LARE LARE Soils and Site Conditions 2 — Questions and Answers
Question 1: Which soil pH range is considered optimal for most landscape plants in the US?
- 4.0–5.0 (strongly acidic)
- 5.5–7.0 (slightly acidic to neutral) (Correct answer)
- 7.5–8.5 (moderately alkaline)
- 8.5–9.5 (strongly alkaline)
Correct answer: 5.5–7.0 (slightly acidic to neutral)
A pH of 5.5–7.0 allows maximum nutrient availability and supports the widest range of landscape plant species in US conditions.
Question 2: What is 'hydric soil,' and why is its presence significant on a project site?
- Soil amended with hydroponics nutrients for intensive planting
- Soil that forms under saturated conditions, indicating potential wetland jurisdiction (Correct answer)
- Highly porous sandy soil used for drainage layer construction
- Soil with high clay content that becomes plastic when wet
Correct answer: Soil that forms under saturated conditions, indicating potential wetland jurisdiction
Hydric soils form in anaerobic, saturated conditions and are one of three indicators used to delineate wetlands under federal jurisdiction, triggering permitting requirements.
Question 3: In slope stability analysis, which factor MOST increases the risk of soil failure on a cut slope?
- Presence of native loam topsoil in the upper horizon
- Seepage of water parallel to the slope face through a permeable layer (Correct answer)
- Compacted subsoil with high internal friction angle
- Dense root networks from established vegetation
Correct answer: Seepage of water parallel to the slope face through a permeable layer
Subsurface water seeping parallel to a slope reduces effective stress and pore water pressure equilibrium, dramatically increasing the risk of shallow translational landslides.
Question 4: What does 'shrink-swell potential' refer to in expansive clay soils, and how does it affect site design?
- The tendency of clay to erode under high-velocity runoff conditions
- Volume changes in clay soil as moisture content changes, causing foundation and pavement movement (Correct answer)
- The rate at which clay soils consolidate under structural loads
- The capacity of clay to retain nutrients for plant growth
Correct answer: Volume changes in clay soil as moisture content changes, causing foundation and pavement movement
Expansive clays (high shrink-swell potential) expand when wet and shrink when dry, causing significant heaving and cracking of pavements, foundations, and retaining structures.
Question 5: Which test method is commonly used on-site to quickly estimate soil drainage for bioretention basin design?
- Sieve analysis (gradation test)
- In-situ percolation test (falling head permeability) (Correct answer)
- Atterberg limits test
- Standard penetration test (SPT)
Correct answer: In-situ percolation test (falling head permeability)
An in-situ falling-head percolation test measures how quickly water infiltrates the native soil, providing the infiltration rate needed to size bioretention cells and rain gardens.
Question 6: What is 'topsoil stripping and stockpiling,' and why is it specified in landscape grading documents?
- Removing contaminated fill and disposing of it off-site before grading begins
- Removing the organic-rich A horizon before grading and replacing it after rough grading is complete (Correct answer)
- Importing topsoil from off-site sources to build up low areas before planting
- Stabilizing stripped areas with erosion control blankets during construction
Correct answer: Removing the organic-rich A horizon before grading and replacing it after rough grading is complete
Stripping and stockpiling topsoil before mass grading preserves the biologically active organic layer so it can be redistributed over finished grades to support plant establishment.
LARE LARE Soils and Site Conditions 2
Which soil pH range is considered optimal for most landscape plants in the US?