Soil Identifications and Classification

Field identification tests are quick hand tests on a small moist sample, used to tell fine-grained soils apart without laboratory equipment. They use the dry strength, dilatancy, toughness, plasticity, and appearance of the soil. They are carried out on the fraction finer than 0.425 mm (No. 40 sieve).

1. Dilatancy (shaking) test

Make a pat of moist soil (soft putty consistency) and shake it in the palm. Then squeeze it.

• Silt: water rises quickly to the surface giving a shiny look; it disappears on squeezing (rapid reaction).
• Clay: no change (no reaction).

2. Dry strength (crushing) test

Dry a moulded pat and crush it between the fingers.

• Clay: high dry strength, cannot be crushed easily.
• Silt: low dry strength, crumbles easily to powder.

3. Toughness (thread) test

Roll the soil into a thread about 3 mm diameter, fold and re-roll repeatedly until it crumbles at the plastic limit.

• Clay: strong, stiff thread; tough lump after crumbling.
• Silt: weak thread, soft lump, loses coherence quickly.

4. Plasticity / sticking

Clay is plastic and sticky in hand and cannot be easily washed off; silt feels gritty or floury and washes off easily.

5. Other tests

• Visual and touch: clay feels smooth, silt feels slightly gritty on the teeth.
• Shine test: a cut surface of clay shows a shiny surface when scratched; silt stays dull.
• Sedimentation test: shaken in water, silt settles in 30 - 60 seconds, clay stays in suspension for hours.

Soil classification groups soils of similar engineering behaviour into the same class, using simple index properties (grain size and plasticity).

Purposes and importance

1. Common language: engineers share a standard name and symbol for a soil (e.g. SC, CH), so reports are understood by everyone.
2. Prediction of behaviour: from the group, the probable strength, compressibility, permeability, compaction and swelling can be estimated.
3. Preliminary design: gives approximate design values (bearing capacity, subgrade suitability, frost susceptibility) before costly tests are done.
4. Selection of construction material: helps to select borrow soil for embankments, dam cores, and road subgrades.
5. Site investigation planning: helps decide what tests are needed.
6. Use of past experience: data from similar soils elsewhere can be applied.
7. Economy: avoids costly lab tests for small or simple projects.
8. Correlation: index properties correlate with engineering properties (e.g. $w_L$ with compression index).

Types of soil classification

1. Textural (particle size) classification: based on percent of sand, silt and clay, e.g. USDA/triangular chart, MIT system.
2. Classification based on plasticity and size (engineering classification): Unified Soil Classification System (USCS), Indian Standard Soil Classification System (ISSCS, IS 1498), AASHTO, Casagrande's airfield system.
3. Classification based on origin and genesis: residual, transported, alluvial, etc.
4. Classification for specific purposes: AASHTO for highways, Federal Aviation Administration (FAA) for airfields, and agricultural (pedological) classification.

Important engineering systems

The MIT (Massachusetts Institute of Technology) classification is a textural system that names a soil only by its particle size. It was proposed by Gilboy and Casagrande and later adopted by the British Standards. The soil is divided into gravel, sand, silt and clay.

• The system has the same limits for sand, silt, and clay as the International classification, except for the sand boundaries.
• The mixture is named by its major fraction, for example "silty sand" or "sandy clay".
• Limitation: it ignores plasticity, so soils with the same size distribution can behave very differently.

A good soil classification system should:

1. Be simple, easy to learn and to use, with a few clear groups.
2. Use standard index tests that are quick and cheap (grain size, Atterberg limits).
3. Place soils of similar engineering behaviour in the same group.
4. Use clear boundaries so that different engineers get the same classification.
5. Use a systematic nomenclature (symbols) that conveys properties of the soil.
6. Be accepted widely and linked to experience, so that behaviour can be predicted.
7. Cover all types of soil, coarse and fine, organic and inorganic.

The plasticity chart (Casagrande) plots plasticity index $I_p$ (vertical) against liquid limit $w_L$ (horizontal). It is used for the fine-grained fraction (passing 75 micron).

