Key details relating to the construction method used for Bridge SAC/20 (Stainforth / Taitlands Tunnel) are provided in a contract document in the collection of the Midland Railway Study Centre (MRSC):
Where the Tunnel is in rock it shall be so worked that the blasting for the general excavation shall leave the top and sides solid and unshaken. The top and sides must be finished by rough pick dressing; no doubtful or loose pieces shall be allowed to remain on the sides or on the top. ... Where arching is required for supporting loose stone near and about the top, and no side walls to spring from, a proper skewback must be prepared and formed in the natural rock, and the Arch turned of the same dimension in brickwork as in the lined portion of the tunnel.
The brickwork is specified as:
... block-in-course masonry in courses not less than 8 inches deep, stretchers no less than 12 inches by 18 inches on the bed.
and
... all cavities behind the masonry ... shall be filled up with good, dry, and hard material, well punned, and brought up as the masonry or brickwork proceeds.
Source: SETTLE TO CARLISLE (Act, 1866.) Contract No. 1. Specification. J.S. CROSSLEY. ENGINEER. SEPTEMBER, 1869. MRSC Item Ref. 27560.
NB: This document might be a draft as some sections have been scribbled-over or altered in pencil.