Quantcast Design Considerations

 
  
 
Surface Armor for Erosion Protection
gradations are sometimes called "quarry run" because little sorting is required after the
blasting operation in the quarry.
The required volume of stone in the trench can be computed according to guidance
provided in Appendix A, after the design depth of toe scour is either computed or estimated
based on previous experience.
Design of the trench is a compromise between economics and performance. A higher
trench bottom elevation reduces the volume of excavation and is less likely to require
expensive dewatering or difficult underwater excavation. Unfortunately, it also requires a
greater total volume of stone because allowing the stone to launch is less efficient than placing
it to the required thickness on a prepared slope above water. Thus, a higher trench elevation
requires a larger volume of stone to protect a given height of bank, because non-uniform
launching of the toe stone must be allowed for. The guidance for required quantity of toe
stone presented in Appendix A allows for this, but the fact remains that pre-placing stone
closer to its final position (that is, to a lower elevation) carries less risk than allowing it to
launch, particularly if the bank contains cohesive material which may retreat by mass failure
rather than eroding uniformly.
Because placing the stone in the trench to the lowest practicable elevation is desirable,
the elevation of the bottom of the trench is sometimes specified to be as much ten feet below
the river stage expected during the construction season, based on the assumption that
groundwater level in the trench will be about the same as the river stage. Ten feet of
underwater excavation is the most that is usually feasible with standard equipment without
dewatering. Careful supervision during construction is required, and the underwater trench
should be filled with stone in a continuous operation immediately behind the excavation
finishing operations.
A useful design refinement is to provide for a variable depth of trench, keyed to the
actual river stage during construction. This permits taking maximum advantage of low river
levels by lowering the trench so that the stone can be placed at a lower elevation. It also
allows the trench bottom to be raised if river levels are unexpectedly high. Construction can
then continue in spite of higher stages, without putting the contractor in an untenable position
by requiring more underwater excavation or dewatering than was anticipated in the original
bid. The specifications should set an upper limit of river stage, above which operations will
be suspended. Setting this upper limit is a subjective decision, determined by the urgency of
completing the work, the hydrologic characteristics of the river, and the experience of design
and construction personnel. The extreme case, if quick completion of the work is mandatory
in spite of high river stages, is to allow for substituting a stone windrow revetment,
constructed without excavation, in place of the trenchfill.
The design slopes of the trench are established by the most critical geotechnical
condition, depending on bank materials. This will usually be the fully launched condition.
The configuration of the riverside slope of the trench is governed only by construction
considerations, the only requirement being that the trench remain stable long enough for the
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