Lines

• A-line: $I_p = 0.73(w_L - 20)$, separating clays (above) from silts and organic soils (below).
• U-line: $I_p = 0.9(w_L - 8)$, the upper limit of natural soils.
• Vertical line at $w_L = 50\%$ (USCS) separates low and high plasticity. IS adds $w_L = 35\%$ for low (L), medium (I), and high (H) plasticity.

 Ip
 60|                       /U
 50|                  CH  /
 40|                    /
 30|        CI    CH  /  A
 20|    CL   /       /
 10|  CL-ML /     MH & OH
  7|--/--------/
  4|ML & OL    MI    MH & OH
  0+---+---+---+---+---+--- wL
    0  10 20 35 50 60 70 80

Group symbols

Sandy soil

• Visual and touch: coarse, gritty, grains visible to the naked eye.
• Dry strength test: crumbles easily; no strength.
• Dilatancy test: quick reaction with water rising to the surface (for fine sand).
• Sedimentation test: settles in less than a minute.
• Does not form a thread (no plasticity).

Clayey soil

• Dry strength test: high strength; hard to crush.
• Dilatancy test: no or very slow reaction.
• Toughness (thread) test: rolls into 3 mm thread; tough and cohesive.
• Stickiness/shine test: sticky, smooth, with shiny cut surface.
• Sedimentation test: stays in suspension for a long time.

Soil classification systems that use both particle size and plasticity are:

1. Unified Soil Classification System (USCS) - Casagrande.
2. Indian Standard Soil Classification System (ISSCS) - IS 1498.
3. AASHTO classification system - uses sieve analysis and Atterberg limits (also gives a group index).
4. Casagrande's airfield classification (the origin of the USCS).
5. British Soil Classification System (BSCS).

Systems based on particle size and plasticity

• Unified Soil Classification System (USCS)
• Indian Standard Soil Classification System (ISSCS, IS 1498)
• AASHTO classification system
• British Soil Classification System (BSCS)

Plasticity chart of ISSCS

The plasticity chart is a graph of plasticity index $I_p$ (y-axis) against liquid limit $w_L$ (x-axis), used for classifying the fine-grained soils (passing 75 micron).

• A-line: $I_p = 0.73(w_L - 20)$. Inorganic clays (C) lie above it; silts (M) and organic soils (O) lie below it.
• Vertical lines at $w_L = 35\%$ and $50\%$ divide the soils into low (L, $w_L < 35$), intermediate (I, 35 - 50) and high plasticity (H, $w_L > 50$).
• The hatched area above the A-line with $I_p$ between 4 and 7 gives dual symbol CL-ML.

The resulting groups are CL, CI, CH (clays), ML, MI, MH (silts), and OL, OI, OH (organic soils).

Unified Soil Classification System (USCS)

Proposed by A. Casagrande (1948) and adopted by the US Army Corps of Engineers and Bureau of Reclamation; it is the most widely used engineering system. Soil is classified by grain size distribution and plasticity.

Procedure

1. Coarse-grained if more than 50% is retained on the 75 micron (No. 200) sieve; fine-grained if 50% or more passes.
2. Coarse-grained: G (gravel) if more than half of the coarse fraction is retained on 4.75 mm sieve, else S (sand).
   • Fines < 5%: GW, GP, SW, SP. Well graded if $C_u \ge 4$ (gravel) or $\ge 6$ (sand) and $1 \le C_c \le 3$.
   • Fines > 12%: GM, GC, SM, SC using Atterberg limits (A-line).
   • Fines 5 - 12%: dual symbols (e.g. GW-GC).
3. Fine-grained: use the plasticity chart. Below A-line: M (silt); above: C (clay); organic: O; $w_L < 50\%$ gives L, $w_L \ge 50\%$ gives H: ML, CL, OL, MH, CH, OH. Pt is peat.

Limitations

• Only the fraction smaller than 75 mm is considered; cobbles and boulders are not classified.
• Gives no direct measure of engineering properties such as strength or compressibility.
• Dual symbols and borderline cases cause ambiguity.
• Particle shape, density and structure (in-situ condition) are not considered.
• Single boundary at $w_L = 50\%$ gives no medium plasticity class (the IS system corrects this).
• Based on the remoulded soil, so natural structure effects are ignored.

