Slope Stability Analysis for an Existing Railway Embankment

Slope Stability Analysis 1
SLOPE STABILITY ANALYSIS
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Slope Stability Analysis 2
Slope Stability Analysis
Part A
Introduction
The purpose of this report is to carry out a stability analysis for an existing railway
embankment which has been in place for just over 80 years. The total length of the embankment
is 500 metres, which is divided to 5 separate sections. The entire section has been assessed for
stability. Soil strength parameters have been derived for each embankment section by carrying
out an intrusive ground investigation using window sampling.
Typically, the strength of the drained case is higher than the strength of undrained case.
But in case of low normal stresses, the strength of the drained case tends to be a lot lower
compared with when normal stresses are high.
Methodology
Stability analysis for the existing embankment has been evaluated using limit equilibrium
method and it has been performed using oasys slope 19.1 software according to Bishop’s
simplified method. The assumption made by this method is that the slip surface is circular and
the forces on the sides of the slices are horizontal (no shear). The stratum coordinates and the
ground water coordinates have been calculated for the 5 sections of the railway embankment
separately using soil strength parameters. The obtained data has been applied in the software for
drained and undrained cases. Drained analysis represent the long term condition when the
embankment without the train whilst undrained analysis represents the short term condition when
the embankment subjected to train’s load. The undrained loading analyses have been performed
using total stress parameters (Cu, φ) while the drained analyses (unloading) were performed by

Slope Stability Analysis 3
applying the effective stress parameters (C', φ'). The input of slip surface data including centres
on grid about local axis (spacing) has been adjusted by trial and error procedures in order to get a
minimum factor of safety ensuring that the critical point is within the grid and the grid is in a
reasonable position.
Results
Th results obtained from the two analyses are presented in Table 1 and 2 below.
Table 1: Results for undrained case
Sections Length (m) Bulk density

Cohesion Cu
(KPa)
Angle of
internal
friction φu
Factor of
safety
Section 1 82 18.50 63 0 3.156
Section 2 120 17.45 57 0 2.681
Section 3 61 17.90 48 0 2.579
Section 4 135 18.75 44 0 2.038
Section 5 102 17.40 54 0 2.395
Table 2: Results for drained case
Sections Length (m) Bulk density
 Cohesion C'
(KPa)
Angle of
internal
friction φ'
Factor of
safety
Section 1 82 18.50 1 28 0.920
Section 2 120 17.45 1 22 0.791
Section 3 61 17.90 0 22 0.890
Section 4 135 18.75 0 24 0.646
Section 5 102 17.40 1 23 0.409
The undrained case is a short term condition when the railway embankment is subjected
to the train’s load. In this case, the rate of loading is much faster than the rate at which the pore
water is able to dissipate. This results to an increase in pore water pressure in the embankment,
which causes a corresponding increase in soil strength. According to the results from Table 1,