In order to obtain a good quality of undisturbed soil the area ratio of the sampling tube should be

Types of sample:

Undisturbed sample:

If the soil structure, moisture content & mineral content remains the same while sampling, the sample is called an undisturbed sample.

Disturbed sample:

These are two types

a) Representative sample:

If only soil structure gets modified but moisture content & mineral content remains the same while sampling, the sample is called a representative sample.

b) Non-representative sample:

If all the 3 i.e. soil structure, mineral content, moisture content get modified by sampling, samples are known as non-representative samples.

Degree of disturbance:

The degree of disturbance of the sample collected by various methods can be expressed by a term called the area ratio, which is given by

\({{\rm{A}}_{\rm{r}}} = \frac{{{\rm{D}}_0^2 - {\rm{D}}_i^2}}{{{\rm{D}}_i^2}}\)

D_i = inner diameter of cutting edge

D_o = outer diameter of cutting edge

A soil sample generally can be considered undisturbed if the area ratio is less than or equal to 10%. The area ratio for the standard spoon sampler is nearly 10% and for Shelby, tubes are 13.5 %.