Lime Treatment in Utility Trenches 1

[vtcivil]

I'm looking for some information on lime treatment in utility trenches- more specifically, for drying of wet soils.
When trench soils are too wet to be properly compacted back into the trench, contractors generally say drying by discing and harrowing is inefficient and costly, and want to be paid for unsuitable.
Has anyone use lime treatment in cold climates (Northern New England) with any success?
How about corrosion of ductile iron pipes?

 

[Ron]

Why not overexcavate and backfill with gravel to stabilize. Makes good pipe bedding material. I would use #89 or #57 stone.

 

[vtcivil]

The problem with overexcavation and backfill with gravel, is we end up with a trench that heaves and settles at a different rate than the existing road- giving the appearance, and same end result, of a trench failure. Idealy we would reconstruct the entire road if unsuitable material is encountered, but funding limitations don't always allow that- instead we would like to find a way to attain proper compaction in poor soils.

 

[vtcivil]

Oh yea.. I spoke with the Ductile Iron Pipe Research Association (DIPRA) and they said that lime won't have any corrosion issues, (clay soils should always send up a red flag though).

 

[TDAA]

Lime will also alter the soils so they will not behave the same as the surrounding. It sounds like you need a better spec to force the contractor to reuse the material once it is dried, or provide a suitable alternative, included in the contract price.

You probably have that, if it calls for the contractor to process the material to a specific moisture and compact.

I am not a fan of water content being the decider on if a material is suitable. Where do you draw the line?

 

[Ron]

vtcivil..you can use the gravel bed then fill up to 1/2 the pipe diameter with CLSM...it will infiltrate the gravel (assuming No. 57 stone), harden and stabilize the whole section as a high modulus bridge to varying soil conditions. Use indigenous soil compacted in lifts up to bottom of pavement base.

 

[msucog]

make sure "unsuitable" is adequately defined in the specs. if it comes out okay then gets rained on, it should NOT be considered "unsuitable" (although it may require drying for reuse as fill). if it's sloppy wet coming out, odds are everything around the trench is not exactly great anyways but i suppose that could partially depend on the particular soil type and geology.

if necessary, backfill immediately around the pipe with good stuff then utilize lime or cement to help dry stockpiled materials...it'll likely be hit or miss when contractors try to do this without specific equipment and oversight. in this case, double up on the testing frequency since bucket by bucket will be different. lots of proofrolling and leave it up to the contractor to give you the product that is required....in other words i'd use caution holding their hand so that it doesn't become your fault since they'll likely point the finger at you in the first place. sometimes owners will pay the additional money for the lime just to speed the project up. again, if the contractor lets it get wet, it should be on them.

 

[Ron]

Soil mixing is difficult on flat ground...in a trench, with assumed clayey materials, it is even worse. You'll be in and out a lot quicker if you'll just overexcavate, use CLSM and/or gravel and move on.

 

[SirAl]

I've never lime or cement modified native backfill in utility trenches. Where deemed impractical to dry or substitute backfill, we have used a one-mold proctor density reference (sample prepared at natural moisture content under standard proctor energy) as the density criterion. This approach is useful to ensure that the voids within the fill have been reduced to prevent settlements but might require a modified structure above due to compensate for weaker subgrade. Also, in some some sensitive soils, a build-up of pore pressure could result in subsidence as pore pressures dissipate with time.

 

[Dozerman56]

VTCivil:

When you say "gravel" do you mean crushed stone or well graded bank run gravel? I would bet they'd behave differently in freeze thaw cycles.

And yes, if the contractor lets the dirt get overly wet it should be on him. But if you're working in existing pavement, I'm sure they're backfilling just as fast as they're digging and laying. Discing/harrowing works fine in warm, dry weather but that would limit your construction season to June July and August.

 

[vtcivil]

Dozerman56: our suitable backfill would be a well graded bank run gravel.

A local contractor took it upon himself to replace the material in a utility crossing in a road constructed across a swampy area with crushed stone a few years back, and that trench is either a 6-inch speed bump or a 3-inch divit depending on if the frost is in or not. It's always been my understanding that this trench is (more) stable, but the surrounding road is heaving and settling.

A geotech suggested finding a pit with material with a similar gradation to the material being replaced, but closer to the optimal moisture content, but as you pointed out, the contractor is usually backfilling as fast as they are digging. I'm looking for a solution that the contractors would be willing to agree to without potential problems, (and hopefully expense) with replacement with a different material.