A History of Research and a Review of Recent Developments

Loads on underground structures 83
where ρ is the mass density of the soil. The strain recovery ratio, M L/M U,
could well lie in the range 0.3 to 0.7 and to take account of this greater range
of conditions Hendron and Auld proposed an amended equation
(4.13)
where V L was in ft/sec. For a given peak overpressure and strain recovery
ratio a scaled as z/w 1/3 · V L. A further discussion of wave propagation in soils
and a review of elementary wave theory was given at about the same time by
Allgood [4.12].
It is generally accepted that when structures are buried very deeply and
subjected only to direct ground shock, the loading action due to this shock is
insignificant when compared with the loads from the dead weight of soil
cover. However, if a huge megaton range nuclear bomb were to burst directly
over a deep structure, it could cause extensive damage at depths of 500 feet
and over.
The ground shock wave from a very deep underground nuclear explosion
gradually weakens into a train of seismic waves that can cause ground motions
and earthquake-type damage a long way from the point of detonation. Structural
loading is influenced by the type of soil in which the structure is buried, and
by the soil-structure interaction effects associated with lateral ground motion.
As with all analytical work on buried structures, care must be taken when
using measurements taken under static loads that they are still applicable in
dynamic and vibrational conditions.
Design guides for underground structures under blast and shock loads, see
for example ref. [4.2], often distinguish between shallow-buried, surface flush
and earth-mounded structures. Surface flush structures have soil covers (h) over
the highest point of their upper surfaces in the range 0