wet sub-base

[s4r4]

this site has a sand soil that we have been working with since august. we are at a 10 foot fill in 11000 sq foot area parking garage ramp (confinded by concrete no sides and sand below) . the sand has been within optimum moisture, density, and compacion of at least 98%. we are using a smooth drum vibratory roller. the site has not had problems with compaction or mosture untill...it was blanketed over night to protect from frost and in the morning when fill resumed the fill has started to roll or turn jello like, no ruting but 8- 12 inch defection. when tested sand again moisture is 2% over optimum. We chose to cover due to snow storm and stay off for a day. what can be done so this dosen't happen. and what happened

 

[s4r4]

i forgot to put in that we are 2-3 miles off lake erie. a high water table but we were 20 feet deep 40 feet away in stalling utillites dry as a bone

 

[Ron]

Sounds like your sand subgrade is pumping. If you blanketed the subgrade to prevent additional water, then the moisture content was at or slightly over optimum as noted by your tests. There's a possibility that with the high moisture content, the sand "bulked" due to the subgrade temperature falling below 40F, but not freezing.

 

[s4r4]

the moisture content has risen over the past couple days. Contractor chose to run a heat system and blankets for a couple of days. the moisture has went from 12.5% to 20% to 33% to 38%. sub surface drains are in prcess now, but i have a feeling this will not work. Right Wronge????

 

[Ron]

Why is the moisture content rising so rapidly? Do you have a broken pipe? Are you below the water table?

Are the underdrains temporary or permanent? Is there going to be pumping from the underdrains or is there a gravity outfall?

 

[oldestguy]

Hey Ron:

It sounds like the method for testing water content is screwy.

38 percent seems way too high for any saturated sand.

Are we looking at some form of moisture migration due to a cold surface zone drawing water there due to vapor pressure differences, cold above and warm below?

I think an experienced geotech is needed on the job.

OG

 

[s4r4]

the underdrains are permanent and gravity tie in to storm completed today. the water table is high in this area but we are on a ramp up starting 5 foot below and raising to 8 foot above out side existing grade. this is a real stumper for eveyone a geo engineer is going to be there Monday.

 

[Ron]

OG...I agree. This is screwy. In a short distance, there's no way capillary rise will go 8 feet.

 

[oldestguy]

Hey Ron: I was talking vapor movement, not capillary action.

OG

 

[Ron]

OG..oops..missed that. Still seems like way too much moisture to be a vapor issue. We deal with vapor migration and condensation a lot here in Florida, but I have not seen that level moisture.

 

[DRC1]

Is it possible ther is a problem with the drains and you are getting an atresian condition? In general, once you have achived compaction subsequent cahnges in water content should not effect it. One exception would be artsin flow.

 

[s4r4]

the underdrains seem to be working the moisture is going down. We all agreed to let the site sit for 3 days so the drains have a chance to work. The engineers seem to think the problem was winter. We did alot of fills in the summer no problems, winter you get frost, snow, and frozen material. eventhough the contractor tried to remove frost and frozen chuncks you can't get everything. the solution is geotextile mat 12 inches of 304 or 57 lime stone then continue fill but be aware of the winter conditions this may happen again. the best is to continue the fill with 57 limestone locking the stone in, in lifts. i think the owner is going to go with sand due to cost