Settlement

Settlement is the vertical downward movement of a foundation or supported structure due to consolidation, compression, or stress redistribution in the underlying soil. Per IS 1904:1986, settlement is classified as: (a) Immediate (elastic) settlement — occurs as soon as load is applied, primarily in sandy/granular soils, typically 70-90% complete during construction; (b) Consolidation settlement — slow time-dependent settlement in cohesive soils (clays, silts) due to expulsion of pore water, typically 20-50 years to reach 90% completion; (c) Secondary compression settlement — additional time-dependent compression in certain organic and high-plasticity soils, may continue for centuries.

Design limits per IS 1904 Cl. 5: (a) total settlement ≤ 75 mm for residential, ≤ 50 mm for industrial. (b) Differential settlement ≤ 1/300 of column spacing for residential frame, ≤ 1/500 for buildings with sensitive partitions. The differential is more critical than total — uniform settlement causes minor architectural issues; differential causes structural distress, cracking, and sometimes failure. (c) Settlement rate is not specified explicitly but should not exceed 5-10 mm/year after construction; sustained higher rates indicate ongoing consolidation that may compromise the structure.

Settlement computation methods: (1) Elastic settlement — Boussinesq stress distribution + soil modulus E from triaxial or pressuremeter test; (2) Consolidation settlement — Terzaghi 1-D consolidation equation with compression index Cc and pre-consolidation pressure σpʹ from oedometer test (IS 2720 Part 15); (3) Plate load test — direct measurement on a small plate, scaled up to foundation size by Bowles or Terzaghi correlations; (4) Standard Penetration Test (SPT) correlations — quick first-pass estimates for sandy soils per IS 6403 Cl. 5. Modern Indian practice uses finite-element analysis (PLAXIS, ABAQUS) for complex foundations, but routine isolated and combined footings are still designed via classical methods.