Slope failures are major natural hazards in many areas throughout the world. Slope failures are also referred to as mass movements. A slope failure is classified based on how it moves and the type of material being moved. Several major types of slope failures have been identified:
- Creep: very slow movement of rock or soil downslope.
- Falls: very rapid fall of rock and earth material from vertical or near vertical slopes.
- Flows: slow to rapid movement of rock, soil, snow, or ice. Types of flows include mud flows, earthflows, debris flows, and snow avalanches.
- Slides: very slow to very rapid movement of soil or rock. This category includes rock slides, earth slides
- Subsidence: slow to very rapid collapse of rock or soil into underlying spaces.
Causes of slope failure
Slope failures can be triggered by a variety of natural events including earthquakes, heavy rainfall from thunderstorms.
Slope failures can also caused by modification of a natural slope by people especially if the slope was not properly designed. This increases the risk of slope movement, especially in areas already susceptible to these natural hazards.
Common cause of Slope failure
The common cause of slope failure is mainly caused by heavy rain which weakened the soil. There is a high correlation between the intensity of rainfall to the number of slope failures occurring in the country.
Other causes of slope failure include steep cutting on the slope where in some instances the slope cutting was almost vertical. Landowner preferred to have steep cutting since this would maximized their land development. However steep cutting and with no slope protection measures can posed a danger to the occupant.
Inadequate and improper drainage systems were also among the common causes of slope failure. There were cases where the house perimeter drain or sewerage pipe outlet were being discharge onto the slope and as a result eroding the slope surface. Sometime the size of the drain was inadequate resulting in overflow of water onto the slope. There were also cases where the drains were not properly maintained and covered with debris and silt and as a result the water overflowed onto the slope causing the slope to erode
Improper dumping of soil from a cut slope onto the surrounding slope surface can also posed danger. It can destabilized the whole slope since the cut soil was not properly compacted and as a result the overburden soil will be in loose condition. Accumulation of water on the loose soil will eventually decreased the amount and friction on the soil mass causing the loose soil to slide.
Method use for slope repair
Various methods can be used to repair slope failure. These include :
Reinforced concrete retaining wall
Bored pile wall
Gabion wall
Sheet pile wall
Construction of reinforced slope using
Geogrid
Geotextile
Surface protection
Cellular confinement
Erosion control mat
Cutting of slope with a gentle gradient
The type of the retaining structure depend on :
Type of failure
Cost
Ease of construction
Site Constraint
Speed of construction
Method of slope protection
Construction of concrete retaining wall
Bored pile wall
Gabion wall
Sheet pile wall
Construction of reinforced slope using Geogrid
Construction of reinforced slope using Geotextile
surface protection using Cellular confinement
Surface protection using Erosion control mat
Cutting of slope with a gentle gradient
Reducing risk of slope failures
In general the reduced number of cases and fatalities could be attributed to the government effort to ensure that land development should be done in a control manner and in a way we are putting some effort on the RISK MANAGEMENT aspect.
RISK is the product of hazard and exposure.Hazard arising from natural fenomena will always be there and therefore is very difficult to control. So one of the ways to reduce our risk is to reduce our exposure.
In the context of risk arising from slope failure we can reduce this by reducing our exposure by implementing development control guideline.
For examples in areas where development is being carried out near to slope , the applicant should indicate that proper measures has been taken to reduce the risk of failures.These include ensuring that the proper slope gradient on the cut areas and fill areas.For natural slopes it should be 1:2 for cut areas and 1:3 slope for fill areas.If the slope gradient is steeper then appropriate measure in term of slope protection should be carried out. These can be done by building retaining structures or reinforced slope
Apart from that applicant should ensure that the slope is vegetated properly and ensure that erosion control measures are being put in place.They must also ensure proper drainage system for the slope and surrounding areas to reduce rainfall runoff on the top surface of the slope.
And finally In areas where slope failures are a hazard, the best solution is not to place any structures on or near a slope that has been determined to be unstable.
Conclusion
We can prevent slope failure from occurring or reduce the risk of slope failure through proper design and ensure guidelines on construction near to slope are met
Appropriate slope protection work being carried out
Not to place structures on unstable slopes
