Square Footing on a Stiff Crust Over Soft Clay
A 1.8 m square isolated footing carries 800 kN dead plus 400 kN live with its base 1.2 m below grade. Profile: 0 to 2.5 m, stiff desiccated clay crust, cu = 90 kPa, unit weight 19 kN/m3; 2.5 m to 11 m, soft to medium clay, cu = 30 kPa, saturated unit weight 17.5 kN/m3, Cc = 0.30, Cr = 0.05, OCR about 1.2, e0 = 0.95; below 11 m, dense sand. Groundwater at 2.0 m. The structural engineer has sized the footing for a net bearing pressure of 370 kPa and points to a plate load test in the crust that gave a very high capacity.
1. The plate load test looks convincing. Why is it not a valid basis, and what actually governs this footing?
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The plate test only loaded the crust. A plate test influences the ground to roughly 2B below the plate - on the order of a meter. It never senses the soft clay starting at 2.5 m, so a high result is expected and irrelevant.
Bearing check against the soft clay at 2.5 m (1.3 m below the base)
Applied gross pressure at the base = 370 + 1.2 x 19 = 393 kPa.
- Lower bound, full load transmitted as a punching block: total stress at 2.5 m = 393 + 1.3 x 19 = 418 kPa. Capacity of the soft clay = 5.14 x 30 x shape/depth factors (about 1.2) + overburden 42 = about 227 kPa. FS = 0.54. Fails.
- Upper bound, 45-degree spread: width at 2.5 m = 1.8 + 1.3 x 2 = 4.4 m, but using the 2:1 approximation, 1200 / (1.8 + 1.3)^2 = 125 kPa, plus 42 kPa overburden = 167 kPa. FS = 227 / 167 = 1.36. Marginal.
- Punching shear of the crust adds perimeter resistance of roughly 4 x 1.8 x 1.3 x 90 = 840 kN, which is why the truth lies between the bounds - but the crust is desiccated, jointed, and weathers quickly, so its full cu should not be relied on in shear.
Settlement governs regardless
At mid-depth of the soft clay (6.75 m): σ'v0 = 2 x 19 + 0.5 x 7.69 + 4.25 x 7.69 = 75 kPa; σ'p = 1.2 x 75 = 90 kPa; Δσ from the footing (2:1, z = 5.55 m) = 1200 / 7.35^2 = 22 kPa; σ'f = 97 kPa.
s = 8.5 / 1.95 x [0.05 log(90/75) + 0.30 log(97/90)] = 4.36 x [0.00396 + 0.01023] = 62 mm, plus 10-15 mm immediate, so roughly 75 mm.
The footing is therefore not confirmed: bearing is marginal at best and settlement is two to three times what an isolated footing should tolerate.
2. Given that settlement and punching control, what are the viable foundation options and how would you choose among them?
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Options
- Deep foundations to the dense sand at 11 m. Most reliable. The crust will cause driving problems with precast piles, so pre-drill or pre-bore through the crust; drilled shafts or micropiles are alternatives. Check downdrag if the fill is ever raised and check group settlement, which will be small since the sand is competent.
- Ground improvement of the soft clay. Stone columns or controlled modulus columns through the crust into the soft clay: they reduce settlement but do not eliminate it, and the crust must be penetrated. Verify with a zone load test.
- A raft or a compensated foundation. A raft spreads the load and reduces the net pressure, which is effective because the soft clay is normally consolidated and responds to net load. A basement that removes 2.5 m of soil would reduce net pressure substantially - but removing the crust is the wrong direction, so the compensation must come from below the crust.
- Surcharge preloading plus a structural slab. Slow, needs time and a fill and a settlement period, and gives no control over differential.
How to choose
- If the structure can tolerate 60-80 mm total with controlled differential, the raft with a reduced net pressure plus a structural slab is the cheapest.
- If column loads are large or the structure is sensitive to differential (brick, glazing, equipment), go to piles or shafts. The 8.5 m of soft clay is too thick for a shallow improvement to be reliable.
- Whatever is chosen, the design must be checked against the net increase at the soft clay, not the crust capacity.
3. The contractor proposes to excavate the crust down to 2.5 m and replace-compact it to make a "better" bearing layer. What is your response?
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Reject the proposal as described.
- The crust is the best material on the site; removing it puts the footing base directly on, or just above, the soft clay and makes bearing and settlement worse, not better.
- Replace-compaction of a 1.3 m lift over soft clay does not change the stress transmitted to the 8.5 m soft layer. The settlement computed above is driven by the soft clay, and no amount of surface compaction fixes it.
- Excavation also destroys the desiccation and aging that give the crust its strength, and it opens the crust to water.
What is acceptable
- Strip only the gravelly fill above the crust and proof-roll the crust; place the footing base at 1.2 m within the crust, with a lean concrete blinding and no disturbance to the base.
- Keep the excavation open for the shortest possible time and protect it from rain; a softened crust base is a real capacity loss.
- If the contractor wants to improve the foundation, redirect the effort into a raft, columns, or piles, as in the previous answer, or into a well-documented proof load test on the actual selected system.