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Field Soil Identification Guide

This document discusses methods for identifying soil composition and types in the field. It describes the particles that make up soil including clay, silt, sand and gravel. Clay particles are small platelets that attract water molecules, giving clay its cohesive properties. Soil containing over 15% clay will behave like clayey soil. The document then outlines simple, quick field tests to identify soils including settling tests to distinguish sand, silt and clay, and methods to differentiate silt from clay based on texture, shine when wet, and appearance of cut surfaces. Field tests are also described to determine consistency of cohesive soils. Color can indicate soil origin and deposition conditions. Preliminary field identification allows estimation of bearing capacity from tables to inform detailed

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477 views3 pages

Field Soil Identification Guide

This document discusses methods for identifying soil composition and types in the field. It describes the particles that make up soil including clay, silt, sand and gravel. Clay particles are small platelets that attract water molecules, giving clay its cohesive properties. Soil containing over 15% clay will behave like clayey soil. The document then outlines simple, quick field tests to identify soils including settling tests to distinguish sand, silt and clay, and methods to differentiate silt from clay based on texture, shine when wet, and appearance of cut surfaces. Field tests are also described to determine consistency of cohesive soils. Color can indicate soil origin and deposition conditions. Preliminary field identification allows estimation of bearing capacity from tables to inform detailed

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FIELD IDENTIFICATION OF SOILS

Composition and Types of Soils

Soil grains consists of inert rock materials (cobble, gravel, sand and silt) often
combined with significant amounts of clay (say more than 5%). While inert silt
grains may be angular or rounded (thus contributing to greater or less angle of
internal friction), particles of clay are small platelets with negative charges on
both faces which attract the positively charged ends of water molecules. Thus
bond is responsible for the cohesion ends of water molecules. This bond is
responsible for the cohesion “C” of clay.

Silt or sand with appreciable amounts of clay (say more than 15%) behaves like
clayey soil since the permeability of clay is of the order of 10-7 cm/s compared
to 10cm/s for sand. This capacity of the clay to hold the water molecules for
long even when the pressure is applied on the soil, greatly influences its
behaviour, i.e. shear strength, compressibility and permeability.

Simple and Quick Methods of Field Identification of Soils:


1. Fine sand is differentiated from silt by placing a spoonful of soil in a glass
of jar or test tube, mixing with water and shaking it to a suspension. Sand
settles first, followed by silt which may take about five minutes. This test may
also be sued for clay which takes more than 10 minutes to start settling. The
percentages of clay, silt and sand are assessed by observing the depths of the
sediments.

2. Silt is differentiated from clay as follows:

a) Clay lumps are more difficult to crush with fingers than silt. When moistened,
the soil lump surface texture is felt with the finger. If it is smooth, it is clay, if
rough it is silt.

b) A ball of the soil is formed and shaken horizontally on the palm of the hand.
If the material becomes shiny from water coming to the surface, it is silt.

c) If soil containing appreciable percent of clay is cut with a knife, the cut
surface appears lustrous. In case of silt, the surface appears dull.

3) Field indication for the consistency of cohesive soils are as follows:

• Stiff consistency: Cannot be moulded with the finger.


• Medium consistency: Can be moulded by the fingers on strong
pressure. Readily indented with thumb nail.
• Soft consistency: Easily moulded with the fingers.

4) Colour of the soil indicates its origin and the condition under which it was
deposited. Sand and gravel deposits may contain lenses of silt, clay or even
organic deposits. If so, the presumptive bearing capacity is reduced.

Based on the field identification of the soil, the presumptive bearing capacity of
the soil can be guessed by referring to Table-2 of IS:1904 – 1986. The
objectives of preliminary soil investigations are to drawn up an appropriate
program for detailed soil investigation and to examine the sketch plans and
preliminary drawings prepared by the Architect from the point of suitability of
the proposed structure.

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