922 



MISCELLANEOUS GEOPHYSICAL METHODS 



[Chap. 12 



factor is proportional to what is known in soil mechanics as the "friction 

 angle." Soils with large damping (or friction) factors are desirable as 

 foundation materials since they lessen not only the amplitude and the cor- 

 responding stress in case of resonance with extraneous vibrations, but also 

 the range of vibratory impulses (due to increased absorption). 

 A second quantity which may be determined from vibrator measure- 

 ments at the source is the p/iase %hijt 

 between impressed force and displace- 

 ment. At low frequencies this phase 

 shift is nearly zero, increases at reso- 

 nance to 90°, and gradually ap- 

 proaches 180*' at the higher frequencies 

 (see Fig. 12-18). It is possible to 

 calculate damping from the phase-fre- 

 quency curve (see Fig. 9-99). 



The variation of 'power with fre- 

 quency may be measured with a watt- 

 meter in the circuit of the vibrator 

 motor. The power rises rapidly with 

 frequency (see Fig. 12-18) and shows 

 a peak at resonance, since at maximum 

 amplitude the ground draws the great- 

 est power. The measured value 

 should be corrected for the no-load 

 variation of power with frequency 

 which can be determined by running 

 the vibrator with balanced masses. 



The compaction, or setting, of the 

 ground is obtainable from readings of 

 a strain gauge inserted between a tri- 

 pod and the vibrator underneath. 

 The variation of compaction with fre- 

 quency runs parallel with the phase 

 curve (see Fig. 12-18), the gradient 

 being a maximum at resonance. 

 Natural ground frequency and damp- 

 ing show a slight decrease with an increase in the mass of the vibrator. 

 The mass partaking in the oscillation .includes a portion of the surface soil 

 and is slightly greater than the vibrator mass. This equivalent mass 



Fig. 12-18. Variation of amplitude, 

 phase shift, power, and compaction 

 with frequency for a resonant surface 

 formation (after Spath). 



Ibid. 

 Ibid. 



