An
illustration of several ways to shore e trench in unstable earth,
using various materials. A 4-piece frame (consisting
of 4 poles, or 4 boards, installed as shown above) should be
installed every 2,5 feet along the length of the trench, including
the horizontal parts of the entryways. All parts of the shoring
should be at least 2 inches below the roof poles, so that the
downward forces on the roof will press only on the earth.[1]
Since
the majority of structures can be destroyed by an overpressure
between 10 and 5 psi generated the shockwave, every
structure capable of withstanding an overpressure of at least
10 psi will be considered an efficacious shelter. Obviously,
assuming an underground shelter, the entryway is a most critical
element which must remain undamaged so that the shelter may
remain uncontaminated and fit for use. It should allow no dust
or water inside the shelter, and preferably it should be some
sort of reinforced door, slant (which
remains closed also through the effect of gravity) towards the
ground and with a cross section at least 20 centimeters larger
and higher than the entrance on all four sides, preferably with
some sort of protective dome-shaped embankment around it.