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Slope FailureSlope failure occurs when gravitational forces exceed the

frictional resistance of the material resting on the slope - massmovement

Whether a slope will fail or remain stable is based on two forces:

1. Magnitude of the driving force: Driving forces promotedownslope movement of material. The primary driving forceis gravity. So movement is dependent on the mass & density ofthe material involved and the slope angle.

2. Magnitude of the resisting force: Resisting forces determovement. Controlled by frictional resistance and response toimposed stress of the material resting on the slope ( shear strength) .

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What controls downslope mass movements?

Ratio of driving forces to resisting forces is called factor of safety(FS)

FS =Sum of resisting forces

Sum of driving forces

If FS = 1, the forces are exactly balanced,

If FS > 1, the slope is stable

If FS < 1 the slope is unstable

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DRIVING FORCESGravity: Slope angle, climate, slope material, and watercontribute to the effect of gravity. Mass movement occurs muchmore frequently on steep slopes than on gentle slopes.

Water plays a key role in producing slope failure. Water erodesthe base of slopes, removing support, which increases slope

angle.

Water can also increase the driving force by loading, i.e.,(weight).

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RESISTING FORCES

Resisting forces act opposite to driving forces.

The resistance to downslope movement is dependent on the shearstrength of the slope material.

Shear strength is a function of cohesion (ability of particles toattract and hold each other together) and internal friction(friction between grains within a material).

Chemical Weathering (interaction of water with surface rock and

soil) slowly weakens slope material (primarily rock), reducing itsshear strength, therefore reducing resisting forces.

The shear strength of the slope material is also decreased byincreasing the pore water pressure (increased amount of water).

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Forces acting on a block at rest

Newtons Third Law: To every action (applied force) there is an equal and oppositereaction (force).

F gravity = F normal

F gravity = Mass x g

= Mass of Block x 9.81m/s 2

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If a force is applied to the block, it must overcome friction in

order to move

Slope Failure

F resistance F nF resistance (F r )= F n x C fCf is the coefficient of friction which depends on surface roughness

When sliding begins: C f =FrFn

= tan (friction angle)

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Friction is dependent on the material surfaces

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Angle of Internal Friction

If the friction that resistsmovement is not along a plane,but instead between the grains,this is internal friction.

Failure occurs when the angle ofinternal friction is exceeded.

For most practical purposes, theangle of internal friction isequivalent to the angle ofrepose.

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Driving & Resisting Forces

= 30

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Driving & Resisting Forces = 45

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Driving & Resisting Forces

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Definitions: Str ess is a force appli ed to a surface area and str ain isany deformation or change in shape or volume of a material caused byapplication of stress

Physical Characteristics of materials

A B

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Driving forces on a hillslope: Shear stress

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Resisting forces on a hillslope: Coulomb Equation