Entire books have been written about landslides, mudslides, and mudflows.  The investigation of landslides and related earth movement requires the specific expertise of a Geotechnical Engineer or Geologist. However, we will add the following points for your perspective, and for a more complete understanding:

The cause of all landslides is that the driving forces (due to gravity) exceeded the restraining forces holding the material in place. Since both the driving forces and restraining forces can be estimated by a qualified professional, it is possible to determine whether or not a landslide is likely to occur. In cases where the landslide has already occurred, it is usually possible to determine why. However, it may also be quite expensive.
We are familiar with cases where leaking pipes, irrigation canals, or drainage ditches added water to the slope below them, thereby simultaneously increasing the driving forces, and reducing the restraining forces. In some cases, the structure carrying water was defective to begin with. In other cases, an initial movement of the slope probably created the defects. Unless the conditions are documented in advance of a complete failure, it may be impossible to determine which failure caused the other one. If you expect to be compensated by whoever is responsible for the failure, then you may need to provide proof of which one happened first.

Not Without Warning

We often hear public officials (and others with a hidden agenda) claim that a disaster was completely unforeseen: “…it came out of nowhere”.  However, in the majority of slope failures, this statement is false.  A perfect example is the death of 34 people in the March 2014 Oso Landslide in Snohomish County, Washington State (US).  In that case, public officials were provided with studies and reports of potential slide activity which provided clear and specific warnings, years in advance of the failure.  Additional landslide failures are guaranteed to happen in the Oso area in the future. When they do, public officials may exclaim once again: “…it came out of nowhere”. One interesting observation about landslides is that each failure usually leaves a steeper face than the one immediately before the failure. A steeper face increases the driving forces, which increases the probability of the next failure.  Thus, future sliding can be predicted based on previous slides at the same location (just like Oso), and nearby adjacent slides in the same type of formation (just like Oso).

Try This:

Many slides will provide advance warning if you know what to look for.  Start by looking for tension cracks in the earth at the top of the slope, or soil movement at the bottom of the slope, which indicates that the slope has started to move, sometimes days or weeks in advance of a complete failure. Also look for flat areas which are in the middle of a slope, or mounded areas at the bottom of a slope.  That material had to come from somewhere.  You may not want to build or inhabit a structure on it, or directly downhill from it.

Resources for Further Research

Evaluating landslides is probably too complex for the average person to tackle.  However, if you want to do your own research, this might be a good place to start:

  • US Army Corps of Engineers EM 1110-2-1902 Slope Stability Engineering Manual.
  • Transportation Research Board (TRB) Special Report #176 Landslides: Analysis and Control.