Soil Segregation

[JKJohn]

What causes gravel segregation? and does it matter as long we reach the specified compaction?[

 

[SoiloftheMonth]

1. In a general sense it seems to me gravel segregation results from a sorting of the particles based on size, mass, shape, or combination thereof and is generally undesirable. A sorting mechanism I have seen is stockpiling of gravel. In the process of stockpiling larger particles tend to "settle out" at the base of the stockpile and finer particles at the top. As a result the gradation of the entire stockpile may be as desired if you could magically measure it all at once in a giant sieve set, however this desired gradation is not uniform throughout the stockpile and thus the real gradation of the material being put into each truck could be different enough from the desired gradation to cause problems.

2. I don't know. There is a point at which the real gradation will become too different from the desired gradation and undesirable effects will be likely. If your "gravel fill" is coming out of the truck as all fines and no gravel that matters. I think gradation bands are used to address this issue.
-SOM

 

[fattdad]

define, "Gravel." In the world of ASTM, that's soil grains less than 3-in diameter. If you are dealing with soils that include sizes from fine sand to coarse gravel, there is a 10-in lift thickness and the material is coming in a truck, or from borrow how would such segregation occur?

In placing mine fills with rocks measured by feet, inches and microns and when end-dumping you get segregation - especially when the, "Lift" thickness is 50 ft! The boulders roll down the slope more than the gravels and sands. So, the lower reaches of valley fills are often coarser.

Just don't see that in conventional earthwork.

f-d

ípapß gordo ainÆt no madre flaca!

 

[cvg]

and does it matter...?

if you are building a dam then it matters.

 

[JKJohn]

Well, we are not building a dam. Just backfilling for future S.O.G. We are only doing 6-inch at a time then checking compaction. The gravel (as defined in ASTM) is coming from off site. Yet, some in some of the lifts we are getting the segregation. If I understood soilofthemonth correctly, the cause is from the source and is not something we are doing on the site?

 

[SoiloftheMonth]

I have seen segregation in aggregate stockpiling operations. I have heard of segregation occurring when material is dumped from too high of height, one instance was a trench for a new sand filter in a dam. I read there is chance of segregation when loading trucks with conveyors. I have not heard of segregation happening, in such a way that it causes issue, once in the truck and when it is offloaded. Bit surprised you are seeing it based on what I think of when I imagine an operation placing gravel in 6" lifts.

A rule of thumb I heard is 2:1, ratio of largest particle to smallest particle. If larger than 2:1 segregation will need to be considered in bulk handling operations, if less than 2:1 it will be small enough to not matter. Would be interested to know how you detected segregation in your lifts, can't be sure I would know it if I saw it. what is the gradation of the material that appears to be experiencing segregation?
-SOM

 

[cvg]

if you are compacting in thin lifts and checking density, than it is easy to see and it can affect your compaction testing results. acceptability really depends on the amount and size of the gravel in relation to the lift thickness. for a light loaded slab on grade (assume this is not pavement) than probably not an issue. ask your soil tech for his opinion and if in doubt, rip it out and redo.

 

[JohnnnyBoy]

I was just a site supervisor for a project and I was having similar issues with segregation of 6" lifts of a 3" graded material. Once consulting the geotech engineer for the project he had brought up a few possibilities as to the segregation I was seeing.

1) End dumping trucks add to gradation issues.
2) Using a mini backhoe to move the gravel around the site added to segregation in the material.
3) Such small lifts using a larger size material added to lift segregation.

Maybe these can also be considered for your project.

 

[EireChch]

@ JB - on a side note, I was always thought that the minimum lift thickness is 2.5 x the largest stone diameter. So a 150mm (6") lift would be too thin, should be a 200mm (or 9" not sure what increments you use in the states) thick lift...IMO.

 

[JohnnnyBoy]

@EireChcm - That is good information to know, this was a design done by a different design firm and we did end up increasing the lift to 9" to reduce segregation. I will keep the 2.5x in my head for the next time I see an issue such as this.

 

[aeoliantexan]

In a "well-graded" granular material the smaller particles tend to fill the spaces between the larger ones, so the maximum density is higher. The strength is increased, but more importantly for road bases and surfaces, the material holds up better under weather and traffic.

 

[cvg]

we did end up increasing the lift to 9" to reduce segregation

increasing lift thickness might require different compaction methods or equipment. for small, hand operated equipment, suggest no more than 6 inches thick. so this might require removal of the larger gravel

 

[BigH]

Segregation many times also occurs in placing subbase and base course materials when the grader is attempting to fine grade in order to bring the layer within the "tolerable" limits of thickness. The blade picks up the larger pieces and keep moving them to the outside - so that the outer wheel path - if not corrected - can sometimes be almost single size stone. Needs to be rectified.