Re: Effects of shockwave on ground

"Crown-Horned Snorkack" <chornedsnorkack@xxxxxxx> wrote in message
news:1192220048.914215.150970@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On 12 okt, 08:41, "Carey Sublette" <carey...@xxxxxxxxx> wrote:
"Crown-Horned Snorkack" <chornedsnork...@xxxxxxx> wrote in message
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On 10 okt, 17:34, "Carey Sublette" <carey...@xxxxxxxxx> wrote:
"Crown-Horned Snorkack" <chornedsnork...@xxxxxxx> wrote in message
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On 9 okt, 15:04, "Carey Sublette" <carey...@xxxxxxxxx> wrote:
....
Indeed it is. The surface was set in motion (that's what happens when
something absorbs momentum). This is separate from spalling however.

If the surface is a cohesive rock mass then the stresses may simply be
transferred laterally in a compressive wave.

But if the surface has spalled, it no longer is cohesive.

(What happens to the momentum of a hammer when you slam it into a steel
beam? A compression wave propagates though the steel carrying off the
momentum to distant regions.)

And eventually, the far end of the beam flies off.
....
As long as the shockwave is travelling in homogenous solid rock away
from surface, it does not care where the surface is. In the front of
the shockwave, the rock is converted from undisturbed rock at its
initial density to compressed rock. Plainly the rock must move at
least slightly to compress the rock ahead, so obviously the rock in
the rear of shockwave is moving forward.

Spalling occurs when the rock decompresses and has
time for the rarefaction wave to move a distance back into the rock.
The
ejection of spalled material is everywhere a local phenomenon (only
the
immediate fractured block is affected) that is normal to the
surface.
There
could conceivably be some horizontal component of motion imposed in
addition
to this by the effect of the earlier lateral stresses, I suppose,
but
probably not since spalling occurs in strong solid materials.

Once a thread devolves into highly fragmented mass of criticisms of
individual sentences, the best way to proceed is to start over.

There are two separate processes involved that occur at different times
involved: upon arrival at the surface the shock wave transfers momentum,
setting the surface in motion and also compressing it. The compressed
material, already set in motion then begins expanding. Although the original
momentum may have horizontal as well as vertical components (and thus induce
horizontal and vertical motion), the expansion of compressed matter is
vertical only, because that is the only direction where pressure is low.
After some short time has past, so that the rarefaction wave can move back
from the surface, fracturing occurs and the material that has acquired
vertical velocity from expansion fies free. This is superimposed on any
shock momentum induced motion, but spalling is normal to the surface only.

Until the surface fragements though, the lateral momentum is being
transferred in a compressive wave along the surface. This tends to carry
these stresses away from the zone of strong compression where spalling
occurs, but even though this compressive wave will be faster than the
rarefaction process it is probably is not complete when spalling occurs
(thus some horizontal component of motion still persists).


.

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