Geologic Hazards - Geotechnical Engineering Investigation - page 712

rock-mass slopes that have remained stable for a very long period of time under a variety
of weather and seismic conditions.
Lateral strains in a slope tend to reduce the peak strength toward the residual, a signifi-
cant factor in the failure of slopes in clay shales and some overconsolidated clays contain-
ing recoverable strain energy (Bjerrum, 1966), as well as in materials where slope
movements have occurred.
Solution of Cementing Agents Reduces Strength.
Leaching of salts from marine clays increases their sensitivity and, therefore, their suscep-
tibility to liquefaction and flow (Bjerrum et al., 1969).
Seepage Forces (Figure 9 .9c)
Significance
Seepage forces may reduce the resisting forces along the failure surface or increase the
driving forces (see also Section 8.3.3).
Factors Causing Increased Seepage Forces
In general, seepage forces are increased by rainfall infiltration or reservoir filling, which
raises the water table or some other phreatic surface (perched water level); sudden draw-
down of a flooded stream or an exceptionally high tide; melting of a frozen slope that had
blocked seepage flow; and earthquake forces.
Rising groundwater level is a common cause. Variables affecting such a rise include rain-
fall accumulation and increase in ground saturation for a given period, the intensity of a
particular storm, the type and density of ground vegetation, drainage characteristics of the
geologic materials, and the slope inclination and other features of topographic expression.
Vegetation, geology, and topography influence the amount of infiltration that can occur,
and careful evaluation of these factors often can provide the reasons for failure to occur at
a particular location along a slope rather than at some other position during a given storm
or weather occurrence.
Earthquake forces (see Section 12.3.4) can cause an increase in pore-air pressures, as well
as porewater pressures. Such an increase is believed to be the cause of the devastating
extent of the massive landslides in loess during the 1920 earthquake in Kansu, China,
which left 200,000 or more dead.
Runoff ( Figure 9 .9d)
Significance
The quantity and velocity of runoff are major factors in erosion, and are a cause of avalanches
and flows. Storm intensity, ground saturation, vegetation, frozen ground, the nature of the
surficial geologic materials, and slope inclination and other topographic features affect runoff.
Hydraulic Excavation
Many avalanches and flows are caused by hydraulic excavation during intense storms, a
common event in tropical and semiarid climates. Water moving downslope picks up soils
loosened by seepage forces, and as the volume and velocity increase, the capacity to
remove more soil and even boulders increases, eventually resulting in a heavy slurry
which removes everything loose in its path as it flows violently downslope. The scar of a
debris avalanche is illustrated in Figure 9.10. Failure could hardly have been foreseen at
that particular location along the slope, since conditions were relatively uniform.