Soil bearing capacity (SBC) is the maximum average pressure the ground under a foundation can carry safely, without shear failure and without settling more than the building can tolerate. It is given in kN/m². For a house foundation, the required footing area is simply the column load (plus footing self-weight) divided by the safe bearing capacity. For example, a column carrying 600 kN on soil with an SBC of 150 kN/m² needs a footing of about 2.1 m x 2.1 m.
In Nepal, soil conditions change a lot over short distances: soft lake clay in parts of Kathmandu Valley, loose sand and silt with a high water table in the Terai, and mixed soil and rock on hill slopes. A footing size that works on one plot can be unsafe on the next. This post explains what SBC means, gives rough values by soil type, shows why a soil test is worth it and works through a footing size example.
Terms used for bearing capacity
- Ultimate bearing capacity: the pressure at which the soil fails in shear.
- Safe bearing capacity: ultimate capacity divided by a factor of safety, commonly 2.5 to 3.
- Allowable bearing pressure: the pressure that is safe against shear failure and also keeps settlement within limits. For footing design, this is the value you actually use. On site it is often just called "SBC".
Older drawings and reports may use t/m². 1 t/m² is about 9.81 kN/m², so 150 kN/m² is about 15.3 t/m². The engineering unit converter handles these conversions.
Typical soil bearing capacity values by soil type
Textbooks and older Indian Standard tables (in the style of IS 1904 and IS 6403 guidance) give presumptive values for preliminary work. The ranges below are rough guidance only. They assume a footing at a reasonable depth, at least about 1 m below ground, on undisturbed soil above the water table. Actual values must come from site investigation.
| Soil type | Indicative safe bearing capacity (kN/m²) |
|---|---|
| Filled or made-up ground | Not reliable, needs testing or deeper foundation |
| Soft clay, soft silt, loose fine sand | About 50 to 100 |
| Medium clay, medium silty sand | About 100 to 150 |
| Stiff clay, medium dense sand | About 150 to 250 |
| Dense sand, compact gravel, sand-gravel mix | About 200 to 450 |
| Soft or weathered rock | About 450 and above |
| Hard sound rock | Much higher, often several thousand |
Many house designs in Nepal assume an SBC between 100 and 150 kN/m² when no test is done. That is a reasonable starting point on average soil, but it can be unsafe on soft clay or filled land, and wasteful on dense gravel.
Soil conditions you may meet in Nepal
- Kathmandu Valley: large parts sit on old lake deposits. Some areas have soft black clay (kalimati) with low bearing capacity and high settlement. Some sandy areas with a high water table showed liquefaction in the 2015 earthquake.
- Terai: mostly river-deposited sand and silt with a shallow water table, especially in the monsoon. Loose saturated sand has reduced bearing capacity and can liquefy in strong shaking.
- Hills: anything from residual soil and old landslide debris to firm rock within a few metres. Slope stability and drainage often matter more than bearing pressure.
- Filled plots: plots raised with dumped soil or construction waste are common near roads and rivers. Footings must go through the fill to natural ground.
Why a soil test matters
A soil investigation usually includes boreholes or test pits, a Standard Penetration Test (SPT) at different depths, groundwater level, and lab tests on samples. For some sites a plate load test is done. The report gives a recommended allowable bearing pressure, foundation depth and type.
For a house, the test cost is small compared with the cost of a foundation failure, cracked walls from uneven settlement, or oversized footings. Many municipalities ask for a soil test report for larger buildings; check your municipality's requirement during the naksa process. Even where it is not required, a test is strongly advised on filled land, near rivers, on slopes and on known soft clay areas.
How footing size is found
The footing area is based on service (unfactored) loads:
Required area = (column load + self-weight of footing and soil over it) / SBC
Self-weight is often taken as about 10% of the column load for preliminary sizing. The structural design of footing thickness and steel then uses factored loads as per IS 456 and NBC 105, which is your engineer's job.
Worked example: isolated footing size
Data: column service load P = 600 kN, safe bearing capacity of soil = 150 kN/m², self-weight of footing taken as 10% of P.
- Self-weight = 10% of 600 = 60 kN.
- Total load = 600 + 60 = 660 kN.
- Required area = 660 / 150 = 4.40 m².
- For a square footing, side = √4.40 = 2.098 m. Adopt 2.1 m x 2.1 m, area = 4.41 m².
- Check: actual gross pressure = 660 / 4.41 = 149.7 kN/m², which is within 150 kN/m². OK.
For the structural design, the engineer uses the net upward pressure from the column load alone, 600 / 4.41 = 136.1 kN/m² at service level. With a load factor of 1.5 for limit state design, the factored upward pressure is 1.5 × 600 / 4.41 = 204.1 kN/m². That value is used to find footing depth and bars, not the size of the footing.
Effect of SBC on footing size
The same 660 kN load on different soils:
| SBC (kN/m²) | Required area (m²) | Calculated side (m) | Adopted size (m) | Actual pressure (kN/m²) |
|---|---|---|---|---|
| 100 | 6.60 | 2.569 | 2.6 x 2.6 | 97.6 |
| 150 | 4.40 | 2.098 | 2.1 x 2.1 | 149.7 |
| 200 | 3.30 | 1.817 | 1.85 x 1.85 | 192.8 |
Going from 200 to 100 kN/m² nearly doubles the footing area. On soft soil, neighbouring footings can start to overlap, and a combined footing, strip footing or raft may be the better solution. On very soft or filled ground, piles or ground improvement may be needed.
Quantities for this footing
With a foundation depth of 1.5 m and 300 mm working space on each side, the pit is 2.7 m x 2.7 m x 1.5 m = 10.935 m³ of excavation. A 75 mm PCC bed projecting 100 mm beyond the footing on each side is 2.3 x 2.3 x 0.075 = 0.397 m³. Use the excavation and earthwork calculator for pit volumes and the concrete calculator for cement, sand and aggregate in the PCC and footing once the engineer fixes the footing depth.
Things that reduce bearing capacity on site
- A rising water table during the monsoon, especially in sandy soil.
- Footings placed on loose fill or on disturbed soil at the bottom of the pit.
- Rainwater left standing in open foundation pits.
- Deep excavation on the neighbouring plot close to your footing.
- Footings near the edge of a slope or a retaining wall.
Frequently asked questions
What SBC should I use for a house in Kathmandu?
There is no single value. Some areas have good gravelly soil and others have soft clay. Use the value from a soil test report. Without one, your engineer may assume a conservative value, but on soft or filled ground a test is strongly advised.
Does a deeper footing increase bearing capacity?
Often yes, because deeper soil is usually firmer and the surrounding soil adds confinement. But going below the water table or into a soft layer can reduce it. The soil test tells you the right depth.
Why add 10% to the column load?
The footing and the soil above it also press on the ground. Taking 10% of the column load is a common preliminary allowance; the engineer can calculate the actual weight once the footing depth is known.
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