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Resin injection instead of underpinning? 3

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milkshakelake

milkshakelake

Structural
Jul 15, 2013
1,106
US
I'm pouring pile caps that are about 4' below an adjacent wall. Normally, we would underpin the adjacent wall to avoid undermining it. Pretty straightforward. But the contractor wants to do pressure grouting (or resin injection, geopolymer injection, whatever it's called). I'm not very familiar with this method but all my research shows that it's used for strengthening and improving bearing capacity of soils, not actual underpinning. Can it be used for underpinning? And does anyone have any articles or drawings showing it can be used or not used in this capacity? (For drawings, I looked around some public works stuff but couldn't come up with anything.)

Sketch below. Ignore the notes, just showing the general idea.
Screenshot_2023-06-06_124427_lgdeo8.png
 
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"It depends."
Questions:
What is the soil - clay, sand? Will the soil be permeable enough to accept the injection?
What is the construction of the wall and its footing - concrete, rubble stone, concrete block (grouted or not)?
What type of piles will be installed and how - driven, drilled?
How deep below the existing wall footing do you need to dig?
How big and thick are the pile caps? Will the tops of the pile caps be above or below the bottom of the wall footing?

 
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Type of soil: uncontrolled fill silty sand and debris with NPT=12
Type of wall: brick. Type of footing: Not sure yet, will dig test pits at some point. It's either concrete or rubble stone based on my experience of footings in this area. Though I don't really do different procedures with standard underpinning whether it's rubble or concrete.
Piles: drilled though soil and socketed into rock. No driving. They are 9 5/8" steel tube in the cased portion (about 10' long) with 5' rock socket.
Depth: bottom of pile cap is about 4' below existing wall. Pile caps are about 2' thick (not my design, I would've made them thicker) so their top is below the existing wall as well, though they'll have concrete wall above.
 
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As you dont want to disturb the wall then you are looking at permeation grouting which essentially fills pores in the soil. All you need is a UCS of at least 2MPa to ensure the material can stand on its on and support the wall. You should easily be able to meet that target with permeation grouting. The silty SAND you have could be grouted but you need to check groutability.

Permeation grouting practical feasibility -i.e. groutability- is a function of soil type, soil density, soil permeability and grout properties. Colloidal silica, sodium silicates, resins, and micro-fine cements are normally employed in medium-to-fine sands or coarse silts where the grouting is almost impossible using cement-based grouts

To evaluate groutability when the grout is a suspension, simple empirical equations are in use. Most developments are based on Terzaghi filter criteria, for instance, the equation proposed by Krizek et al.[45]: N=D10soil/D95grout>8
where d10,soil and d95,grout are the grain sizes corresponding to 10% soil and 95% grout passing fraction, respectively. When this ratio is higher than 8, permeation is deemed to be possible.

the above is stolen from a paper but provides good info.

Although its seems that grouting could be used, I just dont know if it would be quicker or more economical and underpinning the pile caps....
 
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@EireChch Got it, thanks for the advice. I'll look into permeation grouting. Since I'm structural, it might be out of my wheelhouse and I might have to get a geotech to design it. And yeah, I don't see any reason why this would be better than simple underpinning. Concrete underpinning is the tried and true method around here and grouting is rarely, if ever, done. The owner is pushing for it so I had to do some research into an uncomfortable area.
 
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"Type of soil: uncontrolled fill silty sand and debris with NPT=12"

I would not try any type of injection or grouting in uncontrolled fill containing debris. Underpin the wall.

 
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@PEinc Thank you! I think that's all I needed, that's a slam dunk to get away from this crazy idea from the owner.
 
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Good point PEinc - all i seen was silty SAND!

Yes - MSL - that can be your very valid argument to steer away from it. As debris are included, you dont now what you are grouting.
 
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