REP10784

• Permit No: M1988112
• IBM Index Class Name: Report
• Doc Date: 7/14/1995
• Doc Name: APPENDIX B.2 CPT PROBES
• Media Type: D
• Archive: No

' The critical measurement required for accurate velocity determinations is the interval time between
' signals. Shear wave signals are acquired at known depths (in this case, 1 meter vertical separation),
therefore imerval time is the only unknown in the velocity calculation. Using the wave trace from one
depth as a time reference and visually shifting the other wave trace along the time axis, it is possible
' to have the reference trace overlain by the shifted trace. The time offset that is required to achieve
a compatible overlay is the interval time. This technique for interval determination is the
cross-correlation method. The shear waves generated by the hammer and auger source are referred
' to as S-H, since particle motion occurs in the horizontal axis (parallel to the active axis of the
geophone) while wave propagation is vertical and downward. The S-H waves have srmall amplitudes
and are thus representative of a small strain phenomenon.
' Shear wave velocity measurements are unaffected by the percent fines, compressibility, layering or
other factors that infiuence penetration resistance to a lazge degree. Shear wave velocity is a measure
' of the stiffness of the soil skeleton and can be used to directly calculate the elastic parameters of the
soil (Go, Eo and v ). Shear wave velocity is largely controlled by void ratio and stress. Recent work
(Robertson et al, 1992 and Sasitharan, 1993) has shown that shear wave velocity can be used to
' assess the state of sand.
4.0 SPT ENERGY CALIBRATION
' The results of the standard penetrarion test (SPT) energy calibrations aze shown in tabular and
graphical form in Append'a F. The first set of plots show the energy from the blows with the average
energy plotted as a line. The tables summarize the blow counts and the energy for the recorded
sample irttervaLs. The tables also have the equivalem N-60 blow counts for the sample Intervals. The
field blow count, N-60 and percent of theoretical energy are shown on the last set of plots in
' , Appendix F.
' S.0 ASSESSMENT OF STATE FOR LIQUEFIABLE SOILS
5.1 Concept of State
t In 1936, Cassagrande showed that both the peak friction angle and the volume change behavior of
sand depends on the initial void ratio or density of the sand. Since then Critical State Soil Mechanics
' has shown that sand has a critical state, a combination of void ratio and confining pressure, ai which
it can be sheared with esserriially zero volume change. The critical state line separates states that show
increases in volume when sheared (dilative) from states that show decreases in volume when sheared
(contractive). Recently a number of methods have been developed in order to identi8y in situ state,
based on the results of in situ tests. Two of these methods are presented to assist in inCerpretation of
the volume change behavior of potentially liquefiable soils.
' S.2 State Based on Qc Criterion
' Robertson et al (1992) have suggested that the in situ state of clean sands can be determined using
measured cone bearing, Qc which has been normalized to remove the effects of overburden stress.
' Cor>ETec, Ixc. 3 Dmva, Colorado