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Soil Classification and Properties

In the AAI Manager (Civil) syllabus under Geotechnical Engineering · 2 parts

Three-Phase System & Index Properties of Soil · Soil Classification & Field Identification

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📑 Contents (17 sections)

Part 1 of 2

Three-Phase System & Index Properties of Soil

Last reviewed 16 Sept 2026 · 7 min read

The three-phase system

A soil mass consists of solids, water and air. For analysis the phases are separated in a phase diagram:

Volumes: $V = V_{s} + V_{v}$ , $V_{v} = V_{w} + V_{a}$
Masses (weights): $M = M_{s} + M_{w}$ (air has negligible mass)

A dry soil has two phases (solids + air); a saturated soil has two phases (solids + water).

Basic definitions

DefinitionVolume and mass relationships

Water content $w = \frac{M _{w}}{M _{s}}$ (on dry mass; can exceed 100% in soft clays and peat)
Void ratio $e = \frac{V _{v}}{V _{s}}$ (can exceed 1)
Porosity $n = \frac{V _{v}}{V}$ (always < 1)
Degree of saturation $S = \frac{V _{w}}{V _{v}}$ (0 dry, 1 saturated)
Air content $a_{c} = \frac{V _{a}}{V _{v}} = 1 - S$ ; percentage air voids $n_{a} = \frac{V _{a}}{V} = n a_{c}$
Specific gravity of solids $G = \frac{ρ _{s}}{ρ _{w}}$ (sands ≈ 2.65; clays 2.7–2.8; organic soils lower)

DefinitionUnit weights

Bulk unit weight $γ = W / V$
Dry unit weight $γ_{d} = W_{s} / V$
Saturated unit weight $γ_{s a t}$ (all voids full of water)
Submerged (buoyant) unit weight $γ^{'} = γ_{s a t} - γ_{w}$
Unit weight of solids $γ_{s} = G γ_{w}$

Interrelations

FormulaMost-used relations

n = \frac{e}{1 + e}, e = \frac{n}{1 - n}

S e = w G

γ = \frac{( G + S e ) γ _{w}}{1 + e}, γ_{d} = \frac{G γ _{w}}{1 + e} = \frac{γ}{1 + w}

γ_{s a t} = \frac{( G + e ) γ _{w}}{1 + e}, γ^{'} = \frac{( G - 1 ) γ _{w}}{1 + e}

γ_{d} = \frac{( 1 - n _{a} ) G γ _{w}}{1 + w G} (zero-air-voids line when n_{a} = 0)

Determination in the laboratory and field

Water content

Oven-drying method (standard): dry at 105–110 °C for about 24 hours. Organic and gypsum-bearing soils are dried at lower temperature (about 60 °C) to avoid burning or losing water of crystallisation.
Rapid methods: calcium carbide (moisture meter — acetylene gas pressure), sand bath, torsion balance (infrared), pycnometer (for coarse soils), alcohol method.

Specific gravity (pycnometer / density bottle)

G = \frac{M _{2} - M _{1}}{( M _{2} - M _{1} ) - ( M _{3} - M _{4} )}

$M_{1}$ = empty bottle, $M_{2}$ = bottle + dry soil, $M_{3}$ = bottle + soil + water, $M_{4}$ = bottle + water. Density bottle (50 ml) for fine soils; pycnometer (about 1 litre) for coarser soils. Entrapped air must be removed (vacuum or boiling).

Field density

Core cutter method — soft, cohesive soils free of gravel.
Sand replacement method — gravelly and hard soils; the volume of the excavated hole is found from calibrated sand.
Also: water displacement, rubber balloon, nuclear density gauge.

Relative density (density index)

For coarse-grained soils, the state of packing matters more than the water content:

I_{D} = \frac{e _{ma x} - e}{e _{ma x} - e _{min}} \times 100 = \frac{γ _{d, ma x}}{γ _{d}} \cdot \frac{γ _{d} - γ _{d, min}}{γ _{d, ma x} - γ _{d, min}} \times 100

$I_{D}$ (%)	Description
0–15	Very loose
15–35	Loose
35–65	Medium dense
65–85	Dense
85–100	Very dense

Relative compaction $= \frac{γ _{d}}{γ _{d, ma x}} \times 100$ (used in compaction control).

Grain size analysis

Sieve analysis

For particles coarser than 75 µm: shake through a stack of sieves, plot percentage finer (log scale of size) — the particle size distribution curve.

Sedimentation (hydrometer or pipette) analysis

For particles finer than 75 µm, based on Stokes' law for the terminal velocity of a sphere settling in a fluid:

v = \frac{( γ _{s} - γ _{w} ) D ^{2}}{18 μ}

Valid for particles roughly 0.2 µm to 0.2 mm; assumptions: spherical particles, no interference between particles, laminar settling. A dispersing agent (sodium hexametaphosphate) prevents flocculation.

