Posted 27 March 2012 - 11:31 PM
TOTAL percent retained: Sieve 4-4%, Sieve 8-11%, Sieve 16-21%, Sieve 30-22%, Sieve 50-24%, Sieve 100-17%, Dust pan-1%.
If you total all of the TOTAL percentages you get 100.
CUMULATIVE percent retained data: Sieve 4-4%, Sieve 8-15%, Sieve 16-36%, Sieve 30-58%, Sieve 50-82%, Sieve 100-99%.
If you total all of CUMULATIVE data you get 294.
My question is which set of data do I use to classify the soil for USCS and AASHTO? Is it the TOTAL or CUMULATIVE percent retained (or passing depending on which data is provided). I know I need the cumulative data if I want to calc a fineness modulus(unreated to soil classification) but I am unclear on which data set to use for classification.
Please help. I have pulled out many hairs over this as you can see!
Posted 30 March 2012 - 10:21 PM
There is only one data set in regard to sifting soil through a stack of sieves: weight retained on each sieve. Any other information given has been calculated from that data. (I know... it's semantics.)
My question is which set of data do I use to classify the soil for USCS and AASHTO?
Regarding how to approach the test questions, it's typically quicker to use cumulative numbers (if given) to determine % passing (which is what both AASHTO and USCS use in their charts), because the math is very easy ... % passing is just 100% - X% collected on or above sieve N (i.e., 100% - the cumulative total retained).
If you want, you can directly calculate the % passing by simply adding the % retained on each of the individual sieves (and pan) below the one in question. (Everything passing N sieve MUST be retained on the sieves and pan below sieve N.)
But, what happens if the information you're given is weight retained, not percent retained? Can you classify the soil then?
The bottom line is that you need to understand the concept of shaking soil through a stack of sieves. And you also need to understand what information is given, and what the question is asking. Phrases like "coarser than", "finer than", "percent retained", and "percent passing" are used in the soil classification charts. You need to understand what those phrases mean in relation to what's happening in the sieves. When you understand those concepts, the format in which the information is provided won't matter, because the arithmetic is easy.
Edited by ngnrd - PE, 30 March 2012 - 10:23 PM.
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