What Does a Landslide Look Like? (Revisited)

Posted by in Buying/Maintaining a Home, Foundations, Soil Issues | July 16, 2018

A wood deck set at the crest of the hill overlooking a wooded valley with a river.  Mornings drinking a flavored latte, afternoons sipping a lemonade, or evenings toasting friends with the latest creation of Great Lakes Brewery while gazing into the valley.  It sound like a great way to enjoy the day… until the earth begins to shift down the hillside (Figure 1).

Soil sliding down hillside to the left

Figure 1 Soil sliding down hillside to the left.

What happened?

This is commonly known as a landslide.  A landslide may be identified by a scarp, that is, a fracture in the soil (Figure 2), bent tree trunks (Figure 3), shifted mounds of soils, bulging in the sloped surface, dropping of the soil at the top (Figure 4), movement at the toe (bottom) of the hillside, etc.

Scarp (fracture) in soil

Figure 2 Scarp (fracture) in soil.

Bent tree trunks indicate hillside sliding down

Figure 3 Bent tree trunks indicate hillside sliding down.

Soil dropped at top of hillside

Figure 4 Soil dropped at top of hillside.

In summary, the soil on the hillside is sliding down into the valley and taking the wood deck with it.

Why does a landslide occur?

  1. A hillside may be cut to steep such that the remaining slope of soil slides down.
  2. A hillside may be leveled or built-up with fill material resulting in a steeper slope that slides down.
  3. A hillside may be leveled or built-up with a loose fill material that cannot maintain the slope that was placed and the fill slides down.
  4. Water may be introduced into the hillside that changes the properties of the soil, making it less stable such that it can no longer maintain its position and it slides down.
  5. The toe of the hillside may be removed such that the new steep slope at the base of the hill begins to slide causing the upper slope to slide down as well.

And, I am sure there are more causes.  If information regarding the cause or mechanics of the landslide is needed we recommend that a geotechnical (or soils) engineer be hired to evaluate the conditions. The evaluation of a landslide by a soils engineer may require soil sampling through borings and testing of the soil in a laboratory.