Soft Zone Under the Footings, Rain in 48 Hours
You are site superintendent on a 12,000 sf single-story retail building. Design bearing is 2,500 psf on spread footings: 480 lf of 24 in. wide continuous footing plus twelve 5 ft by 5 ft column pads. A test pit at grid C-3 shows 4 ft of loose fill over soft clay with SPT N=4; the other four borings show N=14 to 18 at bearing elevation. The structural engineer directs over-excavation 3 ft below bearing elevation, extending 1 ft beyond each footing edge, backfilled with crushed stone. Your excavation subcontractor proposes instead to "just dig the soft spots" and lay geogrid, arguing the test pit is an anomaly and that full over-excavation will cost the schedule two weeks. Rain is forecast in 48 hours. Excavation is bid at $19/cy, disposal at $22/cy, and crushed stone placed at $54/cy. The sub's bid included 60 cy of unsuitable material.
1. Compute the over-excavation and backfill volume and the cost consequence versus the sub's allowance.
Read the worked answer
Plan area (footing width plus 1 ft each side):
- Continuous: (2 + 1 + 1) = 4 ft wide × 480 lf = 1,920 sf
- Pads: (5 + 1 + 1) = 7 ft square = 49 sf × 12 = 588 sf
- Total = 2,508 sf
Volume: 2,508 × 3 ft = 7,524 cf ÷ 27 = 278.7 cy, call it 279 cy.
Cost: excavate 279 × $19 = $5,301; haul off 279 × $22 = $6,138; stone 279 × $54 = $15,066. Total $26,505.
The bid carried 60 cy, so the delta is 219 cy × ($19 + $22 + $54) = $20,805 of added cost - and the stone is the expensive half. Note the sub is not over-excavating 279 cy for free; every yard removed is a change order in their favor, which is exactly why their "just dig the soft spots" recommendation deserves skepticism rather than deference.
2. Evaluate the geogrid alternative on technical merit. Approve, reject, or conditionally approve?
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Reject as proposed, and say why in writing.
- The anomaly argument cuts the wrong way. A single probe dropping from N=14–18 to N=4 within the same stratum is a signature of an old utility trench, a buried pit, or undocumented fill - features that are linear, not circular. Footing pads are only 5 ft square; differential settlement between an improved pad and a stiff neighbor is what cracks a masonry façade.
- Geogrid is a reinforcement for a competent subgrade, not a cure for no bearing. It bridges over a void; it does not convert N=4 clay into a 2,500 psf bearing stratum at depth. Approving it would substitute your judgment for the engineer's on a life-safety element, without their seal behind it.
- Process: the sealed structural/geotech direction governs. Any alternate requires a written, sealed approval from that engineer, which you are not empowered to grant and the excavation sub is not qualified to propose.
Conditional path if the engineer will entertain it: only after additional probes (two more pits at the footing lines, not one) define the lateral extent, and only with a signed design from the engineer of record and any resulting change order. Do not let schedule pressure decide a foundation question.
3. You have 48 hours before rain on an open, 3 ft deep excavation. What is the sequence of action?
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- Stop water before it reaches bearing. Cut a perimeter diversion berm and swale now, not after the front arrives; a 3 ft deep, 2,500 sf open cut becomes a bathtub and the stone backfill becomes a sponge.
- Determine the excavation's true extent before you dig a yard more. Get the two confirming probes scheduled for tomorrow morning. You want to over-excavate once, to the correct footprint, not chase soft spots through the footprint twice.
- Document before you bury the evidence. Photograph the pit walls, log the elevations of the bearing surface against plan, and record the SPT data. This is the exhibit that supports the $20,805 change order if the sub disputes it.
- Give written notice of differing site conditions within the contract's notice window - often 7 or 10 days, and the clock starts from discovery, not from when you got around to it.
- Sequence the backfill for dry weather. Stone can sit wet; exposed clay subgrade cannot. Do not place structural fill into standing water, and do not leave the bearing surface open over the weekend - a dozer track rut in softened clay is a re-excavation.
- Do not tell the owner "no impact" yet. You have a cost delta of roughly $20,800 and a potential two-week schedule slip. Say what you know, in writing, the day you know it.