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

  • Author(s): Deger, Tonguc Tolga
  • Advisor(s): Seed, Raymond B.
  • et al.
Abstract

The Standard Penetration Test (SPT) has been a staple of geotechnical engineering practice for more than 70 years. Empirical correlations based on in situ SPT data provide an important basis for assessment of a broad range of engineering parameters, and for empirically based analysis and design methods spanning a significant number of areas of geotechnical practice. Despite this longstanding record of usage, the test itself is relatively poorly standardized with regard to the allowable variability of specific details of equipment and procedures, and this requires a number of adjustments or corrections to further standardize this "standard" test. In addition, for many engineering purposes, it is also useful or necessary to "normalize" the penetration resistance measured by SPT in order to account for the influence of effective vertical effective overburden stress at the depth where an individual SPT is performed.

This research addressed two of 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. Considerable additional work has followed, and with ongoing advances in both the availability of improved instrumentation (especially high frequency accelerometers) and of analytical methods, it has become clear that earlier measurement and analytical methods developed prior to about the mid-1980's overestimated to some extent the reduction in hammer energy delivery for conditions wherein rods were "short" (less than about 45 feet in length). With improved modern accelerometers, we are now able to fully track the very high frequency accelerations produced by steel to steel hammer/anvil impacts, and as a result it is now no longer necessary to make simplifying assumptions that had been intrinsic in pre-1986 measurements and analyses. 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.

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