Upon completion at the end of this month, the embankment dam at the San Roque multi-purpose project will rank among the highest in the world. Bill Groth, James R Stevenson and Daniel W Osmun give more details about measures which were taken to bring this Philippine project ahead of its original schedule
Located in Pangasinan Province on the Island of Luzon, Philippines, the San Roque multi-purpose project is being designed and constructed by Washington Group International (formerly Raytheon Engineers & Constructors). Construction started in 1998 with expected completion by the end of 2002.
The main dam, an earth and rockfill structure scheduled for completion in April 2002, will be approximately 1100m long and 200m high, making it the twelfth highest embankment dam in the world. The project will provide a 345MW rated hydroelectric plant, additional flood control for the lower Agno river, and irrigation for 87,000ha of land.
Financing has been obtained from private sources, with partial funding secured from the Philippines state-owned power company, National Power Corporation. Washington Group International companies are co-ordinating their work as the EPC contractor for the San Roque Power Corporation, a joint venture of Marubeni Corporation, Sithe Philippines Holdings and Kansai Electric Power.
One of Washington Group International’s (WGI’s) first tasks was a commitment to reduce the original work schedule for San Roque, which was developed in the 1980s, by almost one year to 58 months. The schedule reduction was accomplished by overlapping first-year site preparatory work with second-year construction work on diversion tunnels, cofferdams and excavation. Furthermore, the schedule was compressed by using two ten-hour work shifts and six-day working weeks.
The main dam embankment at San Roque consists of a central clay core, sand filter, gravel drain, transition zone and rockfill shell. The total volume of fill required for the embankment is approximately 41Mm3. In addition to the embankment, Washington Group is responsible for other facilities, including the diversion works, low level outlet, gates, power tunnel, surge shaft, power house and spillway.
General dam description
The capacity of the reservoir impounded by the main dam is approximately 850Mm3. The maximum height of the dam measured from core trench elevation to the crest elevation is 200m. Both the upstream and the downstream slopes are 2H:1V. The crest width is 12m and the width of the core at the top of the dam is 6m. The central impervious core has upstream and downstream slopes of 0.2H:1.0V.
The core is entirely founded on moderately to slightly weathered rock and most of the upstream and downstream shells are founded on weathered rock excavated to bulldozer blade refusal. Portions of the shells in the river channel are founded on dense, non-liquefiable, alluvial sandy gravel and cobbles.
The dam design initially included an estimated 41Mm3 of embankment material, with about 22% of that material expected to come from excavations necessary for the project. Within the footprint of the dam embankment, spillway and power house, an estimated 19Mm3 of excavation was required. As a result of more efficient use of available construction materials, 34% will come from project excavations, greatly reducing spoil. Use of residual soil material from required excavations resulted in 4Mm3 of excellent core material that would have otherwise been wasted.
Rockfill zones and excavation borrow
Required excavation for the abutments, core trench, tunnels, spillway and spillway approach channel produces rockfill for the shell zones and riprap for slope protection. Two rockfill zones have been designated; zone 7A random rockfill, and zone 7B select rockfill.
Zone 7A consists of excavated rock with a maximum size of 305mm and up to 60% passing the No 4 sieve. Zone 7B material is higher quality rockfill than zone 7A material, and has a maximum size of 750mm with up to 30% passing the No 4 sieve. The splitting of the rockfill shell into two zones with different gradation limits was intended to minimise the amount of spoil generated from the spillway excavation and other required excavations.
Zones 7A and 7B materials are hauled by truck from the excavation area to the dam shell placement area where they are spread into lifts by bulldozers and compacted with smooth-drum vibratory rollers.
Dam construction materials
The original construction plan called for obtaining most of the shell material from the vast downstream alluvial sand, gravel and cobble deposits. Additional rockfill was available from required excavations including the spillway and main dam excavation. Due to the significant depth of rock weathering attributed to the tropical environment, the Agno river alluvial sources were favoured over required excavation material for shell borrow although the required haul distances were much greater. However, a good alternate source of rockfill was developed just upstream of the spillway approach channel area and beyond required excavation limits. This enabled the upstream shell construction schedule to accelerate because of a shorter haul distance.
Main dam construction rates were increased by allowing flexible use of both alluvial borrow and required excavation materials. As an additional benefit to the project, the upstream reservoir storage capacity was increased.
The upstream transition zone 2, filter zone 3, and drain zone 4 materials are produced in the alluvial material processing plant, which also produces aggregates for the project’s concrete structures. The primary borrow source for zones 2, 3, and 4 is the extensive alluvial deposit located downstream of the dam. Rock particles larger than 0.3m are crushed at loading stations and all material is then transported by conveyor belts to the material processing plant. Zones 2, 3, and 4 are produced by sieving and screening operations and are stockpiled in surge piles. Zones 3 and 4 receive an additional washing process to remove fines and achieve the specified gradation. Waste from production of zones 2, 3, and 4 is combined with additional alluvial borrow during the production of the shell zones.
The original dam design included an impervious core constructed of clay, from a nearby source, blended with gravel to enhance its engineering parameters. During early stages of construction it become evident that a significant portion of residual soil from required excavations had sufficient fines to be used as core material. Based on results of laboratory testing of field samples, and the material’s desirable compressibility characteristics, excavated residual soil was designated as zone 1B core. Use of residual zone 1B was favoured over the clay zone 1 because of its engineering properties, its proximity to the dam and elimination of the clay/gravel blending process associated with zone 1. The zone 1B material was placed up to about 80% of the height and comprised about 90% of the dam’s core volume. An unblended version of zone 1 clay was used at the top of the dam.
