ASBOG - Association of State Boards of Geology Engineering Geology Questions and Answers 2 — Questions and Answers
Question 1: What is the primary purpose of a Standard Penetration Test (SPT) in geotechnical investigation?
- To measure in-situ permeability of aquifer materials
- To determine soil bearing capacity and consistency by counting blows per foot (Correct answer)
- To collect undisturbed soil samples for laboratory testing
- To measure pore water pressure in saturated soils
Correct answer: To determine soil bearing capacity and consistency by counting blows per foot
The SPT measures soil resistance by counting the number of hammer blows required to drive a split-spoon sampler 12 inches (the N-value), which correlates to soil consistency, relative density, and bearing capacity.
The Standard Penetration Test is one of the most widely used in-situ tests in geotechnical engineering. A 140-lb hammer is dropped 30 inches to drive a split-barrel sampler 18 inches; blows for the first 6 inches are discarded, and blows for the next 12 inches are recorded as the N-value. The N-value correlates with soil relative density (for sands) or consistency (for clays) and is used in bearing capacity, liquefaction potential, and foundation design calculations.
Question 2: Rock mass quality classification using the Q-system considers which of the following parameters?
- Uniaxial compressive strength and elastic modulus
- Rock Quality Designation (RQD), joint set number, and stress reduction factor (Correct answer)
- Porosity, permeability, and cementation
- Mineral hardness and cleavage orientation
Correct answer: Rock Quality Designation (RQD), joint set number, and stress reduction factor
The Q-system by Barton uses RQD, number of joint sets (Jn), joint roughness (Jr), joint alteration (Ja), joint water reduction (Jw), and stress reduction factor (SRF) to classify rock mass quality for tunneling.
The Q-system (Rock Tunnelling Quality Index) was developed by Barton, Lien, and Lunde for underground excavation design. The Q-value is calculated as (RQD/Jn) × (Jr/Ja) × (Jw/SRF). Values range from 0.001 (exceptionally poor) to 1000 (exceptionally good). The system is used to recommend support measures for underground openings such as tunnels and caverns. It was developed empirically from over 200 case histories.
Question 3: A geologist observes tension cracks at the head of a slope and bulging at the toe. What type of slope failure is most likely developing?
- Rock fall
- Debris flow
- Translational landslide
- Rotational slump (Correct answer)
Correct answer: Rotational slump
Tension cracks at the head and toe bulging are classic precursors of a rotational slump, where a soil or weak rock mass rotates along a curved (concave) failure surface.
Rotational slumps (or rotational landslides) occur along curved, spoon-shaped failure surfaces, most commonly in cohesive soils such as clays. The mass rotates backward as it moves, creating a scarp with tension cracks at the top and compression ridges or toe bulging at the base. They are common in oversteepened cut slopes, river banks, and coastal cliffs. Remediation typically involves drainage, slope regrading, and retaining structures.
Question 4: What does the plasticity index (PI) of a fine-grained soil indicate?
- The grain size distribution across sieve sizes
- The range of moisture content over which the soil behaves plastically (Correct answer)
- The maximum dry density of the soil
- The coefficient of consolidation
Correct answer: The range of moisture content over which the soil behaves plastically
The plasticity index equals the liquid limit minus the plastic limit and represents the range of water content over which a soil behaves as a plastic material, indicating clay content and activity.
The plasticity index (PI = LL - PL) is a fundamental Atterberg limit parameter for fine-grained soils. A high PI indicates a soil with high clay content (especially swelling clays like smectite) that can absorb large amounts of water before flowing. The Unified Soil Classification System (USCS) uses the PI along with the liquid limit to distinguish between CH (high plasticity clay), CL (low plasticity clay), MH, and ML soils. PI is critical for assessing swell potential, slope stability, and compaction characteristics.
Question 5: In the context of dam site investigations, what is the primary concern when a foundation contains soluble carbonate rocks?
- Excessive differential settlement from compression
- Piping and internal erosion through fine-grained seams
- Seepage and potential dam failure due to karst dissolution features (Correct answer)
- Seismic amplification in the soft limestone
Correct answer: Seepage and potential dam failure due to karst dissolution features
Carbonate rocks are susceptible to dissolution by groundwater, creating karst features (sinkholes, caves, conduits) that can cause catastrophic leakage and dam failure if not properly grouted or avoided.
Karst terrains pose severe challenges for dam construction because the dissolution of carbonate minerals (calcite, dolomite) over geologic time creates a network of sinkholes, caves, and conduits that can provide preferential seepage paths under or around a dam. Several dam failures have been attributed to karst, including the Vajont Dam in Italy (though that was a landslide) and Kirindi Oya Dam in Sri Lanka. Site investigations in karst must include extensive borehole programs, dye tracing, and geophysical surveys to identify and treat cavities with grout curtains.
Question 6: Which laboratory test is used to determine the shear strength parameters (cohesion c and friction angle φ) of a soil sample under controlled drainage conditions?
- Proctor compaction test
- Hydrometer analysis test
- Triaxial compression test (Correct answer)
- Consolidation (oedometer) test
Correct answer: Triaxial compression test
The triaxial compression test applies a confining pressure and axial load to a cylindrical soil specimen, allowing measurement of shear strength under drained or undrained conditions to determine c and φ.
The triaxial compression test is the most versatile shear strength test in geotechnical engineering. A cylindrical specimen is placed in a cell filled with fluid at a controlled confining pressure, then loaded axially to failure. Depending on drainage conditions, three variants exist: unconsolidated-undrained (UU), consolidated-undrained (CU), and consolidated-drained (CD). The results are used to construct Mohr-Coulomb failure envelopes, yielding the effective friction angle (φ') and cohesion intercept (c') critical for slope stability and foundation design.
ASBOG - Association of State Boards of Geology Engineering Geology Questions and Answers 2
What is the primary purpose of a Standard Penetration Test (SPT) in geotechnical investigation?