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.