Residential Building Inspector B1Footings and FoundationsMedium
A soil report for a new residential construction identifies the soil as 'fat clay' with a high plasticity index. Based on the Unified Soil Classification System (USCS), which of the following symbols BEST represents this soil type?
- ASP
- BSM
- CML
- DCH
Show answer & explanationAnswer & explanation
Correct answer: D. CH
The Unified Soil Classification System (USCS) uses specific symbols. 'Fat clay' with a high plasticity index is classified as 'CH,' where 'C' denotes clay and 'H' denotes high plasticity. 'SP' is poorly graded sand, 'SM' is silty sand, and 'ML' is lean silt.
Why the other options are wrong
- A. SP refers to poorly graded sand.
- B. SM refers to silty sand.
- C. ML refers to inorganic silts, very fine sands, or lean clays of low plasticity.
USCS Clay Classification
The Unified Soil Classification System (USCS) categorizes fine-grained soils like clay based on their plasticity, with 'CH' representing high plasticity (fat clay) and 'CL' representing low plasticity (lean clay).
- USCS uses a two-letter symbol system.
- First letter denotes primary soil type (G=Gravel, S=Sand, M=Silt, C=Clay, O=Organic).
- Second letter denotes gradation or plasticity (W=Well graded, P=Poorly graded, M=Silty, L=Low plasticity, H=High plasticity).
Memory trick: Gravel, Sand, Silt, Clay, know their letter, come what may.