7 STEPS COMPRISING VISUAL CLASSIFICATION OF SOIL ON SITE

Purpose:

Classify the soil visually.

Standard Reference:

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

Importance:

The first step in any geotechnical engineering project is to identify and describe the condition of the subsurface. For example, as soon as the ground is identified as gravel, the engineer can immediately form some idea on the nature of the problems encountered in a tunnel project. In contrast, other types of design and construction considerations are expected from a soft clay field. Therefore, it is useful to have a systematic process for soil identification 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 gravel and sand. Examples of fine-grained soils are silt and clay. The procedures for visually identifying these two common soil types are described in the following sections.


Device:

magnifying glass (optional)

Identification Process (Step-by-Step)

step 1

Identify soil color (eg brown, brown, grayish brown), odor (if any) and texture (coarse or fine grain).

step 2

Identify the major soil component (>50% by weight) using Table 1 In the form of coarse gravel, fine gravel, coarse sand, medium sand, fine sand or fine sand.

Table 1. Grain Size Distribution
soil component size range familiar example
boulder 12 inches (305 mm) or more bigger than basketball
big coal 3 inch (76 mm) -12 inch (305 mm) Grapefruit
coarse gravel inch (19 mm) – 3 inch (76 mm) orange or lemon
fine gravel 4.75 mm (No. 4 Sieve) – inch (19 mm) grapes or peas
coarse sand 2mm (No.10 Sieve) – 4.75mm (No.4 Sieve) rock salt
medium sand 0.42mm (No. 40 Sieve) – 2mm (No. 10 Sieve) sugar, table salt
fine sand* 0.075mm (No. 200 Sieve) – 0.42mm (No. 40 Sieve) powdered sugar
Fine less than 0.0075 mm (No. 200 sieve) – ,

step 3

Estimate the percentage of all other soil components using Table 1 and the following conditions:

Trace – 0 to 10% by weight

Small – 10 to 20%


Some – 20 to 30%

and – 30 to 50%

(Example: Find fine gravel, little silt, some clay)

step 4

Fig. 1 Size of coarse grained particles
Fig. 1 Size of coarse grained particles

If the major component of the soil is sand or gravel:

Identify the particle distribution. Describe as graded well or graded poorly. Well graded soils have particle sizes in a wide range. Poor grade soil has particles that are roughly the same size.


Identify particle shape (angular, subangular, circular, concave) using figure 1 And Table 2,

Table 2. Criteria for characterizing the size of coarse-grained soil particles
Description Criteria
Angular The particles have relatively flat sides with sharp edges and unpolished surfaces.
subangular The particles are similar to angular details, but their edges are rounded.
underground Particles have roughly flat sides, but well rounded corners and edges.
Round Particles have smoothly curved sides and no edges.

Step-5

If the major components of the soil are finer, then perform the following test:

Dry Strength Test: Mold a sample into a 1/8″ sized ball and allow it to dry. Test the strength of the dried sample by crushing it between fingers. Rate power as none, low, medium, high or very high based on test results as shown Table 3(a),

Table (3a). Criteria for describing dry power
Description Criteria
None The dry sample ball breaks down into powder with slight handling pressure.
Less The dry sample breaks into powder with some pressure with the fingers.
medium The dry specimen breaks or crumbles into pieces with the pressure of the middle finger.
High The dried specimen cannot be broken with finger pressure. The sample will break into pieces between the thumb and a hard surface.
Very high The dried specimen cannot be broken between the thumb and a hard surface.

Delay Test: Sample the consistency of soft putty in your palm. Then observe the reaction during shaking, squeezing (with hands closed) and vigorous tapping. The response is fast, slow or none according to the test results given in Table 3 (b), During the dilution test, vibration condenses the silt and water appears on the surface. Now on squeezing, shear stress is applied on the condensed silt. Dense silt has a tendency to increase or expand in volume due to shear stresses. So the water disappears from the surface. In addition, silt soils have high permeability, so water moves faster. In soil, we see no change, no shiny surface, in other words, no reaction.

Table (3b). Criteria to describe the latency of a soil sample
Description Criteria
None No change is visible in the soil samples.
Slow Water gradually appears during shaking and remains on the surface or water gradually disappears when squeezed.
Fast Water quickly appears on the surface during shaking and quickly disappears when squeezed.

Plasticity (or Toughness) Test: Roll the samples into a thread about 1/8″ in diameter. Twist the thread and twist it repeatedly until the thread breaks at a diameter of 1/8″. Note (a) the pressure required to keep the thread rolling, (b) whether it can support its own weight, (c) whether it can be molded back into a coherent mass, and (d) whether it is difficult to knead. During. Describe plasticity and toughness according to the criteria in Tables 3(c) and 3(d), Low to medium hardness and non-plastic to low plasticity indicate that the soil is silty; Otherwise the soil is smooth.

Table (3c). Criteria for describing soil plasticity
Description Criteria
non plastic A 1/8″ (3-mm) thread cannot be twisted over any water content.
Less The thread is difficult to roll and may not form a cohesive mass if dried beyond the plastic limit.
medium The thread is easier to roll and takes less time to reach the plastic limit. The thread cannot be re-rolled after the plastic limit is reached. The mass tends to crumble when dried beyond the plastic limit.
High To reach the plastic limit, it takes a long time to roll and knead the sample. The thread can be re-twisted and re-worked several times before reaching the plastic limit. A mass may form when the sample dries beyond the plastic limit.

pay attention: The limit of plastic is the amount of water at which clay begins to break down and crumble when rolled into a 1/8″ diameter thread.

Table (3d). Criteria for Describing the Hardness of Soil
Description Criteria
Less Only a slight pressure is needed to roll the thread to the limit of the plastic. The thread and mass are weak and soft.
medium Moderate pressure is required to roll the thread near the plastic limit. The thread and mass have medium hardness.
High Sufficient pressure is needed to roll the thread close to the plastic limit. The thread and mass are very stiff.

Determine soil symbol on the basis of dry strength, tensile strength and hardness Table 4,

Table 4. Identification of Inorganic Fine Grain Soils
clay symbol dry strength latency toughness
ml none or less slow rapid lo or thread cannot be created
chlorine medium to high no one to slow down medium
Maharashtra low to medium no one to slow down low to medium
Chieftain very high from high None High

step rate 6

Identify humidity conditions (dry, moist, wet or saturated) using Table 5,

Table 5. Criteria for describing soil moisture status
Description Criteria
Dry Soil dry to touch, dusty, apparent absence of moisture
Moist the soil is moist, there is little moisture; Soil may begin to retain a molded form
wet the soil is obviously wet; Water appears when the sample is squeezed
soak up Water is easily visible and drains freely from the sample

Step-7

Record the visual classification of soil in the following order:


Colour, major component, minor component, particle distribution and particle size (if major component is coarse-grained), plasticity (if major component is fine-grained), moisture content, soil symbol (if major component 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, medium to fine sand, some silt, trace clay, poorly graded, rounded, saturated.

Examples of fine grained soils:

Soil A: Brown loamy soil, coarse to fine sand, medium plasticity, moist, cl.

Soil B: Trace gray clay silt, fine sand, non-plastic, saturated, ml.

article written by

Pro. Krishna Reddy, UIC

Er. Mukesh Kumar

Photo of author
Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.