Motyginskaya CFRD - creating an impoundment8 May 2009
The following paper reviews the design of Motyginskaya concrete faced rockfill dam on the River Angara in Siberia. By Cesar Alvarado-Ancieta
The 1250MW Motyginskaya hydroelectric power plant is located on the Angara River in Siberia, Russia. The project has been designed to create a reservoir impoundment of approximately 4800 x 106m3 and will consist of the following main structures: i) CFRD with a borepile diaphragm - cut off wall; ii) gated spillway; iii) powerhouse with 10 x 125MW Kaplan type units; iv) switchyard and transmission line; v) navigation lock; vi) fish pass; vii) access roads; and viii) offices and housing. A layout of the project is shown in Figure 1.
Motyginskaya reservoir is expected to have an area of approximately 478km2 (Figure 2). The project’s average width is around 3km and its length is almost 180km, comprised of the Motyginskaya dam site at km 151+600 and the confluence of river Mura into the Angara at km 383+000 (Figure 3). Key parameters of the reservoir are:
• Maximum flood level 127.50m asl
• Normal operating level 127.00 m asl
• Minimum operating level 126.00 m asl
• Flood storage volume (EL. 127.00 - 127.50) 300 x 106m3
• Effective, useful or life storage volume (el. 126.00 - 127.00) 400 x 106m3
• Dead volume (EL. 98.80 - 126.00) 4400 x 106m3
• Total storage volume 4800 x 106m3
• Reservoir area at EL. 127.00 m asl 478.00 km2
• Reservoir area at EL. 126.00 m asl 459.00 km2
As shown above, the normal and minimum reservoir operation levels have been defined at elevations 127 and 126m asl respectively.
The normal reservoir operating level was determined by the water level in the tailrace of the Boguchanskaya hydro power plant, which is currently under construction (Figure 4), and the future construction of a bridge in the township of Yarki, with the superstructure at elevation above 130m asl. This means the operational water level in the Motyginskaya reservoir should not rise above 127m asl as this will reduce the head, and consequently the power output, of the Boguchanskaya plant. It is also essential that the reservoir provides enough safe freeboard to avoid damage to the bridge during a flood events.
The normal and minimum reservoir levels as described above results in an available operational reservoir volume of 400 x 106m3. Although the project may be operated without peaking or storage operations, this volume provides flexibility in operation. This small live storage may be useful in daily or possibly weekly evening releases from the Boguchanskaya hydro power plant. The operational volume of the reservoir is equivalent to the operation of the Motyginskaya hydro power plant at full capacity for 27 hours.
Reservoir operation during extreme flood events
The design criterion for the maximum flood reservoir level has been defined at elevation 127.50m asl. This relatively small volume of approximately 300 x 106m3 is equivalent to the spillway design discharge flowing for about 2.5 hours. Thus, the 0.5m rise in water surface elevation allows for short-term adjustments in spillway discharge capacity [2, 3].
The proposed dam is a concrete face rockfill dam (CRFD) [Figure 5]. The structure is 35m high from the bottom of the excavated river bed and 19m high from the toe of the concrete face elevation 111m asl, where it is connected to a borepile diaphragm 30m high by means of a plinth. The dam is located perpendicular to the Angara river valley in a symmetric canyon with a concrete face of approximately 55000m2, and a total dam body volume of around 2.5Mm3. The dam is comprised of three sections: i) left dam section connecting the powerhouse with the left river bank, with a total length of approximately 810m, ii) main dam section between spillway and navigation lock approximately 960m long; iii) right dam section next to the navigation lock to the right abutment at a length of 90m. The total crest length of the CFRD is 1860m.
The dam shoulders – embankment type cofferdams – will be founded on rock, whereas the central part of the dam will be founded on the alluvium riverbed soil. The dam sealing will be supported by a borepile diaphragm wall, which will penetrate the upstream rockfill shoulder (cofferdam) into the bedrock (central sealed rockfill dam). Hence, it serves as a barrier due to the impermeability of the diaphragm wall through the rockfill into the underlying bedrock. Concrete facing is used for the upper part of the dam as a surface sealing.
