Isolated Footings on Clayey Silt with an Adjacent Deep Excavation

Loads: column 1100 kN + footing self weight 2.6 × 2.6 × 0.6 × 25 = 101 kN; no backfill credit taken on the projection (conservative). Total ≈ 1201 kN.

Pressures: gross = 1201 / 6.76 = 177.7 kPa. Surcharge relieved at founding level = γD ≈ 18 × 1.6 = 28.8 kPa. Net pressure applied to the soil = 177.7 − 28.8 = 148.9 kPa.

Plate load test result: SBC of 160 kPa from a 600 mm plate at 1.5 m is normally a net safe value; so the net applied pressure of 148.9 kPa is about 93% of allowable – bearing capacity is just satisfied, with only 7% margin.

Judgement: bearing capacity is not the governing criterion. With SPT 9–13 in clayey silt and a footing 2.6 m wide, the pressure bulb extends about 2.5–3 m below founding level, and settlement controls. Also, a 600 mm plate in a layered silty clay profile badly underestimates prototype settlement because it stresses only a thin surficial zone; the plate test should not be used to certify settlement. The footing should be redesigned on the basis of settlement, not on the 160 kPa number.

Comparison: total settlement 45 mm is near the upper permissible range for isolated footings on cohesive soil (of the order of 50 mm, IS 1904-type limits – verify against the current table). Differential settlement 22 mm over a 6 m bay gives angular distortion ≈ 22/6000 = 1/273, well beyond the ~1/500 normally tolerated for framed buildings with brittle infill. Cracking of masonry and finishes is likely even before the new loads act.

Effect of the new loads:

• The 3.5 m excavation removes 3.5 × 18 ≈ 63 kPa of overburden within 6 m of the footing edge. That is a stress relief comparable to a third of the applied net pressure; it reduces confinement and will increase heave, then additional settlement as the wall deflects. A 12 mm top tilt of a 3.5 m cantilever already indicates a movement of roughly 1/290, i.e. active soil movement towards the cut.
• The 3 m embankment 15 m away adds about 3 × 18 = 54 kPa over a broad area, which reaches the clayey silt below the footings and will add consolidation settlement.

Decision: a raft, or a piled/ground-improved foundation with a stiffer cap, is warranted. Differential movement, not bearing pressure, is the driver; if piles are chosen, negative skin friction and lateral loading from the embankment must be considered.

Instrumentation: settlement markers on every column (read to ±0.5 mm), inclinometers in the sheet pile and in the soil behind it, standpipe piezometers, and crack gauges on finishes. Take two baseline sets before excavation starts.

Trigger values (proposal, to be confirmed by the geotechnical designer):

• 10 mm differential settlement between adjacent columns → warning; 15 mm → stop and review.
• Sheet pile deflection rate above 2 mm/day, or total 20 mm → stop excavation.
• Piezometric drop exceeding 0.5 m outside the wall → check for drawdown-induced settlement.

Sequence:

1. Complete the piled/raft foundation and allow the structure to take load before the adjacent excavation goes below 2 m.
2. Propped or anchored sheet pile instead of a 3.5 m cantilever; install walings in stages, no more than 1 m excavation below a support.
3. No spoil or plant surcharge within 1.5 times the excavation depth of the wall.
4. Monitor the embankment consolidation separately; if the footings are on piles, design for downdrag.
5. Review monitoring data weekly against triggers; a two-stage response plan (warning / stop) must be written into the contract.