The bottom left image [1] in the preceding page shows, in cross sectional view, an expediency shelter dug in the ground covered by an earth arching protective structure resting on wooden logs. "To attain earth arching, the earth covering the yielding roof must be at least as deep as half the width of the roof between its supports. Then the resultant earth arch above above the roof carries most of the load."[1]
The image shows "... how a flexible roof yields under the weight of 30 inches of earth mounded over it and how earth arching is developed. After the arch is formed, the only weight that the roof supports is the weight of the small thickness of earth between the roof and the bottom of the arch." [1]

In the picture below, [1] we can see the attenuation factor when it comes to shielding from nuclear radiation, i.e. gamma rays. Each layer shown (packed earth, thickness 3.6 inches or ± 9 cm) will reduce the intensity of incoming gamma rays by 50%.
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