Are you trying to figure out soil quality for construction projects but finding it confusing to work with all the variables like sieve analysis, liquid limit, and plasticity index? It can be tricky to juggle all the numbers and understand what they mean for road design or soil classification.
That’s why we built this simple Group Index Calculator. In this article, you’ll not only get a free tool to find your Group Index instantly, but you’ll also learn the formula behind the numbers and how to apply them in real cases.
By the end of this page, you’ll be able to confidently calculate the Group Index and make better decisions in soil evaluation.
How to Use This Calculator:
- Enter P(No.200): Percentage of material passing through the No. 200 sieve.
- Input LL: The Liquid Limit of the soil sample.
- Add PI: The Plasticity Index of the soil.
- Click “Calculate”: Your Group Index will be displayed instantly.
The Formula Explained: How It All Works
For those who like to see the logic, here is the exact formula our calculator uses:
Group Index Formula:
GI = 0.2 × a + 0.005 × a × c + 0.01 × b × d
Where:
- a = P(No.200) − 35
- If P(No.200) < 35, then a = 0. If P(No.200) > 75, then a = 40.
- b = P(No.200) − 15
- If P(No.200) < 15, then b = 0. If P(No.200) > 55, then b = 40.
- c = LL − 40
- If LL < 40, then c = 0. If LL > 60, then c = 20.
- d = PI − 10
- If PI < 10, then d = 0. If PI > 30, then d = 20.
Input Variables:
- P(No.200): Percentage of soil passing 0.075 mm sieve (%)
- LL: Liquid Limit (%)
- PI: Plasticity Index (%)
Practical Example: Let’s Walk Through It
Let’s imagine you want to calculate the Group Index of a soil sample for a road project.
Scenario Data:
- P(No.200): 60%
- LL: 50%
- PI: 20%
Step 1: Calculate a, b, c, d
- a = 60 − 35 = 25
- b = 60 − 15 = 45 (but max 40, so b = 40)
- c = 50 − 40 = 10
- d = 20 − 10 = 10
Step 2: Plug into the formula
GI = 0.2 × 25 + 0.005 × 25 × 10 + 0.01 × 40 × 10
GI = 5 + 1.25 + 4
GI = 10.25 ≈ 10
The Result: The Group Index is 10. This means the soil has a moderate quality for road construction, not too poor but also not ideal.
Group Index Reference Table
| Group Index (GI) | Soil Quality for Roads |
|---|---|
| 0 – 1 | Excellent |
| 2 – 4 | Good |
| 5 – 9 | Fair |
| 10 – 19 | Poor |
| 20+ | Very Poor |
Helpful Tips & Tricks
- Always double-check if your PI and LL values fall within practical limits, as incorrect data can give misleading results.
- Remember that lower GI values mean better soil for road construction—aim for soils with low fines and low plasticity.
- Use this calculator as an initial screening tool, but confirm results with professional soil testing when planning major projects.
FAQs
This calculator follows the official Group Index formula, so it provides highly accurate results when inputs are correct.
A GI of 0 to 4 is considered good for road construction, meaning the soil has excellent to good quality.
These values are obtained from standard laboratory tests such as Atterberg limits testing.