Soil Classification (USCS) – Symbols for Silty Sand and Clayey Sand Under the Unified Soil Classification System (USCS), which pair of group symbols corresponds to silty sand and clayey sand, respectively?

Introduction / Context:
The Unified Soil Classification System (USCS) uses two-letter symbols to classify soils based on grain size and plasticity. Proper identification is crucial for estimating engineering behavior such as permeability, strength, and compaction response in geotechnical design.

Given Data / Assumptions:

• Coarse-grained soils with fines content > 5% are further subdivided by plasticity behavior of fines.
• “M” denotes silt (non-plastic to low-plastic fines), “C” denotes clay (plastic fines).
• “S” denotes sand as the primary coarse fraction.

Concept / Approach:

USCS combines primary grain-size letter with fines-type letter: SM for silty sand (sand with silt fines), SC for clayey sand (sand with clay fines). These classes differ in plasticity index and Atterberg limits, which drive drainage and strength characteristics.

Step-by-Step Solution:

Verification / Alternative check:

Plasticity chart (PI vs LL) confirms whether fines are silt-like (M) or clay-like (C); classification follows accordingly.

Why Other Options Are Wrong:

• SS, CS, MS: Not USCS-standard pairs; “SS/MS” are not valid group symbols.
• SM and CS: “CS” is not a standard USCS symbol; correct is SC.

Common Pitfalls:

Misidentifying fines type without performing Atterberg limit tests; using local nomenclature instead of USCS symbols.

Final Answer:

SM and SC