Buried Topsoil, Two Repair Options, and Four Days of Float
A 22,000 sf, three-story office building. The geotechnical report specifies spread footings bearing 4 ft below finished grade at 3,000 psf, and contains one line of text: undocumented fill and possible buried topsoil may be encountered in the northwest quadrant. The contract states the boring logs are for information only and that the contractor shall verify site conditions. Excavation in the northwest quadrant reveals 18 inches of dark organic silt and brick rubble over roughly 40% of the footprint - about 8,800 sf. Two repairs are on the table: undercut 24 inches below design subgrade and replace with structural fill at $68/cy installed, plus $42/cy to excavate and dispose of the unsuitable material measured in its original 18-inch condition; or lime-stabilize the subgrade in place at $22/sf. Dewatering runs $2,400 per month. Either option adds 6 working days; you have 4 working days of float to the foundation inspection.
1. Price both repair options and recommend one, showing your quantities.
Read the worked answer
Undercut and replace. Affected area 8,800 sf.
- Unsuitable material removed: 18 in. = 1.5 ft; 8,800 x 1.5 = 13,200 cf = 489 cy. At $42/cy = $20,500.
- Structural fill: 24 in. = 2 ft; 8,800 x 2 = 17,600 cf = 652 cy. At $68/cy = $44,300.
- Subtotal about $64,800, plus roughly $500 of extra dewatering for the 6 days.
Lime stabilization as presented. 8,800 sf x $22/sf = $193,600 - about three times the undercut. But the comparison is wrong on engineering grounds. Lime stabilization works by cation exchange and pozzolanic reaction; it does not neutralize organic content, and organic silt is precisely what destroys the reaction and guarantees long-term settlement. The organic layer has to come out either way. The honest alternate is: remove the 18 inches of organics ($20,500) plus stabilize what remains ($193,600) = roughly $214,000.
Recommendation. Undercut and replace with structural fill. It is roughly $130,000 to $150,000 cheaper than the true stabilization alternate, it removes the organics rather than trying to fix them, and the structural engineer can sign off on it without reservation. Lime stabilization is the right answer for plastic clay subgrades, not for buried topsoil.
2. The owner's representative says there is no claim because the report warned you. Assess the entitlement position and recommend how to proceed.
Read the worked answer
A differing site condition claim requires that actual conditions differ materially from what the contract documents indicated and that a reasonable contractor could not have anticipated them. Two facts cut against you:
- The geotechnical report - usually a contract document even when labeled for information only - expressly warned of possible buried topsoil in the northwest quadrant, and the condition appeared exactly where the report placed it.
- The contract tells you the borings are informational and that you must verify site conditions.
What the report did not do is quantify it. Nobody told you 18 inches over 40% of the footprint. Your bid priced normal footing excavation in that quadrant, not 489 cy of disposal. That is the argument: not a surprise condition, but a materially greater quantity of a disclosed condition, which is often compensable as a quantity variation even when a full differing site condition is not.
Recommended path: issue written notice immediately - do not let the notice period run while you debate entitlement - keep the excavation moving, and submit a change order priced at the actual quantity removed above design subgrade, backed by unit prices and haul tickets. Consider a shared-savings or value-engineering proposal rather than a pure claim. Do not stop work, and do not frame this as a full differing site condition; that invites a fight you will likely lose and costs you credibility on the quantity argument, where you are right.
3. Handle the schedule: six working days added against four days of float.
Read the worked answer
Six days against four days of float puts you two days into the foundation inspection and everything downstream.
Options, cheapest first:
- Absorb it. Two days of extended general conditions - field staff, supervision, temporary facilities - may cost less than the argument. Price it in dollars before deciding.
- Accelerate to recover the two days: a Saturday pour or a second excavating crew. Compare the premium to the extended-GC cost; do not accelerate to save a cost smaller than the acceleration.
- Claim the two days as time only, not money, and hold the completion date. Owners settle time far more readily than money.
Whichever you choose, notify the inspection agency now. Most require 24 to 48 hours notice for a foundation inspection, and a missed inspection is a second lost day nobody will credit you for. Also confirm whether the undercut changes the inspector's scope - a footing bearing 24 inches deeper than shown usually needs a re-inspection of the bearing surface, not just the original one.