Soil Bearing & Foundation Selector
Recommend foundation type and minimum footing dimensions from soil type, building load, and water table depth. Preliminary estimate.
How this works
We use IS 1904's presumptive safe bearing capacities to back-figure the spread-footing area, then apply rule-based heuristics for the recommended foundation type:
reqArea = buildingLoadKn / bearingCapacity (m²)
minDim = sqrt(reqArea) (m)
Recommendation:
waterTable < 1 m OR soft-clay
→ raft foundation recommended
dense-sand / gravel / weathered-rock
→ isolated footing
otherwise
→ isolated footing with strap beamsPresumptive bearing capacities (kN/m²) used: soft-clay 75, medium-clay 150, dense-sand 200, gravel 350, weathered-rock 600. These are coarse field-practice values — confirm with SPT or plate-load tests before final design.
Worked example
500 kN column load on medium clay with the water table 3 m below grade:
- Bearing =
150 kN/m² - Required area =
500 / 150 ≈ 3.33 m² - Minimum square side =
√3.33 ≈ 1.83 m - Recommendation: isolated footing with strap beams (medium clay; water table deep enough).
Round up the footing dimension to the next 100 mm increment for detailing — 1.83 m becomes a 1.90 m square footing.
Sources
- IS 1904 (Code of Practice for Design and Construction of Foundations) and IS 6403 (Code of Practice for Determination of Bearing Capacity of Shallow Foundations)
FAQ
How accurate are the IS 1904 presumptive bearing values?
They are coarse field-practice numbers — accurate to about ±30 percent within a soil class. A 'medium clay' at one site can carry 100 kN/m² while another bears 200 kN/m² depending on density, plasticity, water content, and stress history. Use the calculator's recommendation for preliminary sizing and quoting only. Final design needs an SPT (standard penetration test) or plate-load test, with a geotechnical engineer interpreting the results against IS 6403 / IS 8009.
When does the calculator recommend a raft foundation?
Two trigger conditions: water table within 1 m of grade, or soft-clay soil regardless of water table depth. A raft (mat) spreads the building's total load across the entire footprint, lowering the contact pressure to a value the weak soil can carry, and it provides a continuous water barrier when the building sits below the water table. The downsides are higher concrete and steel quantities and a deeper basement excavation.
Why does the recommendation differentiate strap beams?
On medium-strength soil (medium clay, loose sand) isolated column footings can settle differentially when adjacent columns carry uneven loads — one corner of the building sinks more than the others, cracking the structure. Strap beams tie the isolated footings together and force them to settle uniformly, much like a partial raft. The calculator suggests strap beams for medium clay because that is the most common trouble case in residential construction.
What is 'building load per column' and how do I find it?
It is the total vertical load that one structural column transfers to the soil, including the column's own weight, all floors and walls it supports, and live (occupant + furniture) loads. A rough rule of thumb for a 4-storey RCC residential column is 50 kN per floor at typical Indian / Nepali design loads, so 200–300 kN per column for a typical home. For a precise figure use the column-load schedule from the structural design.
Why does the water-table depth matter?
Two reasons. First, a high water table reduces the effective bearing capacity of the soil by 30–50 percent because pore-water pressure offsets the soil's own weight. Second, foundations cast below the water table need waterproofing, dewatering during construction, and uplift checks — a raft handles these concerns more cleanly than separate isolated footings. The 1 m and 2 m thresholds in the recommendation are conservative residential thumb rules; the IS 1904 derating is more nuanced.
Should I round up the calculated footing size?
Yes — always round the minimum footing side up to the next 100 mm or 150 mm detailing increment, then add a small margin for column eccentricity and bearing-capacity uncertainty. A 1.83 m calculated side becomes a 1.90 m or 2.00 m detailed footing in practice. If the load or soil class is borderline (load > 80 percent of bearing capacity × area), bump the side by another 100 mm to give the soil more headroom.