72 SOIL MECHANICS LABORATORY MANUAL
undergone se verakhange s since then, is presently used by federal.... and, departments in the United States. In this soil classification system. soils are in seven major groups: A- A-3. A-4. A-5. A-6. and A-7. Group, I is dis,1.4,/i subgroups (A- I -a and A- I 40. Group A-2 is divided into four subgroups .A.,A.ns, A-2.6. and A-2.7). Soils in gmup A-7 are also divided into two subgroup, ,A..7.;;:i A•7.6). This system is also included in ASTM test designation D•3282. Along with these soil groups and subgroups, another factor called the gm, Hdq is also included in this system..lbe importance of the group index can be explay:_i dforrIlows. Let us assume that two soils fall under the same group. nd:seser. they , Different values of 0/. The soil that has a lower value of group x is likely to per, baler us a highway subgrade material. The procedure for classifying soil under the AASHTO system is outlined in billowing section.
9.3 Step-by-Step Procedure for AASHTO Classification I. I Mountie the percentage of soil passing through a No. 200 U.S. sieve 10.075-, opening). a. If 35% or less passes the No. 200 sieve, it is a coarse-grained material. proceed 10 Steps 2 and 4. b. If more than 35% passes the No. 200 sieve. it is a fine-grained matcrial (i.e.. siky is clayey). For this, go to Steps 3 and 5.
Determination of Groups or Subgroups 2. For coarse-grained soils, determine the percent passing U.S. sieve Nos XXXXXXXXXXand 200 and, in addition, the liquid limit and plasticity index. Then proceed to Table 9-1.. Stan from the top Mir and compare the known soil properties with those given in the table (columns 2 through 6).. down one line at a time until a line is found for which all the properties of the desired soil match. The soil group (or subgroup) is determined from column I. 3. For line•grained soils, determine the liquid limit and the plasticity index. Then , to Table 9-2. Start from the top line. By matching the soil properties from columns 2.3• and 4, determine the proper soil group (or subgroup). Determination of Group Index 4. To determine the group index GI of coarse-grained soils, the following rules need .bc observed. a. G! for soils in groups (or subgroups) A-i-S, A- I -b. A-2-4, A-2-5, and A-3 is