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AASHTO classification of inorganic soil

Short answer

A-4(3), silty soil, fair to poor subgrade. No. 200 passing=58% >35% puts soil in silt-clay group; A-4 is satisfied with wL=30<=40 and Ip=10<=10. Group index =3.45, rounded to 3.

A sample of inorganic soil has the following grain size characteristics:

Size (mm)Percent passing
0.075 (No. 200)58
0.425 (No. 40)80
2 mm (No. 10)100

The liquid limit is 30% and PI is 10%. Classify the soil according to the AASHTO classification system.

Given

Passing No. 200 =58%= 58\%; passing No. 40 =80%= 80\%; passing No. 10 =100%= 100\%; wL=30%w_L = 30\%; Ip=10%I_p = 10\%.

Step 1: Granular or silt-clay?

58% passes the No. 200 sieve, which is more than 35%, so the soil is in the silt-clay group (A-4 to A-7).

Step 2: Check groups from the left of the table (AASHTO)

GroupwLw_L maxIpI_pResult
A-44010 maxSatisfied: wL=30≤40w_L = 30 \le 40, Ip=10≤10I_p = 10 \le 10
A-541 min10 maxNot satisfied (wL<41w_L < 41)
A-640 max11 minNot satisfied (Ip<11I_p < 11)
A-741 min11 minNot satisfied

So the soil is A-4.

Group index

GI=(F−35)[0.2+0.005(wL−40)]+0.01(F−15)(Ip−10)=(58−35)[0.2+0.005(30−40)]+0.01(58−15)(10−10)=23×0.15+0=3.45≈3\begin{aligned} GI &= (F - 35)[0.2 + 0.005(w_L - 40)] + 0.01(F - 15)(I_p - 10) \\ &= (58 - 35)[0.2 + 0.005(30 - 40)] + 0.01(58 - 15)(10 - 10) \\ &= 23 \times 0.15 + 0 = 3.45 \approx 3 \end{aligned}

Answer: A-4(3), silty soil; fair to poor subgrade.

✓ Answer checked by Claude Opus 5.5 (high effort)

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