Construction challenges

18 May 2006

Since construction began in 2002 on the Animas-La Plata scheme, developers have had to overcome several challenges, including the development of a suitable project team, environmental concerns and the geology of the project area itself

Construction of the Animas-La Plata project, now underway in southwestern Colorado, US, near the historic town of Durango, has presented various challenges to the US Bureau of Reclamation (USBR) and its primary construction contractors – Weeminuche Construction Authority and Sky Ute Sand and Gravel. Construction of the dam and its attendant features has been and continues to be the subject of intense public interest and environmental review since construction began in autumn 2002.

When completed, the structural features of the Animas-La Plata project will consist of Ridges Basin dam, a 83.2m clay and earth structure; lake Nighthorse, a 148Mm3 off-stream reservoir at Ridges Basin; the Durango pumping plant, a 7.9m3/sec capacity pumping plant located south of the centre of Durango on the west side of the Animas river; the Ridges Basin inlet conduit, a 3.4km buried pipeline that will carry project water from the pumping plant to lake Nighthorse; and the Navajo Nation Municipal buried pipeline to carry water from the Farmington to Shiprock areas of New Mexico to benefit the Navajo Nation Indian tribe.

Project team

Development of a project team including USBR, the Ute Mountain Ute tribe, the Southern Ute Indian tribe and the various project sponsors has required imaginative and individualistic approaches to the work. The project, although authorised in 1968, was further defined when Congress passed the Colorado Ute Indian Water Rights Settlement Act in 1988. Contained in the 1988 Act were provisions to allow the tribes to utilise Public Law 93-638, the Indian Self Determination and Education Assistance Act (ISDEA) for the design and construction. The spirit and intent of the ISDEA is to provide tribes an opportunity to be self-determining and to take a more active role in those activities that impact their daily lives. The ISDEA is not a sole-source programme; it is a congressionally mandated, direct-source programme that directs the Secretary of the Interior to contract with Native American tribes under certain situations.

Title I of the ISDEA establishes a federal self-determination policy that entitles federally recognised Native American tribes to plan, conduct, and administer programmes and services that have traditionally been performed on their behalf by the federal government. The law provides self-determination contracts in framework PL 93-638, commonly referred to as the ‘638 Process’. Tribal contracting for construction programmes, projects, or activities pursuant to Title I of the law is subject to a proposal and review process that differs significantly from the traditional competitive bidding procurement process under the federal acquisition regulations.

Under the ISDEA, the fixed-price construction contracts are to be comprised of:

• The reasonable costs to the tribe of actually performing the work.

• The costs to the tribe of auditing the general and administrative expenses incurred by the tribe in performing the work.

• The costs of developing the project proposal.

• A fair profit.

The ISDEA regulations state that in a ‘638’ contract, the Secretary shall share all relevant cost information, as the object of the negotiations is to arrive at a fair and equitable price for the award, not to obtain the lowest possible award price. The regulations further state that the agreed upon price does not have to be in strict conformance with either party’s cost estimate.

Following the passing of amendments in 2000 to the 1988 Act, the tribes determined that the Ute Mountain Ute Indian tribe would perform the heavy civil construction and the Southern Ute Indian tribe would supply all ‘batched’ materials. The Ute Mountain Ute Tribal construction company is Weeminuche Construction Authority (WCA) of Towaoc, Colorado, and the Southern Ute Indian tribes ‘batched’ material supplier is Sky Ute Sand and Gravel of Ignacio, Colorado.

USBR is performing the design, project and construction management, land acquisition, mitigation planning, and environmental oversight.

The Ute Mountain Ute tribe has made the P.L. 93-638 process a success by providing an income for the tribe, adding extensive construction experience on a major civil works project, and has provided a substantial training ground having over 60% Native American employment including two apprentice programmes (heavy equipment operators and cement masons).

Ridges basin dam

Ridges Basin dam will be a 4Mm3 zoned earthfill structure containing a core bordered by a downstream filter and drains and supported by pervious upstream and downstream shells. There will be a sand transition protecting the upstream core in the water fluctuation zone. The structural height of the dam will be 83.2m and the hydraulic height will be 62m with a 500.5m crest length (see Figure 1). The clay core will be obtained from an upstream borrow area located approximately 2.4km from the dam. The shell materials will be sand, gravel, and cobbles obtained from a downstream borrow area (Borrow Area B) located approximately 3.2km from the dam. Filter and drain materials will be processed from Borrow Area B. The dam will be founded entirely on rock through which a grout curtain will be extended.

