7 STEPS COMPRISING VISUAL CLASSIFICATION OF SOIL ON SITE

How to Classify Soil Visually on site?

Purpose:

Visually classify the soils.

Standard Reference:

ASTM D 2488 – Standard Practice for Description and Identification of Soils (Visual – Manual Procedure)

Significance:

The first step in any geotechnical engineering project is to identify and describe the subsoil condition. For example, as soon as a ground is identified as gravel, engineer can immediately form some ideas on the nature of problems that might be encountered in a tunneling project. In contrast, a soft clay ground is expected to lead to other types of design and construction considerations. Therefore, it is useful to have a systematic procedure for identification of soils even in the planning stages of a project.

Soils can be classified into two general categories: (1) coarse grained soils and (2) fine grained soils. Examples of coarse-grained soils are gravels and sands. Examples of fine-grained soils are silts and clays. Procedures for visually identifying these two general types of soils are described in the following sections.

Equipment:

Magnifying glass (optional)

Identification Procedure (Step – by – Step)

Step-1

Identify the color (e.g. brown, gray, brownish gray), odor (if any) and texture (coarse or fine-grained) of soil.

Step-2

Identify the major soil constituent (>50% by weight) using Table 1 as coarse gravel, fine gravel, coarse sand, medium sand, fine sand, or fines.

Table 1. Grain Size Distribution
Soil Constituent Size Limits Familiar Example
Boulder 12 in. (305 mm) or more Larger than basketball
Cobbles 3 in (76 mm) -12 in (305 mm) Grapefruit
Coarse Gravel ¾ in. (19 mm) – 3 in. (76 mm) Orange or Lemon
Fine Gravel 4.75 mm (No.4 Sieve) – ¾ in. (19 mm) Grape or Pea
Coarse Sand 2 mm (No.10 Sieve) – 4.75 mm (No. 4 Sieve) Rock salt
Medium Sand 0.42 mm (No. 40 Sieve) – 2 mm (No. 10 Sieve) Sugar, table salt
Fine Sand* 0.075 mm (No. 200 Sieve) – 0.42 mm (No. 40 Sieve) Powdered Sugar
Fines Less than 0.0075 mm (No. 200 Sieve) –

Step-3

Estimate percentages of all other soil constituents using Table 1 and the following terms:

Trace – 0 to 10% by weight

Little – 10 to 20%

Some – 20 to 30%

And – 30 to 50%

(Examples: trace fine gravel, little silt, some clay)

Step-4

Fig-1 Shape of coarse grained particles
Fig-1 Shape of coarse grained particles

If the major soil constituent is sand or gravel:

Identify particle distribution. Describe as well graded or poorly graded. Well-graded soil consists of particle sizes over a wide range. Poorly graded soil consists of particles which are all about the same size.

Identify particle shape (angular, subangular, rounded, subrounded) using Figure 1 and Table 2.

Table 2. Criteria for Describing Shape of Coarse-Grained Soil Particles
Description Criteria
Angular Particles have sharp edges and relatively plane sides with unpolished surfaces.
Subangular Particles are similar to angular description, but have rounded edges.
Subrounded Particles have nearly plane sides, but have well-rounded corners and edges.
Rounded Particles have smoothly curved sides and no edges.

Step-5

If the major soil constituents are fines, perform the following tests:

Dry strength test: Mold a sample into 1/8″ size ball and let it dry. Test the strength of the dry sample by crushing it between the fingers. Describe the strength as none, low, medium, high or very high depending on the results of the test as shown in Table 3(a).

Table (3a). Criteria for Describing Dry Strength
Description Criteria
None The dry specimen ball crumbles into powder with the slightest handling pressure.
Low The dry specimen crumbles into powder with some pressure from fingers.
Medium The dry specimen breaks into pieces or crumbles with moderate finger pressure.
High The dry specimen cannot be broken with finger pressure. Specimen will break into pieces between thumb and a hard surface.
Very High The dry specimen cannot be broken between the thumb and a hard surface.

