Span by span construction - General
For long viaducts where access from below is limited, expensive or practically impossible construction of the deck can be started from one end, one span at a time. Initially abutments and piers are constructed. A gantry, with a length at least two successive spans, is initially positioned between one abutment and first inner support. Prefabricated deck segments are delivered from the abutment and transferred on to trolleys on the gantry for transport to their required position. Each segment is supported on at least three jacks to a predefined vertical alignment.
The joints between the segments are cast-formed using cementitious material or epoxy, that requires match casting. When all the segments are installed at their correct position and their joints are cured, post-tensioning cables are passed through the segments and stressed. At this time the segments are held together and can span between the two supports. The gantry is now moved forward to the next span, ready to receive the next set of segments. If spans are to act in continuous manner, the post-tensioning of subsequent spans could be coupled to the previous span or separate post-tensioning cables that stretch over a couple of spans could be employed for this purpose.
Special supports are normally needed at each pier to receive the gantry. The permanent structure should be temporarily strengthened to take all the loads which are imposed by the gantry.