Blast waves are reflected from solid surfaces but not in the same manner as sound and light waves are reflected. When a bomb is detonated at some distance above the ground, the shock wave spreads out almost spherically until it strikes the ground. This initial wave, called the incident wave, is reflected by the ground surface. At a certain distance along the ground, from the point immediately below the bomb, the reflected wave combines with the original or incident wave to form a third wave which has a vertical front at ground level. This third wave is called a Mach wave and the point where the three waves intersect is called the triple point. At the triple point, where the incident wave is reinforced by the reflected wave, both peak pressure and impulse are at a max and each is considerably higher in value than that exerted by the original shock wave at the same distance from the point of expln. The Fig on next page illustrates the formation of a Mach wave and shows the path of the triple point
Thus by controlling the height of deton, it is practicable, utilizing the phenomenon of Mach reflection to control the region of max blast effect (See Ref 12,p 65ff)
Blast effects are enhanced also by confinement, due to reflection of blast waves by the confining surfaces. For example, a blast wave travelling through a tunnel, corridor, trench or even a street, decreases in intensity much more slowly than the samt wave in the open. If a bomb detonates within a building, there is considerable reflection of the blast wave, even if the walls are demolished. The overall effect of confinement of blast waves is to increase the vulnerable radii of demolition and visible damage(Refs 3,13,19&25)
The underground and underwater blast effects of an expln are more comparable with its open-air effects than with those observed under confinement(Ref 19) Blast Effects in Earth (Underground Blast). An expl chge which is detonated while buried deeply in earth exerts pressure almost entirel) on the earth about it and causes movement effects over a distance known as radius of rupture. If the depth of burial is less than the radius of rupture, the expln products blow through the surface of the ground and form a roughly circular depression known as a crater. An expln on the surface of the ground makes a shallow crater, which may have a greater diam than that produced when the expl is buried. This is due to the scouring action of gases projected downard from the expl chge Important factors in crater formation are the type of earth cratered and the type of expl used. Expln of a moderate chge in soft,
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