SHEAR WAVE VELOCITY AND MODULUS OF A MARINE CLAY

A study was conducted on Boston Blue Clay to develop input for later soil amplification and soil structure interaction studies. The study consisted of (a) direct in situ measurement of the shear wave velocities by seismic cross-hole techniques, (b) obtaining samples of the soil from which to measure the properties necessary to calculate the modulus from empirical formulas, and (c) measurement of the shear modulus by torsional resonant column tests on undisturbed samples. These are all standard techniques in use in earthquake engineering, but, so far as the authors know, they have never before been applied to the Boston Blue Clay. It was the objective of the study to evaluate the accuracy of each of these methods and, if possible, to reconcile the differences in the data provided by each. Insofar as the exprapolated laboratory test results, the calculated results using Hardin's and Black's formula, and the in situ results were in close agreement, the authors concluded that the shear wave velocity of Boston Blue Clay was approximately 800 feet per second, which corresponded to a shear modulus of 17,000 psi. The three techniques for measuring shear wave velocity or shear modulus of the Boston Blue Clay agreed on an average value between 750 and 900 fps. To obtain this agreement it was necessary to recognize that present sampling procedures cause inevitable sample disturbance and that this must be taken into account in evaluating the results of laboratory tests. The most important effect was that of secondary compression, which increased the shear modulus 40 fps per log cycle of time. This was lost when samples were taken from the ground. The laboratory test data indicated that the ground shear wave velocity increased linearly with the logarithm of time and this fact could be used to extrapolate the correct value of shear wave velocity back to the age of the soil deposit. In the present case this was approximately 20,000 years. Failure to incorporate the effects of secondary compression could seriously affect the accuracy of laboratory evaluations of in situ moduli. When proper consideration was given to these effects, all three methods used gave consistent results.

  • Corporate Authors:

    Boston Society of Civil Engineers

    230 Boyston Street
    Boston, MA  USA  02116
  • Authors:
    • Trudeau, P J
    • Whitman, R V
    • Christian, J T
  • Publication Date: 1974-1

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Filing Info

  • Accession Number: 00261645
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Oct 22 1974 12:00AM