Volume 35, Issue 5 (September 2012)
TDR Method for Compaction Quality Control: Multi Evaluation and Sources of Error
The use of the time domain reflectometry (TDR) method, designated ASTM D6780 (ASTM D6780, 2005, “Standard Test Method for Water Content and Density of Soil in Place by Time Domain Reflectometry (TDR),” Annual Book of ASTM Standards, Vol. 04.09 ASTM International, West Conshohocken, PA), to measure water content and soil density is relatively new with both successful and unsatisfactory results being reported. This paper reexamines the TDR method from various aspects, including laboratory investigation of the relation between TDR-measured electrical properties and soil phase parameters, full scale laboratory evaluation of the standardized methods, real world performance evaluation at construction sites of earth dams, and theoretical reexamination. Laboratory calibration reveals the effect of soil type on soil electrical properties, and its implication on the TDR method is discussed. The full scale laboratory evaluation supports the validity of both the one-step and two-step TDR methods. But the field evaluation at construction sites of earth dams indicates otherwise, particularly in the measurement of soil density. The errors in the two-step method are attributed to the penetration disturbance of the field multipole resonance probe (MRP). An additional source of error in the One-Step method is found to be a theoretical flaw in the current empirical adjustment process for correcting the variation of pore-fluid conductivity from the calibration test to field measurements. Remediation for the two-step method and updating of the ASTM D6780 to overcome the shortcomings of the one-step testing are in progress.