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Challenges Overcome in Designing and Building Son La Dam in Vietnam

By moving forward on the design of a number of project structures, Institute Hydroproject of Russia was able to advance work on the 2,400 MW Son La project while still determining the final design. This saved two years in getting the project operational.

By Pavel V. Shestopalov and Alexander N. Volynchikov

One significant problem with regard to water in Vietnam is flood control. The Da River — which runs from its source in China through the northwest area of Northern Vietnam and flows into the Red River near Hanoi — has a catchment area of 52.6 km2 in the territory of three countries: China (48%), Laos (2%) and the Socialist Republic of Vietnam (50%). Floods caused by typhoons occur in the Da River's tributaries during the monsoon season (which runs May through September or October) and merge into a flow of 40,000 m3/sec at the town of Son Tai, west of Hanoi.

In the early 1970s, the government of Vietnam elaborated a master plan for river utilization in the country and began conducting preliminary investigations. To deal with flooding in the region, the government decided to build a cascade of five dams and hydropower facilities on the Da River (see Figure 1). This cascade of developments can provide a significant portion of the power demand in Vietnam: 19.4% of the country's required capacity and 13% of its electric energy output. They also can provide for flood control; irrigation, municipal and industrial water supply; and navigation.

The project furthest downstream in this cascade, 1,920 MW Hoa Binh, began operating in 1994. However, the maximum output of this facility will only be attained after construction of the four upstream projects. These are 2,400 MW Son La and 1,200 MW MW Lai Chau on the Da River, as well as 200 MW Ban Chak and 560 MW Huoi Quang on the Nam Mu River, a first order tributary of the Da River.

Table 1: Parameters of the Da River Cascade

The flood-control storage of the entire cascade is estimated at 7 km3 (see Table 1). The Hoa Binh and Son La projects are supposed to play the main role in flood control measures. Before Son La was built, up to 26% of the river flow during the monsoon season had to be discharged without being run through the turbines of the Hoa Binh facility.

The technical design of the Lai Chau project was approved in May 2012. Preparatory site work and development of detailed drawings has begun. For the other projects, the pre-design work has been completed.

Construction of the Son La project began in 2005. The first unit was commissioned in 2010, and all six units were operational by October 2012. The powerhouse is expected to provide annual power output of 9 GWh.

Designing Son La

To build Son La, by resolution of the government of Vietnam, a project construction management committee was formed within the framework of state-owned utility Electricity of Vietnam (EVN). Development of the technical design for the project was entrusted to a consortium consisting of PECC-1 (Vietnam) and JSC Institute Hydroproject of Russia.

The Son La project is in the Da River valley at Paving-II site, about 215 km upstream of the Hoa-Binh project. The reservoir, impounded by a concrete gravity dam, has a full storage level at Elevation 205 meters to 215 meters. The design studied consisted of installed capacity of 1,970 to 2,400 MW and the average annual output of 7.555 to 9.209 GWh. The number of people to be resettled from the inundation zone of the new reservoir was 79,000 to 91,000.

Challenges to overcome

Many challenges were encountered during design and construction of the Son La project.

During the site investigation stage, comprehensive engineering-geological work performed included drilling of bore holes and laboratory, geomechanical and geophysical investigations. Findings indicated the area of the Da River valley where the project was to be located is composed mainly of basalt porphyrite and is of asymmetrical structure.

The rock mass on the left bank would provide a good foundation for heavy concrete structures. However, the slope is steep, meaning substantial cutting-in would be required, and part of this had to be accomplished in sound, hard-to-excavate rocks.

In the river channel portion, the rocks are eroded, widely developed is highly jointed porphyrite. The river bed topography is somewhat uneven. The surface zone 5 to 10 meters thick had to be removed to reach sound rock suitable for the construction of concrete structures.

Consolidation and seepage control grouting were conducted in the dam foundation. The right bank part of the foundation is composed of basalt porphyrite featuring heterogeneous physical and mechanical properties. This diversity stems from the occurrence of interlayers of schistose basalts, which complicate the engineering-geological conditions of the right bank abutment. The sedimentary rocks occurring on the surface of the basalt strata on the right bank have low characteristics and, therefore, they were removed completely from the foundation.

