• Slope stability and Mass wasting
An exposed ground surface that stands at an angle with the horizontal is called an
The slope can be natural of artificial
If the ground surface is not horizontal, a component of gravity will tend to pull the
soil down slope.
If the component of gravity is large enough and the soils inter shear strength is
small enough, a slope figure can occur. – When failure occurs this is mass wasting –
The main force responsible for mass wasting
Pulls objects in a direction towards the earth
• Slopes and Gravity
On a flat, gravitational force acts to hold objects in place by pulling on them in a
direction perpendicular to the surface
So long as the material remains on the flat surface, it will not move under the force
On a slope, the tangential component of gravity acts down a slope; it causes
objects to move down hill.
Shear stress: the downslope component of the total stress involved
Normal stress: the perpendicular component
• Factors affecting Slope Stability
Increases in shear stress by:
1. Removal of lateral support (rivers, road cuts), steepening
2. Addition of mass
4. Regional tilting (Tectonics)
5. Removal of underlying support (solution, mining)
6. Lateral pressure (swelling, freezing)
Decrease shear strength by:
2. Increased pore water pressure
• Mass Wasting
When the shear stress becomes greater than the combination of forces holding the
object on the slope, the object will move downslope – slope failure.
Mass wasting is the downslope movement of material under the influence of
Slope failure is the collapse of rock or sediment mass. 3 major modes of slope
failure or mass wasting:
Three modes of movement
1. Slides: rapid and accelerating, dry, failure along a well defined plane, consistent
velocity profile moves as a cohesive mass.
2. Flows: rapid and accelerating, wet or dry, velocity profile decreases through the
deforming mass, moves by interparticle shearing – like a fluid. 3. Heaves and Creep: slow imperceptible, involves up and down pattern of
movement of individual particles, up normal to surface, down is vertical, velocity
decreases when depth.
• Weathering – important factor in slope stability
Most rocks and minerals are formed deep within the earth’s crust where
temperatures and pressures differ greatly from the surface.
The physical and chemical nature of materials formed in the earths interior are
characteristically in disequilibrium with conditions occurring on the surface
Because of this disequilibrium, these materials are easily attacked, decomposed,
and eroded by various chemical and physical surface processes
Weathering is the first step for mass wasting processes
At the earths surface, rocks are exposed to the effects of weathering the chemical
alteration and mechanical breakdown of rock, when exposed to air, moisture, and organic
Weathering converts rock to regolith.
Refers to the breakdown and decomposition of rocks and minerals by factors
including air, water, sun and frost.
Physical weathering involves continual breakdown of rocks into smaller and
Chemical weathering involves alteration of the chemical composition of rock
There are a number of slide based mass wasting features
The most basic are simple translational slides where a block of soil or rock slides
along an inclined surface
Slumps (rotational) usually involve a mass of soil moving along a curved failure
• Fall and Topples
Fall and topples involve a block of soil or rock that initially moves along a failure
plane but eventually free falls
Talus is the accumulation of rock debris that collects at the base of a slope.
• Flows (mudflows and debris flows)
Flows involve the downslope movement of material by internal deformation much
like a viscous fluid.
Water is usually the key component
In some cases dry flows involve fine particles moving on a layer/cushion of air
Gradual plastic deformation of soil and rock materials under the influence of
• Sink Holes
Land dissolving underneath our feet.
Carbon dioxide rich water attacks rocks, dissolves some minerals, and at some point, new minerals precipitate
– this case calcite (Calcium carbonate) • Land Subsidence
Most often caused by human activities, mainly from the removal of subsurface
water, also oil and gas extraction.
• Soil Profiles
A soil profile is a vertical cross section of a soil. It is divided into a number of
distinct layers, referred to as horizons
The horizons are normally designated by symbols and letters
The presence or absenece of particular horizons allows pedologists to classify the
In addition the organic or O horizon can form above the mineral soil commonly in
forested areas, resulting from the dead plant and animal remains
• Soil Horizons
The horizons may be furthered subdivided
• Soil forming factors
Soils developed as a result of the interplay of 5 factors: parent material, climate,
organisms, relief, and time.
• Parent Material
This is the material from which the soil has developed and can vary from solid
rock to deposits like alluvium and clay. It has been defined as the ‘initial state of the soil
The parent material can influence the soil in a number of ways (colour, texture,
structure, mineral composition, permeability)
This soil has developed an old red sandstone and so has derived its distinctive
colour from its parent material.
Governs the rate and type of soil formation and is also the main determinant of
Soil climate has 2 major components, moisture and temperature, influencing
evaporation. When precipitation exceeds evaporation, leaching of the soil will occur.
Temperature determines the rate of reactions, chemical and biological decay and so
has an influence on weathering and humification.
• Organisms: vegetation, fauna and soil microbes
Organisms influencing soil development range from microscopic bacteria to large
animals, including man.
Microorganisms, such as bacteria and fungi assist in the decomposition of plant
litter. This litter is mixed into the soil by microorganisms (soil – animals) such as worms
Soil horizons are less distinctive when there is much soil organism activity.
Higher plants influence the soil in many ways. The nature of the soil humus is
determined by the vegetation cover and resultant litter inputs
Roots contribute dead roots to the soil, bind soil particles together and can
redistribute and compress soil
• Relief (Landforms and topography)
Is not static; is a dynamic system Influences soil formation in several ways:
1. Influences soil profile thickness
2. Effect on climate which is also a soil forming factor
3. Gradient affects run off, percolation and mass movement
4. Influences aspect, which creates micro climatic conditions.
• Soils on a Steep Slope
As slopes increase, soils become shallower and have thinner and fewer horizons
This is because the steeper the slope, the greater the runoff and the greater the
Soils develop very slowly. In eastern Canada it takes about 400 years for 10mm to
Young soils retain many of the characteristics of the parent material
Over time they acquire other features resulting from the addition or organic matter
and the activity of organisms.
An important feature of soils is that they pass through a number of stages as they
develop, resulting in a deep profile with many welldifferentiated horizons.
• Decomposition and Humification
Decomposition: the breakdown of plant derived material into its simpler organic
Accomplished by enzymes, earthworms, mites, and other organisms
Humification: the breakdown of plant re