The internal strength of soils can be decreased by groundwater and seepage flows within the bluff.
For instance, rainwater is naturally absorbed and seeps down to lower levels. Soils, such as coarse sand,
which allow rapid and free passage of water are permeable. On the other hand, impermeable soils, such
as clay, do not allow the free flow of water except through cracks or other openings. In the figure, the
large tree's roots penetrate the clay layer and provide a path for seepage to the sand layer beneath.
Likewise, as the clay fails, cracks form at the surface which provide a path for seepage to penetrate the
soil, further weaken it, and accelerate the failure process. Water can also enter the clay along the existing
circular failure surface, leading to further movement.
Once seepage penetrates the clay and reaches the permeable sand layer, it passes freely to the
lower clay layer where it flows along the clay's surface and exits the bluff face. This seepage can
increase the risk of slope failure. In addition, surface runoff can erode the bluff face, causing gullies and
deposits of eroded material on the beach below. The seepage exiting the bluff at the clay layer can also
cause surface erosion.
The added weight or loading of buildings and other structures can increase soil stresses and
contribute to slope failure. Structures located near the top edge of the bluff have the greatest impact. An