The problem of seismic stability and safety of 300 m high Nurek rockfill damFor this project the method of analyses of seismic resistance of Nurek dam under action of the Maximum Credible

Earthquake (MCE) the return period of 1 in 10,000 years, was chosen the accelerogram with the peak acceleration of 1.1g registred in canyon rock ledges on the left abutment of the arch dam under seismic shaking Pocoina 1971 in San Fernando (California, USA) with a magnitude of 6.6. According to US specialists a very high acceleration of the accelerogram was explained by the influence of landscape, such as increased vibration of rock ledgess. Except for this effect the accelerogram was recalculated due to its moving from the raised surface of canyon rock ledges down to the smooth surface thus leading to the decrease of the acceleration from 1.12 to 0.73g in a frequency range of 1 to 15 Hz and from 0.64 to 0.55g in the begining of recording low frequency.

It must be noted that for the credibility of this seismic (dynamic) analysis the earthfill dam (hydraulic type) San Fernando Low, practically was destroyed by the earthquake of 1971. In seismic analysis the non-instrumental accelerogram was used modified the earthquake Pocoima with the maximum acceleration up to (0.5-0.6)g, close to the accelerograms that were recommended by the Institute of Physics of the Earth (Academy of Sciences of the USSR) and other institutes engaged in dynamic analyses of seismic resistance of Nurek dam. So it was concluded that the choice of ground motion with high accelerations Pocoima 1.1g for analysis of seismic resistance of Nurek dam without considering the topographical difference, seismological and other conditions at the point of obtaining seismological readings particularly taking into account the projections of the canyon and place of location of Nurek dam (with an inverted canyon 300 m below the dam crest, with 1300 m wide, comparable with the length of seismic waves its exceeding) is considered unsubstantiated.

Photo 1. Downstream view of 300 m high Nurek rockfill dam & hydropower plant (2700 MW)

Fig. 2. Cross-section of 300 m high Nurek rockfill dam with central clay core:

1 – sandy loam core; 2 – shells of coarse gravel & pebble; 3 - filters (d=0-5 mm & d=0-50 mm); 4 – oversize rockfill surcharge stones on upstream slope (20-40 m thickness), downstream slope (5-10 m thickness) & downstream cofferdam; 5 – downstream cofferdam of oversize rockfill; 6 – upstream cofferdam; 7 - concrete block; 8 – consolidation grout & grout curtain; 9 – anti-seismic belts; 10 – control gallery 2x2 m; 11- control gallery 4x4 m on dam crest; 12 – contour of first-stage dam construction; 13 – temporary clay blanket of first-stage dam construction

For the definition of the characteristics of shear strength of the gravel-pebble materials of shoulders of Nurek dam, landslide tests of 35 m high embankment were performed using explosions of gravel-pebble materials of the upstream slope of the dam with trial vertical horizontal acceleration (Ah) and (Av) measured in the embankment control points (reaching Ah = 2.5-3.0 m/sec, Av = 5.6 m/sec; angle of embankment slope to the horizont (30 grades in water and 20 grades above the water). The angle of soil shear ψ defined by the expression: ψ = arctan[Ah(α/g) + Av(α/g)], where: Ah and Av are components of conventional acceleration (amplitude of harmonic accelerograms, equivalent to actual or instrumental action); α= s/(γs-γa), where: γs & γa - density of soil and water (for the unsaturated slope γa=0 & α=1).

Presented expression is analogous to Mononobe formula (Japan) for the limitt angles (at equilibrium) of the slope of the sandy materials under seismic action. Defined by this formula the ψ angles a number of assumptions was made according to which the duration of the oscillations were activated with explosions took sufficient for performing large displacements of the embankment and to decrease the inclination of the slope until equilibrium condition; cohesion of soil materials under large movements caused by seismic action disappears; to define the angle ψ it was taken into account only horizontal acceleration component of Ah. When two components of acceleration Ah and Av are taken into account the definition of ψ angle was performed using these accelerograms.

