On April 24 1984, in response to an international public tender, a JV consisting of Skanska International Civil Engineering of Sweden (80%) and a Colombian partner, Construcciones Civiles (20%), submitted an offer to build the Urrá 1 Multipurpose Project for the Colombian Ministry of Mines. The Skanska/Conciviles offer was the lowest. After due analysis, on April 17 1985, the JV was awarded the contract, which was signed on October 15 1985. Payment for this unit price contract was designated in Swedish Kronor and Colombia Pesos and was worth about $83.3m in April 1984.
Work could not begin immediately, but discussions continued and, on August 3 1990, an agreement was signed which modified the escalation formula and the value of the contract and which provided compensation for the waiting period. Still the project did not begin. Finally, after further extensive discussions in Bogotá and Washington, an adjusted contract was signed on July 22 1993. This allowed for additional modifications and settled the final price, based on the 1984 offer. From bid date to final contract signing it had taken nine years and three months.
The order to begin the work was also given on July 22 1993, with a duration of 61¼2 years. Financing for the project was provided by the Colombian government and by Swedish and Canadian financing agencies.
The Urrá Multipurpose Project is located on the Sinú River, 115km south west of the city of Montería in Cordoba Province. Urrá is designed to produce 1.4GWH of electricity/year, with an installed capacity of 340MW. It is expected that the dam will serve to regulate the annual downstream flooding which affects several communities along the lower Sinú River.
The project site is in the tropical zone of northwest Colombia, characterised by temperatures of 20°C-38°C, with an average relative humidity of 86%. There are two rainy seasons, which results in an average rainfall of 2 200mm. The source of the Sinú River is in the Nudo de Paramillo in the Andes of Antioquia Province and it flows into the Caribbean near San Bernardo de Viento. The dam site is on the last narrow stretch of the river in an area known as Puerto Pacheco.
Several guerrilla groups operate in the immediate surroundings and paramilitary groups began to appear during the later stages of the project. Site security was a delicate issue and expatriate staff were subjected to severe travel restrictions. The Colombia military and police established a strong presence in the area and secured the jobsite in a satisfactory way.
River Sinú diversion
To enable construction of the dam, the flow of the river was diverted into 2 x 600m long x 6.5 m high horseshoe shaped tunnels. One of the tunnels also serves as the bottom outlet for the reservoir and therefore is equipped with a full concrete lining and two sets of outlet gates. The other tunnel was lined with shotcrete and will be plugged at the end of the project. Upstream and downstream earth and rock cofferdams protected dam construction.
Steel closure gates manufactured in Canada control the flow into the tunnels. An 11m diameter semispherical gate mounted in a circular concrete structure at the intake end of the bottom outlet tunnel was used during the final closure operation while the bottom gates were being installed. The downstream discharge portals have steel ‘stoplogs’ to prevent the river from backing up into the tunnels. Diversion of the Sinú River through these tunnels occurred on January 5 1996 and dam construction in the riverbed started immediately afterwards.
In general, the rock through which the tunnels were driven consists of shale (lutita) and sandstone (arenisca) in alternating layers. The rock was mildly fractured in some areas and additional rock support was needed here. The original project design included two horseshoe shaped 600m long diversion tunnels, four steel lined 215m long penstock tunnels and 610m of access and drainage tunnels.
To save time and money, Skanska proposed several changes to the original design, which involved greater use of tunnelling. On the right abutment of the spillway transition structure, the original design called for slurry walls to be used to stop possible infiltration through this area. After evaluation, the client decided to revise the design, replacing the slurry walls with horseshoe shaped tunnels in rock with an area of 7m2. Drainage holes were drilled upwards from the tunnels to reach soft ground. In this way, the water was drawn into the tunnels in a controlled manner, resulting in a lower overall cost. At the left abutment of the dam itself, the original design called for a grout curtain to be constructed from the surface and toed into the rock. Skanska proposed, and the owner accepted, replacing the grout curtain with drainage galleries with a cross section of about 7m2. In the end, tunnels constructed totalled 2 900m
Both diversion tunnels were originally designed with full concrete lining for their entire length. Here, too, Skanska proposed an alternative. Since one of the tunnels was only to be used during the construction phase and would be sealed at the end of the project, Skanska proposed using shotcrete reinforced with steel fibres for the lining instead of concrete. Only the first and last 50m were concrete lined in order to preserve the hydraulic characteristics. The owner accepted this alternative. After 24 months of use, the tunnel was closed for inspection and the shotcrete lining was found to be in perfect condition.
The rock above the penstock tunnel intake required extensive support. Rock anchors were placed in a tight pattern above the four tunnel portals as excavation proceeded towards tunnel invert level. The penstock tunnels included a short horizontal upstream section, then a 45° decline, leading to a longer downstream section.
All underground excavation was carried out by drill+blast. Skanska used two 2-boom Atlas Copco jumbos for drilling the shot holes in the larger tunnels and jackleg drills in the galleries. At the tunnel portals, steel ribs were placed for the first 15-20m as the rock cover was minimal and the rock unstable. In most areas, rockbolts and wire mesh were required to support the roof of the tunnel as well as two layers of shotcrete. Fortunately, very few rock falls occurred during construction. When they happened, the caverns were filled with concrete during the lining phase and additional grouting was performed as required.
The penstock tunnels were steel lined by Skanska’s subcontractor EMEC. Skanska completed the installation by placing concrete around the liners and contact grouting the cavities between the lining and the concrete and between the shotcrete tunnel support and the concrete around the lining.
The Urrá Multipurpose Project was completed on May 15 1999, four months ahead of schedule. The final value of the civil works is about $3.2bn.
Related Files
Location Map
Cross Section
Longitudinal Section
