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Rock Mechanics.pdf - Mining and Blasting

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TRANSIENT GROUND MOTION<br />

The most generally occurring surface wave is the Rayleigh wave. Particle motion<br />

induced by passage of the wave is backward elliptical, in a vertical plane parallel to the<br />

direction of wave propagation. The motion is equivalent to coupled P <strong>and</strong> vertically<br />

polarised shear (SV) waves. The nature of the ground motion is illustrated in Figure<br />

17.11b. The intensity of the motion dies out rapidly with depth. The propagation<br />

velocity of the R wave is given by the positive real root of the equation<br />

4C 3 2<br />

s Cp − C 2 2<br />

R Cs − C 21/2 2<br />

R = Cp 2Cs − C 22 R<br />

(17.6)<br />

For = 0.25<br />

CR = 0.926 Cs = 0.53 Cp<br />

A Love wave is generated at the ground surface when a layer of low modulus material<br />

overlies a higher modulus material. This is a relatively common condition, which can<br />

arise when a weathered layer overlies fresh rock, or when backfill has been placed to<br />

produce a level surface. In their refraction at the ground surface, waves are effectively<br />

trapped in the upper soft layer. A Love wave consists of coupled P <strong>and</strong> horizontally<br />

polarised shear (SH) waves, as illustrated in Figure 17.11c. A particle undergoes flat<br />

elliptical motion, so that the surface displacement corresponds to horizontal shaking.<br />

The propagation speed of the wave is a function of the wavelength of the motion, <strong>and</strong><br />

is given by the solution of the equation<br />

2<br />

G11 Cs1 − C<br />

G22<br />

2 L<br />

C2 L − C2 2 2H CL = tan<br />

s2<br />

C2 <br />

− 1<br />

(17.7)<br />

s2<br />

where subscripts 1 <strong>and</strong> 2 refer to the hard <strong>and</strong> soft media respectively, <strong>and</strong> H is the<br />

depth of the soft layer.<br />

Equation 17.7 indicates that the CL must lie in the range<br />

Cs2 < CL < Cs1<br />

Also, for the case ≫ H, CL tends to Cs1, indicating that wave propagation is occurring<br />

mainly in the hard layer. For ≪ H, CL tends to Cs2, indicating wave transmission<br />

is occurring mainly in the soft layer.<br />

The generation of surface waves above a blast site is important in that it affects<br />

the range of potential ground disturbance due to transient motion. Body waves are<br />

spherically divergent, so that the amplitude ur (<strong>and</strong> particle velocity) at some range,<br />

r, is related to r by equation 10.43, i.e.<br />

ur ∝ 1/r<br />

Surface waves are cylindrically divergent in the near-surface rock, so that, from equation<br />

10.46, ur, (<strong>and</strong> ˙ur) are related to r by<br />

ur ∝ 1/r 1/2<br />

Thus, body waves are subject to attenuation due to geometric spreading at a greater<br />

rate than surface waves. At points remote from an underground blast site, the induced<br />

surface waves are therefore the main source of transient motion.<br />

533

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