Tools › User guides › SBC Calculator
The Safe Bearing Capacity (SBC) Calculator works out the allowable bearing pressure of a shallow foundation from a borehole log: shear failure by IS 6403, settlement by IS 8009 (Fig. 9) with IS 2131 N-corrections, permissible settlement from IS 1904, and rock by IS 12070 — at several depths at once, with a PDF calculation sheet.
1.5, 2.0, 2.5).| Field | What to enter | Typical value | Tip |
|---|---|---|---|
| Project / structure | Name of the work. | e.g. ROB at Ch. 7.485 | Printed on the PDF only. |
| Borehole / location | Borehole number or location. | BH-01 | |
| Prepared by | Your firm or lab. |
| Field | What to enter | Typical value | Tip |
|---|---|---|---|
| Units for input | MKS (γ in g/cc, c in kg/cm²) or SI (γ in kN/m³, c in kN/m²). | MKS for most Indian reports | Use the units printed in your soil report. |
| Load example | Default example (sand over clay) or Start blank. | Start blank | Loading an example replaces your current entries. |
| Water table depth from top | Depth of ground water below ground level, m. | 0 (worst case) | Enter 0 if water may rise to ground level in the monsoon. If none was met, enter a depth larger than the borehole, e.g. 20. |
| SPT N entered in the table is | Field N (the calculator corrects it) or Corrected N (from the report). | Field N | Field N is corrected for overburden (IS 2131) and dilatancy (N″ = 15 + ½(N′ − 15) for fine sand below water). |
| Column | What to enter | Typical | Tip |
|---|---|---|---|
| To (m) | Depth of the bottom of the layer below ground level. | “From” fills in from the layer above. | |
| Soil type | Sand / gravel, Fine sand / silty sand (c-φ), Clay, or Rock (choose the rock class). | Completely weathered rock that behaves like soil: enter as sand with its N. | |
| γ | Bulk unit weight. | 1.8 – 2.0 g/cc | Below the water table enter the saturated value; the submerged weight is used automatically. |
| c | Cohesion from direct shear / triaxial test. | Clay: cu = ½ × UCS | Sand: 0. |
| φ (°) | Angle of shearing resistance. | Sand 28° – 36° | Use 0 for saturated clay (undrained). |
| SPT N | Average blows per 30 cm for the layer. | Used for settlement of sand. Not needed for clay. | |
| Cc, e0 | Compression index and initial void ratio from the consolidation test. | Only for clay layers (consolidation settlement). |
| Field | What to enter | Typical value | Tip |
|---|---|---|---|
| Shape of footing | Rectangular, Square, Strip or Circular. | Square / rectangular | Sets the shape factors of IS 6403 Table 2. |
| Width B (m) | Shorter plan dimension (diameter for a circle). | 1.5 – 3 m for buildings | |
| Length L (m) | Longer plan dimension (L ≥ B). | ||
| Depths of foundation to check (m) | One or more depths Df, separated by commas. | 1.5, 2.0, 2.5, 3.0 | SBC is worked out at each depth. |
| Factor of safety | On the net ultimate bearing capacity. | 2.5 | Some reports use 3. |
| Type of shear failure | By φ (interpolated), General shear, or Local shear. | By φ | Local ≤ 28°, general ≥ 36°, interpolated in between. |
| Backfill above the footing | Properly compacted (use depth factors) or not. | Compacted | IS 6403 allows depth factors only for compacted backfill. |
| Field | What to enter | Typical value | Tip |
|---|---|---|---|
| Structure and foundation type | e.g. RCC structure — isolated footing; Multistoreyed framed building — raft. | RCC — isolated | Picks the permissible settlement from IS 1904 Table 1. |
| Permissible settlement (mm) | Filled automatically; you can type another value. | 50 mm (sand) | e.g. 25 mm if the project specifies it. |
| Settlement method for sand | IS 8009 Fig. 9 (SPT chart) or Teng / Terzaghi & Peck formula. | IS 8009 | Use Teng to cross-check a report that used it. |
| Foundation rigidity | Rigid (factor 0.8) or Flexible (1.0). | Rigid | IS 8009 Cl. 9.5.2. |
Press Calculate. The table gives, for each depth: the layer at the base, the net safe bearing capacity from shear, the safe pressure for the permissible settlement, the allowable bearing capacity (the lower of the two) and which one governs. Pick a depth in Show detailed calculation for depth to see every step (Nc, Nq, Nγ, shape and depth factors, W′, corrected N, settlement). Download PDF calculation gives the full sheet.
The net safe bearing capacity checks shear failure only. The allowable bearing pressure is the lower of that and the pressure that keeps settlement within the permissible limit — use this one for design.
The depth recorded in the borehole log. If the water may rise to ground level in the monsoon, enter 0 — this is the worst case and what most reports assume.
No. The result depends entirely on the soil parameters. Without a soil investigation, use a conservative value approved by a geotechnical engineer.