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Studies the downslope movement of material under gravity and the triggers for landslides.
Imagine a million tons of rock suddenly behaving like a liquid, racing down a mountainside at 100 km/h. What determines whether a slope stands firm for centuries or collapses in seconds?
Mass movement is the downslope movement of rock, regolith, and soil under the direct influence of gravity. Geographers classify these movements using two primary variables: velocity (speed) and moisture content. At one extreme, we have soil creep, a process so slow ( cm/year) it is only visible through curved tree trunks or tilted fences. At the other, we find rockfalls and debris avalanches, which occur instantaneously with devastating force. Between these lie flows (saturated material moving like fluid) and slides (coherent blocks moving along a distinct failure plane). Understanding where a movement falls on this spectrum is critical for hazard mapping and urban planning.
Quick Check
Which type of mass movement involves saturated material moving downslope in a fluid-like state?
Answer
A flow (such as a mudflow or earthflow).
Slope stability is a tug-of-war between two opposing forces. Shear Stress () is the component of gravity acting parallel to the slope, pulling material down. Shear Strength () is the internal resistance of the material, governed by friction and cohesion. We quantify stability using the Factor of Safety ():
If , the slope is technically stable. If , the shear stress exceeds the strength, and failure is imminent. Factors that increase include steepening the slope or adding weight (loading). Factors that decrease include weathering or the introduction of water, which increases pore water pressure, effectively 'lifting' the particles apart and reducing friction.
Suppose a slope has a calculated internal shear strength of and the current gravitational shear stress acting on it is .
1. Identify the variables: , . 2. Apply the formula: . 3. Result: . 4. Conclusion: Since , the slope is currently stable.
Quick Check
If heavy rain increases the weight of the slope material, which variable in the Factor of Safety equation increases?
Answer
Shear stress ()
Human activity often tips the balance of a marginally stable slope. Deforestation is a primary culprit; roots act as biological 'anchors' that provide apparent cohesion to the soil. When trees are removed, this cohesion vanishes. Furthermore, civil engineering often involves undercutting the 'toe' of a slope to make room for roads or buildings. This removes the physical weight that was previously buttressing the material above. In mountainous regions, the combination of loading (building heavy structures at the top) and undercutting (at the bottom) creates a 'scissors effect' that drastically reduces the Factor of Safety.
In advanced geomorphology, we use the effective stress principle: .
1. is cohesion, is total stress, is pore water pressure, and is the angle of friction. 2. Imagine a slope where (water pressure) increases due to a leaking pipe. 3. As increases, the term decreases. 4. This causes the total shear strength () to drop, even if the slope angle hasn't changed, leading to a 'sudden' landslide.
What is the primary force driving all mass movements?
Which of the following would DECREASE the Factor of Safety ()?
High pore water pressure increases the friction between soil particles, making the slope stronger.
Review Tomorrow
In 24 hours, try to explain the 'Factor of Safety' formula to someone else and list three ways humans decrease it.
Practice Activity
Find a local road cut or construction site. Observe the slope: Are there signs of 'toe' removal or drainage pipes installed to manage pore pressure?