frame (rather than a truss) if the struts (in the attachment in my post on 4 Jan 24 18:38) are longer than 13m.

Will you have more than one level of that type of bracing?

If necessary, then it would be a multistory of this type of bracing.

 

[hoshang]

Hi PEinc,
my proposal on the last post is same as would be for a steel structure frame resisting wind pressure (rather than resisting soil pressure). Is this simulation reasonable?

 

[PEinc]

Final response: You are in way over your head. Get help quickly (not internet help!).

http://www.PeirceEngineering.com

 

[hoshang]

Hi PEinc,
Thanks for your Final response.
My Final query: is my proposal on 6 Jan 24 09:50 doable?

 

[PEinc]

I am sorry, but my opinion is that your multi-level, segmental, truss brace scheme is both unbuildable and uneconomical.

http://www.PeirceEngineering.com

 

[hoshang]

your multi-level, segmental, truss brace scheme is both unbuildable and uneconomical.

Why my truss brace scheme is unbuildable? Can you elaborate more on this, please?

 

[PEinc]

Why don't you discuss your scheme with a local contractor who might bid the project's SOE/Underpinning?

http://www.PeirceEngineering.com

 

[hoshang]

Why don't you discuss your scheme with a local contractor who might bid the project's SOE/Underpinning?

I seek the difficulties in building my truss brace scheme you posted here:

your multi-level, segmental, truss brace scheme is both unbuildable and uneconomical.

I have limited options here since we don't have the underpinning & tieback option. The cantilever soldier pile with this height (8m) must be braced (using either raker or horizontal cross struts). The raker option is omitted since it obstructs the excavation & construction for this limited area. So I have the horizontal cross strut option for bracing the cantilever soldier piles. Since the horizontal cross struts are longer than 13m, I proposed my truss brace scheme. I wonder if one can explain the difficulties in designing & building my truss brace scheme so it can be solved or proposing other options available.

 

[PEinc]

Your truss brace scheme: too big, too heavy, too many pieces, too many connections, needs lateral bracing between trusses, takes up too much vertical space between floor levels, bracing is difficult to remove as floors are being installed.
Braced walls are not cantilevered walls. The sheeting walls will be low, cantilevered walls only until the upper level of bracing is installed.
For 13m long braces, try walers with horizontal pipe braces between opposite sheeting walls. Sometimes, long cross braces need some vertical and/or lateral support to reduce their unbraced length. Install horizontal, corner braces at sheeting corners. This will be easier to build if the new building walls are about 1m off the sheeting walls (the 1m space will need to be backfilled before removing bracing). If you cannot underpin the building, make the sheeting system very stiff and use drilled-in soldier beams, walers, and braces at every other soldier beam. Do a pre-design and a pre-construction survey of the existing buildings and monitor the buildings and sheeting walls during construction.
Again, I ask, why don't you discuss this project with a local contractor who might bid the project's SOE/Underpinning? It does not matter what you design if you cannot find someone, with great experience, to build it.
There is nothing more that I can add.

http://www.PeirceEngineering.com