Various inventors attempted to improve upon the Brunel shield, aware
of the fundamental soundness of the shield principle. Almost all
bypassed the rectangular sectional construction used in the Thames
Tunnel, and took as a starting point a sectional shield of circular
cross section, advanced by Brunel in his original patent of 1818.
James Henry Greathead (1844-1896), rightfully called the father of
modern subaqueous tunneling, surmised in later years that Brunel had
chosen a rectangular configuration for actual use, as one better
adapted to the sectional type of shield. The English civil engineer,
Peter W. Barlow, in 1864 and 1868 patented a circular shield, of one
piece, which was the basis of one used by him in constructing a small
subway of 1350 feet beneath the Thames in 1869, the first work to
follow the lead of Brunel. Greathead, acting as Barlow's contractor,
was the designer of the shield actually used in the work, but it was
obviously inspired by Barlow's patents.
The reduction of the multiplicity of parts in the Brunel shield to
a single rigid unit was of immense advantage and an advance perhaps
equal to the shield concept of tunneling itself. The Barlow-Greathead
shield was like the cap of a telescope with a sharpened circular ring
on the front to assist in penetrating the ground. The diaphragm
functioned, as did Brunel's breasting boards, to resist the
longitudinal earth pressure of the face, and the cylindrical portion
behind the diaphragm bore the radial pressure of roof and walls. Here
also for the first time, a permanent lining formed of cast-iron
segments was used, a second major advancement in soft-ground tunneling
practice. Not only could the segments be placed and bolted together
far more rapidly than masonry lining could be laid up, but unlike the
green masonry, they could immediately bear the full force of the
shield-propelling screws.
Barlow, capitalizing on Brunel's error in burrowing so close to the
riverbed, maintained an average cover of 30 feet over the tunnel,
driving through a solid stratum of firm London clay which was
virtually impervious to water. As the result of this, combined with
the advantages of the solid shield and the rapidly placed iron lining,
the work moved forward at a pace and with a facility in startling
contrast to that of the Thames Tunnel, although in fairness it must be
recalled that the face area was far less.
The clay was found sufficiently sound that it could be readily
excavated without the support of the diaphragm, and normally three
miners worked in front of the shield, digging out the clay and passing
it back through a doorway in the plate. This could be closed in case
of a sudden settlement or break in. Following excavation, the shield
was advanced 18 inches into the excavated area by means of 6 screws,
and a ring of lining segments 18 inches in length bolted to the
previous ring under cover of the overlapping rear skirt of the shield.
The small annular space left between the outside of the lining and the
clay by the thickness and clearance of the skirt--about an inch--was
filled with thin cement grout. The tunnel was advanced 18 inches
during each 8-hour shift. The work continued around the clock, and the
900-foot river section was completed in only 14 weeks.[4] The entire
work was completed almost without incident in just under a year, a
remarkable performance for the world's second subaqueous tunnel.
[Illustration: Figure 21.--ENLARGED DETAIL of Brunel's tunneling
shield, vertical section. The first two and part of the third of the
twelve frames are shown. To the left is the tunnel's completed brick
lining and to the right, the individual breasting boards and screws
for supporting the face. The propelling screws are seen at top and
bottom, bearing against the lining. Three miners worked in each frame,
one above the other. MHT model--3/4" scale. (Smithsonian photo
49260-G.)]
[Illustration: Figure 22.--BROADSIDE PUBLISHED AFTER COMMENCEMENT
OF WORK on the Thames Tunnel, 1827. (MHT collections.)
OPEN TO THE PUBLIC EVERY DAY (_Sundays excepted_) _from Seven in the
Morning, until Eight in the Evening_,