Process Capability Assessment Using SPC and Cpk in the Analysis of Clay Soil Atterberg Limits: A Case Study in Pathum Thani Province

This study evaluated the consistency and capability of the testing process for the Plasticity Index (PI) of fine-grained soils, a parameter critical for soil classification and geotechnical design, particularly in compaction and foundation analyses. Statistical Process Control (SPC) was applied using X–MR control charts to monitor process stability, while process capability was assessed through Cp and Cpk indices under specification limits of LSL = 2.00 and USL = 10.00. Trend analysis and uncertainty evaluation were also conducted to strengthen the assessment framework. Results showed that the PI testing process was statistically stable, with all data points within control limits. However, process capability indices (Cp = 0.702, Cpk = 0.616) were below the benchmark value of 1.33, indicating insufficient performance due to inherent variability. Linear regression revealed no significant time-related trend (R² = 0.1%, p = 0.864), confirming temporal consistency. Uncertainty analysis yielded an expanded uncertainty of ±0.537 at 95% confidence, equivalent to 9.7% of the mean PI. Such uncertainty suggests possible misclassification of results near specification thresholds. In conclusion, although the PI testing process was under statistical control and free of time-related drift, it exhibited substantial variability and high uncertainty. These findings emphasize the need to reduce variation and incorporate uncertainty into quality management practices, providing a more reliable basis for decision-making in geotechnical engineering applications.