Slope instability and methods to prevent it.
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Slope instability and methods to prevent it.

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Slope instability and methods to prevent it. 

We have generally three types of slope instability.

1- Toppling (falling) of rock.

We can see this type of soil instability on vertical rock bluffs, often at the side of oceans or lakes. This type of soil instability happened where water has softened the supporting geologic formation below the base of the rock. As the formation below is softened, the weight of the rock above compresses and squeezes out the formation below and the rock lose its support generally resulting in vertical splits.

When it leans sufficiently, it falls. Toppling failure can also be seen along seashores where wave action erodes the base of cliffs.

2- Gradual down-hill creep.


3- Landslides.

Soil sites with slopes more than 6% may be subject to either down-hill creep or total landslide. The instability could range from gradual down-hill creep which is difficult to detect to an existing or potential landslide which could be devastating to the project. Saturated soils on a slope are always suspect and may fail catastrophically, especially during seismic events.

 

The effect of slope instability.

Instability of a slope can result in gradual movement over a long period of time, which will damage any structure facilities above or on the slope.

If a sudden catastrophic failure occurs, commonly called a landslide, a devastating loss of the structure on the slope, below the slop, or even above such a slope could occur possibly including loss of life. In any event, the failure is generally significant and very difficult and expensive to remediate.

- Methods to prevent slope instability.

We have several methods and techniques to stabilize an unstable slope. They include slope reduction or flattening draining, structure solutions, and chemical stabilization.

Stabilization of unstable slope will require the services of experts and specialists in this type of work.

1- Sub-drained method.

In case of slope becomes unstable because of seepage water, the seepage water softens the soil causing it to lose strength and at the same time cause the affected soil to gain weight because of saturation.

Sub-drain method can frequently be effective in restoring a satisfactory factor of safety to such slopes.

Some factors the engineer of the drainage remediation should consider such as the depth to which the water must be removed, then a subsurface drain using porous media and gravity outfall pipes may be employed.

2- Slope reduction method.

On a soil slope beginning with a vertical cut, the slope at vertical is most unstable and frequently it fails even before the cut is complete. As the face forces on the slope is flattened toward horizontal, the driving forces on the slope are reduced proportionately. There is some angle of the face of the slope at which the slope will remain stable. Nature frequently finds these stable configurations over time by repeatedly failing slopes until they are flattened to the point where the site conditions and the soil strength are in equilibrium. Removing the upper portions or head of a slope or a landslide is a frequently employed technique for reducing the driving forces that cause the failure to occur. The below figure illustrates the effect of slope reduction in increasing the stability of a potential or existing landslide.


3- Structural solutions.

We know now that the reason for unstable slope is an excessive amount of force driving it to become flattened compared to the strength available to resist this force. We have many types of structural solutions such as a retaining wall or other slope-reinforcing procedure may be utilized.

The main aim from structural solutions is add the strength that is missing within the soil mass to provide an adequate safety factor. However, if a slope is actively failing, some engineering creativity is necessary in construction sequencing to enable the structural solution to be built without worsening the slide and destroying the proposed structure before it completed.

Sometimes this can be accomplished by alternating the construction areas or other temporary measures.

Sometimes a slope can be stabilized by pinning it, either by drilling vertical on angled piers well into the ground past the failure plane of the slope or installing drilled-in and grouted tiebacks or other forms of soil reinforcement as per the below figures.


4- Chemical stabilization.

In chemical stabilization method we inject soil by chemicals, such as lime or cement, to strengthen the potential slip surface within the soil. This is a highly specialized field that requires geotechnical studies by specialists who are experienced in the use of this approach for slope stabilization. 









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