Grading coefficients

C_{u} = \frac{D _{60}}{D _{10}}, C_{c} = \frac{D _{30}^{2}}{D _{10} D _{60}}

$D_{10}$ = effective size (10% finer). A soil is well graded when

gravel: $C_{u} > 4$ and $1 \leq C_{c} \leq 3$ ,
sand: $C_{u} > 6$ and $1 \leq C_{c} \leq 3$ .

Otherwise it is poorly graded — uniform ( $C_{u} \approx 1$ ) or gap-graded (missing sizes; $C_{c}$ outside 1–3).

Particle sizes (IS 1498)	Range
Boulder	> 300 mm
Cobble	80–300 mm
Gravel	4.75–80 mm
Sand	0.075–4.75 mm (coarse 2–4.75, medium 0.425–2, fine 0.075–0.425)
Silt	0.002–0.075 mm
Clay	< 0.002 mm

Consistency (Atterberg) limits

Fine-grained soils pass through states as water content falls: liquid → plastic → semi-solid → solid. The boundaries are:

Liquid limit (LL, $w_{L}$ ) — water content at which a groove in the Casagrande cup closes over 12.7 mm (½ inch) in 25 blows; or the cone penetrometer gives 20 mm penetration.
Plastic limit (PL, $w_{P}$ ) — water content at which a soil thread just crumbles when rolled to 3 mm diameter.
Shrinkage limit (SL, $w_{S}$ ) — the water content below which further drying causes no further volume decrease (the soil is saturated at this point).

FormulaConsistency indices

Plasticity index $I_{P} = w_{L} - w_{P}$
Consistency index $I_{C} = \frac{w _{L} - w}{I _{P}}$ (1 at PL, 0 at LL; negative if $w > w_{L}$ )
Liquidity index $I_{L} = \frac{w - w _{P}}{I _{P}}$ , with $I_{C} + I_{L} = 1$
Flow index $I_{F} = \frac{w _{1} - w _{2}}{lo g _{10} ( N _{2} / N _{1} )}$ (slope of the flow curve)
Toughness index $I_{T} = I_{P} / I_{F}$
Shrinkage ratio $S R = \frac{M _{d}}{V _{d} ρ _{w}}$ (at SL); shrinkage limit $w_{S} = (\frac{1}{S R} - \frac{1}{G}) \times 100$
Activity $A = \frac{I _{P}}{% clay fraction ( < 2 μ m )}$

$I_{P}$	Plasticity
0	Non-plastic
< 7	Low
7–17	Medium
> 17	High

Activity	Clay	Typical mineral
< 0.75	Inactive	Kaolinite
0.75–1.25	Normal	Illite
> 1.25	Active	Montmorillonite

Sensitivity and thixotropy

S_{t} = \frac{q _{u} ( undisturbed )}{q _{u} ( remoulded, same water content )}

$S_{t}$	Description
1	Insensitive
1–2	Little sensitive
2–4	Moderately sensitive
4–8	Sensitive
8–16	Extra sensitive
> 16	Quick

Thixotropy — partial regain of strength with time after remoulding, at constant water content (e.g. drilling mud, pile driving in clay: capacity increases after a rest period).

Worked examples

Worked ExampleExample 1 — phase relations

A moist sample has mass 190 g, volume 100 cm³, and dry mass 160 g; $G$ = 2.70. Find $w$ , $ρ$ , $ρ_{d}$ , $e$ , $n$ , $S$ and percentage air voids.

Solution. $w = 30/160 = 18.75%$ ; $ρ = 1.90$ g/cm³; $ρ_{d} = 1.60$ g/cm³ $e = \frac{G ρ _{w}}{ρ _{d}} - 1 = \frac{2.70}{1.60} - 1 = 0.6875$ ; $n = \frac{0.6875}{1.6875} = 0.407$ $S = \frac{w G}{e} = \frac{0.1875 \times 2.70}{0.6875} = 0.736$ ; $n_{a} = n (1 - S) = 0.407 \times 0.264 = 10.7%$

Worked ExampleExample 2 — saturated clay

A saturated clay has $w$ = 40% and $G$ = 2.70. Find $e$ , $γ_{s a t}$ and $γ^{'}$ ( $γ_{w}$ = 9.81 kN/m³).

Solution. $e = w G = 1.08$ $γ_{s a t} = \frac{( 2.70 + 1.08 ) \times 9.81}{2.08} = 17.83 kN/ m^{3}$ ; $γ^{'} = 17.83 - 9.81 = 8.02 kN/ m^{3}$

Worked ExampleExample 3 — consistency and activity

A clay has LL = 55%, PL = 25%, natural water content 35% and 30% clay fraction. Find $I_{P}$ , $I_{C}$ , $I_{L}$ and activity.