Ahead of its time

The Da River valley is not wide, with a river channel width of about 360 meters at the dam site, but discharges are rather high at 38,000 m3/sec. With the full storage level adopted at Elevation 215 meters, the dam crest length is 961.6 meters. To accommodate the narrow site and rather high discharge, seven layout alternatives were considered for the main structures.

The layout alternatives included the powerhouse being located at the toe of the dam, diversion-type, integrated with the surface spillway and low-level outlets. Different methods for surface flow transition, including jet throw by ski-jump, were considered.

Realizing that considering seven alternatives takes considerable time, in the process of designing this facility Institute Hydroproject moved forward on the design of a number of project structures, particularly the river diversion facilities. This made it possible to prepare working drawings and erect structures for river flow diversion, along with preparation of the Detailed Project Report. This advance work resulted in the first unit being commissioned in 2010, two years ahead of the scheduled time.

In addition, six alternatives for river flow diversion were considered. The final arrangement chosen, because it appeared to be the most reliable, was construction of a diversion canal with a bottom width of 90 meters and the bottom at Elevation 110 meters. Water is supplied via a two-opening water conduit (2 by 12 by 12 meters) with the control structure equipped with gates.

Figure 1 Da River Cascade

The selected river flow diversion scheme was built and used from 2005 to 2008 to discharge the river flow and make it possible to perform other civil work, such as foundation treatment, preparing the first-stage grout curtain, and beginning construction on the dam concrete structures and service spillway. In March 2009, during the dry season, the concrete dam was constructed up to Elevation 126 meters at the section of the diversion canal. The high-flow period in 2009 was handled by overtopping the partially constructed dam and sending flow through the diversion conduit. This scheme required that the dam designers and builders ensure the structures were completed in a timely manner.

After this high flow was passed, intensive dam construction in the diversion canal area was continued and was completed up to the design elevation in August 2010. During this period, the water discharges were passed only through the diversion conduit. In May 2010, the gates at the control structure of the diversion conduit were closed, and partial filling of the reservoir was started. At the same time, installation of the concrete plug began in the diversion conduit. By this time, the service low-level outlets were completed, and when the water level reached their sill at Elevation 145 meters, the river flow was passed through them.

Selecting the final layout

To select the final project layout, the consortium of PECC-1 and Institute Hydroproject conducted additional site investigations and hydraulic studies of surface flow transition conditions using physical models. The studies allowed for identification of the most promising layouts. At the stage of the final selection, two layouts were adopted as the most optimal alternatives (see Figures 2 and 3).

Figure 2 Alternative 1 for Son La Project

In the first alternative (see Figure 2), the service spillway with integrated surface and low-level openings and the chute are located on the right bank. Surface flow transition is effected using jet throw-off by ski-jump in the end section of the chute. The powerhouse at the toe of the dam is located in the cutting-in to the left bank and partially in the river channel. The power intake is in the body of the gravity dam, the water being supplied to the spiral casings through penstocks 10.5 meters in diameter.

Figure 3 Alternative 2 for Son La Project

The second alternative (see Figure 3) consists of a surface spillway and bottom outlets located in the river channel to pass the discharge while the plant is operating. The surface flow transition is effected using jet throw-off by ski-jump. The powerhouse intake is in the body of the gravity dam. The powerhouse is located on the right bank of the river at a distance of 100 meters from the dam, and the water to the spiral casings is supplied through open penstocks 10.5 meters in diameter.

When scrutinizing these two alternatives, validation of the structural parameters was done according to specially developed design criteria meeting requirements of design norms and standards of Vietnam, Russia and the USA. Besides taking into account the project importance in terms of flood protection of the Red River valley downstream, safety factors adopted were higher compared to those envisaged in the standards. On the whole, both layout alternatives met the assigned requirements.

Hydraulic studies

Decisive factors in selecting the layout of the main structures were the results of hydraulic studies with the use of physical models simulating a foundation consisting of cohesive and non-cohesive materials. As a result, the following was established:

Alternative 1:

— Surface flow transition conditions do not affect headworks stability and thus will not decrease their safety;

— Plunge pool formed in the full range of discharges does not affect the reinforced concrete structures intended for river flow diversion during construction. It is possible to arrange for the plunge pool upstream edge to spread toward the flip bucket side, and this requires the provision of protective measures; and

— Scouring of 700 meters over the right bank and 1,000 meters over the left bank are forecasted that require special measures to ensure their stability. When passing the flood discharge, no backwater downstream of the powerhouse is expected to occur. Deposition of degradation products from the river bed and banks in the tailwater after passing the flood flows may result in formation of up to 2 meters of backwater at the powerhouse. Under any tailwater conditions, no drift of scour material to the draft tubes takes place.