Both classify soil using grain size distribution and Atterberg limits.

Similarities

• Both use sieve analysis and liquid and plastic limits.
• Both are based on the fraction of soil finer than 75 mm.
• Both separate coarse-grained from fine-grained soil using the 75 micron sieve, though at different percentages.
• Both are used widely in engineering practice.

Differences

The AASHTO classification system (American Association of State Highway and Transportation Officials) is generally used for rating soil as a highway subgrade.

Groups

Granular materials (35% or less passing 75 micron):

• A-1-a: stone fragments, gravel and sand
• A-1-b: stone fragments, gravel and sand
• A-3: fine sand
• A-2-4, A-2-5, A-2-6, A-2-7: silty or clayey gravel and sand

Silt-clay materials (more than 35% passing 75 micron):

• A-4: silty soils
• A-5: elastic (micaceous or diatomaceous) silty soils
• A-6: clayey soils
• A-7-5, A-7-6: elastic clayey soils (A-7-5 moderate plasticity; A-7-6 high change in volume)

General rating as subgrade

Within each group, a lower group index (0 for best soils) indicates a better subgrade.

The plasticity chart plots plasticity index ($I_p$) against liquid limit ($w_L$) and has the A-line ($I_p = 0.73(w_L - 20)$) and a vertical line at $w_L = 50\%$.

It is useful because:

1. It classifies fine-grained soils (more than 50% passing 75 micron) into clay (C), silt (M) or organic (O) by position relative to the A-line (above = clay; below = silt/organic).
2. It divides them into low ($w_L<50$) and high ($w_L>50$) plasticity (L or H), giving CL, CH, ML, MH, OL, OH.
3. The hatched zone above A-line with $I_p = 4$ to 7 gives the borderline class CL-ML.
4. It is also used to classify the fines of coarse-grained soils with more than 12% fines (GM, GC, SM, SC).
5. It indicates relative engineering behaviour: compressibility, swelling and strength rise with position upward and right.

Given

Passing 75 micron $= 4\%$; retained on 4.75 mm $= 35\%$; $C_c = 2$; $C_u = 5$.

Step 1: Coarse or fine?

Only 4% passes 75 micron, so more than 50% is retained: coarse-grained.

Step 2: Gravel or sand?

• Gravel (retained on 4.75 mm) $= 35\%$
• Sand $= 100 - 35 - 4 = 61\%$

Coarse fraction $= 96\%$; gravel is $35/96 = 36\%$ of it, less than half. So the soil is a sand (S).

Step 3: Fines content

4% is less than 5%, so only the grading is needed (symbols SW or SP).

Step 4: Grading

For well-graded sand: $C_u \ge 6$ and $1 \le C_c \le 3$.

• $C_c = 2$ satisfies the condition.
• $C_u = 5 < 6$ fails.

The soil is not well graded.

Answer: SP, poorly graded sand (with gravel).

Given

Passing 75 micron $= 8\%$; passing 4.75 mm $= 42\%$; $C_u = 6$; $C_c = 4$; $I_p = 4$.

Step 1: Coarse or fine?

92% is retained on the 75 micron sieve, so the soil is coarse-grained.

Step 2: Gravel or sand?

• Retained on 4.75 mm (gravel) $= 100 - 42 = 58\%$
• Sand $= 42 - 8 = 34\%$

Gravel is more than half of the coarse fraction ($58/92 = 63\%$), so the soil is gravel (G).

Step 3: Fines content

8% is between 5% and 12%, so a dual symbol is required.

Step 4: Grading

For well-graded gravel: $C_u \ge 4$ and $1 \le C_c \le 3$.

• $C_u = 6 \ge 4$: OK
• $C_c = 4$ is greater than 3: fails

So the gravel is poorly graded (GP).

Step 5: Nature of fines

$I_p = 4$ is at the lower limit of the hatched zone. Since $I_p$ is not above 4 (the fines plot as silt, ML or CL-ML), the fines are taken as silty (M).

Answer: GP-GM, poorly graded gravel with silt and sand.

