Dynamic Analysis of Kaswati Earth Dam

International Journal of Advanced Engineering Technology E-ISSN 0976-3945

IJAET/Vol.III/ Issue II/April-June, 2012/119-123

Research Paper

DYNAMIC ANALYSIS OF KASWATI EARTH DAM 1Patel Samir K.,

2Prof. C.S.Sanghavi

Address for Correspondence

1Applied Mechanics Department,

2Professor , L. D. College of Engineering, Gujarat Technological University,

Ahmedabad, Gujarat (India)

ABSTRACT A large number of water-retaining earthen dams were affected by the earthquake. This paper examines dynamic analysis

with time history methods of kaswati dam are located in Bhuj region by using of geo-studio 2007. The consequences of these

problems were the dams performed reasonably in spite of being shaken by free-field horizontal peak ground acceleration

(PGA) as high as 0.28g. The liquefaction occurred in upstream slope, downstream slope and foundation of dam due to

cohesion-less soil in foundation. The procedure for assessing liquefaction potential uses the Cyclic Stress Ratio (CSR) as the

measure for earthquake load. The procedure for assessing liquefaction potential typically uses the Cyclic Resistance Ratio

(CRR) as a measure of the liquefaction resistance of soils and the Cyclic Stress Ratio (CSR) as a measure of earthquake load.

For cohesion-less soils, CRR has been related to normalized SPT blow count, (N1)60, through correlations that depend on

the fines content of the soil from field performance observations from past earthquakes. Factor of safety is obtained by ratio

of Cyclic stress ratio to the critical stress ratio. For prevention of liquefaction replace liquefied soil with well graded soil in

foundation and get factor of safety above 1 which indicate non liquefied soil.

KEYWORDS Dynamic analysis, Time history method, Kaswati dam, Cyclic stress ratio, Critical stress ratio, Factor of

safety, Liquefaction potential

INTRODUCTION

A Magnitude 7.6 (Mw 7.6) earthquake occurred in

Gujarat state, India on 26 January 2001.The epicenter

of the main shock of the event was near Bachau at

23.36°N and 70.34°E with a focal depth of about 23.6

km. The event, commonly referred to as the Bhuj

Earthquake, was among the most destructive

earthquakes that affected India. A large number of

small-to moderate-size earthen dams and reservoirs,

constructed to fulfill the water demand of the area,

were affected by Bhuj Earthquake. Most of these

dams are embankment dams constructed across

discontinuous ephemeral streams. Although many of

these dams were within 150 km of the epicenter

(Figure 1), the consequences of the damage caused

by the earthquake to these facilities were relatively

light primarily because the reservoirs were nearly

empty during the earthquake.

Fig1. Location of kaswati dam

KASWATI DAM

Kaswati Dam, constructed in 1973, is an earth dam

with a maximum height of 8.8 m and crest length of

1455 m . The dam is underlain by loose to medium-

dense, alluvial, silt-sand mixtures. Limited amount of

subsurface exploration data indicate that the site is

underlain by 2 to 5 m thick granular soils

characterized with an SPT blow count between 13

and 19, below which relatively dense granular soils

with an SPT blow count typically above 25 is found

(Krinitzsky and Hynes 2002). Like the other

impoundments, Kaswati Reservoir was nearly empty

during Bhuj Earthquake. However the alluvium soils

underneath the upstream portion of the dam was

saturated during the earthquake. Bhuj Earthquake

triggered shallow sliding near the bottom portion of

upstream slope, and bulging of ground surface near

the upstream toe . Such distress may have been due to

localized liquefaction near the upstream toe of the

dam. EERI also report relatively narrow, longitudinal

cracks along the crest of the dam running the length

of the dam over which the lower portion of the

upstream slope exhibited distress. It appears that the

problem of development of longitudinal cracks along

the crest was indirectly due to localized liquefaction

of upstream foundation soils. The downstream slope,

on the other hand, remained largely unaffected. ASSESSMENT OF LIQUEFACTION POTENTIAL

The procedure for assessing liquefaction potential

typically uses the Cyclic Resistance Ratio (CRR) as a

measure of the liquefaction resistance of soils and the

Critical Stress Ratio (CSR) as a measure of

earthquake load. For cohesion-less soils, CRR has

been related to normalized SPT blow count, (N1)60,

through correlations that depend on the fines content

of the soil from field performance observations from

past earthquakes. The normalized SPT blow count is

given by

(N1)60 = N× (Pa/ σvo)0.5 × ER

where N is the raw SPT blow count, Pa is the

atmospheric pressure (≈ 100 kP a), σvo is the

effective vertical stress at the depth of testing, and

ER is the energy ratio (≈ 0.92 in a typical Indian SPT

setup).

