Research Paper DYNAMIC ANALYSIS OF KASWATI EARTH DAM 1 Patel Samir K., 2 Prof. C.S.Sanghavi Address for Correspondence 1 Applied Mechanics Department, 2 Professor, 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 mediumdense, 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 (N 1 ) 60 = N (P a / σ vo ) 0.5 ER where N is the raw SPT blow count, Pa is the atmospheric pressure ( 100 kp a), σ vo 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 (a max / g) ( σ vo / σ vo ) r d K -1 m K -1 α K -1 σ CRR = CRR 7.5 K m K α 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): D r = ( ( N 1 ) 60 / 44) 1/2 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 Fig3: Magnitude Correction factor Km Cross-section of kaswati dam with material property Fig4: Stress correction factor Fig5: Correction for initial static shear 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 undrained 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. 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 7 : Response of (a) contdractive and (b) dilative saturated sand to undrained shear Susceptibility of soils to liquefaction in earthquakes 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
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 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.,