Both soils have 59% and 85% passing the 75 micron sieve (more than 50%), so both are fine-grained. Use the plasticity chart with the A-line $I_p = 0.73(w_L - 20)$.

Soil A: $w_L = 45\%$, $I_p = 29\%$

$$I_{p,A\text{-line}} = 0.73(45 - 20) = 18.25\%$$

$I_p = 29 > 18.25$: the soil lies above the A-line (clay), and $w_L = 45 < 50$ (low plasticity).

Soil A: CL, inorganic clay of low plasticity (lean clay). It is also sandy, since 41% is coarse.

Soil B: $w_L = 55\%$, $I_p = 15\%$

$$I_{p,A\text{-line}} = 0.73(55 - 20) = 25.55\%$$

$I_p = 15 < 25.55$: the soil lies below the A-line (silt), and $w_L = 55 > 50$ (high plasticity).

Soil B: MH, inorganic silt of high plasticity (elastic silt).

(Both are taken as inorganic, since no data on organic content is given.)

Given

Passing 75 micron (No. 200) $= 40\%$; retained on 4.75 mm (No. 4) $= 55\%$; $D_{10} = 1.2$ mm, $D_{30} = 2.6$ mm, $D_{60} = 3.8$ mm.

Step 1: Coarse or fine?

60% is retained on the 75 micron sieve, so the soil is coarse-grained.

Step 2: Gravel or sand?

• Gravel $= 55\%$
• Sand $= 100 - 55 - 40 = 5\%$

Gravel is more than half of the coarse fraction, so the soil is gravel (G).

Step 3: Grading parameters

$$C_u = \frac{D_{60}}{D_{10}} = \frac{3.8}{1.2} = 3.17, \qquad C_c = \frac{D_{30}^2}{D_{10}D_{60}} = \frac{2.6^2}{1.2 \times 3.8} = 1.48$$

The gravel fraction is poorly graded ($C_u < 4$), but $C_c$ is in the range 1 - 3.

Step 4: Fines content

The fines are 40%, which is more than 12%. So the grading is not used; the symbol depends on the type of fines (GM or GC) found from $w_L$ and $I_p$ using the A-line. The plasticity data are not given.

Answer: a gravel with fines, GM or GC (silty or clayey gravel). If the fines are silty the symbol is GM; if the fines plot above the A-line with $I_p > 7$ it is GC. The grading ($C_u = 3.17$, $C_c = 1.48$) of the coarse fraction is poor.

Given

Passing No. 4 $= 70\%$; passing No. 200 $= 30\%$; $w_L = 33\%$; $w_P = 11\%$.

$$I_p = w_L - w_P = 33 - 11 = 22\%$$

Step 1: Coarse or fine?

70% is retained on the No. 200 sieve, so the soil is coarse-grained.

Step 2: Gravel or sand?

• Gravel (retained on No. 4) $= 100 - 70 = 30\%$
• Sand $= 70 - 30 = 40\%$

Gravel is 30/70 = 43% of the coarse fraction (less than half), so the soil is sand (S).

Step 3: Fines

Fines are 30% (more than 12%), so classify the fines with the plasticity chart.

$$I_{p,A\text{-line}} = 0.73(33 - 20) = 9.5\%$$

$I_p = 22$ is above the A-line and greater than 7: clayey fines (C).

Answer: SC, clayey sand (with gravel).

Reading the curves

For each soil the larger percent is taken as the percent passing the 4.75 mm sieve and the smaller as the percent passing the 0.075 mm sieve.

Soil A

Fines $= 55\% > 50\%$, so it is fine-grained.

$$I_{p,A\text{-line}} = 0.73(45 - 20) = 18.25\%$$

$I_p = 30 >18.25$: above the A-line (clay); $w_L = 45 < 50$ (low plasticity).

Soil A: CL, lean clay (sandy).

Soil B

Fines $= 13\% > 12\%$ and less than 50%, so it is coarse-grained with fines.

• Gravel $= 100 - 63 = 37\%$
• Sand $= 63 - 13 = 50\%$

Sand is more than half of the coarse fraction ($50/87 = 57\%$), so it is S.

$$I_{p,A\text{-line}} = 0.73(25 - 20) = 3.65\%$$

$I_p = 15 > 7$ and above the A-line: clayey fines.

