Proofrolling a Wet Fat Clay Subgrade
At a building pad, the contractor has scarified and recompacted the upper 12 in. of a fat clay subgrade (CH, LL 62, PI 34). The moisture content at the time of compaction was about 4 percentage points above optimum, and field density is 97% of standard Proctor. After a rain, a 25-ton tandem-axle dump truck proofrolls the area. Ruts 2-3 in. deep and pumping, with mud waves ahead of the tires, appear over about 20% of the pad, mostly near a utility trench. The contractor says the density test passed, so the pad is acceptable, and wants to place 6 in. of crushed base. The geotechnical report requires a stable, non-pumping subgrade and prohibits placing base on softened material. You are the CMT inspector.
1. Does the passing density test establish that the subgrade is acceptable? Explain the soil behavior involved.
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No. Density is only one acceptance criterion, and a high dry density does not guarantee stable shear strength. This is a CH clay compacted wet of optimum, so much of the compaction effort has gone into increasing saturation rather than building a strong, durable soil skeleton. The proofroll is a direct load test and it is showing rutting and pumping, which means the subgrade cannot support construction traffic without shear deformation. The density test may also have been taken at a firm spot rather than in the softened area. A passing test at one location cannot override visible instability across 20% of the pad, especially near a utility trench that may be a water path.
2. What immediate actions and additional evaluations would you require before base placement?
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I would stop base placement and delineate the soft areas by continued proofrolling with a loaded truck. The unstable material should be removed down to firm, non-pumping soil rather than simply capped. Near the utility trench, I would inspect the trench backfill and check for water, a broken pipe, or missing compaction. The contractor should scarify and dry the clay if time and weather permit, or replace the unsuitable material with approved granular fill, then recompact in controlled lifts. I would verify moisture and density after reworking, and use proofrolling again as the acceptance check for stability. If the area remains wet, the designer should review drainage and whether stabilization is needed. Documentation should include the locations and depths of removal.
3. The contractor proposes placing a geotextile over the soft clay and proceeding with the base. How would you respond?
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A geotextile can provide separation and some reinforcement, but it is not a cure for deep pumping or a soft, saturated clay subgrade. If the soft material is left in place, the geotextile may rut, wrinkle, or be punctured, and the base can still settle or fail. The geotechnical report specifically prohibits base on softened material, so the contractor's proposal would require written approval from the designer, not just field acceptance. I would not approve it as a substitute for removing unstable soil unless the designer provides a design and the treated area is proven stable by proofrolling. My recommendation would be remove and replace or dry and recompact first, then use separation only as intended.