Dilatancy Test: Make a sample of soft putty consistency in your palm. Then observe the reaction during shaking, squeezing (by closing hand) and vigorous tapping. The reaction is rapid, slow or none according to the test results given in Table 3(b). During dilatancy test, vibration densifies the silt and water appears on the surface. Now on squeezing, shear stresses are applied on the densified silt. The dense silt has a tendency for volume increase or dilatancy due to shear stresses. So the water disappears from the surface. Moreover, silty soil has a high permeability, so the water moves quickly. In clay, we see no change, no shiny surface, in other words, no reaction.

Table (3b). Criteria for Describing Dilatancy of a Soil Sample
Description Criteria
None There is no visible change in the soil samples.
Slow Water slowly appears and remains on the surface during shaking or water slowly disappears upon squeezing.
Rapid Water quickly appears on the surface during shaking and quickly disappears upon squeezing.

Plasticity (or Toughness) Test: Roll the samples into a thread about 1/8″ in diameter. Fold the thread and reroll it repeatedly until the thread crumbles at a diameter of 1/8″. Note (a) the pressure required to roll the thread when it is near crumbling, (b) whether it can support its own weight, (c) whether it can be molded back into a coherent mass, and (d) whether it is tough during kneading. Describe the plasticity and toughness according to the criteria in Tables 3(c) and 3(d). A low to medium toughness and non-plastic to low plasticity is the indication that the soil is silty; otherwise the soil is clayey.

Table (3c). Criteria for Describing Soil Plasticity
Description Criteria
Non-plastic A 1/8” (3-mm) thread cannot be rolled at any water content.
Low The thread is difficult to roll and a cohesive mass cannot be formed when drier than the plastic limit.
Medium The thread is easy to roll and little time is needed to reach the plastic limit. The thread cannot be re-rolled after the plastic limit is reached. The mass crumbles when it is drier than the plastic limit.
High Considerable time is needed, rolling and kneading the sample, to reach the plastic limit. The thread can be rerolled and reworked several times before reaching the plastic limit. A mass can be formed when the sample is drier than the plastic limit.

Note: The plastic limit is the water content at which the soil begins to break apart and crumbles when rolled into threads 1/8” in diameter.

Table (3d). Criteria for Describing Soil Toughness
Description Criteria
Low Only slight pressure is needed to roll the thread to the plastic limit. The thread and mass are weak and soft.
Medium Moderate pressure is needed to roll the thread to near the plastic limit. The thread and mass have moderate stiffness.
High Substantial pressure is needed to roll the thread to near the plastic limit. The thread and mass are very stiff.

Based on dry strength, dilatancy and toughness, determine soil symbol based on Table 4.

Table 4. Identification of Inorganic Fine-Grained Soils
Soil Symbol Dry Strength Dilatancy Toughness
ML None or Low Slow to Rapid Low or thread cannot be formed
CL Medium to High None to Slow Medium
MH Low to Medium None to Slow Low to Medium
CH High to Very High None High

Step-6

Identify moisture condition (dry, moist, wet or saturated) using Table 5.

Table 5. Criteria for Describing Soil Moisture Conditions
Description Criteria
Dry Soil is dry to the touch, dusty, a clear absence of moisture
Moist Soil is damp, slight moisture; soil may begin to retain molded form
Wet Soil is clearly wet; water is visible when sample is squeezed
Saturated Water is easily visible and drains freely from the sample

Step-7

Record visual classification of the soil in the following order:

color, major constituent, minor constituents, particle distribution and particle shape (if major constituent is coarse-grained), plasticity (if major constituent is fine-grained), moisture content, soil symbol (if major constituent is fine-grained).

Examples of coarse-grained soils:

Soil 1: Brown fine gravel, some coarse to fine sand, trace silt, trace clay, well graded, angular, dry.

Soil 2: Gray coarse sand, trace medium to fine sand, some silt, trace clay, poorly graded, rounded, saturated.

Examples of fine-grained soils:

Soil A: Brown lean clay, trace coarse to fine sand, medium plasticity, moist, CL.

Soil B: Gray clayey silt, trace fine sand, non-plastic, saturated, ML.

Article written by

Prof. Krishna Reddy, UIC

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