Chemical soil stabilization legitimate?

[stewbaby]

We have a facility in the south central Texas area were we are looking to expand the tankfarm (add two additional tanks). The area has fat clays and the geotech report recommends overexcavating 11 feet to replace the fat clays. An expensive process given the tank size and spacing. In preliminary talks with possible contractors, chemical injection was mentioned as being a less costly alternative to the overexcavating. Anyone have experience with this? Trying to get a little more educated before I call our geotech up to discuss. EcSS 3000 was a brand name of chemical specifically mentioned by one contractor but it seems to be more for residential looking at their literature.

 

[JAE]

I did chemical injection in San Antonio back in the 1980's at a large church. Not sure if it worked or not but have not heard anything since.
I might check on it with one of my contacts but it took a lot of closely spaced holes and sequential pumping (starting shallow and going deeper each time).

Some called it snake oil.



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[oldestguy]

Do a search for "hydrated lime" treatment of clays. I'd check around with various contractors as to their injection experience also.

Other alternatives would include keeping the soil moisture constant by one means or another. Designing tanks to take differential support.

I'd be skeptical that 11 feet of replacement is an adequate treatment. Up here in Wisconsin the shrink=swell effect can readily go double that depth or deeper

 

[Terratek]

It has been debatable whether the effects of chemical injection are better than plain old water injection or moisture conditioning (or injection with lime for that matter). The chemical folks claim their chemical mixture changes the soil structure. Basically, they use a potassium based proprietary mixture which is supposed to neutralize the negative charges of the clay platelets which cause water absorption (hence clay expansion). Chemical injection purports to have other advantages such as reduced heaving resulting from the injection process compared to just water injection. This can be useful if moisture conditioning excavations next to an existing structure requires impractical bracing or if the existing structure would be distressed from the heaving effects of water pressure injection (with our without lime).

A good approach to chemical injection is two-fold. First, treat it just like water injection or moisture conditioning insofar as placing moisture barriers after injection to maintain the increased moisture conditions which will occur from the injection process. Keep trees and large bushes away. Second, advise the Client get warranties from the chemical injection contractor. While the contract may not be worth the paper it is written on, it directs liability to to the chemical injector. I happen to know EcSS has a warranty they can offer. This is critical since they use a proprietary process.

Subgrade modifications are alternatives to suspending structures on piers of full depth structural fill solutions. The Client should be educated on the risks associated with alternative methods.

Another point I would add is that based on the depth of improvement (11 ft) it sounds like the PVR is pretty high. In these cases you would likely want to place a 2 ft cap of non-expansive fill atop the 11 ft layer of chemical injection. Being the project is in south central Texas, is there any concern with limestone seams or layers which could cause early refusal to injection rods?

 

[stewbaby]

Thanks for the feedback all. 11' was the excavate and replace recommendation from the geotech. Haven't disuccsed the chemical treatment with him yet (educating myself some first). Not sure either what the proposed chemical injection depth would be (just started discussions with some of these contractors) but as oldestguy was alluding to, it would likely be deeper than the 11' of proposed ecavation. PVR is estiated at 7" by the geotech. With 11' of proposed excavation and replacement,it is estimated to go down to 1". Along the lines of what you recommend Terratek, we have to place fill on the site anyway to get our grade where we want it so the 2' of good select fill could be accomplished with that (vs borrow from onsite). On the lines of warranty, if we go the chemical injection route, I'm hoping we can do some form of retainage till subsequent test are performed to verify the stuff 'took' along with the warranty. Couldn't a post injection soil sample be taken and lab tested to verify the chemical application rate and that absorption / swell went down?

 

[moe333]

How about excavate the fat clay, lime treat (about 5% lime) and compact in lime treated lifts?

 

[Terratek]

Yes, you can and should collect swell tests samples when performing chemical or water modifications to verify swell potential has been reduced. Chemical injected soil samples could be air dried before loading in a swell testing machine to see if they retain the same volume after drying. You could do as moe333 suggests also, but most are unwilling to take this route due to the cost. If they are balking at over-ex and moisture conditioning, they will double balk at adding lime to the expense. I would add that 5 percent lime may not be enough. 7 inches PVR = very fat clay which would likely require 7 or 8 percent lime. You know you've reached the correct amount of lime when the PI of the lime-mixed clay reaches 15 or less (you've basically created select fill). This might require some trial and error in the field, but if you spec 8 percent, for example, and it works, I would not back down, even if you got PI's in the 12 or below range. Lime mixing is far from an exact science. Turn the knob about THIS far to get 7 percent and about THIS far to get 8 percent...

