Highway Research Record

III. GEOTECHNICAL ENGINEERING
ON GOING PROJECTS
Numerical Modeling/ Simulation of Rainfall Induced Landslides

Date of Start: December 2002
Central Road Research Institute, New Delhi (R, I)

Present Status and Progress
Parametric study has been carried out for an idealized slope and for slopes along NH-39. The following studies were made.
(A) Studied the effect of permeability and rainfall intensity on factor of safety for the slopes along NH-39.
(B) Studied the effect of geometry of slope for rainfall induced landslides.
The following points have been observed from the above study
• The pore pressures obtained for all the three cases (i) At steady state condition with rainfall, (ii) after one hour of continuous rainfall and steady state condition, (iii) without rainfall, are different for all the four slopes considering same initial conditions and soil properties.
• The F.S. is changing for all the four slopes with change in rainfall intensity and geometry of the slope.

Findings / Conclusions
Development of computer program to compute the effect of pore-water pressure, suction, permeability of the soil at different depth from the ground surface after a lapse of time from the starting of rainfall by knowing the rainfall intensity, and initial moisture content at different depth and the value of permeability for different value of porosity and moisture content from soil-water characteristic curve.

Limitations
The program that is under development is considering the value of permeability of soil in one dimension only. To overcome this draw-back a FEM based two dimensional solution of unsteady equation of flow of liquid through soil medium work is in progress to compute the pore water pressure (positive and negative) due to infiltration of rain water into a slope.

Recommendations

Due to rainfall, the water will enter into the soil, consequently, the pore water pressure within the slope will be changed and that will change the stability of slope, its stresses within the slope and displacements. The program which is just on the verge of completion will be used to know the pore pressure behaviour of the slope due to rainfall, just knowing the moisture content of the soil with depth. But this is not enough to know the failure condition of the slope. To accomplish this, taking the pore pressure condition of the slope as input, the stresses and displacement behavior of the slope due to rainfall and finally its factor of safety, needs to be computed and on that basis landslide hazard index will be worked out. Development of computer program using FEM for nonlinear Visco-Elastic-Plastic soil model, satisfying Mohr-Coulomb failure criterion with non-associated flow rule is suggested to be persued. This program will compute displacement, stress-strain behavior of the slope and factor of safety for varied piezometric conditions due to rainfall.

Reports/Publications
1 Analysis of Rainfall Induced Landslides along NH-39- IGC-2003-‘Geotechnical Engineering for Infrastructural Development’ pp. 409-412 Dec 18-20, 2003, Roorkee.
2 Analysis of Rainfall Induced Landslides along NH-39 – LS04-Landslides, slope stability and the safety of Infrastructures 24-25 March,2004, Malaysia.