Alternative 2:

— Location and dimensions of the plunge pool depend on many factors. Due consideration was given to stability of the left bank slope depending on the joint effect of the plunge pool formation and downstream water velocities. In design analysis of the spillway dam section, the shear plane is in the immediate vicinity of the plunge pool. Apparently, cohesion of rock in this zone will decrease due to de-compaction and the impact of high dynamic differently oriented forces on the rock mass.

Because it is impossible to objectively determine the changes in the rock parameters with time and definitely determine the size of the plunge pool, it is impossible to assess the real decrease in the safety factor of the structure.

Thus, studies showed that construction of the Son La Project with the powerhouse located in the river channel and the service spillway on the right bank guarantees higher reliability and safety. When passing the operation period discharges, the slope protection against scouring was provided for the downstream banks. For energy dissipation, soil removal from the plunge pool and concrete placement along the right bank were provided.

The alternative proposed by the consortium underwent international expert appraisal (with the participation of Japanese companies) and was adopted for final design when preparation of the Detailed Project Report was complete.

Building the dam

The dam type — concrete gravity — was selected at the stage of the feasibility study approval. Also, several alternatives for the non-overflow dam design were considered for the Son La project. The alternative of the dam having a trapezoidal profile with a continuous crest, internal roller-compacted concrete zone and grout-enriched RCC outer face was adopted. The upstream face was adopted vertical and the downstream one with a slope of 1:0.7275.

The simplicity of the gravity dam design made it possible to use extensive mechanization for concrete placement, advanced reusable types of formwork, and harsh concrete mixes with low cement content. Also, the proposed design of the dam made it possible to create favorable conditions for required temperature regime during the construction period.

Figure 2 shows the typical section of the non-overflow dam design.

The bedding mix at the toe of the dam levels the rock foundation surface and provides safe cohesion with it. The 0.6 meter-wide grout-enriched RCC seepage control element, on the upstream face of the dam, was constructed using grouting to ensure higher strength and seepage control characteristics.

The RCC technology considerably increases the rate of construction. RCC with a higher content of cementitious material (more than 150 kg/m3) was adopted for Son La Dam. Cement and fly ash from the Falai thermal project constructed in the north of Vietnam (a byproduct of fuel combustion) was adopted as a cementitious material.

The power intake consists of six sections, 31.5 meters wide, separated by contraction joints. The six penstocks are 10.5 meters in diameter. The steel lining of the penstocks was designed to take up the full hydrostatic and hydrodynamic loads, and the reinforced concrete coat makes it possible to improve their safety and reduce load on the lining.

A comparatively narrow site conditioned the selection of the spillway with arrangement of the surface spillway and low-level outlets in one section. Location of the spillway structures on the right bank in a separate construction pit made it possible to provide an extended front for civil work planning. The spillway structures comprise: approach channel, spillway dam on the right bank abutment, chute with a flip bucket, plunge pool and outlet channel. The low-level outlets and surface spillway are united into one block. The overall height of the spillway dam is 93 meters.

Conclusion

The Son La plant operates at its full capacity of 2,400 MW. Institute Hydroproject continues to perform work on assessment of safe operation of the water retaining structures. This includes performing a safety analysis of the water retaining structures based on their present condition. The work will include comparison of as-built drawings with the design and analysis of the condition of the main project structures, with consideration of their actual geometry and properties of their construction materials. Institute Hydroproject engineers will also perform preliminary analysis of field observation data.

Owing to the optimal layout solutions, the Institute Hydroproject engineers managed to reduce the period of construction of the Son La project by two years. Only six years passed from the beginning of the design to commissioning of the first unit. In December 2012, the Son La project was put into commercial operation.

Pavel Shestopalov is general director and Alexander Volynchikov, PhD, is chief design engineer and director of production with JSC Institute Hydroproject in Russia.

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