. Mass wasting in coastal Alaska. Mass-wasting Alaska. Figure 7.—Combination debris avalanche-debris flows in a glacial till soil (Karta), Maybeso Creek valley, Prince of Wales Island. A, Aerial view of dissected till-covered slope where active sliding is occurring. Recent sliding, directly correlated with a period of high rainfall and resultant concentration of soil water, has occurred to the right of the leave strip in the center of the photo (1). These are probably associated with clearcutting, but there is also much evidence of prelogging slide occurrence. B, Closeup view of slide at site
. Mass wasting in coastal Alaska. Mass-wasting Alaska. Figure 7.—Combination debris avalanche-debris flows in a glacial till soil (Karta), Maybeso Creek valley, Prince of Wales Island. A, Aerial view of dissected till-covered slope where active sliding is occurring. Recent sliding, directly correlated with a period of high rainfall and resultant concentration of soil water, has occurred to the right of the leave strip in the center of the photo (1). These are probably associated with clearcutting, but there is also much evidence of prelogging slide occurrence. B, Closeup view of slide at site 1. Note the shallowness of the slide trace. The unweathered till which served as the sliding surface is exposed in center foreground as a lighter soil zone next to the long-butted stump. The weathered til! or Karta soil is the darker material above it. configuration in the zone of initial rotational failure, with the slide ma- terial more or less confined within a narrow channel before reaching the valley floor (type A, fig. 6). The slides begin on 34°-40° slopes, with most common occurrence on slopes at or near 37°. This angle corresponds to the laboratory-determined angle of internal friction of the till soil obtained from samples taken in the Hollis o / ' Excess soil water is probably the principal factor in triggering these 8/ Undisturbed pedestal samples tested by triaxial shear at the Civil Engineering Laboratory, Michigan State University, had an effective angle of internal friction () of 37° and an effective co- hesion (C) of 0. till slides. Soil permeability is good,— with subsurface flow concentrated above the unweathered till surface. During periods of extremely high rainfall fol- lowing initial saturation of the soil, excess soil water produces a rising piezometric level. At saturation, maximum soil-water levels produce active pore-water pressures great enough to substantially reduce shear resistance of the soil. Studies relating rainfall to piezometri
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