Solution. $I_{P} = 30%$ ; $I_{C} = \frac{55 - 35}{30} = 0.667$ ; $I_{L} = \frac{35 - 25}{30} = 0.333$ ; $A = 30/30 = 1.0$ (normal clay).

Worked ExampleExample 4 — relative density

A sand has $e_{ma x}$ = 0.90, $e_{min}$ = 0.45 and in-situ $e$ = 0.60. Find $I_{D}$ .

Solution. $I_{D} = \frac{0.90 - 0.60}{0.90 - 0.45} \times 100 = 66.7%$ → dense.

Frequently tested points

$S e = w G$ ; $n = e / (1 + e)$ ; $γ_{d} = γ / (1 + w)$ ; $γ^{'} = (G - 1) γ_{w} / (1 + e)$ .
Water content by oven drying at 105–110 °C for 24 h.
$C_{u} = D_{60} / D_{10}$ , $C_{c} = D_{30}^{2} / (D_{10} D_{60})$ ; well graded sand $C_{u} > 6$ , gravel $C_{u} > 4$ , $1 \leq C_{c} \leq 3$ .
Stokes' law: $v \propto D^{2}$ .
LL at 25 blows (or 20 mm cone penetration); PL at 3 mm thread.
$I_{C} + I_{L} = 1$ ; activity > 1.25 active (montmorillonite).
Sensitivity > 16 → quick clay.

Common MistakeCommon mistakes

Using total mass instead of dry mass in water content.
Confusing porosity (on total volume) with void ratio (on solids volume).
Using $lo g_{e}$ in the flow index.

Revision SummaryChapter summary

Phase relations link water content, voids, saturation and unit weights.
Laboratory and field tests measure $w$ , $G$ and density.
Relative density describes coarse soils; grading curves describe size distribution.
Atterberg limits and indices describe fine soils; activity and sensitivity add mineral and structure information.

Part 2 of 2

Soil Classification & Field Identification

Last reviewed 16 Sept 2026 · 6 min read

Why classify soils

Classification places a soil in a group of similar engineering behaviour using simple index properties (grain size and plasticity), so engineers can communicate, anticipate problems and choose tests and construction methods before detailed testing.

Particle-size and textural classifications

Particle-size classification names soil by size ranges (IS: gravel 4.75–80 mm, sand 0.075–4.75 mm, silt 0.002–0.075 mm, clay < 0.002 mm).
Textural classification (triangular chart of % sand, silt, clay, e.g. US Bureau of Public Roads/USDA) names soils such as sandy loam, silty clay — common in agriculture and highway work but ignores plasticity.

Indian Standard Soil Classification System (IS 1498:1970)

Based on the Unified system, with three plasticity divisions for fine-grained soils.

Major divisions

Division	Criterion	Primary letters
Coarse-grained soils	More than 50% (by mass) retained on the 75 µm sieve	G (gravel), S (sand)
Fine-grained soils	More than 50% passing 75 µm	M (silt), C (clay), O (organic)
Highly organic soils	Fibrous, dark, odorous	Pt (peat)

Gravel vs sand: if more than half of the coarse fraction is retained on the 4.75 mm sieve → gravel; otherwise sand.

Secondary letters

Letter	Meaning
W	Well graded
P	Poorly graded
M	With non-plastic/low-plastic fines (silty)
C	With plastic fines (clayey)
L	Low plasticity/compressibility ( $w_{L}$ < 35%)
I	Intermediate ( $w_{L}$ 35–50%)
H	High ( $w_{L}$ > 50%)

Coarse-grained soils — groups

Fines (passing 75 µm)	Classification
Less than 5%	GW / GP, SW / SP by $C_{u}$ and $C_{c}$ (well graded: gravel $C_{u} > 4$ , sand $C_{u} > 6$ , and $1 \leq C_{c} \leq 3$ )
More than 12%	GM / GC, SM / SC by where the fines plot on the plasticity chart (below A-line or $I_{P} < 4$ → M; above A-line and $I_{P} > 7$ → C)
5% to 12%	Borderline — dual symbols, e.g. GW-GM, SP-SC
$I_{P}$ between 4 and 7 above A-line	Dual symbol such as SM-SC

Fine-grained soils — the plasticity chart

Plot plasticity index ( $I_{P}$ ) against liquid limit ( $w_{L}$ ).

FormulaA-line and U-line

A-line: I_{P} = 0.73 (w_{L} - 20)

U-line (upper limit of natural soils): I_{P} = 0.9 (w_{L} - 8)

Above the A-line → clays (C); below → silts (M) or organic soils (O).
Vertical lines at $w_{L}$ = 35% and 50% separate L, I, H.

