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Aviation History
1962
1962 - 1962.PDF
GOODMANS VIBRATOR SUPPORTING FRAME SURFACE BOLT. GROUTED IN up is amplified and the waveform is displayed on a cathode ray oscilloscope. In order to obtain a fixed reference phase mark, a suitable circuit blanks out the oscilloscope trace for a short period at an identical point in each cycle of the output of the oscillator which is used to drive the power amplifier. The pick-up is moved in contact with the surface of the road, as shewn, radially away from the vibrator along a measuring tape and the positions are noted at which the phase mark occurs at either the peak or the trough of the displayed waveform. The intervals between these positions are then equal to half the wavelength of the signal in its path between the vibrator and the pick-up. The distance moved by the pick-up between the coincidence of one peak or trough and the next with the phase mark on the screen is clearly equal to the wavelength of the transmitted signal. 8O0 SPRING Fig. 2 Bv coupling the end of the plunger of the Goodmans Vibrator to a leaf spring, its output can he increased if the Vibrator is run at approximately the natural frequency of the spring. A series of springs, either singly or in combination, covers the whole range required. Tests for bearing capacity of this nature are lengthy and require considerable care and skill in execution, if reliable results are to be obtained. Moreover, the figure siven is dependent on the weight of overburden met with in the particular case. Because the C.B.R. has been determined for a particular material under certain conditions, it does not necessarily — or even probably — follow that it will be the same under other conditions. It would therefore seem to be highly desirable to use indirect tests to determine the C.B.R. if this is at all possible. The Department of Scientific and Industrial Research are, in fact, working on this and similiar problems at their Road Research Laboratory at Har- mondsworth in Middlesex. As seen from Fig. 1, the typical set-up used employs electronic equipment, in which a power amplifier feeds 40 watts of power at a frequency controllable between 40 and 2,000 c/s into a Goodmans Type 390a Vibrator as manufactured by Goodmans Industries Limited of Wembley. This vibrator is of the moving coil type, having a plunger which oscillates in phase with the applied current which has a magnitude of about 2 amperes. The vibrator is mounted on a base plate as shown in Fig. 2. To amplify the force of vibration transmitted to the road surface, the stiffness of the coil suspension is increased by a steel leaf spring which tunes the mass of the coil and plunger so that the assembly is approximately in resonance at the frequency being applied to the vibrator. One end of this spring is rigidly clamped to the base plate and the other end to the plunger of the Goodmans Vibrator. A series of springs of various stiff- nesse is provided and, by using the springs singly or in comb nations of two or more, a total of four or five s prin , have been found to cover adequately the fre- quen. range from 40 to 500 c/s over which this work is at pn ent being carried out. Th vibration is fed into the road surface through a h °lt! juted into position, or by a similar rigid coupling. ll tra is a certain measured distance and is picked up by a gee ,one or seismic pick-up. The signal from the pick- *» ITY :LO C > 700 600 500 400 •s / /^\ VELOCITY IN SUBGRADE i IOO 200 300 FREQUENCY c/s Fig. 3 Dispersion of velocity of Rayleigh waves in a three-layer road construction. By varying the frequency applied to the vibrator, it is possible to obtain a table of the wavelengths at different frequencies. Multiplying wavelength and frequency gives the actual speed of the wave in the ground. The results obtained show that, at low frequencies, the velocity is fairly constant and is, in fact, the velocity of Ravleigh waves in the sub-grade. As the frequency increases a point is reached where the velocity begins to increase due to the influence of the base materials. The actual frequency at which this change occurs depends upon the relative elasticities of the base and sub-grade and the thickness of the base. At much higher frequen cies the properties of the road carpet affect the dispersion curve and ultimately, at very high frequencies, the velocity is determined solely by the physical properties of the carpet. There is some evidence to indicate a correlation between the Rayleigh wave velocity in the sub-grade and the Californian Bearing Ratio, and work on these lines is proceeding at the Road Research Laboratory. If a definite correlation can be found, important possibilities are at once opened up. A test such as that described takes only a short time and gives information on the properties of all strata making up the road. For information on other seismic applications please contact Goodmans Industries Technical Advice Depart ment.
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