Moisture Conditioning of Soil

[JoelTXCive]

I am working on some circular lift stations in the North Texas area. The lift station will bear at about ~27 ft below grade. From the bearing level upwards, my PI is ~58 and the PVR is estimated to be 6" inches (welcome to Texas)

The geotech is recommending water injection to moisture condition the top 10ft of soil. I have not done this before. (The other option is to remove and replace 10ft of existing soil with select fill)

I understand the moisture conditioning concept. Inject the soil so that it swells in advance and thus reduces the future PVR.

My question is:
Is this a permanent or temporary solution?
Doesn't the soil return to it's in-situ moisture content at some future date? Maybe 1 year later or 5 years later? If that happens, then aren't I back to the original 6" PVR?

I have not called the geotech yet, and would like to get some background info before I talk with him/her.

Thank you!

 

[oldestguy]

Sounds fishy.

 

[MTNClimber]

Water injection can work on some sites to reduce swell. Hayward Baker has a rig in Colorado if you want to give them a call to ask them about questions regarding construction constraints. Typically it's cheaper to have the earth work contractor excavate, moisture condition, and re-compact the soils. We only used water injection on projects that the contractor didn't have enough experience with moisture conditioning and the hand drive samples were still swelling after the contractor attempted to moisture condition.

My question is how is treating the upper 10' going to reduce swell for the lift station? The lift station is too deep (10' of conditioning vs 27' foundation depth) and probably won't move from swelling. Is it meant more for the utilities? If the soils are allowed to dry out, then swell potential will return. Will the site be developed with irrigation? Is there an actual concern of the soil becoming wet?

 

[fattdad]

Wow! I know nothing about such moisture conditioning. MTNClimber's thoughts make sense to me also; however. If there's swell in the upper 10 ft, but the structure has another 17ft of burial, will the tension be mitigated by the depth of burial? I'd think it may just work out?

Interesting.

f-d

ípapß gordo ainÆt no madre flaca!

 

[MTNClimber]

The real question here is: what's the anticipated depth of wetting? Is it 10'? More or less? Once we have that answer, then we can consider tensile stresses but won't have a good answer unless we have load back pressures from swell/consolidation testing.

 

[aeoliantexan]

If your lift station bears at 27 feet, Moisture conditioning won't have any effect on the foundation. You probably should flare the excavation above the depth of seasonal moisture variation (15 feet more or less)and backfill with nonexpansive soil to avoid excessive lateral pressures. If you have a valve platform or other appurtenance at surface level, it probably should be supported over a void space on drilled shafts or piles so it doesn't move relative to the lift station.

Moisture treatment may be useful to reduce movement of a transformer pad or other relatively light structure and to get better long-term performance of access pavements, but 10 feet may be excessive for that.

The piping may move seasonally, so it needs to be somewhat flexible. By the way, the backfill will settle, so provide special support for the piping across the backfill.

Yes, you need to ask your geotech some questions.