However the assumed assumption is unfounded. In reality reading acceleration during test embankment under explosion takes only 2-3 peaks and lasting (4-6 sec), which is not sufficient for the realization of the large displacements and take the slope to a limit equilibrium state, in terms of continuity of action of the harmonic force. The assumption in which under great movements caused by seismic actions cohesion in the soil materials disappears, it is not correct and not checked by the test results. The method discussed in principle does not establish the overall shear strength (cohesive) soils and can not give any information about the presence or absence of cohesion in soils because the formula above mentioned for the definition of ψ angle not gives the possibility to define the angle of internal friction phi (φ) and cohesion C separately.

With the values of θ (tetta)=30 grades, Ah=0.3g, Av=0.6g; γa (gamma)=0 and α (alpha)=1 (the unsaturated slope), mentioned above, we got the shear angles of gravel-pebble soils equal to ψ =40.70 under very adverse conditions of

Av in downward direction; for ψ (ksi) =46.7 grades Av=0 (which is significantly higher than ψ (ksi)=41.6 grades, obtained earlier. Under the same conditions Av=0 and ψ(ksi)=65.8 grades with Av in the upward direction. It can be concluded that the acceleration Av has much more influence on the angles ψ(ksi) than Ah and it can not ignored.

Thus in the safety assessment of Nurek dam the unfavorable upward direction of Av must be considered.

From the above it can make a general conclusion that the right approach and realization field test investigations of the shear strength properties of the gravel-pebble materials, processing and expansion of test results and also authenticity of these results are presented doubtful. The reinforcement antiseismic elements dispersed in Nurek dam are installed with great technical difficulties and it simply resulted in extension of the construction period. Moreover under corresponding indices quality of work placement in the construction and compaction of these dam materials can not be made as necessary with sufficiently high indices of mechanical properties (high density and porosity, low deformability etc.) without using poor means of less costs and time.

In Fig. 3 & Photo 2 the location scheme of antiseismic latticed belts in form of reinforced concrete structures with a height of 3 m, with a distance between them 18-21 m below the dam zone area is presented. The lower belt is located 65 m below the dam crest. Two intermediate belts are located in the middle of the upstream shoulder, the lower and upper belts extend to the upstream slope area and superior also extends to the upstream slope. The three reinforced concrete belts do not reach the upstream transition zone of clay core. This method of discretized bearing structures in the upper part of the upstream dam shoulder covers the 3 m high zones with 18 to 21 m distance between them that leads to considerable heterogeneity in these zones during dam construction, which will be taken into account in the analyses of seismic resistance of the dam. Meanwhile the choice of the size and location of antiseismic belts in the dam body were made very sensible during seismic analysis by the linear spectral theory by the deficit of supportive forces (or moments) for the most dangerous sliding surface of analysis defined using the revision of the seismic slope safety of the outstanding common methods: by the dynamic method with peak ground motion acceleration of 1.1g of earthquake Pocoima in 1971 and using in dynamic analyses with the ideal plastic model with associate plastic flow by Druker-Prager. The limit permissible values the residial (plastic) deformations of soils in the dam the results of the seismic analysis showed that the plastic deformations (up to 3-5 % in contact zones) are spreading in contact zones towards the upstream slope and crest accepted residual deformations of (Fig. 2).

Analogously conducted credible estimates of seismic resistence Nurek dam with antiseismic belts. Obviously, that these estimates of homogeneous dam by the linear spectral method and dynamic theory the seismic resistance of this test dam can not be get a reliable value for the assessment of seismic forces and safety of Nurek dam, taking into account the presence in its upstream shoulder the heterogeneous zones of antiseismic belts. With these belts it can be seen only worsening working conditions during dam construction in thse zones. It is necessary to note that in analysis of seismic safetyof upstream slope of the Nurek dam were considering three antiseismic belts with their bottoms on elevations of 274, 256 and 235 m and were obtained the minimum stability coefficients correspondingly equal to 1.06; 1.05 and 1.05 not enough to provide the seismic safety of Nurek dam according to Soviet design norms.

According to these norms the downstream dam slope should be inclined more to reach the allowable stability coefficient of 1.32.

