EAPS 11100 Study Guide - Midterm Guide: Karst, Mass Wasting, Granular Material
4 Dec 2018
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Department
Course
Professor
Mass Wasting: Ground Collapse and Slope Failure (Lecture 20)
1) What is ground subsidence? How can ground subsidence be related to groundwater or oil
pumping? How can one prevent subsidence when pumping groundwater from an aquifer?
• Subsidence is slow sinking/compression of land after groundwater withdraw (drop in water
table). Oil pumping is another way to cause subsidence. Planting long rooted plants
• subsidence- gradual caving in or sinking of an area of land
• if you pump large volumes of water or oil out of one well it pumps so rapidly that the
groundwater around the well can't flow in fast enough to replenish what is last and it
causes local water table to drop forming a funnel shaped cone of depression around
the well, direction of groundwater flow changes across the entire area
• water table drops, aquifer compacts, land subsidence
• can prevent it by making sure withdrawal rate is less than the recharge rate
2) Why does groundwater dissolve limestone to make caves, while limestone is deposited in
the first place in seawater?
Limestone is soluble
groundwater is slightly acidic b/c of dissolved co2 and sulfuric acid and calcium carbonate
dissolves in acidic conditions
3) What is a sinkhole and how are they related to caves?
• Sinkholes are when caves cave in, creating a prominent depression
• the roof collapse of a cave forms a sinkhole of the surface
• karst topography- surface w/ sinkholes, limestone pillars, disappearing streams
4) What is the primary force acting to cause slope failure events?
gravity
• gravity pulls material downhill, tends to pull everything toward earth's center
o it acts vertically so a block will not move on a flat surface
o it pulls a block at an angle so block can move on an angled slope
5) What is the angle of repose and why is this an important quantity for understanding slope
failure?
• Steepest angle granular material can have before failing by gravity sliding
• angle of repose- steepest angle at which a pile of unconsolidated grains remains
stable
o controlled by frictional contact between grains
o loose dry material about 30 degrees
o higher angle for coarser material, wet stuff or gangular
o angle of resulting slope is at angle of repose, if more is added the bile becomes
wider and higher but angle remains the same
o slope failure occurs when slope is too steep for its material to resist pull of
gravity
6) What other factors besides angle of repose control slope stability?
Moisture, coarseness of grains
angle of repose for the material, discontinuities (fractures, cleavage and bedding reduce
mechanical strength of rock and may allow rock to slip down hill), amount of water (addition
of minor amounts of water increases strength of soil but too much weakens soil by pushing
grans apart, clay can flow)
7) What role does friction and cohesion play in slope stability?
The shear motion is not present because of the frictional force
8) What kinds of things can trigger slope failures? What factors make mass wasting more
likely?
Water, discontinuities (fractures), angle of repose, earthquakes, volcanic activity,
precipitation, changes in land use, the slope
• precipitation
• changes in land use (if you are building houses on areas that were previously stable
but you removed some vegetation so it became less stable)
• overloading slope (if you put too heavy things on a slope that's mass is greater than
the ability of the slope to hold it)
• Undercutting slopes (blasting to make roads, rivers running through, oceans)
• sudden shock like from an earthquake
• volcanic eruption
9) What are the three major characteristics used to classify slope failures? Be able to
describe and/or recognize falls, slides, flows and slumps, and the differences between these
if made of rock or loose earth material
Mechanism of movement, type of material, rate of movement
• mechanism of movement (fall- slope failing in nearly vertical sense), (flow-moving as a
fluid mass down some slope), how the material moved
• type of material (solid rock ie rock fall) or (unconsolidated material ie debris flow)
• rate of movement (fast) or (slow)
10) What kinds of areas near Purdue are prone to slope failures and why?
River bank areas, water can vary
Coastal Processes and Shorelines (Lecture 21)
1) What are the many processes and forces at work at the land/sea interface along
shorelines? Understand in general terms how each contributes to the resulting shape of
shorelines.
Strength of waves, slope of seafloor, orientation of coast, climate, hardness of bedrock,
amount and size of sediment, rivers, amount of precipitation, etc
• swirl away loose pieces of bedrock or break off new pieces
• waves bend (refract) if approach shore at an angle
o part that encounters bottom slows, bending wave
• grind and break clasts, making sand and stones that promote erosion
• waves break directly on promontory from several sides
o bays are protected from largest waves by promontory
• as ground slopes upward the bottom of the waves slows down and wave breaks
Document Summary
How can one prevent subsidence when pumping groundwater from an aquifer: subsidence is slow sinking/compression of land after groundwater withdraw (drop in water table). Oil pumping is another way to cause subsidence. Be able to describe and/or recognize falls, slides, flows and slumps, and the differences between these if made of rock or loose earth material. Understand in general terms how each contributes to the resulting shape of shorelines. What is wave base and how is that related to breaking waves and the depth that wave energy affects the sea floor: wavelength is the distance between two wave crests (the highest point). The trough is the lowest point of wave, and the base is the point underwater when wave action ceases. It is half the distances of the wavelength. It caused the movement of sand laterally across a beach as it moves towards, then away, towards, and away: refraction is the bending of waves because of varying water depths underneath.
