Saturday, June 7, 2014

Geo 730: June 7, Day 524: Rocks 500 Feet

This being late winter/early spring, I fully expected we would see some mass movement (e.g. landslides, mudflows, rockfalls, slumps, etc.) along the coast or in the Coast Range. Winters are very wet along the coast and associated mountains, and in the Cascades as well, much more so than here in the valley. That water adds weight, provides some lubrication, but more importantly than the latter, increases pore pressure. This has the effect of "floating" much of the weight of the overburden- that is, counteracting the normal force. Friction is the product of a constant, "the coefficient of friction" (constant for a given material, but not across varying materials) times the normal force, which is essentially equivalent to the weight of material sitting on a surface times the cosine of the dip (angle of slope) of that surface. Cutting through the math, increasing the pore pressure of water in a soil and rock profile drastically reduces friction, not by "lubrication," but in a sense by decreasing its effective weight. But not its actual weight... when all that stuff falls in the road, it's still a heckuva lot to clean up.

Photo unmodified. March 9, 2012. FlashEarth location (Fairly confident about this one, but not certain).

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