The devastating flood (Box 9.1) associated with the Vaiont Canyon slide could have been avoided by not building the dam. The section of the canyon above the dam was recognized as having a high potential for large-scale, mass movements. Weak sedimentary strata, some highly porous and others rich in clays, dip almost parallel to the north-facing, canyon slope. The site was a "textbook" example of where not to build a dam. As the reservoir was filling, strata low on the slopes became saturated and substantially weakened; unfortunately, these strata provided lateral support to inclined strata higher up the slope. For weeks prior to the catastrophic slide (Oct. 9, 1963), downhill creep of the regolith above the reservoir was observed and documented. Creep rates increased drastically from a few centimeters per day to as high as 80 cm/day just before the slide occurred. Such high, accelerating, pre-slide creep rates should have alerted public officials to the danger; warnings should have been issued and the valley below the dam should have been evacuated. Nothing was done! This famous slide along the south side of the Gros Ventre River occurred in June, 1925. Tilted, sedimentary strata lie roughly parallel to the south slope of the valley; and the surface layer, a fairly hard, resistant sandstone, rests on a much softer, shale stratum. The river had gradually downcut into the shale layer, depriving the inclined, sandstone slab of any lateral (buttressing) support on the downhill side. Water from melting snows and rain seeped into the soil and bedrock, saturating the ground above the shale and weakening the top of the supporting shale layer. These conditions allowed a large, fractured slab of sandstone to break loose and rapidly slide downhill. The slide formed an instant, natural dam and was moving fast enough to climb a short distance up the opposite side of the valley. Two years later the dam burst, causing a tremendous flood on the lower Gros Ventre and upper Snake Rivers The shearing action produced by the moving soil causes weak, partly weathered, bedrock strata beneath the soil to bend in a downslope direction. As most trees grow, their trunks and rootballs develop in straight line continuity at right angles to a horizontal plane, not at right angles to the slope of the land surface. If a tree sprouts in an area of active soil creep, a bend develops between the base of the trunk and the rootball because the deeper roots are anchored in stationary regolith while the shallow roots are moving slowly downhill with the surface soil layer. Thus the tree seems to be growing laterally (sideways) out from the slope. For similar reasons, manmade features such as posts and utility poles, gradually rotate in the downslope direction from their original, vertical orientations; and linear structures, such as walls and fences built at right angles to the slope direction, may show bending and displacement from their original, straight line construction due to different soil creep rates at different locations along the slope. These relations are well illustrated in Figure 9.16.
Wetting and swelling of clay minerals and frost heaving (when water in open pores in the soil expands as it freezes) push overlying soil particles upward and raise the elevation of the soil surface very slightly. As the soil dries, or the ice melts, the soil shrinks, and the particles move a slight distance downslope, under the pull of gravity. Repetitions of these processes (Fig. 9.15) over long periods of time can produce slow, downhill movement of the entire soil layer. In response to gravity, moist, clay-rich soils deform internally and slowly move downhill over the deeper regolith or bedrock.
Permafrost refers to water-bearing soil, weathered rock debris, and porous bedrock that stay frozen year round. Permafrost is common in northern Canada, in most of Alaska north of the panhandle, and in most of northernmost Russia and northern Siberia. Summer melting saturates the "active", thawed surface layer and makes it susceptible to solifluction movements (Fig. 9.17). Heated structures in contact with permafrost can also induce thawing, causing foundation soils to loose weight-bearing capacity and to undergo solifluction flowage. For these reasons, buildings, highways, railroads, and other engineering projects in permafrost regions are subject to special design and maintenance considerations