My experience is that the depth of chemical injection is the same s water modification (moisture conditioning or water injection). A 2 ft cap of non-expansive fill (flex base would be better than select fill, but more expensive) with 10 ft to 12 ft of improved soils should get you to about an 1 inch PVR in 95 percent of cases. There are select geologic formations and geographic locations where you might assume slightly deeper, but I would leave that to the judgment of the geotech. If you subscribe to theory that the active depth of moisture change is about 15 ft, you will have modified 12 to 14 ft of the profile. The remaining portion below the improved depth contributes the least to the overall PVR due to the overburden pressures. Plus you've created a thick pad of relatively impermeable material. I won't get into a debate about the active depth of moisture change, because there are differing opinions and theories, capillary rise,etc. I've never had an issue using the assumption of 15 ft (in most cases) nor have the senior engineers in my firm.

 

[stewbaby]

Thanks all, good stuff. Will update once I know more on our direction. The cap makes sense. We already need to grade and fill the site some so that 2' cap could be part of that.

 

[msucog]

Stabilization with lime and replacing as compacted fill works best in that area...besides replacing with compacted select fill. For the inplace chemical stabilization, it does help to some degree. I would normally run swell tests beforehand as my baseline then sample after a couple passes of chemical treatment...this takes time and mobilization of a drill rig. When it works, it works...when it doesn't then it is a huge time-consuming pain. Also, I have had a couple sites that after inplace stabilization, there is so many holes in the pad and so much additional moisture present that the shear strength starts to go to crap. Here is one approach I took on a tank in that area:
Find out what the minimum level of product in the tank will be...this can help offset the swell potential. Additionally, have a solid estimate of the amount of new fill going on top of the subgrade...again will help offset swell potential and hopefully the geo will adjust the active zone depth from the new (higher grade). This likely won't do away with undercutting but it may reduce it quite a bit. Perhaps it shallows up enough that a few feet of the foundation can be stabilized with lime so import is reduced. If a ringwall is present for the tank, this can also help as a "vertical barrier". Lastly, confirm what the tolerance is for the tank...1" PVR may actually be 2"-3" for the tank bottom...but do also check the perimeter wall PVR at the base. If the perimeter wall is say <1" and the tank bottom is 2" (assuming that is the structural's confort level), then balance out how much select fill and stabilization is required.
Just keep in mind that the scenario is like trying to keep a dried out sponge from expanding in presence of water...more weight on top means less swell potential. Also keep in mind to cut off any select fill from surface water otherwise your active zone resets to the bottom of the select fill.

Talk to the geotech directly and involve the structural. I will gladly refer you to a geotech in that area if needed. Many times, the geo is given too little info to make these types of recommendations...heck, they may not even have actual loading information or have their hands tied if not being compensated for their time to assess the scenario.

In other words, I am not a fan of the inplace chemical stabilization based on my experience in that area.

 

[Ron]

The success of chemical injection is directly related to the soil permeability. Fat clays? Not good permeability!!

You might want to get another opinion on the PVR of 7".

 

[Terratek]

Ron,

If the soil is prone to shrink-swell issues, it will be inherently low perm material. Injection of chemical and water is used all the time in low perm soils.

 

[JAE]

Terratek - well yes - but it is still a challenge. Most techniques that I've seen use a very closely spaced grid of injection points (12" o.c. to 18" o.c.) and inject starting at the top layers and work downward.

I believe that using this method they only get the chemical on about 50% of the material so its effectiveness is limited.

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[Terratek]

I understand it takes work to get the solution in. Yes it takes a grid pattern and usually multiple passes and a curing time to boot.

However, it seems silly to insinuate that you should forgo the option for which it is specifically intended. What high perm soils need injection? None in my area. In fact, it is recommended to NOT use injection techniques in high perm soils since there is no way to verify once refusal is encountered (solution coming up at the ground surface away from the point of injection) because you can loose all the solution below grade.

It's like saying don't use drilled piers in rock because rock is hard to drill.