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an element of a built-up casing for underground structures such as tunnels and mine shafts. The most common type of tubbing is tunnel lining, which is usually circular. Tubbing is made of cast iron, steel, or reinforced concrete. Cast-iron tubbing is made in the form of a cylindrical shell with a flanged border turned inward. The flange is used for making bolted connections between individual elements and whole rings; it imparts to the tubbing the required rigidity and strength. The thickness of tubbing is determined by tension-compression calculations for various combinations of external loads. The width depends on the stability of the rocks and the cross section of the tunnel. Steel tubbing, which is usually welded, is used as a coupling element for cast-iron tubbing, and in some cases (cast or drop-forged tubbing), as a structural element of the main lining. Reinforced-concrete tubbing structures, or units, are used mainly for subway tunnel lining and are of two kinds, one with a curvilinear contour and the other with a flat surface (trough-shaped).
REFERENCETonneli i metropoliteny, 2nd ed. Moscow, 1975.
V. P. VOLKOV