CCIL CCIL - Canadian Council of Independent Laboratories Soil Testing and Geotechnical Analysis Questions and Answers 5 — Questions and Answers
Question 1: In geotechnical engineering, what does the term 'preconsolidation pressure' (σ'p) represent?
- The maximum past effective vertical stress a soil has experienced (Correct answer)
- The current in-situ effective overburden stress
- The stress at which shear failure occurs
- The effective stress at the liquid limit
Correct answer: The maximum past effective vertical stress a soil has experienced
Preconsolidation pressure is the highest effective stress the soil has ever been subjected to, determined from the e-log p' curve using Casagrande's construction.
Question 2: Which CCIL/ASTM test is used to determine the California Bearing Ratio (CBR) of compacted soils in the laboratory?
- ASTM D1883 (Correct answer)
- ASTM D3080
- ASTM D2435
- ASTM D4767
Correct answer: ASTM D1883
ASTM D1883 covers the laboratory CBR test, which measures load-penetration resistance used in pavement subgrade design.
Question 3: What is the significance of the A-line on the Casagrande plasticity chart used in the USCS?
- It separates clays (above) from silts and organic soils (below) at a given liquid limit (Correct answer)
- It marks the boundary between liquid and plastic states
- It identifies the optimum compaction moisture content
- It separates high-plasticity from low-plasticity soils at PI = 7
Correct answer: It separates clays (above) from silts and organic soils (below) at a given liquid limit
The A-line equation PI = 0.73(LL − 20) divides inorganic clays (C soils, plotting above) from silts and organic clays (M/O soils, below).
Question 4: During a Standard Proctor compaction test (ASTM D698), how many layers of soil are compacted into the mold?
- 3 layers, 25 blows each (Correct answer)
- 5 layers, 56 blows each
- 3 layers, 56 blows each
- 5 layers, 25 blows each
Correct answer: 3 layers, 25 blows each
ASTM D698 Method A specifies 3 equal layers compacted with 25 blows of a 5.5-lb hammer dropped 12 inches into a 4-inch diameter mold.
Question 5: What does a negative pore pressure (suction) in an unsaturated soil sample during shearing indicate?
- The soil is dilating, generating negative excess pore pressure (Correct answer)
- The test setup has an air leak
- The sample is over-consolidated beyond the yield surface
- Consolidation is still occurring during shear
Correct answer: The soil is dilating, generating negative excess pore pressure
Dense or heavily over-consolidated soils tend to dilate when sheared, causing negative excess pore pressures that increase effective stress.
Question 6: In a hydrometer analysis, what physical principle is used to determine particle size distribution for fine-grained soils?
- Stokes' Law relating particle settling velocity to particle diameter (Correct answer)
- Bernoulli's principle of fluid pressure variation
- Darcy's Law of flow through porous media
- Archimedes' principle of buoyancy alone
Correct answer: Stokes' Law relating particle settling velocity to particle diameter
Stokes' Law assumes spherical particles settling under gravity through a viscous fluid, relating terminal velocity to particle diameter squared.
Question 7: Which factor most significantly affects the accuracy of a Standard Penetration Test (SPT) N-value in the field?
- Energy efficiency of the hammer delivery system (Correct answer)
- Diameter of the drill casing
- Water table depth relative to the test depth
- Type of drilling fluid used
Correct answer: Energy efficiency of the hammer delivery system
SPT N-values must be corrected to a standard 60% energy efficiency ratio (N60) because hammer systems deliver between 30–95% of theoretical energy.
CCIL CCIL - Canadian Council of Independent Laboratories Soil Testing and Geotechnical Analysis Questions and Answers 5
In geotechnical engineering, what does the term 'preconsolidation pressure' (σ'p) represent?