Design work has been carried out considering the geometry of the area, the minimum dam volume, safety aspects and optimum construction methods. As a result, the following parameters have been applied in the dam design:
• Dam type CFRD dam + borepile diaphragm - cut off wall
• Crest length 1860m
• Maximum height 35m
• Crest elevation 130m asl
• Freeboard 3m
• Top width 8m
• Dam base 95m asl
• Dam base width 160m
• Upstream slope Concrete face slab 1:2, rockfill 1:2.4 (V:H)
• Downstream slope 1:2 (V:H)
• Downstream berm width (for instrumentation, monitoring and inspection) 8m
• Base foundation Rock
• Depth of borepile diaphragm under base foundation 15m
Rockfill shoulders - cofferdams
The are composed of gravel or rockfill for underwater placement. Fines are removed in order to guarantee an adequate control of seepage flow. The filling procedure has to be carried out carefully to accommodate the high velocities which are expected to be present in the narrow gap during the second phase of river diversion .
The faces of both upstream and downstream shoulders will have approximately 1m to 1.5m of riprap measured perpendicular to the slope. The stone size is determined on the basis of depth-averaged local flow velocity, taking into account the width of the river diversion gap . The upstream slope of 1:2.4 is affected by ice, therefore the total thickness of the upstream riprap layer has been increased by approximately 50%. The transition zone from the riprap to the rockfill requires special attention in order to provide graded transitions between impervious and coarser material and a stable base for the riprap.
After excavation, the dredging material is transported in suspension in beach pipes. The mixture is 25-50% solids by volume. The filling procedure will start inside the existing rockfill shoulders. The coarse material will settle close to the discharge point while the finer material is carried to the outlet. The predominant material is sand and gravel, silts and clays will be limited.
Once dredging is complete, vertical geotextile drains will penetrate the hydraulic fill on a layer of gravel material to reduce excess pore pressure development during subsequent dam filling stages. After placing a geotextile layer, the top filling will be undertaken in stages allowing for dissipation of the remaining excess pore pressure.
Borepile diaphragm – cut off wall
The lower part of the dam is waterproofed by the borepile diaphragm wall, which will be constructed from the crest of the upstream rock-fill shoulder. In addition, pipes are to be installed into the secondary reinforced piles in order to allow cement grout to be injected into cracks in the rock below the pile footings, if required.
A reinforced concrete beam connects the piles on the top and serves as a platform for the plinth of the concrete face.
The upper side of the upstream dam shoulder has a concrete facing which consists of monolithic slabs, 10 to 30m2 each. The slab thickness is variable, based on t = 0.40 + 0.002H in metres. Only nominal reinforcement is required, about 0.5% concrete area in each direction. Water tightness is ensured by copper water stops [4, 6, 7].
Concrete faced rockfill dams require a footing or plinth to be constructed around their upstream edge. The plinth is made from concrete and serves as a connection to the borepile diaphragm wall and as a initial point for the concrete face slab [Ref. 4, 6, 7].
Main zones of the dam body
The main elements have been designated according to current trends for rockfill embankments: 1 for soil materials, 2 for processed granular materials and 3 for rockfill zones [4, 5, 6, 7].
Zone 1 – Zone 1A is a silt or fine sand and acts as face slab or perimeter joint healer. Zone 1B supports zone 1 material.
Zone 2 – Zone 2A is a processed fine filter <20mm and will limit leakage when a water stop fails and can heal. Zone 2B, the face support zone, is crusher run <75mm.
Zone 3 – This zone is a quarry run rockfill. The differences in A, B and C are principally in layer thickness, size and type of rock. Zone 3A provides compatibility and limit void size adjacent to zone 2B. Zone 3B provides mass, resists the water load and helps in limiting face deflection. Zone 3C receives little water loading, and settlement is essentially during construction. The 2m-thick layer in zone 3C accepts large size rocks, is more economical to place and its lower density saves rock volume.
Only rockfill will be used in this zone. The fill will be derived from crushing hard, durable, angular rock, free from deleterious matter such as clay lumps and organic matter.
