High Moisture Content Soil

[itsleighton]

I have a jobsite in which the water table is very high (9ft) The project required ecxcavating 38 feet deep and stockpiling the material onsite for 2 months. Even with dewatering wells, the water does not fall out of the soil so it stays vary saturated. The stockpiled material is also still very moist even at the very top. When compacting this material for backfilling, we are getting roughly 90-94% maximum density, but with 6-8% moisture content. What is expected of the behavior of the soil if it is backfilled with that high of moisture if the existing surrounding ground contains an even higher moisture content naturally? The project is a lift station with the bottom of the wet well sitting on shale, so it wont settle. The valve vault is sitting at approximately 9ft down from the top of ground which will be placed on backfilled material. will this ground settle?

 

[BigH]

First, much of this depends on the type(s) of soil that you have. You haven't identified what you have. What is the moisture content of the soil in situ? You indicate that the moisture content on compaction is 6 to 8%. This isn't very high unless you have a medium to fine sand. Please provide this information. Also, how long were the dewatering wells (presumably deep wells) working before you began excavating? Did the soil have a chance to react to the dewatering before excavation?

 

[itsleighton]

the soil is a 30-40 pi clayey / very fine silt. The wells were in place for about a month and the water is holding in the soil- it moves slowly like a glacier and has particles so fine it clogs the wells screens.

 

[Ron]

Why are you using this material for backfill? Trying to compact this material without some modification is going to be a waste of time.

I agree with BigH...6-8 percent is very low....for the material you are using, I would expect optimum to be a bit higher.

Could you post your Proctor curve?

 

[BigH]

Maybe you should have put the wells under a vacuum to draw the water down. I would have thought you'd backfill with a granular soil - much easier to place, level and compact.

 

[aeoliantexan]

Generally, 30 feet of fine-grained soil compacted to 95% maximum standard Proctor can be expected to settle 1% to 2% of its thickness when it becomes saturated. Your soil sounds unusual, and I could not predict its behavior, but I am very cautious about supporting structures on a thick backfill next to a structure that won't settle. Perhaps a highly compacted clean crushed stone backfill could be trusted. I would use flowable fill or move the vault out of the backfill and build a beam to support the pipe.