Foundation on Variable Fill: Bearing Capacity vs Settlement Control
A two-storey office building is to be founded on a site where a 3 m deep layer of loose sandy fill overlies stiff clay. The groundwater table is 1.5 m below ground. The structural engineer has sized isolated pad footings for a allowable bearing pressure of 150 kPa based on the stiff clay, assuming the fill is removed and replaced with compacted granular material. The contractor proposes instead to found the pads on the fill at a reduced bearing pressure of 100 kPa to save excavation. The building has a 6 m grid and the pad sizes are 2.5 m square. The client is concerned about differential settlement affecting the cladding.
1. Compare the two foundation options in terms of bearing capacity and settlement, and recommend one.
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Bearing capacity is rarely the governing issue for a two-storey building on stiff clay; settlement usually is. Founding on the loose fill at 100 kPa transfers the load through a compressible layer 3 m thick. The fill will consolidate under the pad load, and because fill depth varies across the site, differential settlement between adjacent pads is likely. For a 2.5 m pad carrying, say, 600 kN, the stress influence extends well below the pad, so the fill and the clay beneath both contribute to settlement.
Replacing the fill with compacted granular material removes the compressible layer and gives a more uniform stiffness, which controls differential settlement. The cost of excavation and replacement is usually less than the cost of remediating cracked cladding. The recommendation is to replace the fill under the pads, or to use a ground improvement method such as vibro-compaction if the fill is thick. If the contractor's option is retained, it must be supported by a settlement analysis showing differential movement within the cladding tolerance, not just a bearing capacity check.
2. What site investigation would you require before accepting either option?
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The critical unknown is the variability of the fill: its thickness, density, composition and the presence of soft spots or obstructions. Require boreholes or cone penetration tests at each pad location, not just a grid, plus a few deeper boreholes to confirm the clay profile. In-situ density testing of the fill, or plate bearing tests, would give a direct measure of stiffness. Groundwater at 1.5 m means the excavation for replacement will need dewatering and the base will need protection from softening.
Also require a settlement calculation using the measured fill properties, and a monitoring plan during construction if the fill option is retained. The judgement is that a single allowable bearing pressure is not a substitute for understanding the variability of a made ground layer; the investigation should be targeted at the risk, which is differential settlement, not global bearing failure.