Safe Bearing Capacity (SBC) Calculator

About the Safe Bearing Capacity

Safe Bearing Capacity (SBC) is the single most critical geotechnical parameter governing the design of all civil foundations, including isolated column pads, combined footings, raft mats, and continuous strip footings. In geotechnical engineering, a shallow foundation fails in shear when the applied contact pressure exceeds the shear strength of the underlying soil mass. Karl Terzaghi formulated the classic three-term superposition equation for general shear failure of shallow strip footings: qu = c·Nc + q·Nq + 0.5·γ·B·Nγ. The first term (c·Nc) accounts for the cohesive shear strength of the soil; the second term (q·Nq, where q = γ·Df) accounts for the stabilizing surcharge overburden of soil above the foundation base level; and the third term (0.5·γ·B·Nγ) accounts for the self-weight shear resistance of the soil wedge directly beneath the footing width B. Dimensionless factors Nc, Nq, and Nγ depend exclusively on the internal friction angle (φ). While ultimate bearing capacity (qu) represents the theoretical point of catastrophic shear failure, civil engineers apply a Factor of Safety (typically 2.5 to 3.0 per IS 6403) to establish the allowable safe bearing pressure, ensuring both shear safety and limiting total differential settlement to permissible tolerances (typically 25 mm to 40 mm per IS 1904).

Primary Applications

Geotechnical and structural engineers determining permissible column footing sizes for residential and commercial RCC buildings
Site civil engineers validating foundation borehole soil investigation reports against Terzaghi's theoretical equations
Contractors and project managers assessing whether isolated footings, strip footings, or raft foundations are required
Estimators evaluating foundation excavation depths and concrete footings contact area requirements
Civil engineering students studying soil mechanics, shear failure modes, and IS 6403 / IS 1904 foundation design codes

Formula & Method

Terzaghi's Ultimate Capacity qu = c·Nc + γ·Df·Nq + 0.5·γ·B·Nγ Safe Bearing Capacity (SBC) = (qu − γ·Df) ÷ Factor of Safety + γ·Df Factor of Safety (FS) ≈ 2.5 to 3.0

Key Variables & Parameters:

c: Cohesion of soil in kN/m²
γ: Effective bulk unit weight of soil in kN/m³
Df: Foundation embedment depth below ground level in metres
B: Width of footing in metres
Nc, Nq, Nγ: Non-dimensional Terzaghi bearing capacity factors dependent purely on internal friction angle φ

Shallow foundation bearing capacity follows Terzaghi and IS 6403 formulas. Safe bearing capacity is evaluated by dividing net ultimate capacity by a factor of safety (typically 2.5 to 3.0).

How This Calculator Works

Enter project-specific parameters into the designated input fields. The calculation engine standardizes numerical values, verifies boundary conditions, and computes all results in real time. Results update automatically as you change inputs.

Enter soil effective cohesion (c) in kiloPascals (kPa or kN/m²) obtained from laboratory direct shear or triaxial test reports (0 for clean sands/gravels; 10–50 kPa for mixed c-φ soils; >50 kPa for stiff clays).
Enter the moist/bulk unit weight of foundation soil (γ) in kN/m³ (typical ranges: 16 to 19 kN/m³ for sandy/silty soils; default 18 kN/m³).
Enter the proposed foundation depth (Df) in metres (m) below finished ground level (minimum depth 1.0 m to 1.5 m to clear topsoil and seasonal moisture variations).
Enter the planned foundation footing width (B) in metres (m).
Enter the effective internal friction angle (φ) in degrees (°) from standard geotechnical borehole reports (typically 25° to 35° for granular soils; 0° for pure saturated clays in undrained condition).
Set the design Factor of Safety (FoS) (IS 6403 recommends 2.5 for temporary/light structures, and 3.0 for permanent framed building foundations; default 3).
Analyze the computed outputs: Dimensionless factors (Nc, Nq, Nγ), Ultimate bearing capacity (qu in kPa), Gross safe bearing capacity (kPa and metric tonnes per m²), and Net safe bearing capacity (qns).

Worked Example: Safe Bearing Capacity Calculation for an Isolated Pad Footing in c-φ Soil

Scenario: A geotechnical consultant evaluates the allowable bearing capacity for an isolated column footing of width B = 1.50 metres resting at depth Df = 1.50 metres. Soil laboratory results: cohesion c = 20.0 kPa, bulk unit weight γ = 18.0 kN/m³, angle of internal friction φ = 25.0°. Factor of safety FoS = 3.0.

