ASBOG - Association of State Boards of Geology Engineering Geology Questions and Answers — Questions and Answers
Question 1: A geotechnical investigation for a proposed building site identifies a fine-grained soil with a Liquid Limit (LL) of 65 and a Plastic Limit (PL) of 20. According to the Unified Soil Classification System (USCS), how would this soil be classified and what is its Plasticity Index (PI)?
- PI = 45, CH (High-plasticity clay) (Correct answer)
- PI = 85, MH (High-plasticity silt)
- PI = 45, CL (Low-plasticity clay)
- PI = 20, ML (Low-plasticity silt)
Correct answer: PI = 45, CH (High-plasticity clay)
The Plasticity Index (PI) is calculated as the Liquid Limit (LL) minus the Plastic Limit (PL). In this case, PI = 65 - 20 = 45. According to the USCS plasticity chart, a soil with a Liquid Limit greater than 50 and a PI of 45 plots above the 'A-line', classifying it as a high-plasticity clay (CH).
Question 2: An engineering geologist calculates the Rock Quality Designation (RQD) from a 150 cm long core run. The lengths of sound core pieces greater than 10 cm are 25 cm, 15 cm, 30 cm, and 20 cm. What is the RQD value and what does it indicate about the rock mass quality?
- 30%, Poor
- 60%, Fair (Correct answer)
- 90%, Excellent
- 100%, Very Poor
Correct answer: 60%, Fair
RQD is calculated by summing the lengths of all sound core pieces longer than 10 cm and dividing by the total length of the core run, then multiplying by 100. Sum of pieces >10 cm = 25 + 15 + 30 + 20 = 90 cm. Total core run length = 150 cm. RQD = (90 cm / 150 cm) * 100 = 60%. An RQD value between 50% and 75% is generally classified as 'Fair' quality rock.
Question 3: Which of the following site conditions is MOST susceptible to liquefaction during a strong earthquake?
- Dense, well-graded gravels above the water table
- Overconsolidated clay with high plasticity
- Loose, saturated, uniformly-graded fine sand (Correct answer)
- Fractured, competent bedrock
Correct answer: Loose, saturated, uniformly-graded fine sand
Liquefaction requires a specific combination of conditions: (1) loose, granular soil (like uniformly-graded or 'poorly-sorted' sand/silt), (2) saturation with groundwater, and (3) strong seismic shaking. The loose packing allows for pore pressure to build up rapidly during shaking, causing the soil to lose strength and behave like a liquid. Dense gravels, clays, and bedrock do not have this structure and are not susceptible to liquefaction.
Question 4: A geologist is investigating a site for a new highway cut in a hillside. Which of the following conditions would be most likely to DECREASE the stability of the slope?
- Installing rock bolts and drainage systems
- Bedding planes dipping steeply into the hillside, away from the cut face
- Excavating material at the toe of the slope to steepen it (Correct answer)
- Planting deep-rooted vegetation across the slope surface
Correct answer: Excavating material at the toe of the slope to steepen it
Excavating material at the toe of a slope removes support for the overlying material, which increases the driving forces relative to the resisting forces, thereby decreasing stability. Installing drainage and rock bolts, having bedding that dips away from the cut, and adding vegetation are all methods used to increase slope stability.
Question 5: A residential development is experiencing significant foundation damage, including cracked slabs and sticking doors, due to seasonal volume changes in the underlying soil. This type of damage is most commonly associated with soils rich in which clay mineral?
- Kaolinite
- Illite
- Chlorite
- Montmorillonite (Correct answer)
Correct answer: Montmorillonite
Montmorillonite, a member of the smectite group, is the primary mineral responsible for expansive soils. Its crystal structure allows for significant amounts of water to be absorbed between the mineral layers, causing the soil to swell dramatically when wet and shrink when dry. This shrink-swell behavior exerts immense pressure on foundations, leading to structural damage.
Question 6: During a site investigation for a heavy industrial facility founded on rock, the primary objective is to assess the quality, strength, and discontinuity spacing of the bedrock in detail. Which of the following investigation techniques would provide the most direct and quantitative data for this purpose?
- Seismic refraction survey
- Electrical resistivity imaging
- Oriented rock coring and geotechnical logging (Correct answer)
- Excavation of shallow test pits with a backhoe
Correct answer: Oriented rock coring and geotechnical logging
Oriented rock coring provides a physical sample of the bedrock that allows for direct observation and laboratory testing. Geotechnical logging of the core yields quantitative data on rock type, weathering, fracture frequency and orientation, and Rock Quality Designation (RQD). While seismic and resistivity surveys are useful for mapping bedrock depth and general quality, they are indirect methods. Test pits are generally too shallow to adequately characterize bedrock for a heavy foundation.
ASBOG - Association of State Boards of Geology Engineering Geology Questions and Answers
A geotechnical investigation for a proposed building site identifies a fine-grained soil with a Liquid Limit (LL) of 65 and a Plastic Limit (PL) of 20.
According to the Unified Soil Classification System (USCS), how would this soil be classified and what is its Plasticity Index (PI)?