Q: Does this calculator account for settlement?

Bearing capacity and settlement are two separate limit states — both must be satisfied. This calculator computes the shear-failure bearing capacity per IS 6403. IS 1904 limits settlement separately: 25 mm total and 0.0015L differential for isolated footings on sand; 40 mm total and 0.0015L differential on clay. For clay soils in particular, settlement typically governs design even when bearing capacity is adequate. Refer to IS 8009 Part 1 for settlement calculation methodology.

Q: Can I use this for pile foundations?

No — this tool computes shallow foundation bearing capacity only (isolated, strip, combined, raft). For pile foundations, the governing standards are IS 2911 Part 1 (driven and bored piles), IS 2911 Part 3 (under-reamed piles for black cotton soil), and IS 2911 Part 4 (load testing). Pile capacity involves skin friction and end bearing that require separate analysis. Use this tool to determine whether a shallow foundation is viable first; if not, a pile design is needed.

Question 1

What is the difference between ultimate, safe, and net safe bearing capacity?

Accepted Answer

Ultimate bearing capacity (qu) is the maximum pressure the soil can bear before shear failure. Safe bearing capacity (qs) is qu divided by a factor of safety (typically 3) to account for uncertainty. Net safe bearing capacity (qns) subtracts the overburden pressure (γ·Df) from the safe value — this is the net pressure a foundation can transmit over and above the weight of the soil it replaces. IS 1904 and IS 6403 both use net safe bearing capacity for foundation design.

Question 2

When should I use the presumptive values in IS 1904 versus calculating from SPT?

Accepted Answer

IS 1904:1986 Table 1 gives presumptive safe bearing capacity values for preliminary design and small residential buildings (typically up to G+2). IS 1904 explicitly states that for final design and for buildings above G+2, a site-specific geotechnical investigation per IS 6403:1981 is required. Use presumptive values at feasibility or concept stage; always use SPT-based calculation for working drawings.

Question 3

Why does Terzaghi's formula give different values from Meyerhof's?

Accepted Answer

Terzaghi's 1943 formula assumes a general shear failure mode for shallow strip footings with rough base and no ground water above the failure surface. Meyerhof (1963) extended it with shape factors, depth factors, and inclination factors, and revised the Nγ factor. For square/rectangular footings at depth, Meyerhof's method is typically 10–30% higher because it credits the side shear along the footing depth. Most Indian practitioners use Meyerhof's method (adopted in IS 6403:1981 Annex B) for final design.

Question 4

What do I do if my site has black cotton soil?

Accepted Answer

Black cotton (expansive) soil has very low safe bearing capacity (50–130 kN/m²) and swells 20–30% with seasonal moisture changes. Do NOT use isolated or strip footings on black cotton soil. The calculator flags this condition and recommends under-reamed pile foundations per IS 2911 Part III — drilled below the 'active zone' (typically 1.5–3.5 m). Alternatives include a CNS (Cohesive Non-Swelling) soil cushion or raft foundation with specialised detailing.

Question 5

How does the water table affect bearing capacity?

Accepted Answer

Groundwater reduces the effective unit weight of the soil and thus reduces bearing capacity. IS 6403 applies two correction factors — Rw1 for water table above foundation base, and Rw2 for water table below the foundation. The calculator applies these corrections automatically: the closer the water table is to the foundation base, the greater the reduction. In saturated loose sand in seismic zones III–V, liquefaction is also a risk — the tool will warn you.

Question 6

What factor of safety should I use?

Accepted Answer

IS 1904:1986 recommends a factor of safety of 3 against shear failure for shallow foundations in residential and commercial buildings. For critical structures (hospitals, schools, public buildings) use 3.5 or higher. For temporary structures, 2.5 is acceptable. The calculator defaults to FOS = 3 and shows the impact of varying it. Never use FOS < 2.5 for any permanent occupied building.

Question 7

Does this calculator account for settlement?

Accepted Answer

Bearing capacity and settlement are two separate limit states — both must be satisfied. This calculator computes the shear-failure bearing capacity per IS 6403. IS 1904 limits settlement separately: 25 mm total and 0.0015L differential for isolated footings on sand; 40 mm total and 0.0015L differential on clay. For clay soils in particular, settlement typically governs design even when bearing capacity is adequate. Refer to IS 8009 Part 1 for settlement calculation methodology.

Question 8

Can I use this for pile foundations?

Accepted Answer

No — this tool computes shallow foundation bearing capacity only (isolated, strip, combined, raft). For pile foundations, the governing standards are IS 2911 Part 1 (driven and bored piles), IS 2911 Part 3 (under-reamed piles for black cotton soil), and IS 2911 Part 4 (load testing). Pile capacity involves skin friction and end bearing that require separate analysis. Use this tool to determine whether a shallow foundation is viable first; if not, a pile design is needed.

Method	qu (kN/m²)	Safe qs (kN/m²)	Reference
Terzaghi (1943)	535	178	Theoretical Soil Mechanics
Meyerhof (1963)	628	209	IS 6403 Annex B
Presumptive	—	100–245	IS 1904 Table 1

Soil Type	SBC (kN/m²)	Typical Indian Regions
Hard / Sound Rock	3240–3240	Deccan basalt, Karnataka granite, hills of Tamil Nadu
Soft Rock / Laterite	880–1620	Kerala, coastal Karnataka, Goa, Jharkhand, parts of Odisha
Dense Gravel / Sand-Gravel	440–490	Rajasthan plains, river terraces in Punjab and Haryana
Dense / Compact Sand	245–490	Coastal Tamil Nadu, Odisha, Andhra Pradesh; dense river deposits
Stiff Clay	245–440	Inland Karnataka, parts of Madhya Pradesh and Rajasthan
Medium / Loose Sand	100–245	Gangetic plains (UP, Bihar, Bengal), coastal alluvium
Soft / Medium Clay	100–245	Alluvial river basins; Bengal delta; interior plains
Black Cotton Soil (Expansive)	50–130	Maharashtra, Madhya Pradesh, Gujarat, parts of Karnataka, Telangana, Andhra Pradesh
Marine Clay	50–100	Coastal Mumbai (reclaimed areas), Chennai, Kochi, Sundarbans
Filled-up / Made Ground	0–100	Urban reclaimed land, levelled slopes, construction debris fill

Soil Bearing Capacity Estimator

Inputs

Bearing Capacity Results

Bearing Capacity Factors Used (Meyerhof)

Compliance Checks

IS 1904:1986 Presumptive Safe Bearing Capacity

How the bearing capacity is computed

Common questions about bearing capacity