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]

Generally, any underpinning supporting about 2.5m to 3m of unbalanced earth pressure will need lateral support. I expect any underpinning for your project would need tiebacks or bracing.

Thanks for your response. Can underpinning bracing be done by corner struts and horizontal cross struts? or rakers?
Are there any ways rather than underpinning not to lose lot area?

 

[PEinc]

Underpinning can be laterally supported by tiebacks, soil nails, cross braces, inclined raker braces, or corner braces.

Temporary ground freezing (not recommended due to other resulting problems) and permanent jet grouting are other methods of building support. Underpinning can include concrete piers, helical piles, micropiles, jacked piles, jet grouting, chemical grouting (risky voodoo?), bracket piles (which take up room), and slant-augered piles. See any edition of Robert Ratay's Temporary Structures in Construction, preferably his 2012 3rd edition. Concrete piers, jacked piles, jet grouting, chemical grouting, and slant-augered piles are installed under the existing building and encroach very little or not at all into the proposed building. Jet grouting, chemical grouting, and ground freezing will require being trimmed back in order to not encroach into the new building. More underpinning information can be found in Winterkorn & Fang's 1975 Foundation Engineering Handbook and Alan Macnab's 2002 Earth Retention Systems Handbook.

http://www.PeirceEngineering.com

 

[hoshang]

Underpinning can be laterally supported by tiebacks, soil nails, cross braces, inclined raker braces, or corner braces.

I'm not sure, but if tiebacks aren't available here, what's the simplest form of tieback one can use? I mean DIY (Do It Yourself).

 

[PEinc]

If tiebacks are not allowed to cross over the property, you cannot use tiebacks. The same with any support members that could be placed under the existing footings. In my area, if the foundation wall is jointly owned, you would have permission to underpin but not necessarily install tiebacks.

http://www.PeirceEngineering.com

 

[hoshang]

Thanks.
Please review my post on 21 Jan 24 20:38.
So, if tiebacks aren't available in my area, what's the simplest form of tieback one can use? I mean DIY (Do It Yourself).

 

[PEinc]

This project is not one for someone to be learning how to install DIY tieback anchors.
If I remember correctly, you have mostly very stiff to very hard, cohesive soils. Am I correct? If you are allowed to use tiebacks, I would look at using auger-drilled (preferably using a single-stroke, continuous auger), re-groutable, solid bar or strand tieback anchors. I would use an auger with a 150mm to 200mm diameter. Much of what you chose to do will depend on the tieback anchor design loads that you calculate.
Installing helical tieback anchors can be relatively simple BUT your very stiff to very hard cohesive soils will make installing multi-helix anchors difficult. Without multiple helices on an anchor, your anchor capacity will probably be less than you need.

http://www.PeirceEngineering.com

 

[hoshang]

Hi PEinc,
can you elaborate more on auger-drilled (preferably using a single-stroke, continuous auger), re-groutable, solid bar tieback anchors? I searched YouTube for this, is it this:

https://www.youtube.com/watch?v=fAHW0jDfwdo

 

[hoshang]

Also, for 8m pier underpinning, should concreting be a 1_pour process? Is it analyzed as a sheet pile wall?

 

[PEinc]

Your YouTube video shows good information on tieback anchors but not on single-pass anchors or hollow stem auger tieback anchors. You need to know more about tieback anchors than I can type on Eng-Tips. See if you can find online related design manuals on tiebacks and permanent anchored wall. Search for downloadable manuals on the US FHWA Publications web site. Search web sites for tieback drill manufacturers.

Underpinning piers should be excavated full height and then be concreted full depth in one pour. It is poor practice to perform deep underpinning using multiple lifts of shorter piers. Do not underpin the underpinning. Read Ratay's book.

http://www.PeirceEngineering.com

 

[hoshang]

Thanks, PEinc,
what about my query:
Is it analyzed as a sheet pile wall?
I mean is pier underpinning analyzed as a sheet pile wall?
I used Figure 3-4 in USACE EM 1110-2-2504 and found phi to be 30 degrees. Is it reasonable?

 

[PEinc]

If you have continuous concrete underpinning, it can be analyzed as a sheet pile wall to get the bracing loads, bending moments, and shears. Non-continuous (intermittent) underpinning piers with lagging between them can be analyzed as soldier beams. I expect that a 30 degree phi angle could be reasonable for a very dense, drained, cohesive soil. You need to review the project Geotechnical Report, analyze the borings, and choose soil properties you believe to be appropriate. I can't/won't do that for you. Check to see if you have any testing results, such as Atterberg Limits and plasticity indices.
Get some help from an experienced, local, design engineer and sheeting/underpinning/tieback contractor.

http://www.PeirceEngineering.com

 

[hoshang]

Hi, PEinc,
You have been of great help. I uploaded soil reports on 26 Dec 23 18:38 & 26 Dec 23 18:47.

 

[EireChch]

hoshang - man just quit while you are massively ahead

 

[hoshang]

Hi, PEinc,
As you can see in the soil test report, PI is 22 to 25. So using Figure 3-4 in USACE EM 1110-2-2504, phi will be 30 degrees. Is it reasonable?
Another point came into my mind:

Non-continuous (intermittent) underpinning piers with lagging between them can be analyzed as soldier beams.

How the lagging is installed into underpinning piers?

 

[PEinc]

Charts of PI vs. phi shows phi to be about 30 degrees for PI of about 22-25. Use properties that you believe to be correct.
Read the chapter on Underpinning in one of Ratay's books. I have his 1984 and 2012 editions which both show a detail for lagging between underpinning piers.
Again, get some help from an experienced, local, design engineer and sheeting/underpinning/tieback contractor.

http://www.PeirceEngineering.com

 

[hoshang]

Hi PEinc,
does drypack on top of underpinning piers engage at-rest soil pressure on the piers? Or shall they be considered as if they were subject to active soil pressure (without considering drypack engaging at-rest soil pressure)?

 

[PEinc]

Drypack will be subjected to the same lateral pressures and compressive stress that the underpinning pier will see. No special analysis is required for drypck. It is usually much stronger than the soil upon which the footing was originally bearing. The compressive stress or bearing on drypack is usually insignificant (often about 10 to 50 psi or 70 to 350 kPa) for a properly prepared and installed sand-cement mixture. Instead, worry more about the shear and bending stresses in the underpinning concrete and the bearing at the bottom of the underpinning.

http://www.PeirceEngineering.com

 

[hoshang]

Hi PEinc,
Sorry for not wording it correctly.

Or shall they be considered as if they were subject to active soil pressure

Consider the concrete pier without the compressive stress from the building above the concrete pier. The concrete pier will be subject to lateral active soil pressure, correct?
Now, consider the compressive stress from the building above the concrete pier. Doesn't the existence of compressive stress from building above concrete pier change the active soil pressure to at-rest soil pressure?

 

[PEinc]

I have never designed underpinning for at-rest pressure and have never had a problem.

http://www.PeirceEngineering.com

 

[hoshang]

Appreciate your valuable posts.
About drypack, should it be dry? Why? Can one use wedges?