Instability of Soil Masses - Environmental Geomechanics - page 74

Figure 3.3. a) Schematic creep curve and b) creep limits in the p'-q plane
(first stress invariant, second invariant of the stress deviator)

Secondary creep (at constant velocity) is rarely observed in the laboratory,
although it is found in situ . The analysis of any behavior under laboratory conditions
by tests of a short duration leads to difficulty showing real behavior whose duration
is measured in hundreds of years. This is the reason for perfecting in situ methods
for the determination of rheological characteristics (see section 3.2.2).

The geometry of natural, slowly moving slopes is the second key element of the
analysis. The observations made on numerous slides in Switzerland [DUT 85, VUL
86] show that the thickness of the mass in motion is often significantly less than the
length (in the order of 1% to 5%); this can be used advantageously for modeling, a
first-order approximation reducing the complexity of the equation system (similar to
the “shallow mass” approximation, as in glaciology or river hydraulics).

The slope angle can exceed 30°, but in alpine regions is often found to be
between 10° and 20°. The hydrological conditions play a determining role that
influences the seasonal variations of the velocities. In general, however, the phreatic
surface (or the piezometric level) is relatively parallel to the surface of the terrain.
Note that these hydro-geological conditions, although of the greatest importance, are
rarely precisely known.

3.2.2. Determination of the laws of creep in situ

Due to the slenderness ratio of slowly moving slopes, the stress condition of an
infinite mass can be estimated locally by:

()

τ z = ρ g z sin α