Fig2. CRR - (N1)60 Correlations

(from Youd et al. 2001)



Available SPT data from Kaswati Dam however

indicates that the shallow foundation soils underneath

the dam body were characterized with a blow count

between 13 and 19. For assessing liquefaction

potential of foundation soils we assumed that the

fines content of these shallow alluvium layers were

15% or less. The procedure for assessing liquefaction

potential uses the Cyclic Stress Ratio (CSR) as the

measure for earthquake load, where

CSR =

0.65 ×(amax / g) × ( σvo/ σ’vo) ×rd ×K-1m× K

-1α × K

-1σ

CRR = CRR7.5× Km× Kα × Kσ

σ is the total vertical stress, rd is a correction factor to

account for the flexibility of the soil column, and Km,

Kα and Kσ are correction factors to account for the

Magnitude of the earthquake, the presence of initial

static shear (i.e., whether the layers are in a slope)

and the depth of the layer (i.e., the level of initial

overburden pressure), respectively. We estimated the

value of rd for a given depth from Seed et al. (2003)

median relationship. Correction factors Km, Kα and

Kσ were obtained from the relationships

recommended by Youd et al. (2001) using estimates

of relative density obtained from (Olson and Stark

2003b):

Dr = ( ( N1)60/ 44)1/2

Fig3: Magnitude Correction factor Km

Fig4: Stress correction factor

Fig5: Correction for initial static shear

Fig6: Relationship between CRR and (N1)60 for

sand for Mw, 7.5 earthquakes Factor of safety against liquefaction

FS = CRR/ CSR

Table 1 Soil property of kaswati dam

Cross-section of kaswati dam with material

property

Definition of liquefaction of soil

Liquefaction is a phenomenon wherein a mass of soil

losses a large percentage of its shear resistance when

subjected to monotonic , cyclic or shock loading and

flows in a manner resembling a liquid until the shear

stresses acting on the mass are as low as the reduced

shear resistance.



Behavior of saturated, cohesion-less soils in un-

drained shear

During earthquake, the upward propagation of shear

waves through the ground generates shear stresses

and strains that are cyclic in nature. If cohesion-less

soil is saturated, excess pore pressure may

accumulate during seismic shearing and lead to

liquefaction.

The behaviour of a saturated soil under both

monotonic and cyclic shear is depicted in fig. The

response of the same soil loose (contractive) and

dense (dilative) states is indicates part(a) and part(b)

respectively of this fig.

A loose soil tends to compact when sheared and

without drainage, pore water pressure increases As

indicate fig (a), a contractive soil sheared

monotonically reaches a peak shear strength and then

soften, eventually achieving a residual shear

resistance. If the residual shear strength is less than

the static driving shear, a liquefaction flow failure

results.

If the same soil sheared cyclically, also depicted in

fig (a), excess pore pressures are generated with each

cycle of load without drainage , pore pressure

accumulate and effective stress path moves towards

failures. If the shear strength falls below the static

driving stresses a flow failure results and deformation

continue after cyclic loading stops.

Shearing of dense, dilative soils will also produce

some excess pore pressure at small strains. However

at larger strains, the pore pressure decrease and can

become negative as the soil grains, moving up and

over one another, tend to cause an increase in soil

volume (dilation). Consequently as shown in fig (b).

monotonic shearing of a dilative soil results in an

increasing effective stress and shear resistance. Fig (b)

also shows the response of the same dilative soils to

dynamic loading. In this case pore pressures are

generated in each shear cycle resulting in an

accumulation of excess pore pressure and

deformation. However beyond some points the

tendency to dilate and develop negative pore pressure

limits further straining in additional load cycles. As

indicated in the bottom of fig (b), the effective stress

path moves to the left but never reaches the failure

surface.

Fig 7 : Response of (a) contdractive and (b)

dilative saturated sand to undrained shear

Susceptibility of soils to liquefaction in earthquakes

Liquefaction is most commonly observed in shallow,

loose, saturated deposits of cohesion-less soils

subjected to strong ground motions in large

magnitudes earthquakes. Unsaturated soils are not

subjected to liquefaction because volume

compression dose not generate excess pore pressure.

Liquefaction and contractive soils while cyclic

softening and limited deformation are associated with

dilative soils.

