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?

 

[PEinc]

I do not understand your question. Are you concerned that a soldier beam excavation support system between the existing and proposed buildings will interfere with the new building's columns? If so, the existing buildings should be underpinned with concrete piers beneath the existing walls and column footings. A section view could help explain your problem.

http://www.PeirceEngineering.com

 

[hoshang]

Are you concerned that a soldier beam excavation support system between the existing and proposed buildings will interfere with the new building's columns?

Thanks. Can I use soldier beam excavation support system for such a height (3.6+3.6+0.8=8m)?

 

[PEinc]

Soldier beam excavation support systems are considered flexible systems (especially if cantilevered) and, as such, are not normally appropriate for supporting a closely adjacent structure. Flexible means wall movement. Wall movement means building movement and possibly damage. Now, can it work? Maybe, if made stiff enough and if installed carefully. I don't recommend supporting structures with soldier beam and lagging excavation support systems. With an excavation depth of 8m, you should need some type of lateral support, either tieback anchors or raker braces.
You also did not answer my question about having enough room to install the system without interfering with the new building wall and columns. If you can't underpin the building, your better options could be a diaphragm wall, a secant pile wall, or a tangent pile wall - all much more expensive than soldier beams and lagging and they still may need lateral support.
A section view could help explain your problem.

http://www.PeirceEngineering.com

 

[hoshang]

Hi PEinc,
Thanks. I'm sorry I haven't been clear.

Are you concerned that a soldier beam excavation support system between the existing and proposed buildings will interfere with the new building's columns?

I meant the hatched area is the proposed construction area. My main concern is with interfering of the proposed piles (within this construction area to support the neighbor soil and foundations) with the column locations (within this construction area).

 

[PEinc]

As I said in my first response, if you are concerned that a soldier beam excavation support system between the existing and proposed buildings will interfere with the new buildings' columns or walls, then the existing buildings should be underpinned with concrete piers beneath the existing walls and column footings. The underpinning will prevent the existing buildings from settling and the underpinning, with proper lateral bracing or tieback anchors, will resist lateral earth pressure from under the existing buildings and resist any surcharges coming from inside the existing buildings.

http://www.PeirceEngineering.com

 

[hoshang]

The underpinning will prevent the existing buildings from settling and the underpinning, with proper lateral bracing or tieback anchors, will resist lateral earth pressure from under the existing buildings and resist any surcharges coming from inside the existing buildings.

Thanks for your response. I don't think the tieback option is permitted. Will soldier beams (if not interfering with column locations) with struts be a choice? If so, can the walers be located between the flanges of the soldier beams so that the walers will not interfere with column locations?

 

[PEinc]

An 8m high wall generally requires lateral support - tieback anchors or internal bracing. If you try to use soldier beams, they will need to be closely spaced to provide stiffness. Installing a wale between the soldier beams would be difficult and expensive and still need a brace on each wale section or bay between soldier beams. This would be a nightmare for digging and for forming the new foundation walls. Also, the new foundation walls would need to be designed to support the lateral surcharge loads from the existing buildings.

You say you cannot use tieback anchors. Will the neighbors allow you to underpin their buildings? I guess probably not. Plus the underpinning would also need lateral support. I think that your only options may be a very stiff, cantilevered, secant pile wall that also acts as your new foundation wall or a very stiff, closely space, soldier beam wall with one or more levels of braced walers which get removed as the new foundation wall is constructed in shorter height sections which would need to be braced by the new floor slabs prior to removing the braces and walers. The second option would be expensive, tedious, and apply big concentrated loads to the new floor slabs.

If the neighbors are so disagreeable, maybe it is wise to move your building away from the existing buildings.

http://www.PeirceEngineering.com

 

[hoshang]

I think that your only options may be a very stiff, cantilevered, secant pile wall that also acts as your new foundation wall

Thanks PEinc,
Please find the attached image.
Do you think a very stiff, cantilevered, secant pile wall that also acts as my new foundation wall will be applicable for this basement geometry (especially near the stair locations)?

or a very stiff, closely space, soldier beam wall with one or more levels of braced walers which get removed as the new foundation wall is constructed in shorter height sections which would need to be braced by the new floor slabs prior to removing the braces and walers.

Can soldier beam wall be braced with struts without walers? If so, I'm planning on using the struts in the y direction.

 

[PEinc]

You will not know if a system will work until you determine the lateral and vertical loads and the soil and water conditions and then run some design numbers, including deflection calculations. If a cantilevered design can work, it will be a lot easier than working around walerless raker brace at each soldier beam, especially at the corners. I suggest that you talk to a contractor who is experienced in design-build secant pile walls. I don't expect stairs to be a problem if the subgrade is not significantly deeper than the rest of the perimeter walls. The common problem is architects placing deep elevator pits up against existing buildings.
Remember, a proper design is important; but, more so, an experienced contractor is critical.

http://www.PeirceEngineering.com

 

[hoshang]

You will not know if a system will work until you determine the lateral and vertical loads and the soil and water conditions and then run some design numbers, including deflection calculations.

Please find the attached file.

 

[hoshang]

Hi PEinc,
and this attachment

 

[hoshang]

Hi PEinc,
and this:

 

[hoshang]

Hi PEinc,
The soil report evaluation on page 4 of the attached link on 26 Dec 23 18:42, mentioned that:
Due to the fact that no undisturbed samples could be obtained, no compressibility and strength tests performed. The minimum N-values obtained from SPT tests was 30 indicating hard consistency of the soil.
How one can calculate earth pressure coefficients Ka and Kp while no compressibility and strength tests performed?
Can phi be estimated from the SPT N-values?

 

[PEinc]

You have a poor soils report. It does not have much site specific soils information included. There are plenty of geotech/soils books and manuals with estimated soil properties based on soil type and N-values. Your boring shows predominantly clayey silt and clay. Look for estimated undrained and drained properties. I don't know your time frame for providing a design; but you may need to get a few more borings and some lab testing.

http://www.PeirceEngineering.com

 

[hoshang]

There are plenty of geotech/soils books and manuals with estimated soil properties based on soil type and N-values.

Would you recommend Braja M. Das "foundation engineering" textbook for estimated soil properties based on soil type and N-values?

 

[PEinc]

Many people use Das' book. I personally don't have it. I use other books such as Bowles, Winterkorn & Fang, NAVFAC DM 7. Just remember, you may (should?) get greatly different designs for drained and undrained soil conditions. I prefer to use reasonable soil properties with the appropriate safety factor rather than using overly conservative soil properties with overly conservative safety factors.

http://www.PeirceEngineering.com

 

[hoshang]

Many people use Das' book. I personally don't have it. I use other books such as Bowles, Winterkorn & Fang, NAVFAC DM 7.

Can you tell me what will be reasonable soil properties with the appropriate safety factor for my case when you use other books such as Bowles, Winterkorn & Fang, NAVFAC DM 7?

 

[PEinc]

No. That would not be prudent on my part. I know little about your project, your geographical area, or your building codes. It sounds to me like you need to speak to someone in your area who has more experience in designing and building non-gravity, anchored walls. Sorry.

http://www.PeirceEngineering.com

 

[hoshang]

No. That would not be prudent on my part. I know little about your project, your geographical area, or your building codes.

But you recommended using books and manuals in your post on 27 Dec 23 22:58

There are plenty of geotech/soils books and manuals with estimated soil properties based on soil type and N-values.

Your help would be highly appreciated.