An experimental investigation on effect of drawdown rate and drawdown ratios on stability of cohesionless river bank and evaluation of factor of safety by total strength reduction method

ABSTRACT

This paper presents an experimental investigation on the stability of a model river bank composed of homogeneous cohesionless soil under rapid drawdown condition. The effects of major influencing parameters controlling Factor of Safety (FoS) of a river bank under depletion of water level have been considered in the present study. A series of laboratory model studies have been carried out to investigate the cases of drawdown rate and drawdown ratios rendering the bank to maximum damage. Moreover, the effect of water level drawdown on the response variables namely pore water pressure, deformation of the bank profile, and shear strength of bank material have been observed and analysed. Stability analysis of the experimental model banks under drawdown conditions was carried out by evaluating FoS using the principle of reduced shear strength methodology. The in-situ total shear strength after each drawdown was measured using a laboratory vane shear apparatus. The minimum shear strength among these values has been identified. Now the FoS against each drawdown ratio has been computed from the ratio of total shear strength obtained after drawdown for that particular drawdown ratio to the minimum shear strength as obtained. This experimental programme examined the optimum combination of drawdown rate and drawdown ratio causing mass failure of the bank. It was revealed from the variation of pore pressure after drawdown that changes of pressures at points close to the toe of the bank slope is strongly controlled by the stress-state induced by drawdown. From the failure observations it was found that the drawdown rate is the dominating cause of maximum deformation of the bank than that of drawdown ratio. The findings of the present research work may help in predicting the actual failure scenario and stability condition of prototype river under similar boundary conditions.

KEYWORDS:

• Slope stability
• drawdown rate
• drawdown ratio
• river bank model
• strength reduction method
• pore water pressure

Acknowledgements

The authors also acknowledge the physical as well as technical supports of Mr Dibyendu Sengupta, the Laboratory in-charge of Civil Engineering Department, Aliah University; and M.Tech. students Mr Sahin Ahmed, Mr Zeeshan Ahammed and Ms Rituparna Das.

Additional information

Funding

The authorship gratefully acknowledges the financial support from Aliah University for conducting laboratory model experiments.