Safe Bearing Capacity (SBC)

Safe Bearing Capacity (SBC) is the maximum soil pressure that a foundation can sustain without (a) shear failure with adequate factor of safety (typically FoS = 2.5 to 3.0 against ultimate bearing capacity) AND (b) settlement exceeding permissible limits (typically 25-50 mm). SBC is the design-input value for sizing footings — engineers divide the service load by SBC to get the required footing area. IS 6403:1981 (now revised in IS 6403:2024) governs SBC computation; IS 1904:1986 covers settlement and design criteria.

Three determination methods: (1) Plate Load Test (PLT) per IS 1888:1982 — a calibrated steel plate (300/450/600/750 mm) is loaded incrementally and settlement measured; ultimate bearing capacity = 1.5× the load corresponding to the rapid-settlement onset, divided by FoS. (2) SPT correlation per IS 6403 Cl. 5 — Standard Penetration Test N-values are correlated to allowable bearing pressure via Terzaghi-Peck or Meyerhof equations, accounting for foundation width and depth. (3) Triaxial / direct shear test per IS 2720 on extracted samples — gives shear strength parameters c (cohesion) and φ (friction angle) used in Terzaghi's bearing capacity equation. Routine Indian projects typically combine SPT (at-site, 3-5 boreholes) with PLT (at one or two key locations).

Typical Indian SBC values: hard rock 1000+ kN/m², dense sand-gravel 400-600, dense clay 200-400, medium sand 150-300, soft clay 50-150, marine clay / made-up ground 25-100. SBC also varies with foundation depth (deeper = higher capacity for soils with friction angle) and with foundation width (wider = lower capacity for cohesionless soils due to settlement). Black cotton soil (BC) presents special challenges — high SBC in dry condition (200+ kN/m²) but volumetric swell-and-shrink seasonally; foundations must be founded below the active depth (typically 1.5-2 m) and isolated from contact with the active zone via voided clay-cushion construction.