Dames & Moore, 1999 - USDA Forest Service

their more mafic-rich, generally smaller and more weathered nature are not good riprap types. In fact, the
Type C boulders have weathered to the extent that, in many cases, all that remains of the original boulders
are piles of gruss. The Schmidt-hammer tests generally corroborate the field observations, in that the Type
A boulders had higher percentage recoveries (52 to 59 percent), than the Type B (24 to 42 percent) and C
boulders (14 to 28 percent).
Types D (latite) and E (rhyodacite) appear to be better quality than Types B and C but at much smaller
proportions along the tailings. Type F (ferricrete) appears to have been used sparingly along tailings pile 2,
but is generally not a good riprap type because of the potential for the iron cement to break down.
In general, riprap deterioration appears to occur by three primary mechanisms:
grain by grain collapse (grussification), which creates entrainable sediment which is
removed during high flows
, boulders exfoliate into plates or flakes which reduce clast size
boulders fragment into small blocks along pre-existing weathered fracture planes
In terms of the breakdown of ferricrete boulders, the scattered material was observed in the field to be
crumbly and disaggregated, likely due to the breakdown of the cement (primarily iron) binding the
conglomerate. This is consistent with observations of the intact ferricrete, which varied in hardness and
cementation. The observed variation in cementation was apparently not only a result of the lack of cement,
but also in some cases due to the degree to which the cement precipitate had fractured, softened or
disaggregated in the channel environment.
In general, the breakdown of the riprap results in smaller boulder sizes with less resistance to motion under
high stream power, and less protection to the tailings slopes.
Based on the above criteria, the riprap condition was graded for each creek slope reach and is summarized in
Table 4.2-5. Overall, there are several areas with relatively competent boulders (i.e., reach 2-B); most of the
reaches have boulders that are relatively weathered (i.e., grussified). Some of the rocks were observed to be
grussified or broken down by weathering, in particular along tailings pile 1. In addition, there is.field
evidence that much of the gruss has been removed by high waters (i.e., high stream flows have eroded loose
sediments fiom riprap, so that a high water line exists where there is no gruss below the high water line).
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17693-005-019Uuly 19. 1999:4:51 PM:DRAFT FYAL RI REPORT