GEOTECHNICAL ANALYSIS OF RIVER BANK SLOPES IN THE SOUTHERN PART OF MAJULI, ASSAM, INDIA

Jayanta Laskar

Abstract


Majuli is the largest river island in the world and is situated within the Brahmaputra River in the Jorhat District of Assam. It is a seat of Vaishnavite Culture and has been experiencing extensive erosion along its southern margin, which has led to reduction of the land area over the past few decades.

The present study investigates the geotechnical properties of bank materials occurring at a few locations along the southern margin of Majuli, with a view to assess their in-situ stability condition. Field investigations included micro-level survey of river-bank slope, and study of lithofacies types and sequence. Representative samples of bank material collected during field-work were subjected to textural analysis and determination of shear strength parameters, in the laboratory. The Factor of Safety of the slopes was determined following the Bishops method as well as from Circular Failure Charts.

The investigations reveal that materials comprising the slopes range from very fine sand to silt and clay. The lithofacies are represented by fine sands with low angle cross stratification (Sl), horizontal laminated sand (Sh), planar cross stratified sand (Sp) and massive mud (Fm). The slope angle ranges between 17o to 35o , while the slope height varies from 2m to 7m. The values of  cohesion(C) and angle of internal friction (f) indicates the dominance of frictional component.  A near unity value of Factor of Safety of the analyzed slopes indicates prevalence of an unstable to critically stable condition in them.


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