Compaction Control on a Borrow-Source Fill
A 12,000 ft³ embankment lift is to be built from a borrow soil. Lab testing on the borrow gives a maximum dry unit weight of 118 pcf at optimum moisture of 14%. The specification requires 95% of maximum dry unit weight and moisture within ±2% of optimum. Field nuclear gauge readings on the first lift average a dry unit weight of 110 pcf at 17% moisture. The contractor's moisture conditioning consists of a water truck spraying the surface after the lift is spread but before rolling. The density test was taken immediately after the final roller pass. The resident engineer notes the lift is 11 in. loose and is being compacted with a smooth-drum roller in 6 passes.
1. Evaluate whether the first lift meets the specification and identify the controlling issue.
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Required dry unit weight = 0.95 × 118 = 112.1 pcf. Field average is 110 pcf, which is about 1.9 pcf (roughly 2%) below the required value - failing. Moisture is 17% vs. an allowable window of 12–16%, so it is also above the acceptable moisture range. The controlling issue is moisture, not compactive effort: the soil is too wet, so at a given compactive energy the dry unit weight falls. Note that surface spraying after spreading wets the top inch or two but does not condition the lift; water must be mixed into the lift. Also, testing immediately after rolling captures the as-compacted state, which is appropriate, but a wet lift will gain strength/density only after drying. The fix is to aerate/dry the lift (discing, grading) before rolling, or to reduce the applied water and mix it in - not simply to add passes.
2. The contractor proposes adding four more roller passes to reach 95%. Assess whether this will work, using the compaction curve concept.
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On the dry side of optimum, added energy raises dry unit weight substantially. On the wet side (17% > 14% optimum), additional passes produce diminishing returns and can even reduce dry unit weight by pumping/shearing a saturated soil - the curve flattens and rolls over. Since the field point sits wet of optimum, adding passes is the wrong lever. The expected outcome is little density gain, possible surface sealing and a soft, kneading surface. The correct action is to reduce moisture into the 12–16% window, then compact. If drying is impractical, the contractor could request a specification change, but that requires re-verifying the lab max dry unit weight at the higher moisture, which is not the same as the standard Proctor value.
3. Describe how you would verify the next lift so the result is defensible.
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(1) Confirm moisture conditioning is mixed into the lift, not sprayed on the surface - sample at mid-depth. (2) Take density/moisture readings at a minimum of one test per 500 ft³ or per the spec's frequency, at random locations, and compare to 95% × 118 = 112.1 pcf. (3) Use a sand-cone or drive-cylinder check to calibrate the nuclear gauge on this soil, since nuclear gauges are sensitive to chemistry and moisture. (4) Record the loose lift thickness and roller passes so the method spec is also satisfied. (5) If a test fails, retest adjacent to it before accepting or rejecting the whole lift; a single low reading may be a rock or a wet pocket.