Answers to the most common questions from field contractors.
A DOT compaction log must include: test date and time, project number and contract ID, station and offset (example: STA 12+50, 5L), lift number and material type (subgrade, aggregate base, etc.), nuclear gauge serial number and current calibration certificate reference, test method (AASHTO T99 standard proctor or T180 modified proctor), gauge reading depth and mode (direct transmission or backscatter), measured wet density (pcf), measured moisture content (%), calculated dry density (pcf = wet density divided by (1 + moisture/100)), maximum dry density from the proctor test (pcf), percent compaction (field dry density divided by max dry density times 100), pass/fail determination, and QC inspector name and certification number. Missing any required field is grounds for log rejection.
AASHTO T99 is the standard Proctor compaction test, using a 5.5-lb hammer with an 18-inch drop (12,375 ft-lb/ft3 compaction energy). AASHTO T180 is the modified Proctor test, using a 10-lb hammer with an 18-inch drop and 5 layers instead of 3 (56,250 ft-lb/ft3 compaction energy - about 4.5 times more energy). T180 gives a higher maximum dry density than T99 for the same soil. When the specification says "98% modified Proctor," it means 98% of the T180 maximum dry density. T180 is typically specified for base course and subbase where higher compaction is needed; T99 is common for subgrade.
When a test shows compaction below the specification minimum: stop all work on that lift segment immediately, notify the QC manager and inspector, document the failing test with all required fields, determine the root cause (over-wet material, insufficient roller passes, lift too thick, defective material), implement the appropriate correction, and re-test. Do not proceed with the next lift until the failing area passes. The failing test record must be retained - it is not acceptable to destroy or omit failing records from the QC documentation.
Documentation for a failed lift requires three connected records: (1) The original failing test log with all required fields and a note that the area failed and requires correction. (2) A corrective action record showing what correction was made (add moisture and re-compact, additional roller passes, material removal and replacement), who authorized the correction, and when it was done. (3) The re-test log showing results at the same stations plus additional stations if required by the project specifications. Attach all three records together in the QC binder or digital system, clearly showing the failure-correction-re-test sequence.
For aggregate base course on federal-aid projects, 98% of AASHTO T180 (modified Proctor) is the most common specification. For subgrade, 95% of T99 (standard Proctor) is typical. However, specifications vary by state and project type: TxDOT specifies 95% of T99 for embankment, 95% of T180 for Type A base. CDOT specifies 95% of T99 for subgrade, 97% of T180 for aggregate base. Always verify with the project specifications - using the wrong proctor reference makes your test results meaningless.
Nuclear gauge calibration requires: annual recertification by a qualified lab (the gauge goes to the lab, typically $200-500 per year), state radiation safety license for the operator, and daily site verification using the standard count block that comes with the gauge. Before each day of testing, place the gauge on the verification block, take reference readings, and compare to the baseline reference established at calibration. If readings are outside the acceptable range (typically plus or minus 1%), the gauge should not be used until calibrated. Keep calibration certificates on site at all times.
A DOT QC plan for a road construction project must include: an organization chart showing the QC manager, QC inspectors, and lab technicians; inspection frequencies for each work type (compaction, grade, materials); test methods and reference standards (AASHTO, ASTM); calibration schedule for all testing equipment; a nonconformance reporting procedure; the daily QC report format; materials acceptance procedure (how certified test reports and certificates of compliance are processed); subcontractor QC requirements; and the QC manager qualifications and contact information. The plan must be approved by the contracting officer before construction begins.
Typical DOT compaction testing frequencies: one test per 500 square feet per lift for subgrade, one test per 1,000 square feet per lift for aggregate base, one test per 2,500 square feet per lift for embankment. These are minimums - the project specifications may require more. Tests must be distributed across the work area (not all in one location) and must include the areas adjacent to curb, gutter, and structure edges where compaction is most critical. Some DOTs also require one test per each 500 LF of roadway centerline regardless of area.
Superelevation verification documentation must include: date, station, measured cross-slope (percent), design cross-slope (percent) from the superelevation diagram in the plans, deviation (measured minus design), and pass/fail (typically plus or minus 0.3% tolerance on interstates, plus or minus 0.5% on secondary roads). Documentation is typically recorded on a project-specific form or in the QC daily report. The documentation must reference the approved superelevation diagram - if the diagram was revised, verify you are using the current revision.
When you and the DOT inspector have different grade readings: first verify both sets of readings are taken from the same benchmark and datum. If the discrepancy persists, request a witnessed re-test where both parties observe the same measurement simultaneously. If there is still disagreement, request the inspector to take their own independent measurement with their calibrated equipment and document the result. If the dispute cannot be resolved in the field, escalate to the QC manager and the resident engineer. Document everything - the date, the specific station, your readings, the inspector reading, and the resolution.
Sitemark produces lift-by-lift compaction summaries and grade verification reports formatted for DOT submittal.
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