TABLE OF CONTENTS
1.
2.
3.
4.
INTRODUCTION ........................................................................................................ 2
1.1.
General ............................................................................................................... 2
1.2.
Background ....................................................................................................... 2
1.3.
Objectives .......................................................................................................... 2
1.4.
Site Conditions .................................................................................................. 3
METHODOLOGY ....................................................................................................... 1
2.1.
Plaxis 2D Analysis ............................................................................................. 1
2.2.
Slide 2 Validation ............................................................................................... 3
RESULTS AND DISCUSSION ................................................................................... 5
3.1.
Comparison of PLAXIS 2D and Slide 2 Results .............................................. 5
3.2.
Interpretation of Factor of Safety ..................................................................... 7
CONCLUSION ........................................................................................................... 8
1. INTRODUCTION
1.1.
General
Sindh Engro Coal Mining Company (SECMC), hereinafter called as the Client, has been
granted a 30-year Mining Lease for Thar Block-II by Mines & Minerals Development
Department, Government of Sindh that is further extendable to 30 years. Currently,
SECMC is operating a 7.6 Mt/a open pit lignite coal mine in Block II, Tharparkar. SECMC
uses truck and shovel method to remove overburden (waste) and produce lignite coal
The overburden is being dumped in-pit and ex-pit waste dump location. The height of
individual lift is 20 m and slope angle ranges between 37° to 39° (reported by the Client).
Waste dump material includes Clay, silt and sand.
A significant concern has arisen at the Ex-Pit dump, located adjacent to the Mine, due to
increased rainfall spells causing flooding, blowouts, and large-scale water accumulation.
The resulting soil erosion and landslides are damaging critical infrastructure. Currently, the
Ex-Pit dump lacks a surface runoff drainage system, leaving mine infrastructure and local
farmlands vulnerable.
SECMC has hired Berkeley Associates Pvt Ltd. (BAL) for Ex Pit Dump Reclamation Pilot
Project. In this regard, M/s BAL is working on devising an innovative yet efficient system
for reclamation of this Ex Pit Dump by employing methods of Surface Protection and
efficient drainage system. For using various methods of surface protection the existing
slopes of the pit may require some trimming and adjustment and this requires that the
existing slopes must be evaluated for slope stability. This Technical Note provide details of
slope stability analyses carried out by BAL.
1.2.
Background
Slope stability analysis is a critical aspect of geotechnical engineering, ensuring the safety
of natural and man-made slopes. The objective of this study was to evaluate the stability
of a given slope using PLAXIS 2D and validate the results through Slide2.
1.3.
Objectives
The key objectives of this study are outlined below:

Perform numerical slope stability analysis using PLAXIS 2D.

Validate the results using Slide2.

Assess the reliability of both analyses and compare FOS values.
1.4.
Site Conditions
1.4.1. Location
The analysis was conducted for a site within Sindh Engro Coal Mining Company
(SECMC), Pakistan’s leading coal producer. SECMC operates the country’s first openpit lignite mine in Block II of the Tharparkar region, located in Sindh province, Pakistan.
1.4.2. Soil Conditions
The site consists of waste dump material generated from mining activities, forming a
heterogeneous mix of various soil compositions. Due to the variability in material
properties, multiple soil layers exist within the dump. To ensure a conservative stability
assessment, the most critical soil layer was selected based on direct shear test data,
identifying sand as the least stable material among the available soil layers.
1.4.3. Slope Geometry
It is important to mention here that the slopes provided by the Client (37-39 degrees)
are not the true representative of Ex pit Dump Slope i.e., when the slopes were
developed using the DEM data provided by the Client the angle ranges from 32 – 35
degrees as illustrated in the figure 1.4 – 1 & 1.4 – 2
Figure 1.4-1: Existing Slope Geometry 2nd lift
Figure 1.4-2: Existing Slope Geometry 1st lift
To accurately analyze slope stability for the effect of imposed surface protection
features, trimming and adjustment, the following slope geometry has been considered,
with defined coordinates and angles representing the terrain profile
Figure 1.3-1: Designed Slope Geometry
2. METHODOLOGY
The methodology involves numerical modeling of slope stability using PLAXIS 2D and
Slide2. The analysis includes defining material properties, boundary and loading
conditions, and applying the strength reduction method for stability assessment.
2.1.
Plaxis 2D Analysis
2.1.1. Input Parameters
 Material Properties
Based on the available soil layers and their properties, the most critical layer—
identified as the sand lens—was selected for analysis. This layer exhibits a cohesion
of 0 kPa and an internal friction angle of 36°, with a dry unit weight of 14.9 kN/m³.
These parameters were derived from the "Geotechnical Data" file provided by
the Client, specifically from the sheet titled "Direct Shear", which includes data for
various soil layers. The sand lens was determined to be the critical layer based on this
data. To ensure consistency in the analysis, the Mohr-Coulomb constitutive model was
adopted for both PLAXIS 2D and Slide2.
 Boundary Conditions
In the PLAXIS 2D model, appropriate boundary conditions were assigned to ensure
accurate slope stability analysis. The lateral boundaries (Xmin and Xmax) were set as
normally fixed, restricting horizontal movement while allowing vertical displacement.
The bottom boundary (Ymin) was fully fixed, preventing both horizontal and vertical
movement to simulate a rigid base. The top boundary (Ymax) was left free, allowing
natural deformation of the slope surface. These boundary conditions effectively
replicate real-world constraints and ensure reliable numerical results.
 Loading Conditions
No external loading or groundwater conditions were considered in this analysis. The
stability assessment was conducted under dry conditions to evaluate the inherent
strength of the slope material.
2.1.2. Modelling Approach
 Mesh Details
The finite element mesh was generated using a fine element distribution. Enhanced
mesh refinements were enabled to improve numerical accuracy, particularly in regions
where failure mechanisms are expected to develop. A finer mesh provides better
resolution of stress-strain behaviour, ensuring reliable slope stability results.
Figure 2.1.2-1: Finite Element Mesh Configuration
 Strength Reduction Method
The Strength Reduction Method (SRM) was used to determine the Factor of Safety
(FOS) in PLAXIS 2D. This method systematically reduces the shear strength
parameters of the soil (cohesion c and internal friction angle ϕ) until failure occurs. The
FOS is calculated as:
The software iteratively reduces these parameters until the slope reaches a critical
state, where failure is observed through excessive deformations or non-convergence
of the numerical solution. This approach provides an accurate assessment of slope
stability under given conditions.
Figure 2.1.2-2: Plaxis2D Model Setup
2.2.
Slide 2 Validation
2.2.1. Input Parameters
For validation, the same input parameters used in PLAXIS 2D were applied in Slide2,
ensuring consistency in the comparative analysis. These parameters include:

Material Properties (Cohesion: 0 kPa, Friction Angle: 36°, Dry Unit Weight: 14.9
kN/m³)

Boundary Conditions (Same as PLAXIS 2D model)

Loading Conditions (No external loading, dry conditions)
2.2.2. Modelling Approach
The Limit Equilibrium Method (LEM) was employed in Slide2 for slope stability
analysis. Fellenius, Bishop Simplified, Janbu Simplified, Janbu Corrected, Spencer,
and GLE/Morgenstern-Price were used to compute the Factor of Safety (FOS) by
analyzing potential failure surfaces within the slope. A grid search technique was
applied to locate the critical slip surface, ensuring a comprehensive assessment of
slope stability.
By comparing the FOS values from PLAXIS 2D (Finite Element Method) and Slide2
(Limit Equilibrium Method), the results were validated to confirm the reliability of the
analysis.
Figure 2.2.2-1: Slide2 Model Setup
RESULTS AND DISCUSSION
3.
3.1.
Comparison of PLAXIS 2D and Slide 2 Results
3.1.1. PLAXIS 2D FOS
Figure 3.1.1-1: Critical Slip Surface from PLAXIS 2D
Figure 3.1.1-2: Computed Factor of Safety (FOS) from PLAXIS 2D
3.1.2. Slide2 FOS
The Factor of Safety (FOS) values obtained from Slide2 using different limit
equilibrium methods are as follows:

Ordinary/Fellenius: 1.368

Bishop Simplified: 1.424

Janbu Simplified: 1.362

Janbu Corrected: 1.421

Spencer: 1.423

GLE/Morgenstern-Price: 1.421
Fig: Calculated critical slip surface and FOS(1.368) for Fellenious Method
Fig: Calculated critical slip surface and FOS(1.362) for Janbu Simplified
3.2.
Interpretation of Factor of Safety
The Factor of Safety (FOS) obtained from PLAXIS 2D is 1.345, while the values from
Slide2 using various limit equilibrium methods range between 1.362 and 1.424. Both
software accurately capture the same critical slip surface, demonstrating the reliability of
the analysis. The slight variation in FOS values across methods is due to differences in
calculation approaches. Overall, the results confirm that the slope is stable for the
proposed surface protection features.
4.
CONCLUSION
The slope stability analysis conducted using PLAXIS 2D and Slide2 provides consistent
and reliable results, with both software accurately identifying the same critical slip surface.
The Factor of Safety (FOS) values indicate that the slope is marginally stable under current
conditions. While no external loading or groundwater influence was considered, the results
highlight the need for careful monitoring and potential reinforcement if additional loads or
environmental changes occur. If required, slope stabilization measures such as soil
reinforcement, drainage improvements, or slope regrading should be evaluated to
enhance stability.