Behaviour management

5 February 2010

The behaviour of Sayano-Shushenskaya dam after the fatal accident in the project’s powerhouse in August last year is evaluated by N.I. Stefanenko, V.B. Zateev, L.S. Permiakova, E.N. Reshetnikova and E.G. Gaziev

At 242m high, the Sayano-Shushenskaya arch-gravity dam on the Yenisei river in Siberia is one of the largest dams in the world. The structure has a crest length of 1074.4m, crest thickness of 25m and base thickness of 105.7m. The maximum head is 220m, and the full and active storage capacities of the reservoir are 30.71 and 14.71km3 respectively. The 189.6m long spillway has 11 low level openings and is designed to pass 12870m3/sec with the reservoir level at the elevation 539m and 13090m3/sec at the surcharged level.

The region where the dam is located is characterized by a mildly continental climate with a hot summer and cold winter. The minimum recorded ambient temperature is -44°C and the maximum recorded ambient temperature is +40°C.

The dam foundation is composed of igneous rock (orto-paraschist) featuring practically uniform mechanical properties. Therefore, in spite of structural discontinuities found in the rock mass which houses the hydraulic structures, in design the rock mass was treated as a quasi- homogeneous block composed of high strength rock. The modulus of deformation of intact rock tends to vary in the rock mass from 10 to 18 GPa depending on depth. The modulus of deformation drops to 5-9 GPa in the influence zones of major tectonic faults.

After impoundment of the reservoir in 1989 it became evident that the strain-stress state of the dam exceeds the values that were specified at the design stage. Horizontal tensile cracks have developed on the upstream face and disruption of the contact has been found in the base of the upstream face. Seepage through the dam concrete and foundation reached 520 and 546 l/sec respectively [1].

Personnel from the hydroelectric scheme, in cooperation with specialists from the Lenhydroproject (Russia) institute and French company Soletanche, worked out a new procedure for rehabilitation, which involved applying epoxy grouts in the cracks in both the dam and foundation during the period 1996-2003. As a result of this repair, seepage through the dam and foundation has reduced by 99.5% and 78% respectively [1].

Implementation of the remedial measures has allowed for the behaviour of the dam and its rock foundation to be examined. These investigations discovered that:

• 1. Irreversible downstream movements of the dam went on till the year 2005 and amounted to about 78% of the seasonal amplitude of displacements, or 40% of the maximum crown displacement.

• 2. The process of adjustment of the dam-foundation system has not ceased and buildup of residual deformations on the dam contact with its rock abutment continues. In spite of rather low rates of deformation increment with time, their presence indicates the ongoing process of adaptation of the dam to its rock foundation. It is likely that forces are increasingly being transmitted onto the arches, leading to re-distribution of stresses in the dam body with an increase in arch stresses and development of zones of loosening or decompression near the upstream face of the dam, particularly at low elevations on the abutments.

• 3. The study of the results of deformations on the contact between the dam and its foundation (based on the readings of long-base extensometers and deformations measured in the adits of the both dam abutments) indicates that a considerable portion of the section of each abutment on the upstream face stays in the zone of decompression, while resulting forces exerted by the dam are being transferred to the abutments by the remaining part of the section which lies in the compression zone at the downstream face of the dam. The most stressed area of the dam interaction with the abutments occurs in the middle third of the dam height.

• 4. Observations over the last few years point to the trend for stabilization and decay of residual deformations which may relate to a gradual increase in the modulus of deformation in the abutments of the dam due to ongoing “consolidation” of the rock mass in the dam abutments. The process of dam adjustment to its rock foundation is often accompanied with re-distribution of stresses in the dam body and its abutments, which in turn leads to development of new conditions for interaction of the “dam-foundation” system.

• 5. Rates of compression deformation buildup can be regarded as one of the control diagnostic parameters indicating the state of the dam and degree of its safety. Stabilization of the abutment deformation on the contact with the rates of deformation buildup dropping off to zero will point to onset of the normal conditions of the dam function.

The events of 17 August 2009

On 17 August 2009 the seismometers installed in the Sayano-Shushenskaya dam registered dynamic impact on the dam, caused by the tragic accident in the powerhouse. Response of the dam section 33 at elevation 344m (the point closest to the accident location) along the flow has had the following peculiarities:

• 17.7 sec from the beginning of registration before the occurrence of dynamic impact, there were background vibrations of the dam from the generating units operating under the normal conditions.

• During the next 32.5 sec, damping low frequency vibrations of the dam with a duration of 4.5 sec and maximum amplitude of 125 µm were recorded. These were caused by the rotating parts of Unit 2 being forced from their housing and subsequently falling to the floor.

