Lecture 12 - Introduction to landslides Flashcards
When do mass movements occur?
When large volumes of material move downslope under the influence of gravity.
When do we call mass movements landslides?
When the material is rock, soil, mud, or debris.
What components does gravity acting on slopes have?
A component parallel to slope and a component perpendicular to slope.
What can slope instability be increased by?
increasing the slope angle
adding to the mass of material
shaking the slope
reducing friction
What do soils and sedimentary rocks have between their grains?
Soils and sedimentary rocks contain pore spaces between grains. The porosity is 10%–30% in sediments and up to 50% in soils.
What happens if the air in the pore spaces of soil is replaced by water?
If the air in pore space is replaced by water, then its weight increases dramatically.
As water is incompressible, what happens if the rock is buried by more sediments?
The pore pressure increases and strength is reduced.
What is freeze-thaw and how does it affect rock strength?
Water expands when it freezes. When it seeps into cracks and freezes, expansion forces the crack apart. Repeated over several melting and freezing cycles, freeze-thaw weathering disintegrates the rock and reduces strength.
What happens to rock strength when it is dissolved or eroded by groundwater?
Groundwater can dissolve minerals that bind rock together (cement) or erode rock physically, forming caves. This reduces rock strength and increases susceptibility to landsliding.
Why are sinkholes common in Texas and Florida?
Limestones and evaporites are especially prone to dissolution by groundwater. In the US, it is the distribution of these rock types that explains why sinkholes are most common in Florida and Texas.
How does water affect the strength of clay rich rocks?
Clays are minerals composed of layers of “sheet silicates” that are common in near- surface rocks. When wet, water molecules are absorbed between the mineral layers and the clays expand. The increased separation and lubrication between the mineral layers allows them to slide past one another, greatly reducing rock strength.
Why are clays very common in areas like Canada?
Clays are a by-product of the grinding action of ice on bedrock, and are thus very common in formerly-glaciated areas including much of Canada.
Why are landslides common along the Ottawa and St. Lawrence valleys in Ontario and Quebec, despite their moderate topographic relief?
Due to the clay-rich sediments deposited at the end of the Pleistocene in the Champlain Sea — a temporary inlet of the Atlantic Ocean fed by the receding Laurentide ice sheet.
How can bedding planes make a slope more susceptible to landslides?
Sedimentary rocks contain weak bedding planes along which sliding preferentially occurs; the orientation of this layering with respect to topography may create a strong, stable condition where bedding is angled into a hillslope, or a weak, unstable condition where bedding parallels the slope.
What is the role of plate tectonics with regard to landslides?
Tectonics is what ultimately generates topography, and also plays a role through ground shaking in large earthquakes that dislodges material from hillslopes.
