🪨 Calculators

Soil Group Index (HRB/AASHTO)

Computes the group index of the HRB/AASHTO M 145 classification, the number in parentheses that follows the soil symbol in a soil report: GI = 0.2a + 0.005ac + 0.01bd, where a and b come from the percentage passing the No. 200 sieve and c and d come from the liquid limit and the plasticity index. Each term is truncated — a and b from 0 to 40, c and d from 0 to 20 — and the result is rounded to an integer and never negative, which makes the index range from 0 to 20. The higher the group index, the worse the soil as a subgrade: 0 points to clean, well behaved granular material, values above 12 point to plastic clay that only works once replaced or stabilised, and this is the number that feeds the pavement thickness charts. The complete formula with both terms was adopted; for subgroups A-2-6 and A-2-7 the standard calls for the 0.01bd term alone, and the result is the same: every A-2 soil has at most 35% passing the No. 200 sieve, which is exactly where the a term goes to zero, so the first two terms drop out on their own. Enter the percentage passing the No. 200 sieve, the liquid limit and the plasticity index.

Passing the No. 200 sieve (%) Liquid limit (%) Plasticity index (%)

Result

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Group index HRB: rating a subgrade from sieve and limits

The lab report arrives with a grain-size curve on one side and the Atterberg limits on the other, while the pavement design asks for a single thing: the soil symbol with a number in parentheses. That number separates a borrow pit fit for a subbase from one that only works once replaced, and it feeds the thickness charts while CBR results for part of the samples are still pending. Working it out by hand invites mistakes, since every term carries a cutoff of its own.

The AASHTO M 145 formula reads GI = 0.2a + 0.005ac + 0.01bd. Term a is the percentage passing the No. 200 sieve minus 35, clipped between 0 and 40; b is that same percentage minus 15, also clipped between 0 and 40; c is the liquid limit minus 40, between 0 and 20; d is the plasticity index minus 10, between 0 and 20. The output rounds to a whole number, never falls below zero, and tops out at 20. Field reading: 0 means clean granular material, up to 4 the subgrade still behaves, 5 to 9 counts as fair, and above 12 sits plastic clay that only serves once replaced or stabilised. With 60% passing the No. 200, LL 45 and PI 18, the index closes at 9.

The group index describes, it does not size: it ranks soils, yet replaces neither CBR nor resilient modulus in structural design. The cutoffs flatten real differences — two soils at 75% and 95% passing the No. 200 share the same a, since the term locks at 40. Watch the basis of each test as well: the percentage passing the No. 200 comes from the whole sample, while liquid limit and plasticity index get measured on the fraction passing the No. 40. Mixing those two bases is the classic slip. The calculator rejects a percentage outside 0 to 100, a liquid limit above 200, and a plasticity index larger than the liquid limit, which cannot exist by definition.

Frequently asked questions

How does 9 come out of the preloaded values?

With 60% passing the No. 200 sieve, LL 45 and PI 18: a = 60 − 35 = 25; b = 60 − 15 = 45, which locks at 40; c = 45 − 40 = 5; d = 18 − 10 = 8. Adding the three terms, 0.2 × 25 = 5.0, plus 0.005 × 25 × 5 = 0.625, plus 0.01 × 40 × 8 = 3.2, gives 8.825, rounding to 9. That set of numbers describes a textbook A-7-6 (9): a clay subgrade still usable, provided the pavement above it gets thicker.

The standard says subgroups A-2-6 and A-2-7 use the 0.01bd term alone. Is this calculator wrong?

No, it lands on the very same number. Every A-2 soil has at most 35% passing the No. 200 sieve, and that is exactly where term a = P200 − 35 goes to zero. With a at zero the first two terms vanish by themselves and only 0.01bd survives. Try it: 30% passing, LL 45 and PI 18 give a = 0, b = 15 and d = 8, so GI = 0.01 × 15 × 8 = 1.2, rounding to 1. The clause in the standard guards against labelling as A-2 a soil that the grading would place elsewhere.

The message "Check the values you entered" appeared. What gets rejected?

Three things. A percentage passing the No. 200 outside 0 to 100, a liquid limit at zero, negative or above 200, and a plasticity index larger than the liquid limit. The last one shows up most often and usually means two fields swapped places: the plasticity index equals the liquid limit minus the plastic limit, so it never exceeds LL. Non-plastic soil goes in with a plasticity index of zero, a value the calculator accepts and treats as zero.

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Pile Group Efficiency (Converse-Labarre)

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Clay Activity (Skempton)

Computes clay activity as defined by Skempton, A = PI ÷ (% of particles finer than 2 μm), the ratio of the soil's plasticity index to the truly clay-sized fraction. It separates the clay mineral's effect from the mere amount of fines: two soils with the same PI behave very differently if one owes its plasticity to a little highly active clay and the other to a lot of inert clay. The usual classification is A < 0.75 inactive (kaolinite), 0.75 to 1.25 normal (illite) and A > 1.25 active (montmorillonite), the range where the expansive soils that warp pavements and shallow foundations are found. Enter the plasticity index and the clay fraction.

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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.