Gradation requirements

[Throstur]

Hi,

I have a small question with Gradation requirements. I´ve seen a few gradation requirements for roads and structural fills but I´m wondering how is the shape of these curves constructed? Obviously you want the GSD to be well graded to get a high density and, to me, the highest possible density of soil is by having the GSD similar to a Grain size curve known as Fuller grading. The Fuller Grading curve is a GSD curve for an idealized material which assumes the particles are perfectly rounded, i.e. they have a fixed diameter. Hence, the Fuller grading curve is a simple formula where all you have to do is set the largest particle for your soil.

It seems to me that the shape of the Fuller Grading curve does not look like the other Gradation Requirement curves I´ve seen. Why not and do you know what these Gradation Requirements are based on?

regards,
Throstur

 

[Ron]

In general, for structural fill and road base material, the gradation is not "constructed"...it is a result of materials available. Sometimes supplementation to adjust the gradation is done, but that is more expensive. The gradation you have described is idealized and rarely occurs naturally. For roadway base materials, some crushing and scalping of rock materials occurs to create a gradation near the idealized, but it can still be off somewhat.

What we would like to achieve is a granular material with a gradation that is consistent such that the curve is relatively straight from the top size to dust, having no more than about 10 percent fines.

 

[iandig]

I think you also need to bear in mind that the right grading for a particular use, is as much based on what is available, what has worked previously, what the material needs to do, as much from an idealised calcualted value.
In my own experience, I have found that many grading requirements are as much to do with what has always been used and proved to work as what would be the theoretical ideal solution. Often when looking at new gradings of material for design asphalts, or cement stabilised materials, an initial target value for a grading is somewhat modified based upon the findings of performance testing.
I was also told by a very experienced technical person (read old and wise), that many of the original specifications included in the early highway works (UK) were based on what the quarries prodced and what they were used for. This must be a sensible starting point, another piece of advice he gave was "if it works, tell them to do it like that and if it doesn't tell them not to do it like that". This really reflects the use of hard won knowledge, and that there is no substitute for experience!!