Wet season diversion closure
The river diversion system used during dam construction consists of a 55m high upstream cofferdam, and a 10m high downstream cofferdam. Three diversion tunnels re-route the river around the dam site during construction; two have finished dimensions of 10.4m wide by 15m high and a third has finished dimensions of 6m wide by 6m high. All three tunnels are approximately 800m long.
The weather in the Philippines is typical of Southeast Asia and includes a wet season (typhoon) and a dry season. The wet season begins in June and continues through November. The majority of the water for the project is impounded during this wet season in the form of afternoon showers and weekly periods of extreme rain and typhoon storms. During the dry season, flows are typically between 25-50m3/sec. These lower flows can be accommodated in diversion tunnel No 3 – the smallest of the three diversion tunnels. Tunnel No 3 was originally constructed with a set of roller gates gates designed to provide final river closure against flow. The gates will be lowered into place via a mobile crane.
The larger diversion tunnels (Nos 1 and 2) were constructed to add additional capacity for the typhoon rains and to provide protection against a 100-year flood. According to the original plan, plugs for both larger tunnels were to be constructed during the dry season while water passed through the smaller tunnel No 3; this early plan eliminated the requirement for additional intake gates and structures for Nos 1 and 2. The completion of the plug for tunnel No 1 occurred in February 2002 during the dry season.
In order to provide greater scheduling flexibility, a new tunnel No 2 intake structure was designed with gates to withstand the full reservoir head. The gates for diversion intakes 2 and 3 could then be put into place and the tunnel plugs constructed behind them, while the reservoir was filling.
Preliminary designs and price quotations were solicited for a set of large roller gates for the new diversion tunnel No 2 portal. The cost for these gates would have exceeded US$2M, plus the additional costs associated with the intake structures civil works. Therefore, the engineers at Washington Group International decided to explore alternative options for closure of the tunnel No 2 intake.
The approach adopted was to utilise an 8.2m diameter dished pressure head as a bulkhead gate for the closure of this portal. The use of dished heads for flap gates and bulkheads is not a new concept, but the installation of one this large is unusual. The dished head was surplus material from the pressure testing of the lower power tunnel liner manifold. It has been pressure tested to 120% of the maximum reservoir level.
Although the surplus dished head was ‘free’, it was necessary to design and construct new supporting rings, collars and embedded items. The design and fabrication of the dished head and the associated materials was done by va-tech Voest Alpine. The adoption of this closure concept saved both money and several weeks of the construction schedule. In addition, the use of a dished bulkhead reduced the complexity of the intake structure design and the subsequent civil works required.
The estimated diversion closure date is 31 July 2002, but the actual date is expected to depend on the weather. The river can be diverted into tunnel No 3 for several days, making it feasible to install with mobile cranes the dished head onto the tunnel No 2 portal and to lower the roller gates onto tunnel No 3 portal. This will initiate river closure and reservoir filling. The tunnel plugs will then be constructed with access from the downstream portals.
A power house with three 115MW turbine-generators and a 9.45km, 230kV transmission line, will provide power from San Roque into the transmission system of the National Power Corporation
The turbine-generator supply and installation subcontract is with Toshiba. The company is currently working two shifts on critical erection activities for the assembly. This includes the stacking of one of the stators and the assembly of the stator coils for all three units in the generator pits. This allows more room in the erection bay for the stacking and assembly of the rotors.
A permanent power tunnel approximately 1200m long will carry water from the reservoir to the hydro turbines. A second permanent tunnel approximately 1400m long will function as a low-level outlet. A 430m long concrete chute spillway directs flood flows that are released through six radial gates.
In order to overcome the constraints produced by sequential construction of the gate chamber, power tunnel lining, and the power tunnel intake, it was necessary to drive an access adit downstream of the power tunnel intake but upstream of the gate chamber. The addition of this access resulted in a schedule reduction of more than three months on a critical path activity, by allowing work to proceed underground without impeding activities associated with constructing the power tunnel intake.
Lining the power tunnel was accomplished using an 8.5m diameter full circle steel tunnel form, 12m in length, which was cycled ahead for placement regularly. Reinforcing steel for the crown and sides was tied out well ahead of the form, so that only the invert steel had to be placed just prior to setting the form. Concrete was brought into the tunnels using conventional transit mixer trucks and placed using an electric trailer-mounted concrete pump.
The spillway is a gated concrete chute capable of passing probable maximum flood flows of 12,800m3/sec. The structure includes six radial gates, each 15m wide by 19m high. Water flows down the 430m long by 100m wide chute before discharging through a flip bucket into the plunge pool.
In order to reduce the spillway construction schedule, the five piers, each 25m high, 38m long and 4m wide, were constructed using a slipform. Two slipforms were utilised so that work could proceed on two piers concurrently, thus requiring only three horizontal construction joints throughout the height of the slip for each pier. Blockouts for the radial gate guides and stoplog guides were inserted as the slipform progressed. Reinforcing was installed using mechanical threaded reinforcing bar couplers to avoid the long lap splices. The slip progressed at a rate of 10-15cm/hr with a placement rate of 20m3/hr for periods of more than 72hr at a time. The concrete was delivered in conventional transit mixers and pumped into the form using a 52m boom pump. Use of this technique saved an estimated three months on the spillway schedule.
Horizontal and vertical alignment control during the slip was performed using theodolites and targets, along with a backup system of plumb bobs extending from the slipform to control points at the base of the concrete.
Fabrication in the Philippines
All of the major gates on the project were fabricated in a large metal working facility in the city of Batangas, Philippines, utilising imported materials. Designs for the radial gates were done by ABB/alstom in Europe and the designs for the tunnel and draft tube gates were done by Ishikawa Heavy Industries, Japan. The cost savings were significant, due to the relatively lower labour rates at these Philippine fabrication facilities, and representatives from the design firms and from Washington monitored fabrication quality.