Saturday, August 18, 2012

Stabilizing Surficial Slope Failures

 Surficial failures

Surficial failures occur frequently on the slopes of earthen embankments and earth fill dams which are predominantly rainfall induced.

                              Unstable slopes

 Unstable slopes create numerous management and engineering issues as we try to prevent slope failure.
Based on the literature review and interviews conducted with selected state highway engineers, researchers identified three innovative methods of using vertical members to stabilize surficial slope failures:
• Installing small structural members by conventional methods.
• Installing launched soil nails.
• Installing earth anchoring systems.
Investigators also determined that some of these methods are more cost-effective than conventional repair methods.


 Vegetation as an ecotechological solution can assist us in preventing slope failures. 
Vegetation is unlikely to have a significant impact on slope stability where slope planes are deep-seated, due to the shallow nature-of many species. 
However vegetation may protect the ground surface from erosion by wind and water and prevent erosion at the toe of slopes where the slope is being undercut by wave action in water courses.
The stability of the toe of a slope, stabilized by vegetation, may be sufficient to maintain the stability of the slope as a whole (Coppin and Richards 1990: Gray and Sotir 1996).
 Deforestation and wildfires on hillsides and valley slopes may also lead to increased soil erosion. The streams and rivers that meander and flow down theses slopes may undercut the hill-slopes and cause bank instability.  
Ground bio-engineering : the Use of Vegetation to Improve Slope Stability,  is an accepted engineering technique for stabilizing bank erosion.
You may reduce erosion and sedimentation by rapid re-vegetation of areas disturbed by harvesting operations or road construction.

                        Eco-engineering methods

Eco-engineering methods are particularly suited to natural slopes, where management is generally long term and the site is large-scale.
Artificial slopes or earthworks are either cut into natural rock or soil or built up to form embankments, dams, water tips or spoil heapes. 
Vegetation could be used for stabilizing cut slopes, soil embankments , water tips, spoil heaps and terraced slopes.  
On the other hand plant life is less likely to be of value in dams where engineering stabilizability is critical and vegetation could affect soil permeability. 
Ground bioengineering techniques are commonly used on artificial and terraced slopes, as this fast and effective solution should be considered during slope construction and remediation-ecological enhancement of earth structures. 
Vegetation provides protection and stabilization of both natural and man-made slopes along transportation routes.


 Embankments typically occur along highways, railways and canals and are made from materials such as soil or rock excavated from elsewhere and placed on natural ground.

Slope failure in embankments during and after construction is sometimes associated with the interface between the natural ground and the fill material. Pore water pressures and seepage within the embankment and natural ground may exacerbate slope failures.  
A suitable combination of vegetation types, e.g. willow pole, can help to stabilize embankments  that may be prone to the shallow translational slide failure  (Croppinj and Richards 1990). 
Vegetation may help to stabilize the toe of deeper slips. For deep seated slides,  a combination of geosynclines and vegetation may be more  appropriate.

                   Effects of terraces on erosion

Terraced slopes are common features in many parts of Asia (Storey 2002).
Built to conserve soil and water on steep slopes for a variety of agricultural uses, but if not implemented correctly, soil loss may be rapidly increased. 
For example, if hill terraces for the cultivation of crops are poorly constructed or maintained, topsoil erosion and slope instability will be exacerbated  through water collecting on overstepped terraces(Sidle et al. 2006).
Abandonment of terraces can result in the loss of vegetation  and root  reinforcement thus leading to an increase in the rate of soil erosion (Goudie 2000;Commeraat et al 2005;van Beck et al. 2005).
If the terraces collapse, breaches will focus surface runoff leading to gully formation and increase sediment transport down slope (McConchie and Ma 2002)