926 EXPLORATION GEOPHYSICS 



jected to alternating mechanical forces by use of a special type of oscil- 

 lator.t The soil is set vibrating in forced and damped oscillations, 

 having any desired frequency and amplitude, by adjusting the frequency 

 of alternation and the magnitude of the applied forces. Furthermore, 

 by employing seismographs to record the motion of the soil, various 

 characteristics of the motion are measured and the data thus obtained 

 are used to draw inferences on the bearing capacity, composition, natural 

 period, and compressibility of the soil. 



Operating Principles 



The oscillator for producing the artificial vibrations usually com- 

 prises two discs or cylinders revolving in opposite directions and provided 

 with eccentric loading. (Figure 577.) The magnitude of the centrifugal 



>5t^- f jf.- M- '^- 'M 



Fig. 577. — Oscillator for producing artificial vibrations. (Bernhard, 

 A.I.M.E. Geophysical Prospecting, Tech. Pub. 834.) 



forces produced by the rotation of the discs depends on the eccentricity 

 of the loading, and the frequency of alternation of the forces depends upon 

 the speed of rotation. Usually, the horizontal centrifugal forces are neu- 

 tralized and only the vertical forces are utilized in the studies. 



The dynamic properties of the soil may be determined by measuring: 

 (a) the amplitude of the soil motion at the oscillator; (b) the amplitude 

 of the soil motion at various distances from the oscillator and at various 

 depths ; (c) the time phase difference between the applied (oscillator) 

 forces and the vibration on or in the soil; (d) the settlement of the os- 

 cillator on the soil; and (e) the natural period of the site. 



The soil motion characteristics enumerated above are related to the 

 magnitude and frequency of alternation of the exciting forces. To simplify 

 the analysis, the following assumptions will be made:t (1) The oscillator 

 is a small mass vibrating on the surface of a semi-infinite elastic medium. 

 (2) Vertical forces only are effective. (3) The damping is proportional 

 to the speed of propagation of the disturbance. (4) The deformation 

 of the soil varies linearly with load and settlement. 



t R. K. Bernhard, "Geophysical Study of Soil Dynamics," A.I.M.E. Geophysical Prospecting, 

 Tech. Pub. 834. Feb. 1938. 



t Bernhard, loc. cit. See also, A. Hertwig, G. Angenheister, R. Koehler, and A. Ramspeck, 

 "Application of Dynamic Soil Investigation," Trans. German Soc. Research in Soil Mechanics 

 (Degebo) (1936) No. 4. 