There is a large quantity and quality dispersion between seismic analyses results of Nurek dam: by the dynamic method under the same the ground motion of Pocoima (Amax=1.1g) with the use of dynamic models of different soils. As shown in Fig. 2(a, b) at the begining of dam design the its dynamic analysis was made with the use for the soil model with the ideal plastic law of Druker-Prager and it was recommended to strengthen the upsream shoulder below the dam crest with antiseismic belts (Fig. 2-b). Later the dynamic analysis of Nurek dam was performed using Zaretsky plastic soil model with accounting plastic strengthening effect (ICOLD Bulletin 122, 2001). This analysis showed another picture of dam seismic (dynamic) response (Fig. 4): with observed seismic displacements in the downstream slope near the dam crest, in the middle of the core and in the significant part of the upstream shoulder between elevations 235-274 m (Fig. 4).It means that antiseismic belts are not working and have no influence on the the dam seismic stability contrary to results of previous seismic dam analysis. These results indicate to the importance of the proper choice of the soil dynamic model and defining their parametors in dynamic analysis and also the inefficiency of proposed antiseismic measures to ensure the seismic resistance of Nurek dam.

The conclusions about the low seismic resistance of Nurek dam inspite of its antiseismic belts is the result of increase not based on the analyses of seismic actions, accepted in the design stage, the significant reduction of the

characteristics of shear strength of dam materials and using simpler methods to calculate seismic resistance.From the results of the dynamic analyses of the stress-strain state under increased accelerations in dam foundation

with use of two separate dynamic models is seen that the reinforcing belts in the zone under the dam crest do not influence on the volumetric displacements in this zone with the deep sliding surface. (Fig. 4).

Fig. 2. Isolines of plastic deformations (%) in the Nurek dam before (a) & after construction of antiseismic belts (b) according to the results of seismic (dynamic) analysis using the ideal plastic model of Druker-Prager for dam soils

Fig. 3. Location scheme of 3 antiseismic belts in the upstream shoulder of Nurek dam:1 – upper belt (el. 214 m); 2 – upper belt in the zone 3; 3 – rip-rap (rockfill) upstream slope protection; 4 – gravel-pebble

soil of both dam shoulders; 5 – clay core; 6 – fine & coarse filters; 7 – upper control gallery; 8 – gravel

Fig. 4. Scheme of distribution of plastic deformations ( %) in Nurek dam after construction of antiseismic belts according to the results of seismic (dynamic) analysis using the ideal plastic model of Druker-Prager for soils

Photo 2. View from the left bank on construction of the lower antiseismic belt (el. 235 m) in Nurek dam

Fig. 5. Histogram of tension stresses (MPa) in reinforcement of antiseismic belts in upstream shoulder of Nurek dam

Nurek reservoir induced seismicityThe Nurek reservoir extends 70 km upstream with a maximum width of 5 km, depth of 215 m, a volume 10.5 cubic

km. Reservoir filling has taken place in several stages, the water level rising as construction increased the dam height.Nurek is the only reservoir with induced seismicity where a detailed knowledge of the seismological conditions

existed prior to reservoir impoundment. This has made it possible to determine the extent of changes in the rate of seismic activity, related to the filling of the reservoir and furthermore to study changes in other aspects of the seismic regimen a threefold increase in the rate of seismicity related to filling, changes in local mechanisms, a decrease in focal depths, and a migration of activity towards to reservoir.

The filling of a large reservoir changes the stress regimen and decreases effective stresses by increasing the pore pressure. The net effect on fault zones of increased or decreased stability, depends strongly on the orientation of preexisting stresses and the geometry of the reservoir fault system. The Tadjik depression is a broad zone of horizontal compression and focal mechanisms are generally of a thrust type. The induced seismicity at Nurek occurs on a series of thrust or reverse faults showing strike-slip motion. A gradual rise in pore pressure was likely the dominant factor in triggering the earthquake southwest of the reservoir in 1971 at distances of 10-15 km.

Earthquakes as large as magnitude 4.6 have been induced by the filling of Nurek reservoir. Raising the water level above its previous primes the area for increased seismicity. Once primed the timing of activity is controlled by changes in reservoir filling: if filling remains high, seismicity remains low. Decreases in filling rate cause increased

seismicity: the more rapid and sustained, the more intense the increases. The largest earthquakes occurred after the daily rate of filling reportedly was decreased by more than 0.5 m/day.