Bio-engineering is the use of living plants either alone or in combination with small scale civil-engineering structure (Non-living plant materials) for the purpose of reducing shallow seated instability and controlling erosion on slopes.
Interaction between civil engineering system and vegetative systems
Bio-engineering is relatively cheap, reliable and easy to construct because this technique uses local resources and technique.
Use of pure civil engineering in stabilizing the slope is not an effective method because those engineering structures are expensive and require skilled techniques. The use of engineering structures is not reliable in the village due to the lack of construction and skilled manpower. On the other hand, the efficiency of engineering structures is maximum immediately after construction and its efficiency goes on reducing with passes of time. But in bio-engineering, the efficiency is zero initially and becomes maximum later. Therefore the combined use of civil engineering and bio-engineering is the best method for stabilizing the slope. Where stabilizing capacity is transferred from civil engineering to bio-engineering system with the course of time.
Causes of slope failure
Cause of slope failure or slope instability are categorized into two broad class i.e.
i. Those causing an increase in shear stress.
ii. Those causing a decrease in shear strength.
Those causes which increase shear stress or reduces the shear strength of soil are:
i. Force-induced due to the steep slope.
ii. Force due to seepage water.
iii. Force due to the sudden lowering of the water table in the reservoir.
iv. Force due to the earthquake.
v. Force due to the erosion of surface slope by flowing water, etc.
Mechanism of slope failure ( Types of slope failure)
There are two types of slope. They are:
i. Finite slope
A finite slope is limited in extent and the properties of the soil will not be the same at identical depth.
ii. Infinite slope
An infinite slope is very large in extent, theoretically infinite and the properties of the soil will be the same at identical depth.
If a failure occurs in infinite slope such types of failure are called infinite slope failure.
The failure mechanism of slopes
1. Rotational failure
In these types of failure rotation of failure surface (slope) takes place by the downward and outward movement of the soil mass. The slip surface is generally circular for homogeneous soil and non-circular for heterogeneous soil mass. The rotation failure is further divided into:
a. Base failure
When slip surface passes under the base of the slope it is called base failure.
b. Toe failure
When the slip surface intersects the toe of the slope, it is called slope failure.
c. Slope failure
When the slip surface get ended on the slope face, it is called slope failure.
2. Transitional failure
If the failure occurs due to the movement of soil mass along a level surface it is called transitional failure. This type of failure occurs on both finite and infinite slope.
3. Compound failure
A compound failure is the combined failure of rotation and transitional failure as shown. A compound failure surface is a curve at the two ends and plane in the middle portion. A compound failure generally occurs when hard strata exit at considerable depth below the toe.
4. Wedge failure / Block failure / Plane failure
A failure along an infinite plane by separating a triangular wedge is called wedge failure. It occurs when distinct blocks and wedge of soil mass becomes separated.