movement, like your notebook falling to the floor, but slowly your friend is no longer sitting
up straight in the seat. The same thing can happen to rock or even whole parts of a hillside.
This might happen over a period of days or even weeks. In the end, the whole area of soil
or rock has slid to a lower spot.
The last way that gravity moves material along is when it becomes very wet. Saturated
soil flows downhill, often removing trees, homes and bridges that are in the way. To help
you understand how water increases the chances of movement, think about playing in sand
at the beach. If you were making a sand castle with dry sand, you could not build walls very
well. If you add a little bit of water, it helps your walls to stand. A little bit of water helps
to hold grains of sand or soil together. However, if you added lots of water, what would
happen? Too much water causes the sand to flow quickly away. There are a couple of ways
that soil or rock can get very wet and flow. Sometimes this happens if it has been raining
for a very long time or if it rains very hard. In the spring, snow and ice begins to melt and
much of this water moves into the ground. Springtime is a particularly dangerous time for
landslides because there are heavier and more frequent rainfalls at this time of year and it
is also the season when snow and ice melt. Extra water in the soil adds more weight to the
slope and also makes the grains of soil lose contact with each other, allowing them to flow.
Contributing Factors
There are several factors that increase the chance that a landslide will occur. Some of
these we can prevent and some we cannot. Whenever we dig into the base of a slope, this
contributes to the likelihood of a landslide. There are many reasons why we might need to
do this. We may want to build a house on flat ground, so we level out an area by cutting
into a hillside. Roads and railroad tracks also need to be flat and level, so excavation of a
slope could be necessary in areas where we travel frequently. This is particularly dangerous
when the underlying rock layers also slope towards the area that is cut away or when layers
of clay are present. If rock layers slope towards the region that is removed, then the support
for those layers is gone and the overlying rocks can slip away, causing a landslide (Figure
10.46).
Soils rich in clay are water holding types of soils. If you have ever worked with clay in art
class, you know that when clay is wet, it is very slippery. If there is a lot of clay in the soil,
the clays hold onto the water when it rains. This slippery clay layer provides an easy surface
for materials to slide over.
Whenconstructionworkersneedtocutintoslopesforbuildingahomeorroad, itisimportant
to stabilize the slope to help prevent a landslide (Figure10.47). Some ways that you may
haveseenonsteepslopesalongahighwayincludebuildingsupportsintotheslopeorplanting
vegetation to keep the soil in place. Trees have deeper roots than grasses, but each type of
plant produces benefits for particular areas. It is also a good idea to provide drainage for
groundwater so that the slope does not become saturated.