A study on seismic vulnerability of an earth embankment slopes : case study earth embankment on Northern bypass

Abstract

Embankments have a great role in the development of road and railway structures and some of these roles are carrying roads or railways across a low-lying area. Uganda is no stranger to earthquake catastrophes since there is well-documented past seismic activity in the country resulting into mainly structural damage and minimal loss of life and from the fact that Uganda lies between the eastern and western arms of the great rift valley, a seismically active region. Our aim was therefore to assess the seismic vulnerability of an earth embankment on the Northern by-pass road on chainage 18+370 and 18+680 by evaluating the stability of embankment slopes using historical peak ground acceleration of 0.3g, use of future projected peak ground acceleration of 0.352g to determine the possibility of slope failure and evaluation permanent displacement of the mass of the slopes based on the sloping cube slip. This was done by identification a suitable embankment, then gathering all the required information and carrying out analysis using the Pseudo Static Analysis and Newmark Sliding Block Analysis. The main focus of the study was to determine the slope stability of the embankment during a seismic excitation. Pseudo forces have been taken to represent a maximum of the seismic forces. From the analysis carried out using both future and projected accelerations, it was found out that the factor of safety of the embankment for historical peak ground acceleration and future projected peak ground acceleration was 1.05 & 1.15 respectively thus it was low and questionable. The suggested way of improving the F.O.S of the embankment was through reinforcing it with nails of pull-out strength of 100kN/m2 at a spacing of 2m and inclined 150 to the horizontal for a 20m long nail for this specific embankment. This greatly improved the F.O.S of the embankment to 1.5 which is adequate for the embankment with the introduction of the seismic force. The safe settlement distance of the community from the embankment was 20m according to the Newmarks Sliding block Analysis.