Sampler design parameters and significance

A sampler is judged by four geometric ratios (Hvorslev, IS 1892). Let $D_1$ = inside diameter of cutting edge, $D_2$ = inside diameter of sampling tube, $D_3$ = outside diameter of cutting edge, $D_4$ = outside diameter of sampling tube.

1. Area ratio

$$A_r = \frac{D_3^2 - D_1^2}{D_1^2}\times 100\%$$

It is the ratio of soil displaced by the sampler wall to the area of the sample. A high value means a thick wall, more displacement and more disturbance. For undisturbed samples $A_r \le 10\%$ (up to 20% is tolerated for stiff soils; thin-walled Shelby tube is about 10% or less).

2. Inside clearance

$$C_i = \frac{D_2 - D_1}{D_1}\times 100\%$$

The cutting edge is made slightly smaller than the tube so the sample does not grip the tube wall (reduces friction and drag on the sample) and can expand slightly. Recommended: 0.5 to 3%. Short samples in clay can use about 0.5-1.5%; too large a value lets the sample loosen.

3. Outside clearance

$$C_o = \frac{D_3 - D_4}{D_4}\times 100\%$$

The cutting edge is slightly larger than the outside of the tube so the tube moves through the hole with less friction during driving and withdrawal. Recommended: 0 to 2% (more than this increases area ratio and disturbance).

4. Recovery ratio

$$R = \frac{L}{H}$$

$L$ is the length of the sample recovered and $H$ the length of sampler penetration. It shows how much of the pushed length is actually recovered; $R$ close to 1 means little compression or loss. For an undisturbed sample $R$ should be 96-98% or more (about 0.96 to 1.0).

Also a sharp cutting edge (angle about 10-12 degrees, or less) and slow steady pushing are used.