Overburden Stress Normalization and Rod Length Corrections for the Standard Penetration Test (SPT)

• Main Author: Deger, Tonguc Tolga
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2014
• Subjects: Civil engineering; Environmental engineering; Geotechnology
• ISBN: 1321258682; 9781321258684

The Standard Penetration Test (SPT) has been a staple of geotechnical engineering practice for more than 70 years. This research addressed two corrections and adjustments that can be applied to SPT: (1) normalization of measured penetration resistances to account for effective overburden stress, and (2) corrections of SPT data for the effects of variations in hammer energy successfully transferred into the rods during driving due to "short rod" effects. The development of procedures and relationships for normalization of SPT penetration data for effects of effective overburden stress dates back more than 60 years, and a number of top geotechnical experts of the past six decades have weighed in on this issue. Despite this long history, current normalization relationships are not based on very extensive data sets, and they are based largely on data for clean sands only (SW and SP) and so do not necessarily represent a suitable basis for overburden normalization of SPT for silty soils (SP-SM, SP-SC, SM and ML) which can be of significant interest in a number of problem areas including, but not limited to, soil liquefaction engineering. The approach taken here was to first re-evaluate the important data developed by Marcuson and Bieganousky (1977) based on large-scale laboratory calibration chamber tests of SPT performed on three clean sands. Then the resulting overburden normalization relationships developed were further examined by cross-comparison with several additional sets of field (in situ) SPT data. To extend these types of relationships to silty soils, data were next gathered for six silty foundation soil strata beneath major dams, where the overlying (largely trapezoidal) earthen dam embankments provided the needed broad ranges of effective overburden stresses within foundation strata that were judged to be suitably laterally geologically continuous as to provide a basis for development of these types of relationships. In the end, it was found that the new relationships developed for "silty" soils, with suitable fines adjustments (similar to the fines adjustments currently employed in widely used SPT-based liquefaction triggering correlations), match relatively well with the newly developed relationships for overburden stress normalization of SPT data for cleaner sands. The issue of short rod effects has a shorter history, having first been broached by Schmertmann and his colleagues in the early 1970's. It is now possible to trace hammer energy (and wave travel) up and down the rods for multiple cycles of wave travel and to account for additional hammer energy transferred into and out from the rods until the entire "event" had been concluded. Debate has ensued as to the magnitude of actual hammer energy correction that is appropriate now that this improved measurement data is becoming available. The approach taken here was to gather instrumented hammer energy data from SPT performed, under closely controlled conditions, for purposes of "calibration" of automatic (mechanical) hammers. The resulting data show that short rods effects are indeed somewhat less significant then had previously been postulated, but that this reduction in short rods effects is less pronounced that has been postulated by a number of recent papers. An additional finding, also of engineering significance, is that "common practice" with regard to performing of SPT for purposes of calibration of automatic hammers often does not properly account for short rod effects, and that this can produce a conservative bias in SPT data when these hammers are subsequently used for actual engineering investigations. Recommendations are presented for (a) new short rod corrections, and (b) for hammer energy calibration testing of automatic SPT hammer systems. (Abstract shortened by UMI.)