A satisfactory design of a foundation depends upon the accuracy with which the various soil parameters required for the design are obtained. The accuracy of the soil parameters depends upon the accuracy with which representative soil samples are obtained from the field.
Sampling is carried out in order that soil and rock description, and laboratory testing can be carried out.
Laboratory tests typically consist of:
Index tests (for example, specific gravity, water content)
Classification tests (for example, Atterberg’s limit tests on clayey soil); and
Tests to determine engineering design parameters (for example strength, compressibility, and permeability).
Factors to be considered while sampling soil
i) Samples should be representative of the ground from which they are taken.
ii) They should be large enough to contain representative particles sizes, fabric, and fissuring and fracturing.
iii) They should be taken in such a way that they have not lost fractions of the in situ soil (for example, coarse or fine particles) and, where strength and compressibility tests are planned, they should be subject to as little disturbance as possible.
Type of Soil Samples
Non-Representative soil samples are those in which neither the in-situ soil structure, moisture content nor the soil particles are preserved.
They are not representative
They cannot be used for any tests as the soil particles either gets mixed up or some particles may be lost.
Samples that are obtained through wash boring or percussion drilling are examples of non-representative samples
Disturbed soil samples
Disturbed soil samples are those in which the in-situ soil structure and moisture content are lost, but the soil particles are intact.
They are representative
They can be used for grain size analysis, liquid and plastic limit, specific gravity, compaction tests, moisture content, organic content determination and soil classification test performed in the lab
e.g., Examples of such sample are those obtained through cuttings while auguring, grab, split spoon (SPT), etc.
Undisturbed soil samples
Undisturbed soil samples are those in which the in-situ soil structure and moisture content are preserved.
They are representative and also intact
These are used for consolidation, permeability or shear strengths test (Engineering properties)
More complex jobs or where clay exists
In sand is very difficult to obtain undisturbed sample
Obtained by using Shelby tube (thin wall), piston sampler, surface (box), vacuum, freezing, etc.,