Quantcast SEQUENCE OF CONSTRUCTION (cont.)

 
  
 
Construction of Stabilization Works
(b) In all cases, it is advisable to construct the toe protection component as early in the
operation as is practicable.
(c) For projects involving large quantities of armor protection and a long construction
period, it may be advisable to place the armor in strips proceeding up the bank slope,
with the lowest strip following immediately behind the slope preparation. This reduces
the probability of a rapid rise in water level overtopping and eroding the unarmored
portion of the slope. A similar precaution is to limit the distance that slope preparation
operations can precede completion of slope armoring. That limit may also be stated in
terms of maximum allowable time interval between the two operations.
(d) The most economical procedure for construction of stone dikes in flowing water is to
construct them in vertical increments, or "lifts," if a significant amount of flow will be
intercepted by the structure during construction. If the dike is constructed to full grade
in one lift, a significant scour hole is likely to develop ahead of stone placement, which
will increase the amount of stone substantially over the original estimate.
Even if the lift procedure is used, some overrun due to scour and stone
displacement will occur, and should be provided for in the estimate of required stone
quantity. The amount of overrun will vary widely due to the following factors:
Amount of flow intercepted by the structure;
Erodibility of the bed material;
Rate of stone placement (rapid placement reduces overrun);
Gradation of the stone (larger stone reduces overrun); and
Height of the dike (higher structures increase overrun).
No quantitative guidance exists for estimating this overrun for a given situation;
therefore, it is usually based on experience. As an example, for dikes on the lower
Mississippi River, which are normally constructed in lifts of about 10 feet in height,
common practice for estimating the overrun is to assume an average bed scour of 5 feet
in front of stone placement in the initial lift only.
Placing a "blanket" of stone ahead of the placement of the initial lift will also
reduce overrun due to scour. However, placement of stone in a blanket takes longer
than placing an equivalent amount of stone in a "peaked" section within the ultimate dike
cross-section. Therefore, the reduction in scour provided by the blanket is offset to
some degree by the longer period of time required to complete the initial lift.
(e) For multi-stage construction of dikes, stone should be placed in the downstream part of
the dike cross-section first, because a scour hole will occur downstream of the stone
almost immediately as a "plunge pool" forms from flow over the stone. Placing
subsequent lifts of stone in the upstream part of the dike cross-section will require less
stone than if subsequent lifts were placed in the downstream scour hole.
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