Interface stability of granular filter structures


Interface stability of granular filter structures

Theoretical design methods for currents C233

In hydraulic engineering granular filter structures are used for protection of structures against scour and erosion. For a proper functioning of a granular filter structure it is necessary that the interfaces between the filter structures, the subsoil and the water flowing above the filter structure are stable. Stability of the interfaces means that there is no transport of subsoil material through the filter to the water above the filter, and there is no filter material removed by the currents above the filter.

The publication Interface stability of granular filter structures presents an overview of the existing filter criteria. It focusses on two main points:

1.  interface stability as function of the thickness of the filter layer consisting of standard armourstone and gravel gradings, and
2.  interface stability of gravel mixtures with a wide gradation.

New formulas are also presented including their derivation. These formulas for geometrically-open granular filter structures subject to currents describe the interface stability as a function of the filter layer thickness and include the influence of the material grading and the influence of turbulence. Finally, examples of possible approaches are presented to design a wide-graded filter layer subject to armouring.

In the past many protective structures have been built using granular material, for example around bridge piers and spur dikes. In general, the structures consist of one or more layers. The filter structure should prevent erosion of base material and, therefore, sand tightness is required. This means that the internal stability (within the filter layer) as well as the interface stability (at the interfaces between the layers) has to be guaranteed. Due to larger design loads than assumed in the past, the interface stability is in some instances no longer guaranteed. Increasing the layer thickness might solve this problem.

Also large amounts of wide-graded gravel mixtures are becoming available on the market because of various projects in the flood plains of rivers. These mixtures may be useful to be applied in (bed) protection structures. However, the strength against currents is known insufficiently, viz in the armouring phenomenon in relation to the layer thickness.

For both typical situations it is assumed that the resistance of the material in the top layer i.e. the armour layer stability – against the erosive forces of the flow velocities is sufficient.

The publication presents an overview of the existing design formulas for interface stability, including remarks which formulas need improvement or are even missing. On the basis of the inventory recommendations are presented for further research including a list of priorities and quick wins. Four quick wins have been carried out and included in this report: a sensitivity analysis of the new equations, damping of the turbulence in a granular filter, adjustment of the Wörman formula for a bed protection, and a design method to assess the required thickness of a bed protection consisting of wide-graded material – armouring.

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Artikelnummer C233
Auteur(s) CUR B&I
Aantal pagina's 82
Jaar van uitgave 2010