Expansive Black Cotton Soil at Dewas: Footing Capacity versus Heave
A two-storey school building near Dewas is to be founded on black cotton soil. Each interior column carries 700 kN at service. The trial pit shows 1.2 m of loose fill, then stiff expansive clay to at least 6 m depth. Free swell index is 70 per cent, water table is 4.5 m and is not expected to rise above 4 m, and SPT N values in the clay are 6 to 9. Undrained shear strength from triaxial tests is 30 kPa and unit weight is 18 kN/m³. The proposed detail is an isolated footing 2.0 m × 2.0 m founded at 1.5 m depth, with a 300 mm sand cushion below it. The contractor cites a nearby eight-year-old building with the same detail that has not cracked, and argues that N above 5 means the ground is stiff and no problem exists. You must verify the footing, judge the sand-cushion argument and recommend a foundation.
1. Compute the net safe bearing capacity and check the proposed 2.0 m × 2.0 m footing.
Read the worked answer
For a square footing on saturated clay, treat the undrained case as φ = 0, Nc = 5.7, Nq = 1, Nγ = 0:
- Gross ultimate qu = 1.3 c Nc = 1.3 × 30 × 5.7 = 222 kPa.
- Net ultimate = 222 - γD = 222 - (18 × 1.5) = 195 kPa.
- Net safe with FOS 2.5 = 195/2.5 = 78 kPa.
Applied pressure = 700/(2.0 × 2.0) = 175 kPa, which is 2.2 times the permissible value. The proposed footing is grossly inadequate on bearing capacity alone.
Required area = 700/78 = 9.0 m², i.e. a minimum 3.0 m × 3.0 m footing if a shallow foundation were retained (with FOS 3 the value drops to about 65 kPa and the size rises to 3.3 m). Note also that with N = 6 to 9 and c = 30 kPa, settlement will probably govern before bearing capacity, so a full consolidation settlement check, not just a capacity check, is mandatory before any shallow option is accepted. Because the clay is expansive, even a satisfactory bearing capacity would not control seasonal heave, which is the real design issue.
2. Evaluate the contractor's sand-cushion proposal and the argument based on the nearby building.
Read the worked answer
A 300 mm sand cushion is not a remedy, for four reasons.
- The expansive clay is not removed. Heave is generated in the clay itself, 2 to 4 m deep, from seasonal moisture change. A cushion above it changes neither the swell potential nor the swelling pressure under the footing.
- The cushion can worsen the problem. Sand is a permeable layer; it collects water from rain, garden taps and broken drains and feeds it directly to the clay under the footing, which can intensify local heave and produce differential movement between footings.
- Bearing capacity gain is negligible. The cushion only acts as an added layer of a stronger material; at 300 mm thickness and 2 m width, the load spreads too little to change the 78 kPa allowable value appreciably, and the 1.2 m of loose fill above the clay remains.
- The evidence is anecdotal. An eight-year-old building is a single sample, with different trees, drainage, fill and loading, and may simply not yet have passed through an unusual drought-wetting cycle; heave damage commonly appears 10 to 15 years later. N = 6 to 9 means a stiff consistency, not a non-expansive stratum.
The proposal must be rejected and the nearby building cannot be used as justification.
3. Recommend a foundation scheme and list the checks you would insist on before construction.
Read the worked answer
Adopt under-reamed piles (IS 2911 Part 3) with a 300 mm shaft, bulb diameter 2.5 times the shaft (about 750 mm), taken to about 3.5 m depth, with at least one bulb below the zone of seasonal moisture change and a rigid pile cap tied by plinth beams. Under-reamed piles are the standard solution here because the bulb anchors the pile against uplift from swelling and because the shaft bypasses the active zone; a raft on expansive clay would still heave unless a thick non-swelling cushion were placed, which is costlier at this load level.
Checks to insist on:
- Capacity in the stiff clay using measured c (side friction ignored over the top 1.5 m and end bearing of the bulb from IS 2911); with c of 30-50 kPa and two bulbs, an economical group of 4 to 6 piles per 700 kN column is typical, but the number must come from the actual computation, not from this estimate.
- Uplift capacity against swelling pressure, and structural design of the bulb and reinforcement for tension.
- Consolidation settlement of the clay group and differential settlement between adjacent footings (limit about 20 mm differential and 40 mm total for framed buildings).
- Reject the 1.2 m of loose fill as a founding medium; remove or treat it, provide a 1.5 m paved apron sloping away from the building, and keep trees at least 3 m away to limit moisture variation.