Groups: CL, CI, CH (inorganic clays); ML, MI, MH (inorganic silts); OL, OI, OH (organic silts and clays, identified by odour, colour and a sharp drop in LL after oven drying); Pt (peat).

The hatched zone just above the A-line with $I_{P}$ between 4 and 7 at low LL is designated CL-ML.

FigurePlasticity chart (IS 1498)

Scale: horizontal axis 0–100% liquid limit; the A-line meets the axis at $w_{L}$ = 20%.

Field identification of fine-grained soils

Test	Procedure	Clay	Silt
Dilatancy (shaking)	Shake a wet pat; watch water appear/disappear	None to slow	Quick reaction
Toughness	Roll a thread near the plastic limit	Tough, stiff thread	Weak, crumbly
Dry strength	Crush a dried lump between fingers	High	Low (powders easily)
Feel / washing	Rub in water	Smooth, sticky, hard to wash off	Gritty, washes off

Organic soils: dark colour, musty odour, spongy feel.

Other classification systems

Unified Soil Classification System (USCS)

Same letter symbols as IS 1498, but fine-grained soils have only two plasticity divisions: L ( $w_{L}$ < 50%) and H ( $w_{L}$ ≥ 50%).

AASHTO (HRB) system — for highway subgrades

Soils are grouped A-1 to A-7 (A-1 best as subgrade; A-4 to A-7 silt–clay materials; A-8 peat) using sieve analysis (2 mm, 425 µm, 75 µm) and Atterberg limits. Within groups, the group index ranks quality:

FormulaGroup index

G I = 0.2 a + 0.005 a c + 0.01 b d

$a$ = % passing 75 µm in excess of 35 (0 to 40)
$b$ = % passing 75 µm in excess of 15 (0 to 40)
$c$ = liquid limit in excess of 40 (0 to 20)
$d$ = plasticity index in excess of 10 (0 to 20)

GI ranges from 0 (good subgrade) to 20 (very poor); higher GI → thicker pavement.

Worked examples

Worked ExampleExample 1 — coarse-grained soil

A soil has 3% passing 75 µm; 70% of the coarse fraction passes 4.75 mm; $D_{10}$ = 0.15 mm, $D_{30}$ = 0.40 mm, $D_{60}$ = 1.10 mm. Classify.

Solution. Coarse-grained (97% retained on 75 µm). More than half the coarse fraction passes 4.75 mm → sand. Fines < 5% → W or P.

$C_{u} = 1.10/0.15 = 7.33$ (> 6); $C_{c} = \frac{0.4 0 ^{2}}{0.15 \times 1.10} = 0.97$ (< 1) → not well graded → SP (poorly graded sand).

Worked ExampleExample 2 — fine-grained soil

A soil has 78% passing 75 µm, $w_{L}$ = 45%, $w_{P}$ = 22%. Classify.

Solution. Fine-grained. $I_{P} = 23%$ . A-line at $w_{L}$ = 45: $0.73 \times 25 = 18.25$ → 23 > 18.25 → above A-line → clay. $w_{L}$ between 35 and 50 → CI (clay of intermediate plasticity).

Worked ExampleExample 3 — group index

A subgrade soil: 60% passing 75 µm, $w_{L}$ = 48%, $I_{P}$ = 18%. Find GI.

Solution. $a = 60 - 35 = 25$ ; $b = 60 - 15 = 45 \to 40$ (max); $c = 48 - 40 = 8$ ; $d = 18 - 10 = 8$ .

$G I = 0.2 \times 25 + 0.005 \times 25 \times 8 + 0.01 \times 40 \times 8 = 5 + 1 + 3.2 = 9.2 \approx 9$

Frequently tested points

IS 1498: coarse-grained if > 50% retained on 75 µm; gravel vs sand at 4.75 mm on the coarse fraction.
Fines < 5%: W/P; > 12%: M/C; 5–12%: dual symbols.
A-line $I_{P} = 0.73 (w_{L} - 20)$ ; above = clay, below = silt/organic.
IS divisions L < 35%, I 35–50%, H > 50%; USCS uses only L/H at 50%.
Dilatancy quick for silts; dry strength high for clays.
$G I = 0.2 a + 0.005 a c + 0.01 b d$ ; AASHTO A-1 best, A-7 poorest (A-8 peat).

Common MistakeCommon mistakes

Using the whole sample (instead of the coarse fraction) to decide gravel vs sand.
Classifying fine soils by LL alone without checking the A-line.
Forgetting the limits (0–40, 0–20) on $a$ , $b$ , $c$ , $d$ in the group index.

Revision SummaryChapter summary

Classification groups soils by grain size and plasticity.
IS 1498 uses G, S, M, C, O, Pt with W, P, M, C, L, I, H qualifiers.
The plasticity chart and A-line separate clays from silts.
Field tests identify fine soils quickly; AASHTO and group index serve highway work.

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