SM soil, in the context of soil classification, refers to silty sands and poorly graded sand-silt mixtures. These are coarse-grained soils that contain a significant amount of non-plastic fines (silt) passing through the No. 200 sieve (0.075 mm).
Here's a breakdown:
- Coarse-grained: The soil primarily consists of sand-sized particles.
- Silty: It contains a notable amount of silt, which are fine-grained soil particles larger than clay but smaller than sand. The percentage passing the No. 200 sieve is greater than 12%.
- Poorly Graded: This means the soil lacks a good distribution of particle sizes. A poorly graded soil typically has a narrow range of particle sizes, leading to higher void ratios and potentially weaker engineering properties.
- Sand-Silt Mixtures: This simply emphasizes the composition of the soil being a mixture of sand and silt particles.
Key Characteristics of SM Soils:
- Non-plastic or Slightly Plastic: The silt content generally imparts little to no plasticity to the soil.
- Moderate Permeability: The permeability (ability to transmit water) is typically lower than clean sands but higher than silts or clays.
- Compressibility: SM soils are generally more compressible than clean sands, particularly when loosely compacted.
- Fairly Low Strength: Their strength characteristics are influenced by the amount of silt content, grading, and degree of compaction.
- Susceptible to Frost Heave: The presence of silt can make SM soils susceptible to frost heave in freezing climates.
Engineering Considerations:
The engineering behavior of SM soils is complex and depends on factors such as silt content, density, and confining pressure. Geotechnical engineers need to carefully evaluate these properties for foundation design, slope stability analysis, and other construction-related applications. Proper compaction and drainage are often essential to improve the performance of SM soils.
In summary, SM soils are sandy soils with a significant amount of silt that can influence their engineering properties, requiring careful consideration in construction and geotechnical applications.
