This work is supported by the National Science Foundation’s Transforming Undergraduate Education in STEM program within the Directorate for Education and Human Resources (DUE-1245025).

WHAT DETERMINES WHETHER A SLOPE WILL STAY IN PLACE OR FAIL?

Introduction to driving forces and resisting forces

DRIVING FORCES VS RESISTING FORCES

GRAVITY FRICTION

promotes sliding resistance to sliding

Which of the following Earth system scenarios will increase the risk of slope failure? (a) Increase gravity(b) Increase friction(c) Reduce gravity(d) Reduce friction

DRIVING FORCES

Force = mass x acceleration Force: A push or pull in a particular direction

measured in Newtons (kg m/s2 = N)

F = ma

Weight (force of gravity) = mass x acceleration due to gravity Gravity is a mass-dependent force directed downward towards the center of the Earth. Acceleration due to gravity = 9.8m/s2

W = mg

W = FG

The downward-directed force is dependent on the mass of block.

DRIVING FORCES

10 kg block 100 kg block

Which block exerts the greater downward directed force? Why?

DRIVING FORCES – ON A SLOPE!

FN = Normal Force = perpendicular to the sloping surfaceFS = Shear Force = parallel to the sloping surfaceq = Slope Angle

On a slope, the force due to gravity consists of two components: Shear Force (FS) Normal Force (FN)W = FG

FS

FN q

In the diagram above:Which force increases friction and keeps the block in place?

Which force pulls the block down the slope?

CALCULATING DRIVING FORCES– ON A SLOPE!

FN = Normal Force = perpendicular to the sloping surfaceFS = Shear Force = parallel to the sloping surfaceq = Slope Angle

W = FG

FS

FN q

FS = W sin qFN = W cos q

FORCE VS STRESS

sN = Normal Stress

sS = Shear Stress

q = Slope Angle

Stress is the force acting over a specific area. In other words: Stress (s) = Force/Area

W = FG

FS

FN q

sS = FS/AsN = FN/A

A area between the block and sloping plane

l w