Cut-and-Fill Balance, Failing Compaction, and Perched Water
You are building a 6-acre building pad for a distribution warehouse. The geotechnical report requires 12,000 compacted cubic yards (CCY) of structural fill at 95% of Standard Proctor maximum dry density. The site cut yields 8,400 bank cubic yards (BCY) of sandy lean clay. The soil has 18% swell and 12% shrinkage. The excavating contractor claims it needs to import only 3,600 loose cubic yards (LCY), based on the cut volume and a 25% factor it calls “fluff.” Compaction tests at seven locations show four failing at 91% to 93%; the contractor blames rain and asks you to accept 90% because “the building will consolidate it.” Groundwater is at 6 feet; footings bear at 5 feet. A perched water layer is entering the excavation. The schedule allows five working days for remedial work. You must evaluate the import quantity, the compaction dispute, and the dewatering plan.
1. Calculate the correct net import and compare it with the contractor's 3,600 LCY.
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Use the shrinkage factor to convert required compacted fill back to bank volume. A 12% shrinkage means 12% of the bank volume is lost during excavation, moisture conditioning, and compaction, so the yield is 88%:
- Required bank volume = 12,000 CCY ÷ 0.88 = 13,636 BCY
- Available cut = 8,400 BCY
- Net import = 13,636 − 8,400 = 5,236 BCY
Convert bank import to loose volume using the 18% swell:
- 5,236 BCY × 1.18 = 6,178 LCY
The contractor's 3,600 LCY is short by about 2,578 LCY, or about 2,185 BCY. Even if the contractor meant 3,600 BCY, it is short by 1,636 BCY. The “25% fluff” factor is not a substitute for the shrinkage and swell relationship. Also confirm whether the cut is all suitable; if any is rejected, import increases. The correct order of magnitude is roughly 5,200 BCY, not 3,600 LCY.
2. How should you respond to the failing compaction tests and the request to accept 90%?
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The specification is 95% Standard Proctor, and four of seven tests are below it. The contractor cannot unilaterally lower the acceptance criterion. Rain may explain a moisture problem and may support a time extension, but it does not convert failing density into acceptable fill. Structural fill under footings and slabs must meet the geotechnical requirement.
Direct the contractor to:
- Scarify, aerate, or dry the affected lift, adjust moisture to within the specified range, and recompact.
- Retest the same failing locations plus random locations at the required frequency.
- If retesting still fails, undercut and replace the material or use a geotechnical-approved stabilization method.
- Hold the next lift until the failed lift is accepted.
Ask the geotechnical engineer to evaluate whether the failures are isolated surface conditions or a deeper loose layer. Do not accept 90% “because the building will consolidate it.” Consolidation under load is not the same as meeting compaction, and differential settlement under a warehouse slab or footing is costly to repair later. Document the rejection, the remediation, and the retest results.
3. What dewatering and bearing verification should you require before placing fill or footings?
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Groundwater at 6 feet with footings at 5 feet leaves little margin, and the perched water entering the excavation means the bottom is unstable. Require a dewatering plan before further excavation: sumps and pumps, perimeter swales or cutoff trenches, and, if flows are significant, wellpoints or deep wells. The goal is to lower the water table below the excavation bottom and keep it there during fill placement and concrete work.
Do not place structural fill in standing water or on a pumped, disturbed bottom. After dewatering, proofroll and test the subgrade. If the perched water has softened the bearing stratum, the geotechnical engineer should evaluate over-excavation and replacement with granular material. Verify footing bearing at the actual bottom elevation, not the design elevation. Require the contractor to maintain dewatering until the footings and slab are structurally capable of resisting hydrostatic uplift. Document water levels, pump logs, and bearing inspections.