Local and National Scale Energy Calibration of Standard Penetration Test Hammers

Abstract

A Standard Penetration Test (SPT) energy testing program was developed for the Alabama Department of Transportation. Six Central Mine Equipment (CME) automatic hammers were calibrated using force and velocity measurements. The energy transfer ratio (ETR) for each hammer system was from 82.2% to 96.1%, with an overall average of approximately 91%. The coefficients of variance (COV) ranged from 2.2% to 5.7%. In addition to the testing program, a database for automatic hammers was created and included energy records from approximately 19,000 SPT hammer blows. 90% of the data were obtained from CME automatic hammers, and over two-thirds of the CME data represented repeat testing. The database records were acquired under Nuclear Quality Assurance Level I standards (NQA-1), which were provided by a private sector consultant for research purposes. The database records were used to determine a broad-based value of transfer efficiency for CME automatic hammers. The overall ETR was determined to be 82.7% ± 5.5% with a COV of 6.7%. The range of COVs between CME groups was from 1.1% to 10.5%. The overall average COV was 4.3%. These COV results compared well to the expected COV range for CME hammers documented in historical studies, which had a maximum value of 10%. iii Supplemental multiple regression analyses were performed on the CME database records for rod lengths less than 50 ft. The variables evaluated include hammer operation rate, rod length, penetration resistance, and rod type. The first regression model was used to predict transfer efficiency. The second model, which was regressed through the origin, was used to estimate the full effect of each variable without the intercept. The prediction accuracy between the two models was approximately 2% ETR. Hypothesis testing was performed on the independent variables.