headwall

plural headwalls

1. geographyThe highest cliff of a glacial cirque.
2. geographyThe steep cliff at the back of a cirque.
3. civil engineeringA retaining wall at the outlet of a drain, culvert or tunnel.

   The platforms of all stations from Shepherds Bush to Liverpool Street have been lengthened from 325 ft. to 427 ft. between tunnel headwalls, to take longer trains.

   1950 September, “Central London Railway Jubilee”, in Railway Magazine, page 622

   1 more example

   HS2 Ltd has built a 17-metre-high headwall and reinforced the ground at what will become the south portal of the ten-mile Chiltern Tunnel, as part of preparations for the arrival next year of tunnel boring machines (TBMs).

   2020 August 12, “Network News: HS2 tunnel portal readied for TBMs”, in Rail, page 10

   Synonyms: endwall