Fill Beneath a New Warehouse on Former Agricultural Land
A logistics company is building a single-storey warehouse near Bijeljina on a 1.2 m thick fill layer over soft alluvial clay. The groundwater table is about 1.5 m below grade. A geotechnical report commissioned by the seller recommends strip footings with a presumed allowable bearing pressure of 150 kPa, based on two boreholes to 6 m depth and no laboratory testing. The warehouse slab will carry forklift traffic and racking loads, and the client wants to avoid piling for cost reasons. During a site visit you notice a nearby irrigation channel, cracks in an adjacent access road, and that the fill contains construction debris. The contractor has already begun excavating footing trenches to the depth given in the report.
1. Assess the adequacy of the existing geotechnical report and list the gaps that matter for this project.
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The report is not adequate for design:
- Insufficient investigation: two boreholes to 6 m on a site with 1.2 m of fill over soft clay do not establish stratigraphic continuity or the depth of compressible layers.
- No laboratory testing: the 150 kPa value has no measured basis; shear strength, consolidation parameters, and fill composition are unknown.
- Fill character: debris-bearing fill is variable and may be compressible or contain voids; it cannot be treated as engineered fill.
- Groundwater and channel: the irrigation channel implies seasonal water-level variation and possible seepage; the water table at 1.5 m affects excavation stability and slab design.
- Settlement: soft clay under fill is a settlement problem, not only a bearing-capacity problem; differential settlement will distress the slab and racking.
A supplemental investigation with additional boreholes, sampling, and laboratory testing is required before design proceeds.
2. Recommend a foundation and slab strategy, and explain how you would justify it to a cost-conscious client.
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Options to evaluate:
- Ground improvement (e.g. removal and replacement of fill, or controlled modulus columns) plus a stiffened ground-bearing slab, if settlement can be controlled.
- Piles or CFA to a competent stratum with a suspended slab, if soft clay is deep or settlement intolerable.
- Raft where loads are moderate and the clay thickness is limited.
Selection must follow from settlement calculations using measured parameters, not from a target bearing pressure. For the client, frame the choice in lifecycle terms: slab cracking and racking misalignment from differential settlement cost more to repair than the foundation upgrade, and racking tolerances are strict. Present a table of options with estimated settlement, cost, and risk, and state the design assumptions explicitly. Also require the contractor to stop trench excavation until the revised design is issued, and to protect open trenches from water ingress.