Geologic Hazards - Geotechnical Engineering Investigation - page 722

9.2.4 Rotational Slides in Rock

General
In the rotational slide form, a spoon-shaped mass begins failure by rotation along a cylin-
drical rupture surface; cracks appear at the head of the unstable area, and bulging appears
at the toe as the mass slumps ( Figure 9.2a) . At final failure, the mass has displaced sub-
stantially, and a scarp remains at the head (see Section 9.2.5 for nomenclature). The major
causes are an increase in slope inclination, weathering, and seepage forces.

Recognition
Rotational slides are essentially unknown in hard-rock formations, but are common in
marine shales and other soft rocks, and in heavily jointed stratified sedimentary rocks
with weak beds.
Marine shales , with their characteristic expansive properties and highly fractured struc-
ture, are very susceptible to slump failures ( Figure 6.89), and their wide geographic dis-
tribution makes such failures common (see Section 6.7.3). Natural slope angles are low,
about 8 to 15°, and stabilization is often difficult. Failure is often progressive and can
develop into large moving masses (see Section 9.2.6) .
Stratified sedimentary rocks can on occasion result in large slides, and in humid climates
slope failures can be common (Hamel, 1980) (see Example below).
Surface features before total failure are tension cracks; after total failure, a head scarp
remains along with spoon-shaped slump topography (see Section 9.2.5).

Example of Major Failure
Event
At the Brilliant cut, Pittsburgh, Pennsylvania, on March 20, 1941, a rotational slide involv-
ing 120,000 yd 3 of material displaced three sets of railroad tracks and caused a train to be
derailed (Hamel, 1972). A plan of the slide area is given in Figure 9.21b.
Geological conditions are illustrated on the section given in Figure 9.21a. The basal stra-
tum, Zone 1, is described as “soft clay shale and indurated clay (a massive slickensided
claystone).” The Birmingham shale of Zone 4 is heavily jointed vertically.

Slide History
In the 1930s, a large tension crack opened at the top of the slope. Sealing with concrete to
prevent infiltration was unsuccessful in stopping movement and the crack continued to
open over a period of several years. The rainfall that entered the slope through the verti-
cal fractures normally drained from the slope along pervious horizontal beds. On the day
of failure, which followed a week of rainfall, the horizontal passages were blocked with
ice. Hamel (1972) concluded that final failure was caused by water pressure in the mass,
and the failure surface was largely defined by the existing crack at the top of the slope and
the weak basal stratum.

9.2.5 Rotational Slides in Soils

General
A common form of sliding in soil formations is the rotation about some axis of one or more
blocks bounded by a more or less cylindrical failure surface ( Figures 9.3a-c) . The major causes
are seepage forces and increased slope inclination, and relict structures in residual soils.