slopes and slope processes

Classification of slope processes

Mass movement (mass wasting)- movement of material under the influence of GRAVITY, without a transporting medium.

Mass transport- The movement of material within a transporting medium, such as water, air or ice.

DENUDATION- Moving mass from a generally high area of sediment to an area of low sediment. The potential energy from being suspended at the top of a mountain makes the material want to move under gravity, as low as it can get to the geoid surface.

uplifted landscape- When we add mass/ elevation to a mountain range e.g. erupting volcanoes more energy is added in the form of potential energy for the erosional process to break the mass down. therefore, there are faster erosion rates.

Slope stability and factors of safety

F = sum of resisting force (shear strength/ sum of aplied force (shear strength). If F>1 likely stable slope. If F<1 failure occurs.

Force varies due to external and internal forces.

External

  1. Undercutting (stream undercutting can lead to failure but also to more stable slopes.
  1. Loading- houses, dams
  1. Rainfall events- Lubrication and loading.
  1. shock - earthquake, or eruption.

Internal

  1. Weathering
  1. Progressive failure.

Trigger mechanisms

Shear stress- The amount of force per unit area (or along a plane of weakness) due to a shear force. i.e. Pressure acting on the system.

Shear Strain- The amount of displacement (in the directiton of a force ) that the material is subjected to. Leads to deformation of the material. The inherent resilience to strain affects whether it has a low or high resistance.

Factors contributing to high shear stress

  1. Removal of lateral support (streams, frost action)
  1. Overloaded by (rain, snow, structures)
  1. Transitory stress (Earthquakes- ground motion and tilt- immediate fail).
  1. Lateral pressure (water in interstices (small places) Freezing and swelling of water).
  1. Increase in slope angle (regional tectonic tilting. volcanic processes).

Factors contributing to low shear strength

  1. Composition and texture (weak materials, loosely packed material, smooth and uniform grains).
  1. Physico-chemical reactions (Cation (+ charged ion) exchange , hydration/ drying of clays).
  1. Effects of pore water (bouyance effects, reduction in capillary tension).
  1. Changes in structures (spontaneous liquification)
  1. Vegetation (removal of trees, increased soil cracking).
  1. Relict structures (joints and other planes of weakness).

Slope failure processes

progressive block failure- Early warning signs followed by an instantaneous failure.

Block toppling- Joints are exploited by freeze thaw and giant blocks fall off.

Exacerbation of frost shattering- water gets into joints, progressively widening the joint. making the bottom of the rock under tension as it's being pulled by gravity downwards.

Causes of rockfalls

weathering - wearing away due to long exposure of the atmosphere.

Rainfall- pore pressure from infiltration, additional loading and lubrication.

Earthquakes- Displacement of ground

Undercutting- cutting away at the base material.

Stress release (confining pressure removed and rock forced to exist in different conditions.

De-vegetation- no veg binding the soil together so it becomes dry.

Wave quarrying- repeated battering, chemical and physical weathering. steep waves have considerable energy affecting loose and unconsolidated rock fragments. Air is trapped and compressed in a joints breaking the rock apart.

Example: Frank slide, Canada 1903

30 million cubic meters of rock buried the town Frank, along with 2km of Canadian railway.

in 100 seconds, 70 people were killed.

A long run out landslide like this has, HIGH velocities, and flows as FLEXIBLE debris sheets with ripple like flow structures.

Whole mountain side was taken off- caused by a plane of weakness- possible a fault.

Types of landslides

Rotational slides- deep seated with concave curved failure planes.
occurs in cohesive soils- clay or soil with a high clay content- doesn't crumble.
Often associated with surface creep- wind erosion process whereby grains are moved along the ground by impact of other grains in saltation.
There are 3 different types; single, multiple and successive.

Flows (mid to fast, intermediate)

Occur when coarse debris, fine grained soil and clay are liquefied.

Debris flows

Debris flows- requires debris (regolith loose material), water and STEEP Slopes (moves under gravity).

Mix of granular soils, clays, air (allows pressure to increase, acts like a battering ram) and water.

No distinct shoar plane at base (shoar evenly distributed through depth).

Velocity variation through profile. Laminar flow- fluid in parallel layers, no disruption between the 2, although they can slide past one and other. Internal turbulance- Mixing

Capable of moving large clasts due to high pore pressure (pore water pressure- refers to the pressure of groundwater (water held beneath earth's surface in soil pore spaces) held within a rock or soil, in gaps between particles (pores)) (water under a lot of stress allowing boulders to be buoyant).

Mudflows

Fine grined material (clay sized)

High fluidity, lots of water (up to 60%)

can occur at very low angled slopes (5-15%) due to low viscosity.

Failure that occurs over a discrete boundary.

Elongated as they are so fluid they can travel for a long time.

Three distinct elements- supply scar (where it is from), frack (run out) and toe.

Heaves

Particles lifted up in a direction normal to the slope.

Fine grained material (often soils) lifted (often due to growth of ice or addition of water that swells sediment lifting it away from the surface.then falls down the slope by gravity.

Particles settle in a versicle direction to a location downslope from an original location.

2 main types of creep- continuous creep and seasonal creep.

Seasonal creep- Movement of material due to seaosnal expansions and contractions due to seaonal change. Water undergoes volume changes (freezing= expansion of 9%).

solifluction- slow downslope flow of sediment saturated with water.
Gelifluction- same as solification but moving material slides over a slick permafrost layer.

Leads to burrows being closed off and curved tree roots (as trees always wishing to face the sunlight).