The outlet works will be located in the left abutment of the dam and will consist of an intake approach channel, intake structure, upstream tunnel, gate chamber, downstream tunnel, gatehouse, and discharge channel. The upstream portion will be a concrete lined 2.3m diameter 202m long tunnel. The gate chamber will be located within the left abutment about 1m upstream of the dam axis and will contain an emergency gate. The downstream tunnel will be concrete lined horseshoe-shaped containing a 168cm diameter steel pipe which will connect to a control house. A 152cm jet flow valve will be used for emergency releases and normal releases will be through a 122cm diameter sleeve valve. The control house will also contain other valves and piping for future pressure releases.

Construction of the dam, outlet works and related features will require the excavation of approximately 2.3Mm3 of sand, gravel and clay materials. Five separate contracts have been awarded since 2002 to perform this work ‘in pieces’. Though this is not an ideal scenario for any of the project’s primary contractors, close coordination between all parties is being maintained to ensure continual progress toward meeting the construction schedule and to utilise design-build opportunities.

Upon completion of the entire project, water stored in lake Nighthorse will be released from the dam as necessary, falling back to the Animas river for use by municipal and industrial users within Colorado and New Mexico. Lake Nighthorse will impound approximately 14.8Mm3 of water and include an inactive pool of approximately 37Mm3 for recreational, fishery and water quality purposes.

Progress on the dam

In the past three years, significant progress has been made on the dam. In order to begin the construction as quickly as possible, work began around three active high pressure gas lines which traversed the dam footprint. Balancing available funding and working safely around the gas lines, the first two dam construction contracts began. They were the Outlet Works Portal Excavation and Right Abutment Excavation contracts (see Figure 2). Once the gas lines were relocated around the dam site, excavation throughout the remainder of the dam foundation began.

The foundation excavation work could not be fully completed until the tunnel was excavated and prepared for stream diversion. The Tunnel Excavation contract was completed in the summer of 2005 and stream diversion was possible in the late spring of 2005. The foundation excavation work was completed in the summer of 2005.

A materials processing plant was purchased and erected on site to provide the sand filter and gravel drain material for use in the construction of the dam. Manufacture of the sand filter and gravel drain materials was started in the spring of 2005. A substantial portion of the sand filter and gravel drain was required to be manufactured prior to the start of embankment placing operations.

Excavation at the site has included the removal of alluvial materials throughout the entire footprint of the dam to expose a bedrock foundation. The clayey alluvium reaches a maximum depth of approximately 27.4m within the valley floor with the lower two thirds existing below the groundwater piezometric surface. Approximately 1-1.5m of slopewash and colluvium were removed to expose bedrock on the upper dam abutments. Weathered bedrock, averaging about 0.61m in sandstone units and about 1-2.5m in siltstone and shale units, was also removed beneath the Zone 1 core of the dam to provide a firm foundation.

Bedrock at the site is sedimentary rock which includes the Cretaceous age lewis shale and the pictured cliffs sandstone. The lewis shale is composed primarily of siltstone with interbedded shale. The lewis shale is exposed on the lower abutments and the upstream portion of the maximum section of the dam. The pictured cliffs sandstone is composed of interbedded sandstone, siltstone, and shale exposed on the upper abutments and the downstream portion of the maximum section. Bedrock units dip approximately 25º to the southeast (downstream). Bedding thickness throughout the dam site is variable up to about 6.1m in the thicker sandstone units exposed on the upper portions of the abutments.

Foundation cleanup is being performed in two separate operations. The initial cleanup is for geologic mapping beneath the embankment Zones 1, 1A, 2, and 2A, including the entire filter blanket area downstream of centreline. It is cleaned with compressed air as the initial excavation is complete. A second foundation cleanup using compressed air is specified following foundation grouting and immediately prior to placement of embankment within the areas beneath Zones 1, 1A, and beneath the Zone 2 and Zone 3 chimney. Compressed air is being used for cleanup instead of water, this is because water deteriorates many of the shaley and silty units causing rapid slaking.

In addition to dental concrete and slush grouting, foundation treatment includes blanket and curtain grouting. A grout curtain to reduce seepage through the foundation extends along a single line across the entire dam approximately 30.5m upstream of the dam centreline to a depth of approximately 33.5m and as deep as 61m beneath foundation grade. Blanket grouting is being performed on the upper abutments within the exposures of the pictured cliffs sandstone between the dam centreline and the grout curtain. Blanket grouting extends to a depth of approximately 9.1m below foundation grade on a 3.05m minimum grid pattern.

Foundation grouting started in the spring of 2005. The final foundation cleanup and placement of the Zone 1 impervious core of the dam were started in August 2005. By the time of the winter shutdown in November 2005, foundation grouting and cleanup operations had been completed across the bottom of the dam foundation and approximately 105,515m3 of Zone 1 material, or 9-10m of embankment, and 214,853m3 of other zoned embankment had been placed. A frost cap consisting of a 1m thick layer of the pit-run shell material was placed over the Zone 1 core during the winter months. This frost cap has been removed and placement of embankment has resumed for the 2006 construction season.

