Soil classification

A classification system turns the index tests of lesson 3 into a short label that every engineer understands. Calling the site clay a CL tells a reader at once that it is a fine-grained soil of low to intermediate plasticity, likely to be compressible, of low permeability and strength sensitive to water. Classification does not replace testing, but it organizes experience: soils in the same group tend to behave alike, so a label carries decades of lessons from other projects. This lesson applies the Unified Soil Classification System to the soils from the operations center and explains how classification fits with field description.

Why classify

Classification serves three purposes. It is a common language for logs, reports and specifications. It groups soils for correlations, so that a reader can look up typical permeabilities, strengths and compaction behavior for each group. And it flags problems early: a symbol such as OH or Pt, or a high-plasticity CH, warns of difficult ground before any detailed test is run. Most systems use only the grain size distribution and the Atterberg limits, which are cheap and quick, and deliberately ignore properties such as density and structure, which depend on the state of the soil in the ground rather than on what it is made of.

The Unified Soil Classification System

The USCS was developed by Arthur Casagrande for airfield construction during the Second World War and later adopted by the US Army Corps of Engineers and the Bureau of Reclamation. It is standardized as ASTM D2487 for laboratory classification and ASTM D2488 for visual-manual field description, and it is the standard engineering classification for foundations and earthworks across the United States.

Each group has a two-letter symbol. The first letter is the main soil type: G for gravel, S for sand, M for silt (from the Swedish mo), C for clay, O for organic and Pt for peat. The second letter is a modifier: W for well graded, P for poorly graded, M for silty fines, C for clayey fines, L for low plasticity and H for high plasticity. Each group also has a name, such as poorly graded sand with silt or lean clay, which the standard builds from the symbol and the percentages of secondary components.

Coarse-grained soils

A soil is coarse grained if more than half of it by mass is retained on the No. 200 sieve. It is a gravel if more of the coarse fraction is gravel than sand, and a sand otherwise. The fines content then decides the next step:

• Less than 5 % fines: the soil is clean and is classified by its grading, GW or GP, SW or SP, using \(C_u\) and \(C_c\) from lesson 3.
• More than 12 % fines: the fines control the behavior and the soil is GM or GC, SM or SC, depending on whether the fines plot as silt or clay on the plasticity chart.
• From 5 to 12 % fines: the soil is a borderline case and gets a dual symbol that combines the grading and the fines, such as SP-SM or GW-GC.

Fine-grained soils and the plasticity chart

A soil is fine grained if half or more passes the finest sieve. It is classified by plotting its plasticity index against its liquid limit on Casagrande's plasticity chart.

Two lines divide the chart. The A-line, \(PI = 0.73(LL - 20)\), separates clays above from silts and organic soils below. The vertical line at \(LL = 50\) separates low plasticity (L) from high plasticity (H). The U-line, \(PI = 0.9(LL - 8)\), is an upper limit found empirically for natural soils; a point above it suggests a testing error. A small hatched zone between \(PI = 4\) and 7 above the A-line gives the dual symbol CL-ML.

Organic soils are identified by a drop in the liquid limit after oven drying: if the liquid limit of the oven-dried soil is less than three quarters of that of the undried soil, the soil is organic (OL or OH). They are also recognized by their dark color, odor and spongy feel. Peat, Pt, is mostly plant matter and is identified by eye. Organic soils and peat are very compressible and usually removed from beneath structures.

Classifying the site soils

Owen classifies the soils tested in lesson 3, together with a silt lens found in B-2 and the crushed stone proposed as aggregate base for the yard. Sofia codes a simplified version of the ASTM D2487 flowchart for inorganic soils. It ignores the group names and the rules for cobbles and organic soils, but it reproduces the symbols. Because the yard pavement will be designed by Minnesota highway methods, she also computes the AASHTO classification.

# A simplified Unified Soil Classification System (after ASTM D2487) for inorganic soils
def a_line(LL):
    return 0.73 * (LL - 20)

def fine_symbol(LL, PI):
    """Group symbol for fines from the plasticity chart; PI None means nonplastic."""
    if PI is None:
        return "ML"
    if 4 <= PI <= 7 and PI >= a_line(LL):
        return "CL-ML"
    if PI < 4 or PI < a_line(LL):
        return "MH" if LL >= 50 else "ML"
    return "CH" if LL >= 50 else "CL"

def uscs(gravel, sand, fines, Cu=None, Cc=None, LL=None, PI=None):
    if fines >= 50:
        return fine_symbol(LL, PI)
    G = gravel > sand
    first = "G" if G else "S"
    well = Cu is not None and Cc is not None and 1 <= Cc <= 3 and Cu >= (4 if G else 6)
    grade = first + ("W" if well else "P")
    if fines < 5:
        return grade
    fsym = fine_symbol(LL, PI)
    plastic = first + ("C" if fsym in ("CL", "CH") else "M")
    if fsym == "CL-ML":
        plastic = first + "C-" + first + "M"
    if fines <= 12:
        return grade + "-" + plastic[:2]
    return plastic

samples = [
    ("Brown sand, B-3 5 ft",          dict(gravel=2, sand=91, fines=7, Cu=5.0, Cc=1.32)),
    ("Gray varved clay, B-3 20 ft",      dict(gravel=0, sand=1, fines=99, LL=45, PI=23)),
    ("Gray till, B-3 40 ft",            dict(gravel=31, sand=37, fines=32, LL=18, PI=3)),
    ("Silt lens, B-2 25 ft",           dict(gravel=0, sand=15, fines=85, LL=32, PI=6)),
    ("Crushed stone base, stockpile",   dict(gravel=55, sand=41, fines=4, Cu=30, Cc=1.4)),
]
for name, d in samples:
    sym = uscs(**d)
    extra = ""
    if d.get("LL") is not None:
        extra = f"  (LL {d['LL']}, PI {d['PI']}, A-line PI {a_line(d['LL']):.1f})"
    print(f"{name:32s} {sym:7s}{extra}")

