By Christopher Wilhite, chief engineer and co-owner of Engineered Soil Repairs, Inc.
What causes landslides?
Ninety-nine percent of the time, landslides are caused by water affecting the stability of a slope. There are two basic forces that determine slope stability:
1. Driving Force. This force wants to push the soil mass downhill. It’s a function of the weight of the soil and the steepness of the slope.
2. Resisting Force. This force tries to keep the soil from moving downhill. It’s the internal friction between the soil mass that wants to slide and the mass of soil below, which is stable. When the driving force is greater than the resisting force, you have a landslide.
Why does water cause landslides?
When water is introduced to the soil (i.e. from rain or a water pipe break), it fills the tiny void spaces between soil particles and adds weight to the soil. This weight increases the driving force. At the same time, water reduces the resisting force by reducing friction. Essentially, the soil gets slick at the point of contact between the soil mass that wants to slide and the soil mass that’s stable. When certain types of soil become saturated, they lose all strength and behave like a liquid, flowing down the slope. This is what’s known as a mudslide... Read more