USCS soil classification calculator (ASTM D2487)

Enter what the sieve analysis and the Atterberg limits gave for a soil. The calculator classifies it under the Unified Soil Classification System, with the group symbol, its name and the criteria that decided it, and places the soil on the plasticity chart. It follows ASTM D2487, and it’s free and needs no account.

How it calculates

The first split is the percent passing the No. 200 sieve, $P_{200}$: with 50 % or more, the soil is fine-grained, and otherwise coarse-grained. A coarse soil is a gravel when more than half of its coarse fraction stays on the No. 4 sieve, and a sand when it doesn’t:

Then its fines decide how it’s classified:

• With less than 5 % fines, by its grading. A gravel is well graded, GW, with $C_u \ge 4$ and $1 \le C_c \le 3$, and poorly graded, GP, otherwise. A sand needs $C_u \ge 6$ and the same $C_c$ to be SW, and is SP otherwise.
• From 5 to 12 %, with a dual symbol that adds the kind of fines to the grading, as in GW-GM, GP-GC, SW-SM or SP-SC.
• Over 12 %, by its fines alone: GM, GC or GC-GM for a gravel, and SM, SC or SC-SM for a sand.

A fine soil, and the fines of a coarse one, go on the plasticity chart, where the A-line separates the clays, above it, from the silts, below:

With a liquid limit under 50, the soil is CL if $PI > 7$ and it plots on or above the A-line, ML if $PI < 4$ or it plots below, and CL-ML in between. With a liquid limit of 50 or more, it’s CH on or above the A-line and MH below. An organic soil is OL with a liquid limit under 50 and OH with 50 or more. The chart also draws the U-line, $PI = 0.9\,(LL - 8)$, which doesn’t classify: a point above it suggests an error in the limits.

The name the calculator gives is the group’s basic name, such as lean clay or silty sand. The full group name in ASTM D2487 also counts the sand or gravel in the soil, as in “sandy lean clay”, and takes the whole grading.

The classification, step by step, is in the Academy lesson: Unified Soil Classification System.

How to use it

1. Enter the percent passing the No. 200 and No. 4 sieves, from the sieve analysis.
2. Fill in what your soil needs. A coarse soil with 12 % fines or less takes $C_u$ and $C_c$, and one with 5 % fines or more, the liquid limit and plasticity index of its fines. A fine soil takes its own liquid limit and plasticity index, from the Atterberg limits calculator, and whether it’s organic.
3. The fields your soil doesn’t use fade, each with the reason.
4. Read the group symbol, its name and the criteria that decided it, with the soil’s point on the plasticity chart.

Restore the example brings back the worked example, a lean clay (CL).

Frequently asked questions

What do the letters of the USCS mean?

The first letter is the main kind of soil: G for gravel, S for sand, M for silt, C for clay, O for organic soil and Pt for peat. The second describes it: W and P for well and poorly graded, L and H for low and high plasticity (a liquid limit under 50, or 50 and over), and M or C for a coarse soil’s silty or clayey fines.

What is the difference between a well-graded and a poorly graded soil?

A well-graded soil has a wide, even range of sizes, so the smaller grains fill the gaps between the larger ones and the soil packs dense. A poorly graded soil is either uniform, with most grains about the same size, or gap-graded, with a range of sizes missing. The USCS tells them apart with $C_u$ and $C_c$.

What does SM mean in soil classification?

SM is a silty sand: mostly sand, with more than 12 % fines that classify as silt (ML or MH) on the plasticity chart. When the fines classify as clay (CL or CH), the soil is SC, a clayey sand.

What is the difference between the USCS and AASHTO?

Both classify soils by grading and plasticity. The USCS (ASTM D2487) is the general system of geotechnical engineering, used for foundations, earthworks and dams. The AASHTO system (AASHTO M 145) rates soils as highway subgrade, in groups A-1 to A-7, and adds a group index to rank them within a group.