Compaction 1

[ohengineer]

I have a client who has placed fill material and it initiall passed density and moisture requirements, but during installation of utilities it was noticed that the soil was dry. Now this most likely happened due to the dry weather. The testing firm is saying to scarify the soils to a depth of 12 inches, add water, and then recompact. The only problem is that utilities are in and doing so will most likely cause several utilities to be damaged. The standard Proctor has a optimum density of 104 pcf and moisture of 17% with a +3% and -2% spec window. The moisture samples obtained say the moisture content is 12%. From my conversations, only the top 12 inches is effected by the dry conditions. If left in place how much settlement if any could be expected. They are contemplating on putting a water truck on the subgrade and letting the water permeate into the ground. The soils are a silty loess material with a LL of 40 and a PI of 19. Thanks

 

[civilperson]

Before filling or paving on the dry materials, moisture needs to be elevated, otherwise leave alone to protect the underground utilities.

 

[fattdad]

If the backfill was initially placed and compacted with an appropriate moisture content and the fill realized the specified DRY density, then subsequent drying should not be an issue. Just re-check the dry density and see if it remains in specification. In other words, compaction moisture content is not a factor if your not doing compaction.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

fattdad - would agree for most soils subject to the soil not being disturbed for some reason or another (say people walking all over the "fine sand"). However, the material that was to be addressed is a loess. Loess is a different animal and when dried out (and 12% moisture does not indicate to me that it has "dried out") can be very fluffy. We placed loess in China and after drying, a single step would "fluff" out some 8 inches or more. I'm not sure that "watering the top" and letting it sink in is a good idea - depending on how dry is really dry - as the act of the water being poured on can "fluff" a loess.

 

[fattdad]

I would also agree with BigH. That said, I'm o.k. with my earlier post. If the dry density remains in spec, what's the worry. For the issues raised by BigH (good issues), I would argue (not that we're arguing) that you'd not have specified dry density. Under that case, you'd have to add water to return the formerly compacted fill to specification.

There are really two issues at play here: The behavior of fill when the moisture content changes AND the bahavior of fill under dynamic loading with various moisture contents. In static conditions, dry density is not affected by moisture content. In dynamic conditions it is - ALOT!

f-d

¡papá gordo ain’t no madre flaca!

 

[ohengineer]

Thanks for the input. My thinking was that once it was put in at the specified dry density it would be ok, just wanted to bounce it off some other people. In its current state, the material is very hard and passess a proofroll, so it should be ok.

 

[fattdad]

In its current state, the material is very hard and passess a proofroll, so it should be ok.

Providing it's not "very hard" by virtue of dry strength. Do a density test and see whether it remains in spec.

f-d

¡papá gordo ain’t no madre flaca!

 

[namdac]

I agree with all but, one question looms over my head why would the utilities be in any danger of being disturbed. Where I live utilities are at a minimum of 36" below the surface. If they are not then I feel that the compaction requirements are secondary to your problem.
Regards,
Namdac

 

[LKS93]

I tend to agree that moisture content is not necessarily an issue as long as the compaction requirement is met, with the caveat that you need to verify that the geotech has the right proctor for the in-place soils. We don't have "loess" soils in Richmond, Virginia (and researching it a little bit leads me to say Thank God, the stuff looks nasty to work with) but SILTS are typically very moisture sensitive. I know in my area with some of the typical silts, it's very difficult to achieve adequate compaction if the moisture content is even a little bit out of specs (we typically use 3 percentage points either way.

If the m/c is 5% low, you probably aren't getting compaction results much above 95%, which I assume is the minimum.

I'd evaluate it carefully. We've had soils that have "baked" during the summer months that look fine during proofrolling and meet the dry density requirements, but are out of spec for moisture. Then, when it rains, the soils go to hell in a hurry.


And yes, I just noticed I'm a month out on this.

 

[fattdad]

in Richmond, Virginia

I'm in Richmond, Virginia also. Honk next time you pass a brown station wagon with licence plate "fatt dad".

f-d

¡papá gordo ain’t no madre flaca!