Construction of Stabilization Works
construct most works. If the stream is not navigable, but sufficient water for floatation exists
at the worksite, portable barges can be used as access ways or working platforms for
equipment and materials. This may allow the use of smaller machines than if all work is
constructed with shore-based equipment, and will provide environmental benefits by reducing
site disturbance from on-shore working and staging areas.
The procurement of rights of way should also consider the need for permanent access
for monitoring and maintaining the completed work.
(a) For projects which are not likely to have significant channel changes during construction,
sequence of operations can be left to the discretion of construction personnel so that the
most efficient procedure can be used.
However, a common practice is to specify that construction begin at the upstream
end of each worksite. This may seem to be inconsistent with the principle stated in 6.1.1
that the downstream end of the work is usually the most vulnerable to damage.
However, the apparent discrepancy is resolved by noting that vulnerability of the
downstream end is a relatively long-term process, related to the opposite bar moving
downvalley and encroaching into the channel. Conversely, a construction operation will
be of relatively short duration, during which the attack on the downstream end is unlikely
to increase. Thus, at least for armor protection, the tradition of beginning work at the
upstream end probably stems from a fear that bank erosion and channel migration of the
upstream portion of a bend while construction is underway might upset channel
alignment more severely than similar erosion downstream during the work. For indirect
protection, a more tangible case can be made for beginning upstream. Completed or
partially completed indirect protection structures upstream often reduce streamflow
attack on the structures downstream as they are being built, sometimes to the point of
inducing deposition, thus reducing the quantity of materials required for the downstream
structures. The result is a de facto "incremental construction" approach. However,
significant amounts of induced deposition at the site of uncompleted structures
downstream may dictate changes in design or construction procedures, which may in
turn require extra contract administrative effort, but with the reward that total cost of
the work is often reduced.
Theoretically, a case can be made for beginning construction at the point on the
channel that maximum erosion is occurring, then completing construction as rapidly as
possible in both directions. This is rarely done in practice because it requires two
concurrent operations, which may limit the number of contractors capable of the work
and increase the cost of the work. For emergency jobs, though, or situations where rapid
erosion is occurring, such an approach may be advisable.
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