Soil B: SC, clayey sand.

Plasticity chart

 Ip
 30|  * A
 20|        / A-line
 15|   * B /
  7|----/-----------
  4| /
  0+----+----+----+---- wL
    0   25   45   50

Point A (45, 30) lies above the A-line; B (25, 15) lies above it as well.

Given

Passing No. 200 $= 55\%$; $w_L = 68\%$; $w_P = 22\%$ (the percentage of coarse fraction passing No. 4 is not needed).

$$I_p = 68 - 22 = 46\%$$

Step 1: Coarse or fine?

Since 55% passes the 75 micron sieve (more than 50%), the soil is fine-grained.

Step 2: Plasticity group (IS 1498)

• $w_L < 35\%$: low (L); $35 - 50\%$: medium (I); $> 50\%$: high (H).
• $w_L = 68\% > 50\%$: high plasticity (H).

Step 3: Position relative to the A-line

$$I_{p,A\text{-line}} = 0.73(w_L - 20) = 0.73(68 - 20) = 35.0\%$$

$I_p = 46 > 35.0$: the soil plots above the A-line, so it is a clay (C) (inorganic).

Answer: CH, inorganic clay of high plasticity (fat clay). Because 45% is coarse, it may be described as a sandy or gravelly clay.

Given

Passing No. 200 $= 58\%$; passing No. 40 $= 80\%$; passing No. 10 $= 100\%$; $w_L = 30\%$; $I_p = 10\%$.

Step 1: Granular or silt-clay?

58% passes the No. 200 sieve, which is more than 35%, so the soil is in the silt-clay group (A-4 to A-7).

Step 2: Check groups from the left of the table (AASHTO)

So the soil is A-4.

Group index

$$\begin{aligned} GI &= (F - 35)[0.2 + 0.005(w_L - 40)] + 0.01(F - 15)(I_p - 10) \\ &= (58 - 35)[0.2 + 0.005(30 - 40)] + 0.01(58 - 15)(10 - 10) \\ &= 23 \times 0.15 + 0 = 3.45 \approx 3 \end{aligned}$$

Answer: A-4(3), silty soil; fair to poor subgrade.

Given

Passing 75 micron $= 4\%$; passing 4.75 mm $= 62\%$; $C_u = 5$; $C_c = 2.6$.

• Fines are only 4%, so the soil is coarse-grained.
• Gravel (retained on 4.75 mm) $= 100 - 62 = 38\%$; sand $= 62 - 4 = 58\%$. Sand is more than half of the coarse fraction (58/96 = 60%), so it is sand (S).
• Fines are < 5%, so grading decides: well-graded sand needs $C_u \ge 6$ and $1 \le C_c \le 3$. Here $C_c = 2.6$ passes, but $C_u = 5 < 6$ fails.

Answer: SP, poorly graded sand (with gravel).

Given

Passing 75 micron $= 62\%$; $w_L = 54\%$; $w_P = 23\%$.

$$I_p = 54 - 23 = 31\%$$

• 62% passes the 75 micron sieve (more than 50%), so the soil is fine-grained.
• $w_L = 54 > 50$: high plasticity (H).
• A-line at $w_L = 54$: $I_p = 0.73(54 - 20) = 24.8\%$. Since $I_p = 31 > 24.8$, the soil is above the A-line: clay (C).

Answer: CH, inorganic clay of high plasticity (fat clay).

Given

Passing 75 micron $= 39\%$; $w_L = 33\%$; $w_P = 18\%$.

$$I_p = 33 - 18 = 15\%$$

• Only 39% is finer than 75 micron (less than 50%), so the soil is coarse-grained, with more than 12% fines.
• Fines classification: A-line at $w_L = 33$: $I_p = 0.73(33-20) = 9.5\%$. $I_p = 15$ lies above the A-line and is greater than 7, so the fines are clayey (C).
• The gravel/sand split is not given. Assuming the coarse fraction is mainly sand (the usual case), the symbol is SC; if gravel dominates it would be GC.

Answer: SC, clayey sand (GC if gravel exceeds sand).