--- title: "Sampler design parameters and significance" subject: "Foundation Engineering" chapter: 2 key: "semester_5/foundation_engineering:sampler-design-parameters" source: https://www.bctengineer.com/q/semester_5/foundation_engineering/sampler-design-parameters site: BCT Engineering --- # Sampler design parameters and significance Foundation Engineering · Chapter 2: Soil Exploration - Asked in: 2080 Bhadra, 2+4 marks; 2079 Bhadra, 4 marks; 2075 Chaitra, 6+2 marks; 2072 Chaitra, 5 marks; 2076 Chaitra, 2 marks; 2081 Bhadra, 4 marks (asked 6 times) - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/sampler-design-parameters - Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-2 ## Question What are the sampler design parameters (area ratio, inside clearance, outside clearance, recovery ratio)? Explain the physical significance of each, with the recommended values to obtain least disturbed (undisturbed) samples. ## Short answer Area ratio (displaced soil area/sample area) must be ≤10% to limit disturbance. Inside clearance (tube expansion gap) of 0.5-3% reduces friction. Outside clearance (cutting edge oversize) 0-2% eases driving. Recovery ratio (sample length/penetration) should exceed 96% for a good undisturbed sample. ## Answer 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). | Parameter | Value for undisturbed sample | |---|---| | Area ratio | $\le 10\%$ | | Inside clearance | 0.5 to 3% | | Outside clearance | 0 to 2% | | Recovery ratio | $\ge 96\%$ | Also a sharp cutting edge (angle about 10-12 degrees, or less) and slow steady pushing are used. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The area ratio of a sampler is A. (D2 - D1)/D1 x 100% B. (D3^2 - D1^2)/D1^2 x 100%, the soil displaced relative to the sample area C. (D3 - D4)/D4 x 100% D. L/H Answer: B. Area ratio measures wall volume displaced; high values mean more disturbance. (Key point: Area ratio) 2. For undisturbed samples the area ratio should be A. at least 30% B. 10% or less C. about 50% D. exactly 100% Answer: B. Thin walls displace less soil; Shelby tubes are about 10% or less. (Key point: Area ratio limit) 3. Why is the cutting edge made slightly smaller than the sampling tube (inside clearance)? A. To increase the area ratio B. To make the sample compress C. To reduce friction and drag so the sample does not grip the tube wall D. To make driving harder Answer: C. Inside clearance (0.5-3%) lets the sample enter and expand slightly with less wall friction. (Key point: Inside clearance) 4. Which pair of values is recommended for an undisturbed sample? A. Outside clearance 10% and recovery 50% B. Outside clearance 5% and recovery 80% C. Outside clearance 0-2% and recovery below 90% D. Outside clearance 0-2% and recovery ratio 96% or more Answer: D. Outside clearance should be small (0-2%) and recovery ratio L/H should be about 0.96-1.0. 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