Ecuadorian dam meets growing water demands

QUITO in Ecuador, located at an elevation of 2896m in the Andes mountains, is one of the highest cities in the world. The area is surrounded by active and inactive volcanoes that have constrained its growth.

All the same, the rural population has continued to move into Quito and its environs, creating a challenging water supply problem for the municipal water and wastewater agency responsible for water supply, sewerage, and drainage (EMAAP).

An area of particular development in recent decades has been Quito Sur, located south of Quito along the floor and flanks of a wide valley known as the Avenue of the Volcanoes. For more than 20 years, EMAAP had planned to divert water from La Mica lake, set even higher in the Andes – about 60km from Quito – to meet the growing needs of the Quito Sur area. That goal was achieved recently with the completion of the La Mica-Quito Sur water supply project, on which TAMS Consultants was the lead firm of a joint venture that included the US consulting company Hazen and Sawyer and local consulting engineering company CIC.

The water source, Lake La Mica, and its associated tributaries and streams, is situated well above the tree line in an area of ecological importance containing high altitude vegetation. The climate in this high, remote area is cold and wet. La Mica dam, a central feature of the project, was constructed along the lake outlet to increase the level of the lake and regulate the highly variable flows that feed it.

The regulated flow of water from La Mica is conveyed to Quito Sur via a steel pipeline that traverses the high plain for about 20km before plunging for the next 20km down the steep valley sides to the bottom of the Río San Pedro, almost 1500m below La Mica reservoir level.

The pipeline then climbs back about 700m over the next 7km to El Troje treatment plant, which commands the target delivery area of Quito Sur.

The treatment plant is designed in two equal phases, the first of which has been completed, with a flow capacity that is the equivalent to 88M litres of water per day. The second phase will be implemented to meet growing water demand in the future. Water from El Troje plant is conveyed by steel pipelines to a series of water tanks that feed the distribution zones of the Quito Sur area. The design takes advantage of the large drop in elevation between La Mica reservoir and El Troje treatment plan to generate electricity at El Carmen hydro power plant, located about midway between the dam and the treatment plant.

Key elements of project

The major elements of the project are:

• La Mica dam, a 1.4km long embankment dam, 20m high at the deepest section, equipped with tunnel spillway and outlet works feeding the conveyance system.

• An upper steel pipeline system, about 18km in length, with a diameter of 1.08m.

• A steel penstock system and surge tank feeding the power house, about 4km long and 1.08m in diameter.

• El Carmen hydroelectric power plant, equipped with a two-jet Pelton turbine with an installed capacity of about 10MW, operating under a head of 613m.

• El Troje treatment plant which includes settling tanks and filters and six distribution tanks – five with 2300m3 capacity and one with 1000m capacity – each fed by gravity from El Troje by a series of steel pipelines with a total length of 27km.

Technological challenges

The most challenging technical feature of the project was the construction of the dam. Although relatively low at 20m, the design of the dam was daunting because of the extremely low strength of the black volcanic ash material (black clay) that formed the structure's foundation. In addition, it was impossible to obtain earth fill materials at sufficiently low water contents to achieve the degree of compaction normally specified for embankment dam construction. The most suitable materials available in the area were well-graded fluvio-glacial soils.

To counter such difficulties and achieve the necessary stability, the dam design was finalised during construction. It was based on the densities that could be achieved in test embankments in the field with the available earthfill material. The basic design concept was an homogeneous embankment dam with a vertical filter-drain connected to a drainage blanket laid at foundation level beneath the downstream shoulder of the dam. Designated borrow areas were located close to the dam site. Extensive test pitting and laboratory testing were undertaken in the borrow areas to select the areas that gave the driest material.

Despite these measures, placing and compacting this material was extremely difficult because of its high water content and frequent rain at the site. In practice, the solution was to place the materials using the lightest equipment. Trafficability was the limiting factor. The slopes of the dam were set very flat – one on three downstream and one on 2 1/2 upstream – dictated by the low shear strength of the foundation materials. Tests on the embankment fill material confirmed that the slopes were also appropriate for the wet, lightly compacted fill materials. Piezometers placed in the dam foundation and fill material have confirmed the effectiveness of the design.

Pipeline design and construction

Design and construction of the 47km long main pipeline system also presented a technical challenge. The pipeline contractor elected to manufacture a good portion of the pipeline at site and imported a sophisticated manufacturing plant from Scotland to do so. The centrepiece of the plant was a unit to produce pipeline from rolled coils of steel using spiral welding techniques. This approach imposed a major quality control problem on the joint venture. All pipes were inspected and tested at the manufacturing plant, including hydrostatic testing at 80% of the steel-yield strength. The field welds were subject to 100% radiographic inspection, and special care was taken in qualifying the field welding crew.

In situ hydrostatic testing was undertaken for the completed length of pipeline including valves. Because of the marked changes in elevation along the pipeline – exceeding 1300m – the pipeline thickness was varied to match the differing hydrostatic and dynamic loading conditions. A careful analysis was therefore required of the pipeline’s subdivision into reaches for the in situ testing to ensure that all pipe sections were subject to the appropriate minimum test pressure, without other sections of the pipeline being subject to stresses in excess of 75% of the yield strength of the steel.

The in situ hydrostatic pressure testing was completed in the first five months of operation and no leaks were found in the pipeline itself. Some minor leaks were found at valves but these were easily remedied and the hydrostatic testing programme was completed.

Now in operation, the La Mica-Quito Sur water supply project is recognised as a major step in meeting the future water needs of people in the Quito Sur area, where the population is eventually expected to reach more than 600,000 people.