M3 Hillside: Stability and Mass Movement

Topic

Slope stability is essentially a balance between two competing sets of forces within the slope itself: driving forces pushing the material downslope—such as the material's own weight, accumulated pore pressure, and seismic shaking—and resisting forces opposing this movement—such as soil or rock cohesion and internal friction between particles.

The slope's factor of safety encapsulates this balance as a single ratio of resisting forces to driving forces, and the value of this ratio determines the slope's state: a factor of safety greater than one indicates stability (though the actual margin of stability depends on data uncertainty); a factor of safety of one marks the threshold of instability; and a factor of safety less than one indicates active movement. Slope stability analysis used to calculate this factor relies on classical Mohr-Coulomb soil mechanics; it can be modeled as planar—when studying sliding along a single plane—or circular—using Bishop’s method—when the mass being evaluated is large enough to slide along a curved surface.

Of all the factors involved in this balance, pore pressure is the most common trigger for landslides: when heavy rainfall saturates the slope's soil, pore pressure rises; this increase reduces effective stress—defined by Terzaghi’s principle as the difference between total stress borne by the soil and internal pore pressure—thereby diminishing available shear strength and, consequently, the factor of safety itself. A transient rainfall infiltration model, coupled with a regional slope stability analysis, combines the one-dimensional Richards equation—used to track pore-pressure diffusion through the soil profile—with the aforementioned stability analysis; this allows for the calculation of how the factor of safety varies across both space and time along the entire slope.

Soil fluidization represents the extreme outcome of this same mechanism: it occurs when pore pressure fully equals the total stress borne by the soil, thereby nullifying effective stress; this loss of effective stress causes the soil's shear strength to vanish completely, leaving the slope with no capacity to support itself.

The spectrum of mass movements triggered by an unstable slope encompasses a vast range of velocities: at one end lies soil creep—a slow movement of merely a few millimeters per year, driven by repeated freeze-thaw cycles or fluctuations in soil moisture—while at the other end are catastrophic debris flows, which travel at speeds of several meters per second and can cover distances of several kilometers from their source. Debris flows channeled through mountain watercourses are particularly dangerous because they concentrate large volumes of moving mass that advance at high speed, offering very little advance warning to those living downstream.

The expected behavior of this entire slope system involves a distinct acceleration of mass movements during periods of heavy rainfall, a reduction in the factor of safety as the soil becomes saturated, and the propagation of the debris flow wave along the entire channel conveying it. Thus, from the balance of forces with which this document begins to the propagation of the debris wave with which it concludes—encompassing the factor of safety, pore pressure, soil fluidization, and the continuum of mass movements—slope stability dictates that the fate of all that material (whether it remains stationary or plunges down the slope) depends, ultimately, on how close its factor of safety is to losing the balance that sustains it.

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