pilot-tube

from The Century Dictionary.

• noun In subaqueous tunneling, a wrought-iron or steel cylindrical shell whose axis coincides with the axis of the tunnel, and whose diameter is less than that of the tunnel, open at both ends, and forced into the soil in advance of the finished tunnel, for the purpose of providing a protection to the men employed in excavating the material at the heading inside the tube, and also of furnishing a central support for the inner ends of radial struts placed outside the tube to support the material forming the roof and sides of the tunnel until permanently supported by the lining. The rear end of the pilot-tube is also used as a support for the radial struts, or the centering, used in constructing the permanent tunnel lining.