Hammer Efficiency and Correction Factors for the TxDOT Texas Cone Penetration Test

Abstract

This study analyzes blowcount data from instrumented Texas Cone Penetration (TCP) tests. TCP hammer efficiency, rod length influence on the hammer efficiency, and overburden pressure correction factors for the TCP blowcounts (NTCP) are explored. Results are compared to published correction factors for the standard penetration test (SPT). The final dataset analyzed for this study consisted of 293 TCP tests from which 135 tests were instrumented. TCP hammer efficiency values for automatic trip hammers ranged from 74 to 101% with an average of 89%. Analyses showed a statistically-significant relationship between the TCP hammer efficiency and the rod length below ground surface. Statistical models were developed for undifferentiated soils, and corresponding rod length correction factors for the TCP test (CR-TCP) were obtained ranging from 0.90 to 1.00. In a second analysis, the relationship between the overburden pressure and NTCP was explored and a mathematical expression for the overburden correction factor for the TCP blowcount value (CN-TCP) was determined. This work represents the first study where corrections to NTCP are explored, and the outcome of this research benefits the geotechnical engineering community using the TCP test and its associated foundation design method.