Civil Engineering Reference
In-Depth Information
Figure 12.4
Screeding frame used on the Fort McHenry Tunnel. (Photo courtesy of
W. Grantz.)
under water. The finished level created was to a coarse tolerance and, in
this instance, the tunnel elements were temporarily supported and the gap
between the tunnel and the gravel was filled with grout.
Scrading™
A different approach to forming a gravel bed was developed for the
Øresund Tunnel project and has subsequently been used on a number of
immersed tube schemes. The technique achieves a close tolerance finish on
a gravel bed. The Øresund Tunnel structure was originally designed for
a sand foundation and the design therefore did not take account of any
significant imposed deformations due to surface tolerance in the founda-
tion layer. The foundation solution was changed to gravel and this then
became an important design consideration as the gravel was overlying
limestone and any variations in gravel thickness would impose deforma-
tions on the tunnel structure. The gravel-laying procedure had to achieve
a certain tolerance so that the design of the structure would not need to be
changed. This resulted in the development by Boskalis of the multipurpose
pontoon (MPP), a floating barge that could span the width of the tunnel
trench, held in position using spuds on either side of the tunnel trench, as
seen in Figure 12.5. The barge was not fully fixed to the spuds and could
