Roadway Fill 1

[obrien0403]

I have a project that requires all the roadway fill to have less than 35% passing the 200 sieve. The plant asphalt roadway pavement design is a CBR of 1. The fill depth is about 5' and is compacted to 100% standard.

The on site materials have about 50% passing the 200 sieve. The geotechnical engineer has approved the deviation and will allow the 50% passing material. The owner wants the 35% passing material (or a credit).

One option I am considering is to mix the on-site material with sand to lower the 50% to 35%. It would be blended with a dozer.

By doing this is the material any better ?

What are the engineering concerns with too much material passing the 200 sieve ?

 

[Ron]

There are several good reasons to limit the amount of "fines" in the fill. Though you don't say whether the fines are clay or silt, I would not want 50% fines in any material used for fill under a roadway.

The more fines, the more susceptible to moisture the material will be, particularly during the compaction phase. This makes for tenuous stability, particularly on the wet side of optimum. Since they are using a design CBR of 1 (which is patently absurd....if you're bringing in material, why not bring in good material), they are considering that, theoretically, the material offers almost no support to the pavement structure. This is not the way to build a flexible pavement. It should be built from the bottom up, not the top down.

Material with 50% fines is hard to compact and since your requirement is for a standard proctor comparison, a 5-foot thick layer will likely consolidate a bit more with time, thus causing an undulating pavement. Not good.

Your idea to dilute the fines with sand is good, but I wouldn't use a dozer to mix. Use a "Reclaimer" or similar rotary "tilling" machine to achieve a proper mix. Since you're mixing, I would bring the fines content down to below 15%.

 

[GeoPaveTraffic]

I have to disagree with Ron with respect to using material with 50% fines for a roadway fill. I would have no problem with this fill, assuming that there was a proper base and subbase below the pavement. However, I agree with him that using a CBR of 1 when you are placing 5 feet of fill is absurd.

Now to your direct question. I would agree with the owner, either you place material that meets the specifications, i.e. has less than 35% fines or you give him a credit. Now there is nothing to say that the credit must be in cash, it could be in the form of extra work, etc.

Also, don't try to mix the sand using a dozer, it will not do a complete job and could lead to problems.

 

[layfieldman]

I would agree with both the previous in that normally I would not want to use a material with that much fines content but it would depend on what the sub base is. I also agree not to use a dozer to mix the material as you don't want to create pockets of fines.

 

[cvg]

did the owner conduct a geotechnical investigation prior to bidding the job? If so, why wasn't it discovered that the onsite materials wouldn't meet the fill spec? The owner should have required import material and it should have been bid that way. Sounds like change order time for the contractor...

 

[SirAl]

In my region, roadbeds are comprised, predominately, of fine grained soils. The vast majority of these are comprised of medium to high plastic clays. Here we simply have little choice. Despite this, we have been sucessful in building highways to a relatively high level of service and performance.

In the long term, fine grain soils are more suscepable to embankment settlements and frost related movements. These will also offer poorer subgrade drainage. As such, I would expect an increase in maintenance and rehabilitation costs over the service period of a road constructed over a fine grained soil vs. one that constructed over a coarse grained subsoil environment. The fine grained soils are weaker than coarse grained soils although we have frequently used a soaked CBR value in the order of 3.0 for the clays in our region . Unless there is something very unique about the soils, a CBR of 1.0 appears excessively low.

Forget blending with a dozer. As others have pointed out, this is not a proper method of mixing soils.

 

[obrien0403]

This is what happened prior to bid.

The documents can out with a 20% maximum passing the 200 sieve and included a geotechnical report indicating the native soils had around 50%. I contacted them and brought the conflict to their attention. They changed the documents via addendum to 35%. After receiving the addendum I contacted them satating the still did not solve the problem.

They said that the best they can do right now pending further review. After the bid I hired a geotechnical consultants who review the material, wrote a letter stating the material is acceptable and stamped it. The engineer of record concurs with the letter. The owner wants a credit.

Sand meets the spec. Whould the road fail if we build it on 5' of sand ? This is what they want.

Also, they are fine with the dozer mixing the material. Their goal is just to make me spend some money.

 

[Ron]

Assuming you have an adequate base and pavement section, building on sand is fine....it's about all we have where I practice!

Blending with a dozer is a waste of time.