Every layer of rockfill material will be compacted by a vibrating roller or equivalent compaction method. Compaction features such as water content limits, layer thickness, compaction equipment and number of passes will be evaluated based on the data of existing rockfill material prior to commencement of works.
Gradations of materials to be placed in the embankment zones are defined by roughly parallel curves on the standard grain-size plot. Lift thicknesses are in the range from 50-80cm, depending on the zone and the number of compaction passes.
In Zone 3B the maximum lift height will be 50cm and the maximum grain size will be 35cm. The maximum lift height in zone 3C will equal 80cm, with a maximum grain size of 60cm.
An appropriate range of grain-size distribution is given in Figure 7 taking into account the material of existing quarries shown in Figure 6.
The top of the dam should have sufficient freeboard to prevent waves from splashing over the dam at the highest flood water level. The required minimum freeboard was determined as the sum of the maxima for floodwater rise, wind tide, wave run-up and possible Seiche effect. The freeboard was determined to be equal to 4.5m from the maximum flood level.
The camber has been conceived to cover expected settlements allowing a certain margin of safety. For well-compacted fills of good quality rock, founded in rock, a camber of about 0.5-1% is found to be satisfactory. The top of the core is given the same camber as the top of the dam. The camber adopted was Dh = 0.5m.
As defined above, the top of the dam is at elevation 130m asl, but the reach of the dam at the main body should be at the elevation 130.5m asl in a length of approximately 300m. From the edges of this reach the dam should reach the elevation 130m with a slope of 0.001 and then remain at 130m at the abutments .
As a design parameter, the dam crest will reach el. 130m asl, taking into account the freeboard. A crest 8m wide, with a stretch for restrictive and slow traffic, will help provide the freeboard. The supporting layers of this stretch consist of 30cm-thick coarse granular material as a sub-base above the rockfill body, and a well-compacted surfacing layer 20cm-thick as a supporting surface of the stretch.
In the berms of the crest, up and downstream and along the traffic stretch, coarse material and selected rocks will act as a support transition to the slopes and the rip-rap protection. Taking into account settlement of the dam, a 0.5m over-heightening of the rockfill body along the axis, prior to the execution of the specific-detail crest operation, has been considered .
The crest access will be designed to allow for operation and maintenance, and surveillance, of the dam and installations.
Access to the berm, located at el. 111m asl, will be restricted to necessary personnel for inspection and instrumentation operation purposes. A concrete coping wall will be constructed at the upstream slope to provide protection against maximum wave height.
Upstream and Downstream Cofferdams
The construction of the dam requires the implementation of two cofferdams, one upstream and the other downstream of the projected dam-axis, which will have a crest width of 8m over the elevations 111m asl. The upstream and downstream slopes for both cofferdams will be 1:2.4 and 1:2 respectively.
Both cofferdams will be constructed in phases to reach the area of the gated spillway, which would be constructed previously to allow the routing of the design flood through a 190m-wide section. The design flood has been estimated at 14,000m3/sec for a 10-year return period. The powerhouse would be also implemented and protected with cofferdams of similar elevation in a preliminary river diversion stage.
To allow construction of the powerhouse and navigation locks, the river will be diverted by temporary cofferdams. At the time of cofferdam closure, construction of the main dam can start with end-dumping of the gravel/rock shoulders under water for 1/3 of the length of the dam. Dredging material will be used within the rockfill shoulders. The remaining 2/3 of the dam will be constructed following drawdown of the river water through the gated spillway and the navigation lock. After closure of the gravel/rock shoulders and completion of dredging in the central part, the main dam body will be constructed using rockfill in predetermined lift heights dumped by the usual methods using the roller compaction method .
Recent experiences in CFRD have focused on strengthening concrete face slabs in the central side taking an asymmetric canyon into consideration. However, for Motyginskaya, this is not so important due to its symmetric cross profile and length. The implementation of soft vertical and horizontal joints are expected to minimize compressive stresses in the face slabs.
César Adolfo Alvarado-Ancieta, Civil Engineer, Dipl.- Ing., M. Sc., Hydropower, Dams, Hydraulic Engineering, Strauss 799, San Borja, Lima 41, Peru. Email: [email protected]