Geologic Hazards - Geotechnical Engineering Investigation - page 714

as illustrated in Figure 9.11 , and tilted fence posts, poles, and tree trunks. Straight tilted
tree trunks indicate recent movement ( Figure 9.12) , whereas bent tree trunks indicate old
continuing movement ( Figure 9.13) (see Section 9.5.2, Dating Relict Slide Movements).

9.2.2 Falls

General
Falls are the sudden failures of vertical or near-vertical slopes involving single or multiple
blocks wherein the material descends essentially in free fall. Toppling, or overturning of
rock blocks, often results in a fall.
In soils, falls are caused by the undercutting of slopes due to stream or wave erosion,
usually assisted by seepage forces. In rock masses, falls result from undercutting by
erosion or human excavation; increased pressures in joints from frost, water, or
expanding materials; weathering along joints combined with seepage forces; and differ-
ential weathering wherein less-resistant beds remove support from stronger beds (see
Figure 6.48).
Their engineering significance lies normally in the occurrence of a single or a few blocks
falling on a roadway, or occasionally encountering structures on slopes. At times, how-
ever, they can be massive and very destructive as shown in Figure 9.14.

Recognition
Falls are characteristic of vertical to near-vertical slopes in weak to moderately strong soils
and jointed rock masses. Before total failure some displacement often occurs, as indicated
by tension cracks; after total failure, a fresh rock surface remains and talus debris accu-
mulates at the toe (see Figure 6.67).

FIGURE 9.11
Creep ridges and erosion in residual soils after removal of vegetation (state of Rio de Janeiro, Brazil).