Purpose and basis of soil classification systems: Select the correct statement(s) about why soils are classified and how common systems are organized.

Introduction / Context:
Soil classification is the first step in geotechnical characterization. It standardizes descriptions to communicate likely behavior and to guide selection of tests and design parameters before detailed laboratory testing is completed.

Given Data / Assumptions:

• Common systems include the Unified Soil Classification System (USCS) and AASHTO.
• Observations include grain-size distribution and plasticity indices.
• Field descriptors (color, structure, consistency) supplement formal symbols.

Concept / Approach:

Classification reduces complex natural variability to practical groups (e.g., GW, CL, CH) using particle size limits (gravel, sand, silt, clay) and plasticity behavior (LL, PL, PI). This supports preliminary correlations for permeability, compressibility, and strength and allows consistent communication among engineers.

Step-by-Step Solution:

Verification / Alternative check:

USCS employs grain-size boundaries and the plasticity chart (A-line) to classify fines; AASHTO uses group index incorporating both fractions.

Why Other Options Are Wrong:

Each individual statement (a–d) is accurate; hence “All the above” is correct.

Common Pitfalls:

Assuming classification alone determines design parameters without confirmatory testing; misinterpreting “silt” and “clay” as mineralogy instead of size/behavior in USCS context.

Final Answer:

All the above