Soil Classification 4

[wasjay]

When we talk about a soil classified as GW (Graded Well), what does that type of soil really look like? Does that mean it has a broad or narrow spectrum of grain sizes?

 

[ctmtwilliams]

I pretty sure GW stands for Gravel, Well Graded. By definition a GW is gravel with little to no fines and more than half of the coarse fraction is larger than the No. 200 sieve. A GW can include gravel and sand mixtures as long as it meets the above criteria.

 

[cvg]

a "well graded" soil has an even distribution of different grain sizes. The opposite would be a "uniformly graded" soil which has a predominance of only one grain size.

 

[BigH]

GW is the designation in the Unified Soil Classification System for a well graded "gravel". The characteristics are that:
a. the overall material is more than 50% coarser than the #200 sieve size.
b. 50% of the coarse size (i.e., a. above) is gravel sized - retained on #4 sieve or 12.5mm).
c. The percent of fines passing the #200 sieve is less than 5%.
d. The coefficient of uniformity (D60/D10) is greater than 4.
e. The coefficient of curvature ( (D30*D30)/(D60*D10) ) lies between 1 and 3.

For completeness, the material would be called GP if points d. and e. are not met. If more than 12% passes the #200 sieve, the "gravelly" material would be classified as MH or MC depending on where the Atterberg limits fall on the Plasticity chart. If the material has between 5 and 12% fines, then there would be a dual symbol use (GW-GM, GP-GC, etc.)

This is given in almost all soil mechanics books as well as in ASTM.

 

[CarlB]

Good detail on the GW description BigH.

One point of correction; if more than 12% passes the #200 sieve, the material is GM or GC - the "M" for silty and "C" for clayey fines (as determined by Atterberg limits).

 

[BigH]

I know that, but . . . my fingers done hit the wrong keys! So, more than 12%, it is GM or GC.

 

[kidCivil]

wasjay

well graded means (in figurative terms) that the curve on the particle size distribution graph is smooth and concave (think of it as a pizza with the lot - there's a bit of everything on it)

the flatter the curve - the broader the spectrum of particle sizes, the steeper the curve (usually with kinks in it) means that the particle sizes are concentrated in a narrow spectrum only... if the spectrum is too narrow, we call it a 'uniform soil'

the term 'well-graded' is a bit subjective as it depends which type of soil youre talking about but the curves on the particle size plot are semi-logs so it makes the comparison of two different types of soils with the same degree of uniformity easy

the opposite of 'well-graded' is 'poorly-graded' and this can be one of two things, 'gap-graded' = the curve has a gap in it indicating that there's probably heaps of fines and heaps of gravels but nothing in between or 'uniform soil' as mention above

hope this helps, albeit late mail

 

[BigH]

Be careful on "curves" as being flat or steep. It all depends on the scale of the logrithmic particle sizes and your "trained perception" is based on the style of plots of which you are used to. If you plot, say, 5-10 stone on a normal soil engineering particle size plot, it will plot steep; if you plot it on an aggregate or base course particle size plot, it will appear flatter (because on the plot, the latter doesn't include the silt and clay sizes. It took me a long time to accept particle size plots where the gravel sizes were at the right rather than the traditional geotechnical methods of being at the left. I still prefer the geotechnical orientation.

 

[kidCivil]

good point bigH

I have never been exposed to different log particle size scales because here in Australia, we always use the USCS particle sizes (each chart displays the full spectrum) - thats what makes it easier to compare 'em

but it's useful to be aware that a difference can exist and to be cautious of it - well spotted

 

[BigH]

kidCivil - it does make more sense to me for coarse aggregate plotting to do it on a plot without the silt and clay sizes - by stretching the width of a log cycle, you can, perhaps gain a bit more appreciation of the nature of the sizes in the narrow, say 12-20mm range - especially if you were to have an intermediate sieve. On normal plot is dang near vertical - and even variations in gradation appear to be the same.

 

[Rockjoint]

..."the opposite of 'well-graded' is 'poorly-graded'..."

Also, the opposite of well-graded is poorly sorted, to the geologist, indicating a wide range of grain sizes.

 

[vtgeotech]

Rockjoint,

The opposite of well graded is well sorted, indicating that the majority of the grains are of simular size, i.e. poorly-graded.

Poorly sort means there isa range of grain sizes. The same as well graded.

 

[cvg]

actually, a gap graded material would also be considered as not being "well graded", even though there is quite a range of sizes

 

[LHA]

Wow, this thread taught me more than I have learned in my other 40 yrs on earth about grading of soils. Very well explained by all.

The only question is: was it explained well, poorly, uniformly or just plain-ol' gap-explained...sorry, couldn't resist a little humor after another day at the grind.



Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve

 

[BigH]

Mainly, the explanations were just plaine 'dirty' - though the ideas were well sorted and the range of diversity was well graded.