1. Convert angle of friction to radians: φ = 25° × (π / 180) = 0.43633 rad.
2. Calculate bearing capacity factor Nq: Nq = e^(π · tan φ) × tan²(45° + φ/2) = e^(3.1416 × tan 25°) × tan²(45° + 12.5°) = e^(1.4649) × tan²(57.5°) = 4.327 × (1.5697)² = 4.327 × 2.4639 = 10.66.
3. Calculate bearing capacity factor Nc: Nc = (Nq - 1) ÷ tan φ = (10.66 - 1) ÷ tan 25° = 9.66 ÷ 0.4663 = 20.72.
4. Calculate bearing capacity factor Nγ: Nγ = 2 × (Nq + 1) × tan φ = 2 × (10.66 + 1) × tan 25° = 2 × 11.66 × 0.4663 = 10.88.
5. Calculate surcharge overburden pressure (q): q = γ × Df = 18.0 kN/m³ × 1.50 m = 27.0 kPa.
6. Calculate ultimate bearing capacity (qu) using Terzaghi's equation: qu = (c × Nc) + (q × Nq) + (0.5 × γ × B × Nγ) = (20.0 × 20.72) + (27.0 × 10.66) + (0.5 × 18.0 × 1.50 × 10.88) = 414.4 + 287.8 + 146.9 = 849.1 kPa.
7. Compute gross safe bearing capacity (qs) with FoS = 3: qs = qu ÷ FoS = 849.1 ÷ 3.0 = 283.03 kPa (~283 kPa). In metric engineering units: 283.03 ÷ 9.81 = 28.85 tonnes/m².
8. Compute net safe bearing capacity (qns): qns = qs - q = 283.03 - 27.0 = 256.03 kPa (~256 kPa or 26.1 tonnes/m²).

Result Summary: For the 1.5 m footing in c-φ soil, ultimate bearing capacity is 849 kPa, yielding a gross safe bearing pressure of 283 kPa (28.8 t/m²) and a net safe bearing capacity of 256 kPa (26.1 t/m²) with FoS = 3.0.

Inputs and Units to Verify

Reliable results require verified input data and strict consistency of units. Review all measurements, dimensions, rate benchmarks, and underlying assumptions before relying on the calculated outputs.

Soil Cohesion (c): Enter cohesion from lab direct shear or triaxial test (0 for clean sands).
Internal Friction Angle (φ): Enter friction angle from geotechnical laboratory reports.
Soil Density (γ): Enter moist unit weight in kN/m³ (typically 17–19 kN/m³).
Footing Depth (Df) & Width (B): Enter proposed footing embedment depth and minimum width in metres.
Water Table Depth: Apply water table correction factors if groundwater is within depth Df + B.

Key Checks / Assumptions

General shear vs local shear failure: Terzaghi's standard equations assume general shear failure (valid for dense sand or stiff clay where φ > 28°). For loose sand or soft clay (φ < 28° or N-value < 10), IS 6403 requires reducing parameters for local shear: mobilized c' = 2/3 c, and tan φ' = 2/3 tan φ.
Water table correction: If the groundwater table rises to or above the foundation base level, the effective unit weight drops from bulk γ (~18 kN/m³) to submerged γ' (γsat - 9.81 ≈ 8 to 10 kN/m³), reducing the third bearing term (0.5·γ·B·Nγ) by nearly 50%.
Net Safe vs Gross Safe: The Net Safe Bearing Capacity (qns = (qu - q)/FoS) represents the net additional structural contact pressure that can be imposed by the column, whereas Gross Safe (qs = qu/FoS) includes the soil overburden weight.
Settlement control criterion per IS 1904: In fine-grained clays or silts, foundation sizing is almost always governed by permissible settlement (25 mm to 40 mm for isolated pads; 40 mm to 75 mm for rafts on clay) rather than shear failure.
Minimum foundation depth: Per IS 1904 Clause 12.1, shallow foundations must be placed at a minimum depth of 0.50 m, but practically 1.2 m to 1.8 m in expansive black cotton soil zones to pass below the seasonal active shrink-swell zone.

Understanding the Result

Outputs Ultimate Bearing Capacity, Net Safe Bearing Capacity, and allowable gross foundation pressure in kN/m² and tonnes/m².

