---
title: "AASHTO classification of inorganic soil"
subject: "Soil Mechanics"
chapter: 3
key: "semester_4/soil_mechanics:aashto-classify-inorganic-soil"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/aashto-classify-inorganic-soil
site: BCT Engineering
---

# AASHTO classification of inorganic soil

Soil Mechanics · Chapter 3: Soil Identifications and Classification

- Asked in: 2076 Baisakh, 6 marks
- Video: [[TAGLISH] Problem: AASHTO Soil Classification System](https://www.youtube.com/watch?v=Bk55qmCe0sA) — Marvin Bartido Channel
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/aashto-classify-inorganic-soil
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-3

## Question

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.

## 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.

## Answer

#### Given
Passing No. 200 $= 58\%$; passing No. 40 $= 80\%$; passing No. 10 $= 100\%$; $w_L = 30\%$; $I_p = 10\%$.

#### 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)
| Group | $w_L$ max | $I_p$ | Result |
|---|---|---|---|
| A-4 | 40 | 10 max | Satisfied: $w_L = 30 \le 40$, $I_p = 10 \le 10$ |
| A-5 | 41 min | 10 max | Not satisfied ($w_L < 41$) |
| A-6 | 40 max | 11 min | Not satisfied ($I_p < 11$) |
| A-7 | 41 min | 11 min | Not satisfied |

So the soil is **A-4**.

#### Group index
$$
\begin{aligned}
GI &= (F - 35)[0.2 + 0.005(w_L - 40)] + 0.01(F - 15)(I_p - 10) \\
&= (58 - 35)[0.2 + 0.005(30 - 40)] + 0.01(58 - 15)(10 - 10) \\
&= 23 \times 0.15 + 0 = 3.45 \approx 3
\end{aligned}
$$

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

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. 58% passing No. 200 means
   A. silt-clay group (A-4 to A-7), more than 35%
   B. granular group
   C. A-1
   D. A-3
   Answer: A. > 35% fines. (Key point: Granular or silt-clay)

2. wL=30 and Ip=10 fit
   A. A-5
   B. A-4 (wL max 40, Ip max 10)
   C. A-6
   D. A-7
   Answer: B. A-5 needs wL >= 41; A-6 needs Ip >= 11. (Key point: Why A-4)

3. GI formula first term is
   A. (F-15)[0.2+0.005(wL-40)]
   B. (F-35)(wL-40)
   C. (F-35)[0.2+0.005(wL-40)]
   D. 0.01(F-35)(Ip-10)
   Answer: C. Second term is 0.01(F-15)(Ip-10). (Key point: Group index)

4. GI = 23x0.15 + 0 =
   A. about 3 (3.45)
   B. 0
   C. 10
   D. 34
   Answer: A. Second term zero as Ip = 10. (Key point: GI)

## Related in Soil Mechanics

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