(call it simply
) would be realized if the weight of any arch is entirely held up by the friction on the sides (induced by the lateral thrust) and the cohesion acting there; for then this same condition of affairs would exist for all lower arches,[Footnote 29] ] and thus
is transmitted vertically downward, unchanged, to the tunnel. Stating this condition in algebraic form,
This equation strictly holds for large values of
, but it is practically true for much smaller values. For such values,