69 5.0 Stability Analysis of Dam Embankment Stability analysis of dam embankment is carried out base on the recommendations made for the stability analysis for the embankment dams by United States Society on Dams (2007) as given in this report sub chapter 2.2.4. The embankment stability is assessed on the basis of the factor of safety against sliding failure by means of, Circular slip analysis by Computer software- Geo studio 2004 version 6.02 Wedge analysis and Infinite slope analysis of external slope Wedge analysis and Infinite slope analysis of external slope were carried out manually. 5.1 Geo Studio 2004(Version 6.02) Numerous slope stability programs are available. Virtually all have facilities for both circular and non-circular analyses by various limit equilibrium methods, and can handle applied external forces and distributed loads, multiple soil layers, specification of groundwater conditions by water table, pore pressures or Ru values, seismic loading coefficients and tension cracks. In most cases the circular slip surfaces may be defined as a grid of circle centers, a tangent to a common surface or as passing through a common point. Non-circular slip surfaces are individually defined by the user. The various programs are distinguished mainly by the limit equilibrium methods that they employ and the user interface. With the more modern programs the geometry can also be input graphically in a CAD style interface. In the Stability analysis of this study Slope/W of Geo studio 2004(Version 6.02) is used. 5.1.1 Loading conditions on the Dam The stability of the upstream and downstream slopes of the dam embankment is analyzed for the most critical or severe loading conditions that may occur during the life of the dam using the shear parameters which were estimated in previous chapter for different grade of weathering of rocks. The following loading conditions as mentioned in the sub chapter 2.2.4 are used to analyze the stability with slope/W.
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69
5.0 Stability Analysis of Dam Embankment
Stability analysis of dam embankment is carried out base on the recommendations made for
the stability analysis for the embankment dams by United States Society on Dams (2007) as
given in this report sub chapter 2.2.4.
The embankment stability is assessed on the basis of the factor of safety against sliding
failure by means of,
Circular slip analysis by Computer software- Geo studio 2004 version 6.02
Wedge analysis and
Infinite slope analysis of external slope
Wedge analysis and Infinite slope analysis of external slope were carried out manually.
5.1 Geo Studio 2004(Version 6.02)
Numerous slope stability programs are available. Virtually all have facilities for both
circular and non-circular analyses by various limit equilibrium methods, and can handle
applied external forces and distributed loads, multiple soil layers, specification of
groundwater conditions by water table, pore pressures or Ru values, seismic loading
coefficients and tension cracks. In most cases the circular slip surfaces may be defined as a
grid of circle centers, a tangent to a common surface or as passing through a common point.
Non-circular slip surfaces are individually defined by the user. The various programs are
distinguished mainly by the limit equilibrium methods that they employ and the user
interface. With the more modern programs the geometry can also be input graphically in a
CAD style interface.
In the Stability analysis of this study Slope/W of Geo studio 2004(Version 6.02) is used.
5.1.1 Loading conditions on the Dam
The stability of the upstream and downstream slopes of the dam embankment is analyzed
for the most critical or severe loading conditions that may occur during the life of the dam
using the shear parameters which were estimated in previous chapter for different grade of
weathering of rocks.
The following loading conditions as mentioned in the sub chapter 2.2.4 are used to analyze
the stability with slope/W.
70
5.1.1(a)Steady State Seepage
When the long-term phreatic surface within the embankment has been established, the
following events are considered for analyzing.
Steady state Seepage at normal pool level or Normal High Water Level( NHWL)
460m-masl
Steady state seepage at flood surcharge or embankment crest level 463.5m-masl
Steady state seepage at Partial pool Level or Minimum Operating Level(MOL) 424 m-
masl
5.1.1(b) Rapid (or Sudden) Drawdown
When the reservoir is drawn down faster than the pore pressures can dissipate within the
embankment after the establishment of steady-state seepage conditions, the following
events are considered for this loading condition.
From Normal pool (460m-masl) to Minimum Operating Level (424m-masl)
From Normal pool (460m- masl) to Elevation 385m-masl
5.1.2 Determination of Shear Strengths
Shear Strength parameters estimated from empirical equation by Barton and Kjǽrnsh
(1981) as in the sub chapter 4.5.1.6 are effective stress parameters. However, the effective
stress analysis can be carried out for all loading conditions as per the recommendation
made by USBR as explained in the sub chapter 2.2.4.2. Hence, all analysis was carried out
as effective stress analysis with the computer software slope/w 2004.
5.1.3 Material Properties
The following figure 5.1 shows different zones of the dam and table 5.1 shows the material
properties used for slope/w analysis for the different weathering grades considered. It had
been assumed that the cohesion of all material (c´) is equal to zero at the design stage. This
assumption is conserved the analysis but it may need some value for clay core zone for
cohesion. However this analysis is also considered with same assumption due to there is no
any reliable values found to use for cohesion of the clay core zone in report of design
calculations (Sir Alexander Gibb& Partners, 1987).