• Over the next 74 sec polychromic vibrations occurred ranging in duration from 0.05 to 0.18 sec and with a maximum amplitude of up to 120 µm. This was caused by the devastating impact of the water flow from the Unit 2 turbine chamber on the powerhouse structure.

• Up to the end of record, vibrations with durations of 0.07 to 0.15 sec increasing to a maximum amplitude of up to 424 µm were registered, caused by the impact of Unit 7 and Unit 9 running at the runway speed.

Turbine 2 with its shaft mounting the rotor was thrown out vertically from its housing, causing extensive damage and flooding to the powerhouse. All 10 units were shut down, the spillway was opened and the entire Yenisei flow was diverted to the spillway dam.

The cast-in place reinforced concrete structural floors in the area of Units 2, 7 and 9 (elevation 327m), the columns under the floors of these units, and the walls of the generator pits have been completely destroyed. The columns supporting the floors below elevation 327m were broken or sustained damaged. The enclosed structure of the power house has been completely destroyed in the area of Unit 2, 3 and 4.

The heaviest damage was inflicted to the powerhouse at elevations 327m and 320m. Elevation 315 of the powerhouse sustained less damage, with the condition of the load bearing structures considered satisfactory. The equipment that suffered the most damage were the turbine-generator units 2, 7 and 9 (Figures 4 and 5).

After the water was pumped out, the condition of the joints between the turbine bays was investigated. Based on the readings of geodetic instruments (three-axial joint meters and hydrolevels) the condition of joints at elevation 306.2m corresponds to the pre-accident state.

Behaviour of the dam following the accident

Immediately after the accident the structural behaviour surveillance unit of the Sayano-Shushenskaya hydro power station began detailed examinations of the dam, producing daily reports.

Over the period 11-18 August 2009, the reservoir level rose by 22cm. The mean daily ambient temperature dropped from 21.60C to 11.1°C. The dam has sustained considerable dynamic impact caused by the accident in the powerhouse. Because of the shutdown of all generating units, all 11 spillway gates were opened to release water from the reservoir starting from 13:00 hours on 17 August.

From 2007 to September 2009, the strain-stress state of the dam was governed by variations in hydrostatic loads (WL) and temperature effects. No abnormal displacements have been detected in the crown cantilever of the dam (section 33) following the accident (Figure 6).

The accident took place with HWL at elevation 537m. The post-accident period from 17 August to 10 September is characterized by an inconsiderable rise of the reservoir level to the maximum elevation 537.68m (22 August) and its subsequent drop to elevation 536.9m (10 September). In the post-accident period, readings were taken at closer intervals. In particular, eight cycles of measurements have been carried on the plumblines during this period.

From 18-24 September radial displacements of the dam have increased from 0.3mm at elevation 344m to 1.2-2mm at the crest. The maximum increase in displacements (2mm at elevation 542m) was recorded in central section 33.

No irreversible components of downstream displacements of the dam have been detected over the period from 2007 to 2009. Maximum displacement recorded at the crest of crown section 33 reached 132.6mm (2008).

In the zones of the dam for which quantified safety criteria are specified, maximum compression stress remained below its critical values and worked out to be: 11.9 MPa (section 33, elevation 504m) and 9.7 MPa (section 33 elevation 504m) in the arch direction at the upstream and downstream faces respectively and 12.8 MPa (section 45, elevation 322m) in the cantilever direction. The given stresses have been computed considering initial creep during the first year. Consideration of long-term creep gives a reduction in measured stresses of 20-25% by the clusters of strain meters.

In the zone of compression stresses in each abutment there developed zones in which long-base deformation meters continue to register residual deformations. Out of total number of 45 deformation meters available in the left abutment, 21 instruments (46%) continue to record an increase in compression deformations, in the right bank abutment the residual compression deformations are still being recorded by 13 instruments (26%) out of 50 available deformation meters.

In sections 18 and 25 in the dam foundation (94-105m from the upstream face), a daily increase in compression was recorded by five gages out of the six vibrating wire instruments installed during construction, and which are still in working condition at present. The maximum value of compression deformation increment recorded over the last nine years worked out to be 1×10-5 relative units per year.

Consolidation of the rock foundation goes on under the downstream wedge of separate dam sections. In recent years compression deformations in the foundation of central section 33 continue to increase at a low but constant rate of about 1.6×10-5 relative units per year. .Over the period from 1990 to 2009, the total deformation under the downstream face of section 18 worked out to be 9.2× 10-5 relative units with the tendency for decay displayed over the recent years. Decay of deformations, decrease in their rates with time take place gradually at a number of deformation meters which are being monitored.

Despite the small deformation values obtained, their presence does suggest that the process of the dam adjustment to its rock foundation still goes on.

