slope stability and seepage by slide software (Teton dam)

Table of content
• purpose
• Background
• Dam Geometry
• Failure process
• Causes of failure
• Solutions models
• Software Models Result
• Discussion
• Conclusion

purpose
Analysis of Teton dam has been performed to
1. To predict the cause of it’s failure
2. To design Teton dam by slide software

Background
• The Teton Dam was situated on the snake River, three miles northeast of Newdale, Idaho.
• It was designed to provide recreation, flood control, power generation, and irrigation for over
40,000 hectares.
• Design and Construction by U.S. Bureau of Reclamation (USBR).
• Teton Dam, a large earthen dam in eastern Idaho that failed on June 5, 1976 during it’s first filling.
• The failure of Teton Dam killed fourteen people and caused damage to hundreds of million
downstream.

Introduction to failure
Sequence of event
 Began storing water on October 3,1975.
 rising at a rate of about 1 meter (3 feet) per day
 On June 3, 1976 several small seepages were noticed in the north abutment wall.
 On June 4, 1976 small springs were beginning to appear.
 On June 5, 1976
 Between 7:30 and 8:00 a.m. the leak was flowing at about 500 to 800 liters per second at right
abutment.
 By 9:00 a.m. the flow had increased to 1,100 to 1,400 liters per second .
 At 11:00 a.m. a whirlpool was observed in the reservoir .
 Between 11:15 and 11:30 a.m. a 6 by 6 meter chunk of dam fell into the whirlpool and within minutes
the entire dam collapsed (Independent Panel, 1976).

Right abutment failure sequence

Right abutment failure sequenceRight abutment failure sequence

Dam materials
Zone Material
1 Silt with some clay , sand and gravel
2 Selected sand, gravel and cobbles
3 Miscellaneous fill
4 Select silt, sand ,gravel and cobbles
5 Rockfill

Causes of failure
 An Independent panel reviewed the
cause of failure of Teton dam and reach
to the conclusion that the dam failed
due to the high hydraulic gradient in the
key of the dam. Fracturing occurred in
the key of the dam as shown in the
figure.

Causes of failure
 The upper foundation rock was grouted and the
atturites consider it safe but latter on during
investigation water pressure tests were
performed at about 50 ft and they reported a
very high permeability rock so it mean that the
rock was highly fractured this could have lead to
piping through the foundation.
 While these modes of failure are possible, all
physical evidence was washed away on 5 June
1976. The exact mode of failure will never be
known. However, inadequate protection of the
zone one core from internal erosion remains the
undeniable cause of failure.

Process inside the damProcess inside the dam

• Two type of analysis is used for dams
1. Seepage analysis
2. Slope stability
Analysis type

Seepage analysis is performed using slide software. The following data
is look for
• Horizonal gradient
• Factor of safety against exit gradient
Seepage analysis

Horizontal gradient
The following criteria has been used to check for the horizonal gradient

Vertical gradient
Cohesionless soil
For the case of cohesionless soils, the factor of safety (FS) with respect to vertical exit gradients
(against boiling or heave) is generally defined as the ratio of the critical gradient (Ic) to the predicted or
measured exit gradient (Ie):

vertical gradient
Cohesive soil
• For the case of cohesive soils, the factor of safety (FS) with respect to vertical exit gradients

Relief well
• Relief wells are frequently used to reduce subsurface
hydrostatic pressures in the downstream foundations
of dams and levees.
• Relief wells act like valves to relieve the water pressure and
allow excess water to be diverted safely, for example, to a
canal. Relief wells can prevent sand boils from occurring by
relieving the water pressure as described. In all the model’s
relief well is provided.

Slide the software of rocksciences has been used to perform the
following type of analysis
1. Slope stability
2. Transient groundwater analysis
3. Steady state seepage analysis
Slide Software

• Auto grid has been used to search for the critical slip surface
• The following method are reported
1. Bishop simplified
2. Janbu simplified
3. Janbu corrected
4. Morgenstern-Price
5. spencer
Slope stability

• A transient groundwater analysis may be important when there is a time-dependent change in
pore pressure
• As the Teton dam failed in the first filling so it is important to anylisize it for various water level at
various time.
Transient groundwater analysis

• The steady state seepage analysis used when the water level is kept constant and the pore
pressure in the soil become constant mean the water come to the soil element become equal to
the water going out of the soil element.
• For all the model this analysis is performed at maximum water level so that we can check the
effect of maximum water level after a considerable amount of time.
• Data like the horizontal, vertical hydraulic gradient, vertical and horizontal seepage velocity has
been noted.
Steady state groundwater analysis

Slope stability result
Method Factor of safety Factor of safety
Upstream Downstream
Bishop simplified 1.93 1.62
Janbu simplified 1.91 1.59
Janbu corrected 1.94 1.62
Morgenstern-Price 1.93 1.62
spencer 1.93 1.62

