Horseshoe section

What a horseshoe section is

A horseshoe section is a tunnel profile with a near-semicircular arch above and near-vertical side walls below. The outline resembles a horseshoe, which is where both the Japanese and the English name come from.

Japan's government railways adopted it as the standard profile for single-track tunnels in the Meiji period. The former Chuo Line Tamano Third tunnel in Aichi Prefecture, built in 1900, is described as following that standard, with a brick shell and Refuge recess inside, on a stretch through the gorge of the Shonai river where many tunnels were needed.

The Tamano Fourth tunnel on the same line, also from 1900, is described as a straight tunnel 75 metres long with a single-track profile of this kind and side walls laid in Stretcher bond.

The former Hokuriku Line Funagatani tunnel in Fukui Prefecture, from about 1895, is described as a straight tunnel 64 metres long with the same single-track profile as the others on the line, its side walls in dressed stone and its arch in brick stretcher bond. The phrase acknowledging that the others match indicates a profile standardised across the route.

Profile and material are separate facts

Those three entries show the point. The profile stays constant while the materials change: one tunnel has brick side walls, another dressed stone with a brick arch. The relationship is the same as that between the shape of a roof and its covering.

The lining is the shell built against the excavated rock. Side wall and arch are often built of different materials, and the records follow that division.

Standing at the mouth and looking through settles it. A ceiling curving as a half circle over walls that rise almost vertically is this profile. Rectangular sections also exist: the former Chuo Line Kasaishibora Culvert in Aichi Prefecture is described as rectangular in section, formed by a shaft of about nine metres meeting a horizontal drift of about eleven.