In addition, seasonal variations in the reservoir stages cause consolidation of the bank abutments and displacement of the dam sections towards the banks, which entails an additional inclination of the dam in the downstream direction.

The pattern of deformation diagrams for the right bank rock abutment is much calmer than that for the left bank, which is most likely due to more intensive jointing of the left bank.

The total seepage flow through the foundation and banks at WL = 537.44m (as on 20 August 2009) worked out to be 80.1 l/sec which is 5 l/sec higher than the seepage flow before the accident at the hydro power plant. No new fissures, new sources of seepage, or local increase in the seepage flows through the impounding structures have been detected.

From 13-20 August, the seepage flow through the impounding structures slightly increased, from 14.3 to 14.9 l/sec. In practically all monitored zones of the impoundment, the seepage flow has slightly exceeded the values recorded in 2007 but remained below the values recorded in 2006.

Uplift pressure under the dam base has not exceeded the specified values, i.e. efficiency of the grout curtain is secured. When measuring the seepage pressure in the foundation of the river channel portion of the dam (sections 16-17) on the 20 August 2009, a small rise of the piezometric levels was recorded under monoliths III and IV of the dam. This did not negatively affect the condition of the grout curtain. As a result, it may be concluded that the dam-foundation system of the Sayano-Shushenskyaya hydroelectric scheme is in the normal working condition.

Handling the Yenissei Flows

During the winter season of 2009-2010 one of the biggest challenges to be addressed is the handling of the Yenissei flows through the spillway with 10 generating units shut down. This is challenging as, historically, the project’s spillway has never operated in winter. During this period water flow used to pass through the operating turbines.

The release of water flow through the 161m high spillway at a total head of 210-220m always causes formation of dense water mist, with some water spilling over the side walls of the spillway chutes (Figure 7). The operation of the spillway in winter will bring up a number of other problems including: icing up of the gate guides, affecting their handling; icing up of chute walls; a drop in downstream face temperature because of the penstock dewatering, which may lead to an increase in the downstream inclination of the dam (with operating units, water flowing through the dam conduits has the temperature not lower than +20C); and drop of temperature in the powerhouse due to ingress of cold air through the emptied penstocks.

At the full supply level of 539m, the discharge capacity of the service spillway permits handling the maximum spring flood flow of 0.01%, provided that at least one unit is running. However, computational studies and practical experience confirm that in passing the flows exceeding 5000m3/sec, the stilling basin does not possess the required strength and its prolonged operation may lead to partial destruction of the floor slabs.

The most efficient solution is to set the spillway gates to open, which will permit drafting of the reservoir to dead storage level of 500m by 1 May 2010 (to accommodate the spring flood inflow) ensuring minimum environmental flow release for riparian water supply and for the Maina re-regulating dam, with due account of restrictions imposed on maximum winter flows in the vicinity of the town of Minusinsk. The scheme has been worked out for operation of the Sayano-Shushekaya spillway dam from November 2009 to May 2010 based on the handling of average year flows. Under such a scheme of spillway operation, the Sayano-Shushenskaya reservoir is to be drafted from elevation 536m to the dead storage level of 500m.

In October 2009, the flow of 1700m3/sec was being passed. When the inflow to the reservoir drops to 1000m3/sec, the gate opening will be reduced to 1.2m (half of the first step). These measures will ensure release of 1100m3/sec and, on drawdown of the reservoir level, flow release will drop to 800-900m3/sec which would provide flow for operation of two generating units at the Maina re-regulating dam.

Stefanenko N.I. - Head of Structural behaviour surveillance unit at Sayano-Shushenskaya HPS;
Zateev V.B., Ph D (Tch Sc) - Specialist of I-category of Structural behaviour surveillance unit at Sayano-Shushenskaya HPS;
Permiakova L.S., Ph D (Tch Sc) - Specialist of I-category of Structural behaviour surveillance unit at Sayano-Shushenskaya HPS;
Reshetnikova E.N., - Head engineer of Structural behaviour surveillance unit at Sayano-Shushenskaya HPS;
Gaziev E.G., Dr. (Tch Sc) - Chief specialist of Geodynamic Research Center of Power Industry, Moscow.

Author Info:

1. Bryzgalov V.I. “Rehabilitation of the Sayano-Shushenskaya HPS dam-foundation system”. Proceedings of the International Congress on Conservation and Rehabilitation of Dams, Madrid, Spain 11-13 November 2002, pp695-697.

2. Bryzgalov V.I., Gaziev E.G.. “ Behaviour of Sayano-Shushenskaya Dam before and after Rehabilitation”, Vestnik (Proceedings) of Krasnoyarsk State Architectural and Civil Academy, 6, Krasnoyarsk, 2003, pp 77-86.

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