Seepage result
Dam that failed
Dam part Vertical hydraulic
gradient
Horizontal hydraulic
gradient
Vertical discharge
velocity (ft/day)
Horizontal discharge
velocity (ft/day)
zone 1 (core) -8 5.6 0 .003
zone 5 0 0 -0.01 0.01
Rylie foundation 0.12 0.17 0.012 0.087
Zone 2 -28 27 0 0.01
Zone 4 12 12 0 0.05

Factor of safety against vertical
gradient
Material type x-coordinate y-coordinate Factor of safety
Cohesive -102 5205 4
Cohesionless Zero vertical
gradient at exit
points

Method Factor of safety Factor of safety
Upstream Downstream
spencer 3.66 2.07

Seepage result
Zone dam model 1
gradient
gradient
Vertical discharge
velocity (ft/day)
velocity (ft/day)
Core 0.3 1.3 0.4 0.003
Shell 0.09 0.25 0.09 0.8
Rylite foundation 0.2 0.5 0.1 1
Filter 0.3 0.1 0.45 0.2
Relief well 0 0.15 -10 0

gradient
Cohesionless 236 5219 2
Cohesionless -10 5330 3.1
Cohesionless 55 5220 7.0

Factor of safety Factor of safety
Upstream Downstream
spencer 4.23 1.54

Seepage result
Model with Drainage Blanket
gradient
Horizontal
hydraulic gradient
Vertical discharge
velocity (ft/day)
Horizontal
discharge velocity
(ft/day)
Core 0 2.25 -0.001 0
Drain Blanket 0 0.001 0.002 0.114
Rylite foundation 1 0.375 0.002 0
Relief well 0 0 -0.135 0.34

gradient
Cohesive -14 5309 3.28
Cohesive -50 5312 5
Cohesionless 83.5 5250 2

Upstream Downstream
Spencer 7.85 2.25

Seepage result
Dam with Toe Drain
gradient
gradient
Vertical discharge
velocity (ft/day)
velocity (ft/day)
Core -0.167 0.8 0 0.01
Toe Drain -0.009 0.036 -0.01 0.004
Rylite foundation 0 0.08 0.02 0.2
Relief well -0.02 0.3 -.26 2

gradient
cohesive -147 5277 2.83
cohesive -93 5396 2
cohesive -12 5532 Head lower then elevation
cohesive 38 5317 Head lower then elevation
cohesive 21 5324 Head lower then elevation

Upstream Downstream
spencer 7.15 2.06

Seepage result
Dam with chimney drain
gradient
gradient
Vertical discharge
velocity (ft/day)
velocity (ft/day)
Core -10 8 0 0
Chimney Drain -0.66 0.2 -0.014 0.6
Rylite foundation -0.66 0.2 0.013 0.087
Relief well 0 0 0.006 0.166

gradient
Cohesive At all Pressure head below
elevation
coheshionless 106 5135 400

Discussion
 The Teton dam constructed in 1970s failed due to internal erosion. From this study as shown in the result
section that high internal gradient of up to 7 was developed in the key of the dam. Secondly high internal
gradient was also noted in filter material (zone 2) of the dam.
 Various methods in this study are used to model the dam. Like we used the zoned dam, drainage blanket,
toe drain and chimney drain. Each method has its own pros and cons.
 Zoned dam is the most appropriate solution as it give the drainage facility on both the sides and due to
rapid drawdown, it will be also stable as the shell will dissipate the water quickly as the water level goes
down.

Slope stability downstream
1.3
1.5
1.7
1.9
2.1
2.3
2.5
Factor of safety for downstream slope
Core Dam Drainage Blanket Toe drain Chimney Drain Constructed dam

Slope stability upstream
1.3
2.3
3.3
4.3
5.3
6.3
7.3
8.3
Factor of safety upstream slope
Core Dam Drainage Blanket Toe drain Chimney Drain Constructed dam
Slope stability upstream

Summery of horizonal gradient check
 For cohesionless soil the horizontal hydraulic gradient has been compared with the chugaev
criteria (as mention in the above slide) and has been satisfied for all cohesionless soil of all the
models.

Conclusion
• In the present study the slope stability and steady state seepage analysis of the right abutment of Teton
dam has been performed.
• Case of the failure of dam has been analyzed which has been confirmed that it was due to high internal
gradient.
• Four model of Teton dam have been analyzed and their factor of safety has been calculated. The safety has
also been checked against the horizonal gradient as it led to internal erosion.
• The analysis of an earthen dam is not a very simple process. There are many factors that effect the result
which cannot be modeled in the software that must be checked out in the laboratory. Properties like the
plasticity of the core of the dam should be properly evaluated.
• Many thinks during construction should be look for like the gradation of the material.

slope stability and seepage by slide software (Teton dam)

slope stability and seepage by slide software (Teton dam)

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