Glossary of terms and symbols relating to soil engineering
1972 Edition

IS 2809 (1972) establishes a detailed set of soil property definitions vital for engineering applications. These include dry density (mass of solids per unit volume excluding air), moisture content (ratio of water weight to dry soil weight), and the zero air voids curve which relates dry density to moisture content at full saturation. Other critical terms comprise subsoil (the soil beneath topsoil layers), porosity (void volume relative to total volume), void ratio (void volume over solids volume), degree of saturation (water volume to void volume ratio), and plasticity indices such as the plasticity index and liquid limit. These parameters are fundamental to soil classification, compaction control, bearing capacity, and settlement predictions. The zero air voids curve formula is given by ρ_d = (G × ρ_w) / (1 + w × G), where ρ_d is dry density, G is soil solids' specific gravity, ρ_w is water density, and w is moisture content.

According to IS 2809, soil failure by shear is categorized primarily into general shear failure and local shear failure. General shear failure involves mobilization of ultimate soil strength throughout the entire potential slip surface, characterized by a distinct rupture plane and significant soil displacement. Local shear failure, on the other hand, is where ultimate strength is engaged only in localized zones along the slip surface, resulting in less distinct failure planes and more gradual soil yielding. This differentiation is vital for slope stability assessment and foundation design strategies.

IS 2809 standardizes several key symbols and terms related to soil moisture. The symbol w(D) denotes moisture content or water content, representing the percentage ratio of water weight to soil solids weight, calculated as w = (W_w / W_s) × 100, where W_w is water weight and W_s is solids weight. Hygroscopic water content is indicated by WH(D), referring to water held in air-dried soil. Hygroscopic capacity, or hygroscopic coefficient, symbolized as w.(D), defines the ratio of water absorbed by dry soil exposed to a saturated atmosphere at a specified temperature relative to the oven-dried soil weight. These standardized terms ensure consistency in reporting soil moisture measurements.

The IS 2809 glossary clarifies consolidation and compressibility by defining consolidation as the gradual volume reduction of saturated soil under applied stress due to pore water expulsion. It further distinguishes initial consolidation as the rapid volume decrease caused by gas compression or escape before primary consolidation begins. Primary consolidation refers to the volume decrease from sustained load as pore water is expelled, transferring stress from water to soil solids. Secondary consolidation describes the continued slow volume reduction over time attributable to soil structure adjustment under constant load. These classifications are essential for understanding time-dependent soil settlement relevant to foundation engineering.

IS 2809 defines an undisturbed soil sample as one collected using techniques that preserve the soil's natural structure and moisture content with minimal alteration, ensuring reliable representation of in-situ conditions. Conversely, a disturbed sample is one where the soil's structure or moisture has been modified during collection, potentially impacting the accuracy of test results. The standard aligns with ASTM D 653-67 for soil and rock mechanics terminology to maintain international consistency. Proper distinction between these sample types is critical for accurate soil behavior evaluation in foundation design.