Effects of Injection Grouting (Below Slab) on Soil Subgrade Modulus

[kjvmha]

I have looked into repair methodologies for filling voids below an industrial slab on grade for the purpose of providing continuous support under the slab. Injection grouting is one possibility that is being considered. My question is, does the grout (cementitious) provide an increase in the subgrade modulus (k-value of the soil)? Or, does the grout just help provide a more uniform surface to evenly distribute the load onto the subgrade? If it does provide an increase, is there a way to quantify the increase to the k-value in order to calculate load carrying capacity of the slab after the repair is completed? Any thoughts, or recommended references would be much appreciated!

Thanks!

 

[MiketheEngineer]

My gut reaction is that it just fills the voids. Why would it strengthen the soil?? Often called "mud jacking"

 

[kjvmha]

I don't necessarily mean strengthen the soil -- I may not have phrased that appropriately. My thoughts are that the properties of the cementitious grout would allow it to resist vertical deflection (of a slab for example) the same way soil does, but to a lesser degree because the amount of grout would be minimal in comparison to the amount of soil. In effect I would think the grout reacts in a similar fashion as soil, for which we use the modulus of subgrade reaction (k-value) in slab design calculations. If these things are true, then would you be able to compute a composite k-value (so-to-speak) that would combine the reaction of the soil and the reaction of the grout, thus increasing the overall strength of the material below the slab? Or would it be so minimal that any increase in strength due to the grout would be basically negligible?

 

[dicksewerrat]

Unless you had to fill a void more than a foot deep, I don't see where you could consider this a subgrade improvement.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[fattdad]

There'd be no calculated benefit to the soil modulus. There may be some structural benefit as the "thickness" of the concrete slab would be greater and the risk to punching failure for the soil's modulus value would be reduced.

f-d

¡papá gordo ain’t no madre flaca!

 

[kjvmha]

Thanks all.

 

[Ron]

You will have 4 relatively distinct conditions under the slab after grouting:

1. In areas where your grouting compacts but does not infuse the existing soil, you will get a change in the modulus as the modulus is somewhat dependent on density (though its effect can be relatively small)

2. You will have voids filled directly under the slab with all grout material which has a much higher modulus than the surrounding soil (a hard spot)

3. You will have soil that is infused with cementitious material that will have a modulus somewhere between the grout modulus and the original soil modulus

4. You will have soil that gets no treatment at all, thus no change in its modulus.

These conditions will serve to create an irregular bearing condition. This will induce differing subgrade restaint and thus more random cracking, and if the slab is heavily loaded, will respond differently to the loading based on the variable hard and soft spots.

 

[Prepakt1]

You can drill into the underlying soil in a regular pattern and compaction grout. This will strengthen the soil base and force the soil up to support the slab in a more consistent manner. This will be a much more expensive project than mud jacking. Mud-jacking is always a compromise as it supports and lifts the settled slab but the increased thickness of concrete can also add to the load on the soil and cause future failure.

Intrusion Prepakt /marineconcrete.com