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An exposed ground surface that stands at an angle with the horizontal is called slope. Slopes are required in the construction of highway and railway embankments, earth dams, levees and canals. These are constructed by sloping the lateral faces of the soil because slopes are generally less expensive than constructing walls. Slopes can be natural or manmade. When the ground surface is not horizontal a component of gravity will try to move the sloping soil mass downwards. Failure of natural slopes (landslides) and man-made slopes has resulted in much death and destruction. Some failures are sudden and catastrophic; others are widespread; some are localized. Civil Engineers are expected to check the safety of natural and slopes of excavation. Slope stability analysis consists of determining and comparing the shear stress developed along the potential rupture surface with the shear strength of the soil. Attention has to be paid to geology, surface drainage, groundwater, and the shear strength of soils in assessing slope stability.
Manmade slopes are used in:
- Highways
- Railways
- Earthdams
- River Training works
Slope Failure Triggering Mechanisms:
- Intense Rain-Fall
- Water-Level Change
- Seepage Water Flow
- Volcanic Eruption
- Earthquake Shaking
- Human activity
Causes of Slope failure:
1. Erosion: The wind and flowing water causes erosion of top surface of slope and makes the slope steep and thereby increase the tangential component of driving force.
2. Steady Seepage: Seepage forces in the sloping direction add to gravity forces and make the slope susceptible to instability. The pore water pressure decrease the shear strength. This condition is critical for the downstream slope.
3. Sudden Drawdown: in this case there is reversal in the direction flow and results in instability of side slope. Due to sudden drawdown the shear stresses are more due to saturated unit weight while the shearing resistance decreases due to pore water pressure that does not dissipate quickly.
4. Rainfall: Long periods of rainfall saturate, soften, and erode soils. Water enters into existing cracks and may weaken underlying soil layers, leading to failure, for example, mud slides.
5. Earthquakes: They induce dynamic shear forces. In addition there is sudden buildup of pore water pressure that reduces available shear strength.
6. External Loading: Additional loads placed on top of the slope increases the gravitational forces that may cause the slope to fail.
7. Construction activities at the toe of the slope: Excavation at the bottom of the sloping surface will make the slopes steep and thereby increase the gravitational forces which may result in slope failure.
