Code of practice for calculation of settlements of foundations, Part 1: Shallow foundations subjected to symmetrical static vertical loads
1976 Edition

IS 8009 Part 1 (1976) outlines practical approaches to estimate vertical stresses under shallow foundations subjected to symmetrical static vertical loads. It recommends employing Boussinesq’s theory to determine stress distribution beneath point or uniformly loaded areas, using influence charts or factors to calculate vertical stress increments at a given depth beneath the footing. Typical formulas involve evaluating stress at depth by considering the radial distance from the load axis and the depth below the foundation base. This methodology supports subsequent settlement assessments through soil compressibility parameters and also includes provisions for computing time-dependent settlements.

Settlement computations under IS 8009 Part 1 distinguish between soil categories. For cohesionless soils like sands, settlement is treated as immediate elastic deformation, estimated semi-empirically using data from static cone penetration tests (SCPT), dynamic penetration tests (DPT), and plate load tests, given the difficulty of laboratory compressibility tests. For clayey soils, particularly those sandwiched between cohesionless layers or rock, settlement is calculated based on consolidation theory, considering compressibility parameters, drainage conditions, initial effective stress, and overconsolidation ratio. This differentiated approach ensures settlement estimates accurately reflect the soil’s mechanical response.

The static cone penetration test (SCPT) is integral for evaluating settlements in cohesionless soils where direct laboratory compressibility testing is impractical. As per IS 8009 Part 1, SCPT data—conducted following IS 4968 (Part III)-1971—provides a profile of cone resistance with depth. This profile is segmented into layers of relatively uniform resistance, and each layer’s compressibility constant is derived accordingly. Settlement for each layer is estimated based on the increase in stress and soil properties, and the total settlement is obtained by summing these layer-wise contributions. This method offers a practical, semi-empirical means to estimate immediate settlements from in-situ soil characteristics.

IS 8009 Part 1 accounts for preloading and overconsolidation by incorporating the soil's stress history into settlement calculations. Initial effective stress and pore pressure, reflecting prior loading conditions, are fundamental to establishing the starting point for settlement estimation. The code identifies several preloading scenarios—simple static, residual hydrostatic, artesian, and overconsolidated soils—and provides formulas to calculate settlement accordingly. For non-precompressed clays, settlement is computed using the compression index and logarithmic stress ratio. Overconsolidated soils require adjustments using recompression indices for stress increments below the preconsolidation pressure. Accurate data from consolidation tests and knowledge of groundwater conditions are essential for reliable assessments.

The code assumes shallow foundations subjected to symmetrical static vertical loads and excludes failures caused by excessive or catastrophic loading. It considers settlements due to immediate elastic deformation and primary consolidation but does not provide theoretical solutions for secondary compression, which must be addressed by engineering judgment especially in organic or highly plastic soils. The analysis presumes soil homogeneity, isotropy, and elastic behavior for stress calculations, though real soils may deviate from these conditions, making results approximate. It also assumes uniform settlement is generally harmless structurally but could affect utilities. Engineers should be aware of these constraints and adjust foundation design if computed settlements exceed permissible limits.