Evaluation of Earth-covered Structures.
Aircraft missiles impacting earth media will, to varying degrees, disintegrate and/or penetrate
the ground, depending on the characteristics of the missile (such as impact velocity, size,
shape, weight, rigidity, and material properties) and the earth medium (clay, rock, etc.). A
missile can directly impact an earth-covered structure if the earth cover is not sufficient to stop
it. Such missiles also generate a pressure pulse, which is propagated through the earth media
and acts on the buried structures.
Analysis and experiments performed to evaluate missile penetration into earth media and
reported in the literature primarily deal with rigid or nondeformable missiles (e.g., ballistic). On
the other hand, aircraft impact forcing functions reported in the literature have all been derived
assuming a deformable missile impacting a rigid barrier. In the case of aircraft crash, two types
of missiles impacting the ground can potentially be generated. First, the fuselage, which is a
hollow, stiffened, thin shell structure, deforms upon impacting the ground. Second, the engines
and other solid components, which are relatively rigid, essentially act as nondeformable
missiles, although engines have been shown to deform on impact with rigid barriers
In the case of nondeformable missiles, the primary concern for buried structures is direct impact
if the depth of the earth is insufficient to stop the missile. The depth of penetration will depend
on the missile characteristics (shape, weight, velocity, material properties, etc.) and the
resistance offered by the ground. For the various classes of aircraft under consideration (i.e.,
general aviation, commercial aviation, and military aircraft), their impact velocities, and the
strengths of the earth media (stiff soil, sand, clay, etc.), a set of conservative bounding earth
media depths can be determined. Structures embedded beyond these depths will not be
impacted by the rigid aircraft missiles.
Penetration of a rigid missile into an earth medium has been addressed empirically
have been compared with a limited number of tests performed with ballistic-type projectiles.
Formulas have also been presented (Reference 14) to calculate the velocity decay and the