# AASHTO group and group index for the yard pavement subgrade
def aashto(F200, LL, PI, P40=None):
    if F200 <= 35:
        if P40 is not None and P40 >= 51 and F200 <= 10 and (PI or 0) == 0:
            return "A-3", 0
        grp = "A-2-" + ("4" if LL <= 40 and PI <= 10 else "5" if PI <= 10 else "6" if LL <= 40 else "7")
        gi = 0.01 * (F200 - 15) * (PI - 10) if grp in ("A-2-6", "A-2-7") else 0
        return grp, max(0, round(gi))
    if PI <= 10:
        grp = "A-4" if LL <= 40 else "A-5"
    else:
        grp = "A-6" if LL <= 40 else ("A-7-5" if PI <= LL - 30 else "A-7-6")
    gi = (F200 - 35) * (0.2 + 0.005 * (LL - 40)) + 0.01 * (F200 - 15) * (PI - 10)
    return grp, max(0, round(gi))

print()
for name, F, LL, PI, P40 in (("sand", 7, 0, 0, 55), ("till", 32, 18, 3, 46), ("clay", 99, 45, 23, 100)):
    g, gi = aashto(F, LL, PI, P40)
    print(f"AASHTO {name:5s}: {g}({gi})")

Brown sand, B-3 5 ft             SP-SM  
Gray varved clay, B-3 20 ft      CL       (LL 45, PI 23, A-line PI 18.2)
Gray till, B-3 40 ft             SM       (LL 18, PI 3, A-line PI -1.5)
Silt lens, B-2 25 ft             ML       (LL 32, PI 6, A-line PI 8.8)
Crushed stone base, stockpile    GW     

AASHTO sand : A-3(0)
AASHTO till : A-2-4(0)
AASHTO clay : A-7-6(25)

The sand, with 7 % nonplastic fines and \(C_u\) below 6, is SP-SM, poorly graded sand with silt. The clay is CL, lean clay: its point lies above the A-line and to the left of \(LL = 50\). The silt lens has a PI of 6, which would put it in the CL-ML zone if it lay above the A-line, but at \(LL = 32\) the A-line is at a PI of 8.8, so it is ML, silt. The crushed stone, well graded with only 4 % fines, is GW.

The silt lens deserves attention even though it is thin. It is more permeable than the clay around it, so it drains the clay and speeds consolidation, and in an excavation it can bring water and fines in through the sides. Dr. Kowalczyk asks for the CPT records to be checked for other lenses between the borings.

The AASHTO system

The American Association of State Highway and Transportation Officials (AASHTO) system, standardized as AASHTO M 145 and ASTM D3282, rates soils as highway subgrades. It places soils in groups A-1 to A-7: A-1 to A-3 are granular materials with 35 % or less passing the No. 200 sieve, rated excellent to good as subgrade, and A-4 to A-7 are silt-clay materials, rated fair to poor. A group index, computed from the fines content, liquid limit and plasticity index, refines the rating within a group; the higher the index, the poorer the subgrade:

where \(F\) is the percentage passing the No. 200 sieve. Negative terms are taken as zero and the result is rounded to a whole number. The script places the site sand in A-3, the till in A-2-4 and the clay in A-7-6 with a group index of 25, a very poor subgrade. The yard pavement will therefore need a thick granular section over the clay areas, or the clay will be undercut and replaced. Minnesota and most other state DOTs now design pavements with mechanistic-empirical methods that use the resilient modulus of the subgrade, but AASHTO groups are still reported on soil surveys and used to screen borrow.

Field identification

In the field, without a laboratory, an engineer or technician can classify soil well enough for logging by simple tests. Coarse and fine soils are separated by eye: individual sand grains are visible, silt and clay particles are not. Fine soils are distinguished by four manual tests from ASTM D2488:

• Dry strength: a dried lump of clay is hard to crush between the fingers; silt crumbles easily.
• Dilatancy: a pat of wet silt shaken in the palm turns glossy as water comes to the surface and dull when squeezed; clay shows little or no reaction.
• Toughness: a thread of clay rolled near the plastic limit is stiff and tough; a silt thread is weak and crumbles.
• Plasticity: the ease of rolling a thread and the water content range over which it can be rolled.

A trained eye and these tests usually place a soil within one group of its laboratory classification. The laboratory then confirms the classification on representative samples, and the logs are corrected where needed.

What classification cannot tell you

Two soils in the same group can behave very differently. A loose SP sand and a dense SP sand share a symbol but differ in strength, stiffness and liquefaction potential. A soft, normally consolidated CL and a stiff, overconsolidated CL share a symbol but differ several times over in compressibility and strength. Classification describes what a soil is made of; its state, density, stress history and structure must be measured separately, by the in situ tests of lesson 1 and the consolidation and strength tests of lessons 10 and 12. Sensitive clays, such as the Leda clay of the Ottawa and St. Lawrence valleys studied in CIV-241, are a striking example: they classify as ordinary CL or CH soils but can turn to liquid when disturbed.

Practical points

• Report the group name and the percentages, not just the symbol; the name carries the secondary components.
• Use dual symbols for borderline gradings (5 to 12 % fines) and the CL-ML zone, as the standard requires.
• Classify every sample used in design, and correlate field descriptions with laboratory results.
• Treat thin lenses and seams as their own soil type in design if they control drainage or stability.