Civil Engineering Reference
In-Depth Information
construction and the type of deck may need to be chosen for their ability to adapt to
such variations.
16.8.2 Cast-in-situ
The mechanised methods of deck erection for both the twin decks and the single
decks described above are still applicable for this length of viaduct. However, the
construction heads become remote from the site establishment, making it more
diffi cult to supply construction materials and supervise the work force. Also, the
rate of construction of two spans per week is most probably a maximum that cannot
be accelerated. If twin decks with 40 m spans were adopted and one falsework rig
was used to erect the 500 spans, the deck erection time would be some 70 months,
including the learning curve plus the time to reposition the rig. Thus it would
probably be necessary to mobilise at least four rigs, reducing the overall erection
time to some 17 months. Multiple rigs would be best served by a site situated at the
mid-point of the viaduct, so that they can all work away from their base, optimising
their supply lines.
If the deck is of constant width, the logical choice would be to opt for a single
wide deck, as a super twin rib. Two falsework rigs would build the deck in 27 months,
assuming the spans were 45 m, and that the learning curve involves a 12 week addition
to the programme for one rig.
16.8.3 Precast Tee beams
Precast beams remain a viable option for very long viaducts. They can cope well with
increases of width of the deck at interchanges, as both the spacing and the number
of beams can be changed from one span to another, even if the result is not very
elegant.
With six beams across the cross section and with spans of 35 m, there would be 286
spans and 1,716 beams. As the rate of erection of precast beams is not very fast, it will
be necessary to use several beam launchers. To avoid excessive transport of the beams
that weigh upwards of 80 tons, it is likely to be cost effective to mobilise several casting
yards along the length of the site.
For the GSZ project (
10.3.8
), which included 28 km of 15 m wide decks, there was
one main casting yard with 24 steel moulds equipped for steam curing located near
the centre of the viaduct, and several secondary yards at strategic positions along the
route. This method of building a long viaduct is relatively labour intensive, and can
achieve speed only by mobilising very large resources. This is likely to be cost effective
in developing countries where labour is cheap compared with the cost of plant.
16.8.4 Precast segmental
If twin boxes are adopted, 5,700 segments 3.5 m long are required. At a rate of erection
of 75 segments per month on average, 76 gantry/months are needed. Two erection
gantries per deck would complete the deck erection in 19 months.
As it would not be feasible to store thousands of segments, the number of moulds
needs to be calculated to match the rate of erection. Four gantries place 300 segments
per month, requiring a minimum of 15 moulds working at 20 segments per month.
