Strip Footings on Uncontrolled Fill Above Frost Depth
A single-storey warehouse, 24 m × 60 m, is to be built on the edge of Ulaanbaatar. The site has about 2.0 m of uncontrolled fill over silty clay; groundwater is at 4.0 m and the design frost depth is 2.2 m. No site investigation was done. The contractor proposes 600 mm wide strip footings at 1.2 m depth, arguing that the soil 'looks hard' after compaction by the excavator bucket. Last winter, the adjacent access road heaved and cracked, and a neighbouring workshop floor slab lifted near a downpipe. The warehouse will store heavy palletised goods and needs a flat floor. The client wants to start footing excavation next week. The engineer must review the foundation proposal and decide what investigation and design changes are needed before construction.
1. Should the engineer approve the proposed strip footings? Justify your answer using the site conditions.
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The proposal is not acceptable. A 1.2 m footing depth is above the 2.2 m frost depth, and the footing would bear on 2.0 m of uncontrolled fill. 'Looks hard' after bucket compaction is not a bearing-capacity test. The fill can settle differentially, the silty clay may be frost-susceptible, and the floor slab and footings can heave. The road heave and neighbour's slab lifting are evidence that water and frost are already causing movement. Heavy palletised storage increases the consequences of differential settlement. The engineer should not approve excavation for footings until a site investigation and foundation design are complete.
2. Outline the site investigation and testing programme needed before redesign.
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Investigation: boreholes and test pits to below the fill and into the clay, with SPT or CPT; groundwater monitoring; fill thickness and composition; compaction and consolidation tests; grain size and Atterberg limits; frost-susceptibility testing; and a topographic/drainage survey. The neighbour's cracks should be recorded. The investigation should define a suitable bearing stratum and the frost design approach.
3. What foundation and floor-slab options would you require the designer to compare?
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Options: remove the uncontrolled fill and replace it with engineered, compacted fill; use deep foundations such as piles to a competent stratum; or use ground improvement with a designed load-transfer layer. Footings should bear below frost depth or be designed as frost-protected with insulation, drainage and a capillary break. The floor slab needs a vapour barrier, insulation and perimeter drainage. The final choice must be supported by geotechnical calculations and settlement estimates, not by a rule of thumb.