effects on earth dams of drawdown scenarios imposed after ... · stefania sica, federica rotili...

TC210 Embankment DamsWorkshop on Embankment Dams

Effects on earth dams of drawdown scenarios imposed after a strong earthquake

Stefania Sica, Federica Rotili(University of Sannio, Italy)

Luca Pagano (University of Naples Federico II, Italy)

XVII ECSMGE-2019 Geotechnical Engineering foundation of the future

Reykjavik, 1 September 2019

Lowering the water level of a reservoir may be critical for the stability of earthdams. In literature, several cases of slope failures caused by rapid drawdownwere described (Sherard et al. 1963; Morgenstern 1963, etc.). An iconic case-historyis San Luis Dam in California in 1981.

OVERVIEW

To date the combined effects of an earthquake first and a rapid drawdown later have never been investigated.

Effects on earth dams of drawdown scenarios imposed after a strong earthquake Sica et al.

Embankment construction – Reservoir impounding – Steady state – Earthquake - Drawdown

1 2 3 4 5

Numerical study

(mod. from Alonso and Pinyol, 2009)

DRAWDOWN EFFECTS: HYDROSTATIC PRESSURES ALONG THE SLOPE SURFACE

L

reservoirsubmerged slope


drawdown

L


(mod. from Alonso and Pinyol, 2009)

The external stabilizing hydrostatic pressures reduce but the internal-to-slope pore waterpressures could delay their decrease to the steady-state values associated to the newreservoir level.

DRAWDOWN EFFECTS: HYDROSTATIC PRESSURES ALONG THE SLOPE SURFACE

L



drawdown

L


DRAWDOWN EFFECTS: CHANGES IN TOTAL STRESS AND PORE PRESSURE INSIDE THE SLOPE

The resulting pore water pressures will not be in equilibrium with the newboundary conditions and a transient regime will develop.

R/k = Rate of drawdown/Hydraulic conductivity

During the drawdown, the resulting pore pressures will be affected by:

• soil permeability• rate of water level lowering• initial pore pressures (and initial stress field),• soil skeleton behavior (controlled by soil stiffness and change in saturation).

The change in hydrostatic pressures along the slope surface induces a change intotal stress and pore water pressures inside the slope.


Pore

wat

er

Pre

ssu

re (

uw

)

Uw1 at steady state 1

Drawdown rate/Hydraulic conductivity

undraineddrawdown

partially draineddrawdown

draineddrawdown

Uw1U*w2

1

2

U*w2

DUw2

GLOBAL INSTABILITY – DRAWDOWN

Uw2∞ at steady state 2

LHP

Pore

wat

er

Pre

ssu

re (

uw

)

Drawdown rate/Hydraulic conductivity

undraineddrawdown

partially draineddrawdown

draineddrawdown

(Uw1+DUw1 ) U*w2

1

2

DUw2

Duw1

GLOBAL INSTABILITY – DRAWDOWN AFTER AN EARTHQUAKE

Uw1 at steady state 1

U*w2

Uw2∞ at steady state 2

(Uw1 +Duw1 )

HP

Earthquake effect

• Coupled 2-phase formulation from construction up to the end of the seismic stage.• Non-linear elastic soil behaviour combined to Mohr-Coulomb yield criterion.• Cyclic soil behavior described through a hysteretic model combined to Masing (1926) rules.• Excess pore water pressure model (Byrne, 1991) activated during the seismic loading.

THE CAMPOLATTARO DAM (ITALY)

core

Foundation

Parameter γd[kN/m3] PI% n c' [kPa] φ' [°] k [m/s]

Core 17.38 30 0.35 50 23 1.49 ∙ 10-9

Shells 21.09 0 0.25 0 43 1.68 ∙ 10-5

Drains 19.75 0 0.25 0 30 5.27 ∙ 10-6

Foundation 17.21 33 0.30 130 22 10-9

ID Earthquake Date M Epicentral

distance [km] PGA [g]

FS

Scaled

PGA

[g]

Drms

7142ya Bingol 01/05/2003 6.3 14 0.297 1.40 0.415 0.037

6277xa South Iceland 17/06/2000 6.5 15 0.518 0.80 0.415 0.062

4674xa South Iceland 17/06/2000 6.5 5 0.318 1.31 0.415 0.067

182ya Tabas 16/09/1978 7.3 12 0.385 1.08 0.415 0.085

182xa Tabas 16/09/1978 7.3 12 0.338 1.23 0.415 0.127

Rock outcrop

Bedrock

H = 63 mBuilt 1986 -1992 Maximum water storage 125 Mm3

0

0.2

0.4

0.6

0.8

1

0.01 0.1 1 10 100 1000 10000

Sw

ug - uw [kPa]

core

shells

drains

SIMULATED DRAWDOWN SCENARIOS

Coupled unsaturated approach (3-phase formulation) during the drawdwon stage.

Soil-water retention curves defined through the Van Genuchten (1980) model.

0

10

20

30

40

50

60

0 20 40 60 80 100 120

Wat

er l

evel

[m

]

days

0.5 m/day

1 m/day

2 m/day

4 m/day

HL

Initial reservoir level

Drawdown level


E

F

D

a)

b)

c)

e)

d)

Pore water pressures at different stages of the dam lifetime

end of the construction stage

water level at +42.36 m

water level +52.62 m

steady-state at the maximum water level

end of the dynamic stage (4674xa input signal)

Static drawdown

Post-seismic drawdown

The shear strength reduction technique proposed by Duncan (1996) was adopted following the specific procedure implemented in FLAC2D.

DRAWDOWN and GLOBAL INSTABILITY

LH

Drawdown ratio (L/H)

SAFE

TY F

AC

TOR

4 m/day

Drawdown rate 0,5 m/day

Post-seismic drawdown

‘static’ drawdown

1

1,2

1,4

1,6

1,8

2

2,2

2,4

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

4 m/day

a)

b)

Figure 17. Critical slip surfaces corresponding to: a) L/H = 0.2, b) L/H = 0.4.

DRAWDOWN and GLOBAL INSTABILITY

For L/H<0.3, the global instability phenomenon develops entirely within the upstream shell.

With increasing the drawdown ratio, the critical slip surface partly crosses the core where theexcess pore water pressures are higher.

HL

Initial reservoir level

Drawdown level

L/H<0.3

L/H>0.3


CONCLUSIONS

Different drawdown scenarios were simulated on a zoned earth damconsidering the combined effects of a strong earthquake first and adrawdown later.

For the case-history considered in this study, the earthquake stageaffected dam stability during the reservoir lowering.

The global safety factor (FOS) dropped to one when a very fastdrawdown was superimposed to a previous severe seismic stage.

Dam managers should be aware of the risk of rapidly emptying thereservoir immediately after a major earthquake, in the unjustifiedbelief that they are proceeding in the safest way.


effects on earth dams of drawdown scenarios imposed after ... · stefania sica, federica rotili...

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