Practical Tips

Always rely on laboratory triaxial shear tests and Standard Penetration Test (SPT N-values per IS 2131) from calibrated geotechnical boreholes rather than visual assumptions.
For pure clays (φ = 0° in undrained condition), Nq = 1.0, Nγ = 0, and Nc = 5.14 (or 5.7 for strip footings), simplifying the ultimate capacity to qu = 5.14 cu + q.
When constructing in black cotton (expansive) soil with low SBC (<100 kPa), avoid isolated shallow footings; use under-reamed bored compaction piles (IS 2911 Part 3) anchored into stable non-expansive strata.
Compact the bottom of the excavation pit using plate vibrators and pour a 75 mm to 100 mm thick mud mat of M10/M15 plain cement concrete (PCC) immediately to prevent soil slaking and softening.

Limitations

Applies to shallow foundations where foundation depth Df does not exceed the footing width B (Df / B ≤ 1.0 per classical shallow foundation criteria).
Assumes homogeneous, isotropic soil strata; multilayered soil profiles, sloping ground surfaces, or eccentric column loading require advanced IS 6403 shape and inclination modification factors.
Does not evaluate consolidation settlement, differential settlement, or elastic settlement; settlement must be analyzed separately to verify compliance with IS 1904 limits.
Submerged groundwater table requires reducing soil density to buoyant submerged unit weight (γ'), which must be entered explicitly.

Practical Workflow

Extract soil shear parameters (c, φ, γ) from certified geotechnical soil test reports.
Input parameters alongside trial footing depth and width.
Determine safe bearing capacity (SBC) for sizing foundation pads.
Check settlement criteria (IS 1904) to confirm total and differential settlements remain within permissible limits.
Inspect foundation trench base on site to ensure strata matches bore log descriptions.

Frequently Asked Questions

What is the difference between Ultimate, Safe, and Net Safe Bearing Capacity?

Ultimate Bearing Capacity (qu) is the theoretical maximum contact pressure at which the foundation soil fails catastrophically in shear. Safe Bearing Capacity (qs) is the ultimate capacity divided by a Factor of Safety (typically 3.0), representing total permissible pressure including soil overburden. Net Safe Bearing Capacity (qns) is the net maximum structural load pressure that can be placed above the foundation level after subtracting the existing soil surcharge (qns = (qu - q) / FoS).

What typical Safe Bearing Capacity (SBC) values are found in Indian soils?

Typical indicative SBC values across India: Soft clay or organic soil: 50–80 kPa (5–8 t/m²); Medium stiff clay: 100–150 kPa (10–15 t/m²); Loose fine sand: 100–150 kPa (10–15 t/m²); Medium dense sand: 200–300 kPa (20–30 t/m²); Dense gravelly sand: 300–450 kPa (30–45 t/m²); Weathered soft rock / moorum: 250–400 kPa; Hard basalt/granite bedrock: 1,000–3,000+ kPa (100–300+ t/m²). Exact values must always be confirmed by soil borehole tests.

How does the groundwater table affect Safe Bearing Capacity?

When the groundwater table rises to or above the foundation base level, soil buoyancy reduces effective unit weight by about 50% (from bulk γ of ~18 kN/m³ to submerged γ' of ~8–9 kN/m³). In sandy soils, this buoyancy cuts the self-weight bearing resistance term (0.5·γ·B·Nγ) nearly in half, resulting in an overall SBC reduction of 30% to 50%. Foundational design must always consider the highest seasonal monsoon water table level.

Why is a Factor of Safety of 3.0 standard in foundation design?

A Factor of Safety of 3.0 accounts for natural geotechnical uncertainties: non-homogeneous soil strata, variations in moisture content between summer and monsoon seasons, sampling disturbance during drilling, laboratory testing tolerances, and long-term creep settlement. An FoS of 3.0 ensures that operating stresses remain well within the elastic range of the soil, preventing both shear rupture and excessive settlement.

When is a soil bearing capacity test considered settlement-governed rather than shear-governed?

For footings on clays, silts, or wide footings (>2 metres wide) on medium sands, allowable bearing pressure is almost always governed by permissible settlement rather than shear failure. While the soil might safely support 350 kPa without shear collapse, that pressure might induce 60 mm of settlement, exceeding the 25 mm permissible limit of IS 1904 and cracking the superstructure. In such cases, allowable bearing pressure is capped by settlement analysis.

Important Professional-Use Note

Bearing capacity calculations must follow IS 6403 (Determination of Bearing Capacity of Shallow Foundations). Foundation settlement must also be verified per IS 8009 to prevent structural distress.

Safe Bearing Capacity