Geologic Hazards - Geotechnical Engineering Investigation - page 701

Slope Failure Forms Related to Geologic Conditions
Anticipation of the form of slope failure often can be based on geologic conditions as
summarized in Table 9.3. Detailed descriptions of the various forms are given in Section
9.2. Some forms of falls and slides in rock masses are illustrated in Figure 9.1. and Figure
9.2, slides in soil formations in Figure 9.3, and avalanches and flows in rock, soil, and mix-
tures in Figure 9.4.

Slope Activity
Slope activity relates to the amount and rate of slope movement that occur. Some failure
forms occur suddenly on stable slopes without warning, although many forms occur
slowly through a number of stages. Failure implies only that movement has occurred, but
not necessarily that it has terminated; therefore, it is necessary to establish descriptive cri-
teria for failure, or stability, in terms of stages. The amount and rate of movement vary
with the failure stage for some failure forms.
Slide forms of failure may be classified by five stages of activity:

1. Stable slope : No movement has occurred in the past, or is occurring now.

2. Early failure stage : Creep occurs, with or without the development of tension

cracks on the surface (see Figure 9.22) . Slump form movement velocities are gen-
erally of the order of a few inches per year.

TABLE 9.3
Geologic Conditions and Typical Forms of Slope Failures

Geologic Condition Typical Movement Forma

Rock masses: general Falls and topples from support loss

Wedge failure along joints, or joints, shears, and bedding
Block glides along joints and shears
Planar slide along joints and shears
Multiplanar failure along joint sets
Dry rock flow

Metamorphic rocks Slides along foliations

Sedimentary rocks Weathering degree has strong affect

Horizontal beds Rotational, or a general wedge through joints and along bedding planes

Dipping beds Planar along bedding contacts; block glides on beds from joint separation

Marine shales, clay shales Rotational, general wedge, or progressive through joints and along

mylonite seams

Residual and colluvial soils Depends on stratum thickness

Thick deposit Rotational, often progressive

Thin deposit over rock Debris slide, planar; debris avalanche or flow

Alluvial soils Depends on soil type and structure

Cohesionless Runs and flows

Cohesive Rotational or planar wedge

Stratified Rotational or wedges, becoming lateral spreading in fine-grained soils

Aeolian deposits Variable

Sand dunes or sheets Runs and flows

Loess Block glides: flows during earthquakes

Glacial deposits Variable

Till Rotational

Stratified drift Rotational

Lacustrine Rotational becoming progressive

Marine Rotational to progressive: rotational becoming lateral spreading: flows