Area Ratio, Inside Clearance, Outside Clearance, Recovery Ratio

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The sample disturbance depends upon the design of the samplers and the method of sampling.
The design features governing the degree of disturbance are

Cutting edge,
Inside wall friction,
Non – return valve.

Area Ratio

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

The penetration resistance of the sampler,
The possibility of entrance of excess soil
Danger of disturbance of sample

The allowable area ratio intended for obtaining undisturbed samples depends on
diameter,
design
method of operation of the sampler.

The area ratio should be preferably be less than 10 %.

The greater area ratio can be tolerated when the sampler is provided with
A stationary piston
The cutting edge having very small angle of taper (𝜽).

Inside clearance

The soil is under great stress as it enters the sampler and has a tendency to laterally expand.

The inside clearance should be large enough to allow a part of lateral expansion to take place

It should not be so large that it permits excessive deformations and causes disturbances of the sample.

For good sampling process

Outside clearance

For good sampling process, the ratio should be within 0 – 2 %.

Guide lines for good Sampling

Minimum inside diameter = 𝑫_𝒊=75𝑚𝑚.

The length (𝐿) should be at least equal to (𝑡ℎ𝑒 𝑖𝑛𝑡𝑒𝑛𝑑𝑒𝑑 𝑙𝑒𝑛𝑔𝑡ℎ+100𝑚𝑚) for residual soils.
For soft soils depending upon the sensitivity, (𝑳/𝑫_𝒊 ) ratios are given in table

The cutting edge angle should be within 20°.

The tube should be uniform and should not have any bump or irregularities.
The inside of the tube should be clean and smooth.

Recovery Ratio

It represents the disturbance of the soil sample.
For good sampling the recovery ratio should be 96 to 98 %.
Wall friction can be reduced by suitable inside clearance, smooth finish and oiling.
The non – returned wall should have large orifice to allow air and water to escape.

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