SPT Hammer Energy Ratio versus Drop Height

Abstract

Automatic trip hammers have advantages for standard penetration test (SPT) of consistent drop height and low friction loss during hammer fall. These advantages, however, generate high energy transfer ratios (ER), typically about 90%. This efficiency causes lower sensitivity and higher energy correction coefficients, $C_E$ . To reduce ER and $C_E$ and to increase the sensitivity of SPT conducted at the Wildlife Liquefaction Array (WLA) and the Garner Valley Downhole Array, instrumented Network for Earthquake Engineering Simulation sites, a $127\mathrm{mm}$ $\left(5.00\mathrm{in.}\right)$ long sleeve was placed in the hammer mechanism to reduce the drop height from $762\mathrm{mm}$ $\left(30\mathrm{in.}\right)$ to $635\mathrm{mm}$ $\left(25\mathrm{in.}\right)$ . To calibrate the energy for these drop heights, measurements were made for a series of SPT tests in Borehole X2 at WLA on November 21, 2003. For these SPT, sleeves were inserted with lengths of $50\mathrm{mm}$ $\left(2\mathrm{in.}\right)$ , $127\mathrm{mm}$ $\left(5\mathrm{in.}\right)$ $177\mathrm{mm}$ $\left(7\mathrm{in.}\right)$ , and no sleeve. Resulting drop heights were $762\mathrm{mm}$ $\left(30\mathrm{in.}\right)$ , $711\mathrm{mm}$ $\left(28\mathrm{in.}\right)$ , $635\mathrm{mm}$ $\left(25\mathrm{in.}\right)$ , and $584\mathrm{mm}$ $\left(23\mathrm{in.}\right)$ . Results indicate that: (1) ER increases with rod length as expected; (2) corrections for rod length, $C_R$ , increased with rod length in accordance with $C_R$ published in 2001 by Youd et al.; and (3) for lengths greater than $6\mathrm{m}$ , ER increased approximately linearly with drop height. Average ${\mathrm{ER}}_{30}$ [ER based on a $762\mathrm{mm}$ $\left(30\mathrm{in.}\right)$ drop height] were 43% for a $584\mathrm{mm}$ $\left(23\mathrm{in.}\right)$ drop, 60% for a $635\mathrm{mm}$ $\left(25\mathrm{in.}\right)$ drop, 84% for $711\mathrm{mm}$ $\left(28\mathrm{in.}\right)$ drop, and 89% for a $762\mathrm{mm}$ $\left(30\mathrm{in.}\right)$ drop.