Steel underpinning vs. concrete grouting

[lgrady]

I have been told that the most secure way to make repairs is by both grouting and underpinning. Could you explain this more?

[admin]

Ok,

Grouting is used to do two things:

1) First is to seal the top of the limestone layer in order to stop the sinkhole from raveling anymore sand into it.

2) Second is to add density to the soil matrix in order to stabilze the ground.



Piering (underpinning) is used to stabilze the load bearing walls (where the most load is).

The determination of which (or both) in each individual circumsatance is determined by the engineer overseeing the remediation.

It is my opinion (for what it's worth) that grouting is to be used to fill voids within the soil matrix and piers should be used to stabilize the house.Grouting to stabilize a structure is problematic at best and piering does nothing for the soil matrix.

In essence, grouting stabilizes the soil and piering stabilizes the stucture.

[lgrady]

Ok, so if the insurance company's geo engineer's report does not call for piering, am I to assume that it's not needed?

[admin]

Not nessecarily, sometimes the engineering firm doing the report is instructed not to make a pier reccomendation in the report due to cost restrictions. I believe that piering should be recommended when there is a high possiblity that the property might move more from soil modification.

[kg]

1. What exactly would constitute a high possibility that the property might move more from soil modification?
2. Does the cement grouting process work where you have shallow plastic clays? If not, why not?

[admin]

1) Steel piers should be used anytime you grout or if the limestone is under 30 feet then piers should be used as a primary method.

2) Grout and plastic clays do not bond together like grout and sand. If the clays are active or can still expand and contract, you risk more damage to the structure by grouting. If you have highly plastic clays, the best remediation is to stabilize the property using steel piers and form a moisture barrier under the foundation or add a moisture control system to constantly keep the clays at a controled degree of wetness.

[kg]

I have talked w/ some geotechnical engs who believe that grouting is an effective remedial measure even when there is highly plastic clay at depths of 6 - 7 ft when the loose soil zone is apprx 20 ft
The premise is that the grouting will be at the limestone depth and then the compaction grouting would be at the 20 ft depth but would not come up to the 6 -7 ft depth where the expansive clay is.
Any thoughts on this?

[admin]

The proposed plan is a sound one, providing that the the grout is injected underneath the active clay zone. This will allow the grout to compact the loose soils below the clay layer. Grout and clay do not mix.

The real question here is what is actually moving the property, the loose soils at 20 feet or the active clay layer at 1-7 feet. While the grout will take care of the problem at the deeper level, if the actual problem is at the shallow level, the grouting will not solve the problem.

Piers will solve the problem of the active soils by transfering the weight of the structure to the deeper soils hence bypassing the shrink/swell activity of the surficial clays.