steady-state vibratory pavement responses and compression wave veloci-

ties are measured from impulse (seismic) tests. The former usually

follows procedures developed by researchers at the Royal Dutch Shell

Laboratory (43,53,93), the British Road Research Laboratory (79), and

the Waterways Experiment Station (79). They utilized a mechanical

vibrator for low-frequency vibrations (5-100 Hz) and a small electro-

magnetic vibrator for the high-frequency work (43,53). The general

procedure currently in use is to place the vibrator on the pavement

surface and set the equipment in operation at a constant frequency.

Details of the procedure can be found in Reference 79.

 Seismic tests may be conducted to determine the velocity of com-

pression waves, which can be used with the shear wave (or Rayleigh wave)

velocity to compute Poisson's ratio. One such method is the hammer-

impulse technique in which the pavement is struck with a light hammer

and the resultant ground motion is observed at one or more points with

horizontal motion geophones. However, this method is only good for

soils where the velocity of the materials increases with depth. It is

not applicable to layered pavement systems where strong, high velocity

layers occur at the top and grow progressively weaker with depth. How-

ever, Moore et al. (79) report that this procedure has been used to

obtain compression wave velocities of pavement layers during construc-

tion.

 A method of using surface waves to structurally characterize pave-

ments is currently in the research stage at the University of Texas at

Austin (80,81,82). The technique, called Spectral Analysis of Surface

Waves (SASW), determines shear wave velocity at soil or pavement sites.

The elastic shear and Young's moduli profiles are then calculated under