Free · Geotechnical Engineering

Safe Bearing Capacity (SBC) Calculator — IS 6403, IS 8009 & IS 1904

Enter the borehole log, the water table and the footing size. The calculator finds the net safe bearing capacity from shear (IS 6403:1981) and the safe pressure for the permissible settlement (IS 8009 Part 1 with IS 1904 Table 1) at each depth you choose. The allowable value is the lower of the two. You can download a calculation sheet as a PDF.

1. Report details optional · printed on the PDF

Project / structure

Borehole / location

Prepared by

2. Borehole log

Units for input

Use the units printed in your soil report. Indian reports usually give bulk density in g/cc (= t/m³) and cohesion in kg/cm².

Load example

The page opens with the default example so you can see how data is entered. Choose “Start blank” to enter your own borehole log (your current entries are replaced).

💧 Water table depth from top (ground level), m

Depth of the ground water level below existing ground level, as recorded in the borehole log. Enter 0 if the water may rise to ground level in the monsoon (worst case — most reports assume this). If no water was met, enter a depth larger than the borehole, e.g. 20. It is used for the effective overburden, the water table factor W′ (IS 6403 Cl. 5.1.2.4) and the correction W′ of IS 8009 Fig. 9.

Layer	From (m)	To (m)	Soil type	γ	c	φ (°)	SPT N	C_c	e₀

SPT N entered in the table is

Field N is corrected for overburden (IS 2131 Fig. 1) and, for fine sand / silt below the water table with N′ > 15, for dilatancy: N″ = 15 + ½(N′ − 15) (IS 2131 Cl. 3.6). Choose “already corrected” if the report table gives corrected N.

What to enter in each column

To (m): Depth of the bottom of the layer below ground level. “From” fills in by itself from the layer above.
Soil type: Sand / gravel — clean granular soil (c = 0). Fine sand / silty sand (c-φ) — fine or silty sand, or soil with both c and φ. Clay — clay or clayey silt. Rock — choose the rock class (IS 12070). Completely weathered rock that behaves like soil should be entered as sand with its N value.
γ: Bulk unit weight from the report. For a layer below the water table enter the saturated value; the calculator uses the submerged weight γ − 1 t/m³ below the water table.
c: Cohesion from the shear test (direct shear / triaxial). For clay enter the undrained cohesion c_u (half the unconfined compressive strength).
φ: Angle of shearing resistance in degrees. Use 0 for saturated clay in undrained condition.
SPT N: Average standard penetration value of the layer (blows per 30 cm). Used for settlement of sand (IS 8009 Fig. 9). Not needed for clay.
C_c, e₀: Compression index and initial void ratio from the consolidation test. Needed only for clay layers, for consolidation settlement.

Cells that a soil type does not need are hidden, so you only fill what the calculation uses.

3. Footing

Shape of footing

Sets the shape factors of IS 6403 Table 2 (strip 1, square 1.3/1.2/0.8, circle 1.3/1.2/0.6, rectangle 1 + 0.2B/L …).

Width B (m)

The shorter plan dimension of the footing (diameter for a circle). Bearing capacity and settlement both depend on it.

Length L (m)

The longer plan dimension (L ≥ B).

Depths of foundation to check (m)

Depth of the footing base below ground level, D_f. Give one or more values separated by commas; the calculator works out the SBC at each.

Factor of safety

Applied on the net ultimate bearing capacity to get the net safe bearing capacity. 2.5 is the usual value; some reports use 3.

Type of shear failure

IS 6403 Cl. 5.1.2: general shear for dense soil, local shear for loose soil (Table 3, by relative density). The usual practice in soil reports is to interpolate by φ between 28° and 36°.

Backfill above the footing

IS 6403 Cl. 5.1.2.2 allows the depth factors d_c, d_q, d_γ only when the backfill is properly compacted.

Settlement criterion

Structure and foundation type

Picks the permissible maximum settlement from IS 1904 Table 1. The value for plastic clay is used when the footing rests on clay; otherwise the value for sand and hard clay is used.

Permissible settlement (mm)

Filled from IS 1904 Table 1. You can type another value, e.g. 25 mm if the project specifies it.

Settlement method for sand

IS 8009 Cl. 9.1.4 reads the settlement per unit pressure from Fig. 9 for the corrected N and width B, divides it by the water table factor W′ and applies the depth factor (Fig. 12) and the rigidity factor (Cl. 9.5). The Teng formula q_a = 3.5(N−3)[(B+0.3)/2B]²R_wR_d is used in many soil reports; pick it to cross-check such a report.

Foundation rigidity

IS 8009 Cl. 9.5.2: the settlement of a rigid foundation is 0.8 times the centre settlement of a flexible one. Choose flexible to match a report that did not apply this factor.

4. Results at each depth

Show detailed calculation for depth

Enter the data and click Calculate.

How the calculator works

Step	Rule used
Effective surcharge q	Overburden at founding level, with submerged unit weight (γ − 1) below the water table (IS 6403 Cl. 2.2.2).
Bearing capacity factors	N_q = e^{π tanφ} tan²(45°+φ/2), N_c = (N_q−1) cotφ, N_γ = 2(N_q+1) tanφ — the values of IS 6403 Table 1. Local shear uses φ′ = tan⁻¹(0.67 tanφ) and c′ = 2c/3.
Net ultimate	q_d = c N_c s_c d_c + q (N_q−1) s_q d_q + ½ B γ N_γ s_γ d_γ W′ (Cl. 5.1.2), shape factors from Table 2, depth factors from Cl. 5.1.2.2 and W′ from Cl. 5.1.2.4. The soil parameters are those of the layer at founding level.
Net safe bearing capacity	Net ultimate (general, local or interpolated by φ) ÷ factor of safety.
Corrected N	Field N × overburden correction of IS 2131 Fig. 1 at each depth, then the dilatancy correction for fine sand / silt below the water table. N is averaged from the base to 1.5B below it, leaving out values more than 50 % above the average (IS 6403 Cl. 5.2.2).
Settlement — sand	IS 8009 Part 1 Cl. 9.1.4: settlement per kg/cm² read from Fig. 9 for the average corrected N and B, divided by W′ = 0.5 + 0.5 d/B ≤ 1 (d = depth of the water table below the base; Fig. 9 inset), proportional to pressure, times the Fox depth factor (Fig. 12) and the rigidity factor 0.8 for a rigid foundation (Cl. 9.5).
Settlement — clay	Consolidation S = Σ C_c H/(1+e₀) log[(σ′₀+Δσ)/σ′₀] to 2B below the base, Δσ by 2:1 spread, times the depth and rigidity factors (IS 8009 Part 1).
Rock at founding level	Net safe bearing pressure from IS 12070 Table 2 (as amended) for the rock class chosen.
Allowable bearing capacity	The lesser of the net safe bearing capacity and the safe pressure for the permissible settlement (IS 6403 Cl. 6.1; IS 1904 Table 1).

Engineering aid Results depend on the soil parameters you enter. Check them against the soil investigation report and use engineering judgement before adopting a value. Immediate (elastic) settlement of clay, eccentric and inclined loads are not included.