Flow liquefaction

Flow liquefaction can occur when the static shear

stresses in a liquefiable soil deposit is grater the

steady-state strength of the soil. In can produce

devastating flow slide failures during and after an

earthquake shaking. Flow liquefaction can occur only

in loose soil.

Cyclic mobility

Cyclic mobility can occur when the static shear stress

is less than the steady-state(residual) shear strength

and the cyclic shear stress large enough that the

steady-state strength is exceeded momentarily.

Deformations produced by cyclic mobility develop

incrementally but become substantial at the end of a

strong and/ or long-duration earthquake. Cyclic

mobility can occur in both loose and dense soils but

deformation decreases markedly with increased

density.

In the contractive region, an un-drained stress path

will tend to move to the left as the tendency for

contraction causes pore pressure to increase and p’ to

decrease. As the stress path approaches the

PTL(Phase transformation line), the tendency for

contraction reduces and the stress path become more

vertical. When the stress path reaches the PTL, there

is no tendency for contraction or dilation, hence p’ is

constant and the stress path is vertical. After the

stress path crosses the PTL, the tendency for dilation

causes the pore pressure to decrease and p’ to

increase, and the stress path moves to the right. Note

that, because the stiffness of soil depends on p’, the

stiffness decreases (While the stress path is below the

PTL) but then increases (when the stress path moves

above the PTL).

q/p’ stress ratio under earthquake shaking

Figure shows contours of q/p΄ stress ratios under the

initial static stresses. A point of significance is the

high q/p΄ ratios in the central part of the hydraulic fill.

This means that there is a zone where the initial q/p΄

points are above the collapse surface. The soil

strength in this zone could easily fall down to the

steady-state strength with a small amount of shaking.

The yellow shaded area in Figure is the zone where

the stress ratios are initially above or on the collapse

surface. In QUAKE/W this is flagged as a liquefied

zone.

Fig 8. Zone of liquefaction based at the end of

shaking cohesion-less soil in foundation



Fig 9. Zone of liquefaction based at the end of

shaking well graded compacted soil in foundation

Fig 10. Excess pore water pressure contour



CONCLUSION:-

Damaging effects of Bhuj Earthquake on

embankment dams have been considered in this

paper. This paper present dynamic analysis by time

history method of Kaswati Dam. Under earthquake

shaking earthen dam subjected cyclic motion. Due

to Saturated cohesion-less soil under oscillatory

motion during earthquake, loses all its shear strength

due to pore water pressure increased and q/p’ ratio

increased and cyclic stress ratio increased so that soil

behave as a liquid. In this analysis factor of safety

below 1, which indicate liquefaction occur in given

earthen dam. For prevention of liquefaction potential

replace liquefied soil with well graded compacted

soil so that pore water pressure, q/p’ ratio and cyclic

stress ratio decreased while mean effective stress

increased and get factor of safety above 1 which

indicate non-liquefied soil in earthen.

REFERENCES:- 1. Adalier, K., and Sharp, M. K. (2002b). “Embankment dam

on liquefiablefoundation—Dynamic behavior and

ensification remediation.” J.Geotech. Eng., in press. 2. Beaty, M.H. (2003). “A Synthesized Approach for imating

Liquefaction-Induced Displacements of Geotechnical Structures”. Ph.D. Dissertation. University of British

Columbia, Vancouver, Canada.

3. Idriss I.M. 1990. Response of soft soil sites during earthquakes. Proceedings, H. Bolton Seed Memorial

Symposium, BiTech Publishers, Vancouver, 2, 273-289.

4. Lee, K.L., Idriss, I.M. and Makadisi, F.I. (1975). The Slides in the San Fernando Damsduring the Earthquake of

February 9, 1971 – ASCE, J of the Geotechnical

Engineering Division, GT7, pp. 651-688 Lee, K.L.,

5. Olson, S.M. and Stark, T.D. 2003b. Use of laboratory data

to confirm yield and liquefied strength ratio concepts. Canadian Geotechnical Journal, 40, 1164-1184.

6. Youd, T.L., Idriss, I.M., Andrus, R.D., Arango, I., Castro,

G., Christian, J.T., Dobry, R., Finn,W.D.L., Harder, L.F., Jr., Hynes, M.E., Ishihara, K., Koester, J.P., Liao,

S.S.C.,Marcuson, W.F., III, Martin G.R., Mitchell, J.K.,

Moriwaki, Y., Power, M.S., Robertson,P.K., Seed, R.B. and Stokoe, K.H., II. 2001. Liquefaction resistance of soils

Documents

Dynamic Analysis of Kaswati Earth Dam