Tt is rather surprising to 1 galleries below ground need walls twice as thi those above ground. Thi is due to the ce: ment of the explosion when it oe: below ground. If a bomb explodes beside a wall above ground, the explosion away from the well will be into the air and will be virtually unresisted. But if it explodes beside a wall below. gran the explosion away from the wall will be resisted by the earth. This will make the effect of the explosion against the wall much more violent. When the explosion cannot move fras bribed as tamped, its local violence is increased.
The success of the Anderson shelter is due especially to its good behaviour against ea movements. It is a flexible structure, and in many instances has been shifted one foot by a near explosion without the occu!
A concrete or brick undercrourd will resist such a big earth movexaent must be made much more strongly, because it is more rigid and less flexible and is unable to "give." One remedy for this drawback is to ngo that underground concrete or brick shelters shelters should be surrounded by an air space one or two feet wide, so that the earth movement due to local sions will not act directly on the wall but will expend itself in fi ing earth into the cave But to return to the bomb-resisting shelter. Its underground walls should be 6ft, Gin, thick and the floor should be Sft. thick. TI to keep out bombs that have penetrated the ground sideways and burrowed under the floor of the shelter.
It is possible to make a rough estimate of how much material would be neeeed to make such shelters for a substantial fraction of the population in the most important and dangerous places, say for 5,000,000 persons. For this, some 530,000,000 cubic feet of concrete, 4.975.000 tons of cement, and 625,000 tons of steel would be needed. These quantities of extraordinary The annual output of the Gritish cement in dustry is about 7,000,000 tons, and there seems to be no good reason why its production should not have been expanded by 60 per cent for such an important object.
Text read from the printed page and cleaned; old type leaves the occasional error.