Reconciling Quota-Based and Field-Derived CO2 Emissions in Subgrade Earthwork Construction: A Retrospective Analysis of Productivity and Operating Conditions

Objective: To examine the differences between quota-based and field-derived CO2 emissions from highway subgrade earthworks while assessing the robustness of procedure attribution. Methods: Quota workdays were compared with equipment CO2 concentrations and exhaust-flow observations from selected operations in one Chinese expressway project. The productivity-duration factor η and emission-intensity factor λ were used to decompose, retrospectively, the reported field-to-quota emission ratio. Nineteen procedures were described by four mutually exclusive η–λ quadrants. Two-class Jenks thresholds were recalculated, and their stability was explored by leave-one-out analysis and 5, 000 procedure-level bootstrap resamples. Results: Soft-rock excavation generated 360.91 kg CO2 during a representative day (0.435 kg/m3), versus 448.19 kg (0.351 kg/m3) during moderately hard-rock excavation. The workday reductions for rock compaction, soft-rock crushing, and soil grading were 68.7%, 64.4%, and 62.2%, corresponding to productivity increases of approximately 219%, 181%, and 165%, respectively. The four nominal quadrants contain 6, 6, 4, and 3 procedures. Recomputed Jenks midpoints were η = 0.668 and λ = 1.300, differing from the original heuristic boundaries of 0.65 and 1.15. One crushing-procedure record is internally inconsistent, and the evidence is insufficient to verify how all 19 procedure totals were extrapolated. Conclusions: The two-factor identity is useful for retrospective accounting diagnostics but is not independently predictive. Classification thresholds and procedure-level totals must be regarded as exploratory until the monitoring-to-inventory links, source data, and cross-project calibration have been validated.