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<br />
<br />the slope divided by the total shear force ex-
<br />erted, f. Thus, s=r/f, and failure occurred when
<br />the ratio fell below LO - that is, when shear
<br />force exceeded shear resistance, Moisture, which
<br />reduces r, was (and is) the chief cause of fail-
<br />ure, although many other factors generally con-
<br />tributed to the failure. The addition of moisture
<br />to marginally stable slopes on May 5-6 reduced
<br />the safety factor to LO or less by (1) weaken-
<br />ing the cementing bonds between soil and rock
<br />particles and, hence, reducing the internal shear
<br />resistance of the slopes, (2) adding weight to
<br />the potential slide mass, and (3) increasing the
<br />pore-water pressure in the interstices of the
<br />rock or soiL The effect of increased pore-water
<br />pressure was to "float" or suspend soil or rock
<br />grains relative to one another, thereby drasti-
<br />cally reducing the shear strength of the mass
<br />and precipitating failure,
<br />Still another effect of adding moisture to the
<br />ground on May 5-6 was related to the behavior
<br />of soils that contain plastic clays having high
<br />swell-shrink ratios. Such clays, often called
<br />"bentonite," are common soil constituents in
<br />many parts of Greater Denver. They contain
<br />large proportions of certain clay minerals, such
<br />as montmorillonite, which have the property of
<br />greatly increasing their volume by the adsorp-
<br />tion of water, or of shrinking with the loss of
<br />water. The adsorption of water not only changes
<br />
<br />the vol ume of the soil but, at the same time,
<br />adds weight and reduces shear strength which
<br />reduce the safety factor and lead to slope fail-
<br />ure.
<br />SLUMPS AND EARTHFLOWS
<br />Slumps and earthflows were among the more
<br />prevalent kinds of landslides (fig, 12), Many
<br />existed in the western part of Greater Denver
<br />before the May 5-6 storm, many of these were
<br />further activated by the storm, and many new
<br />ones appeared during or after the storm. In
<br />the Greater Denver area sloping ground that is
<br />underlain by the Lykins, Morrison, Benton,
<br />Pierre, Laramie, or Denver Formation is more
<br />prone to landsliding than is other ground, be-
<br />cause of the high proportion of plastic clay in
<br />these formations. Landslides occurred, however,
<br />even on the Precambrian terrane back in the
<br />mountains.
<br />A slump moves as a coherent block by dis-
<br />placement along a curved shear surface. The
<br />block rotates and slides about a horizontal axis
<br />on its center of gravity. Motion is arrested
<br />when the rebalancing of weight or an increase
<br />in friction returns the safety factor to more
<br />than LO. Because of their rotational habit,
<br />slumps tilt backward at their heads and bulge
<br />outward at their toes, and even slight movement
<br />is disruptive to a superincumbent structure, Most
<br />slumps in the Greater Denver area were com-
<br />
<br />
<br />A
<br />
<br />B
<br />
<br />'~f~"".
<br />tY,
<br />I
<br />---- ~=./
<br />~/
<br />'~
<br />/
<br />
<br />c
<br />
<br />D
<br />
<br />FIGURE 12. - Landslide types common in the western part of the Greater Denver area. From left to right: A,
<br />Rockfall moves mostly by free faU, bounding, and rolling; B, slump by rotational slippage on concave-up.
<br />\vard shear surfaces; C, debris slide by complex internal adjustments of highly deformed, sheared slide mass;
<br />D, earthfiow by displacements and velocities similar to those of viscous fluids (Varnes, 1958, pI. 1). Illustra-
<br />tions by Natalie J. Miller, from ~i1sen (1972).
<br />
<br />13
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