Civil Engineering Reference
In-Depth Information
and if it is too light, then it may be difficult to find room for sufficient
permanent ballast to provide the required safety factor against uplift. An
associated issue is the placing equipment that is going to be used. The
Conwy Tunnel in the U.K. was designed on the basis that a catamaran-
type placing rig would be used, which places no additional load on the
element while it is floating. However, the contractor decided to use two
pontoons to place the element. These pontoons were placed on top of the
element, so initially the element had to be sufficiently buoyant to sup-
port them while it was being floated to the tunnel site. The two pon-
toons weighed a total of 7000 kN, which had not been allowed for in the
design. Fortunately, the decision was made early on during the construc-
tion of the elements and it was possible to save an equivalent amount of
weight in the construction. The element had a concrete protective layer
on the roof, and by using lightweight concrete for the protection, suf-
ficient weight was saved to compensate for the additional weight of the
pontoons. If the method of placing has not been decided at the outset,
then the element should be designed to cope with the additional loads of
the pontoon method of placing. If they are not subsequently used and the
element freeboard is too large, then some permanent ballast concrete can
be placed inside the element before floatation.
Positioning of the element should be accurate. The primary end is posi-
tively located on the previous element, so the only variable should be the
position of the secondary end. This is subject to some variation as the initial
position of the secondary end depends, to a large extent, on the accuracy
with which the end frames have been set out. The Gina seal bears against
the steel end frame on the previous element. If the plane of this end frame
is not correct, then the secondary end will be misaligned during the initial
placing as the “direction” of the element is determined by the plane of the
end frames. If, for example, the end frame is 25 mm out of plane horizon-
tally, then as the aspect ratio of the element will usually be about 5:1, the
secondary end will be 125 mm off line. This can be rectified after the initial
positioning by simply lifting the secondary end and pulling it across to the
correct position. This will, however, result in differential compression in
the Gina gasket, which will try to even itself out and push the element off
line again. A more positive method is to install jacks in the element joint.
Then, if the position of the secondary end has to be corrected, it can be
lifted and swung across and the primary end can be locked into that posi-
tion with the jacks. Once the element has been backfilled, the frictional
forces from the backfill will prevent it going off line again and the jacks
between the elements can be released so that the long-term thermal move-
ment of the joint is not compromised.
Water density has to be assessed carefully, with consideration given to
extreme conditions while the element is in a temporary state. A storm
occurred on one project shortly after a concrete tunnel element had been