Placement of embankment materials will continue through the 2006 and 2007 construction seasons and is anticipated to be completed in the 2008 construction season. Construction of the outlet works for the dam will be competed in late 2007. Filling of the reservoir is anticipated to start in 2009 and will require two years to finish; this is based on assuming normal hydrologic conditions in the Animas river.

Durango pumping plant

Located on a parcel of land directly across from Santa Rita Park along the banks of the Animas river, the Durango pumping plant will lift a maximum of 7.9m3/sec of water from the river up 168.3m through the Ridges Basin Inlet Conduit into lake Nighthorse, formed by Ridges Basin dam. The pumping plant will be placed about 61m from the river and will include an intake structure, a fish screen structure, and a 11.9m diameter air chamber. The pumping plant structure itself is approximately 70.1m in length and 20m in width, and 18.3m of the structure is constructed below the ground surface. The majority of the structure will extend only 5.5m above the ground surface. In the service bay, the high point will extend to 12.2m above the ground surface.

Construction of the pumping plant and appurtenant structures will require the placement of approximately 16,821m3 of concrete. Concrete for the project is being provided by a portable batch plant owned by Sky Ute Sand and Gravel and located at the south end of the pumping plant site. Two concrete mixes are being used in the construction of the plant. A mass structure concrete mix with a 5cm maximum size aggregate (MSA) is being used in placements that are greater than 1m in thickness. A structural concrete mix with a 2.5cm MSA is being used in all other placements. The mass concrete mix has a total of 307.5kg of cementitious material with 25% pozzolan and a W/(C+P) ratio of 0.39. The structural concrete mix has a total of 298.4kg of cementitious material with 20% pozzolan and a W/(C+P) ratio of 0.42. Both mixes are designed to have a compressive strength of 4000psi in 28 days.

The intake structure is located on the west bank of the Animas river and is approximately 15.2m long. The project does not utilise a diversion structure across the Animas river; therefore, the actual pumping rate will begin to drop below 7.9m3/sec as the river flows drop below 17m3/sec. Obermeyer crest gates are utilised in the intake structure to minimise the amount of sediment pulled into the plant.

Fish protection

A fish screen is included in the pumping plant design to prevent fish from being pumped from the river to the reservoir. The fish screen is a flat plate, wedge wire style screen with panels 2.4m tall totalling 30.5m in length. The fish screen is designed such that a majority of the screen area will be submerged at the low water elevation of the river. Blank steel panels are utilised above the fish screen panels up to the maximum water surface elevation. Openings in the fish screen are a maximum of 1.75mm with a design maximum approach velocity of 0.1m/sec and a minimum sweeping velocity of 0.2m/sec. Adjustable baffles will be installed behind the fish screens to control flow through the screens and eliminate any localised areas of high approach velocity. Fish entrained in the intake structure will be returned to the Animas river via a 91.4cm outside diameter HDPE pipeline.

A 213cm diameter steel pipeline will convey water from the fish screen structure to the pumping plant. Inside the pumping plant, a total of eight horizontal shaft, split case, single stage, double suction, double-volute centrifugal pumps will be utilised. The horizontal centrifugal pumps were selected, in large part, based on their ability to handle high sediment loads. The pumps are being manufactured by Flowserve. Four of the pumping units will have 5000hp motors and have a capacity of 1.6m3/sec, two of the units will have 2500hp motors and have a capacity of 0.8m3/sec. The last two units will have 1250hp motors and have a capacity of 0.4m3/sec. The pumping units will be capable of being operated in various combinations to pump within a range of 0.4m3/sec to 8m3/sec, in 0.4m3/sec increments. Manifold piping within the plant is designed so that the 0.4m3/sec pumps can pump to either lake Nighthorse or directly to the city of Durangos water storage reservoir.

Water will be conveyed from the plant to lake Nighthorse via the Ridges Basin inlet conduit, a 183cm diameter steel pipeline around 3.4km in length. A 11.9m diameter air chamber is utilised adjacent to the pumping plant to protect against hydraulic transient pressure waves generated due to drastic changes in pumping rates in emergency situations. The air chamber is a pressurised vessel connected to the inlet conduit that contains a volume of air over water. It is designed in accordance with requirements of Section VIII, Division 1, of the ASME Boiler and Pressure Vessel code.

Progress on durango pumping plant

Construction of the pumping plant will take approximately five years. Work commenced on the excavation for the facility in April 2003. Approximately 45,876m3 of common material (dirt and loose rock) were removed from the plant footprint. The first blasting operation, to remove solid rock, took place on 13 May 2003 and the last blast was on 10 November 2003. A total of 108 shots were used during this period to aid in the excavation of approximately 71,108m3 of rock from the pumping plant site. Blasting operations required the closure and detour of the major highway through Durango during the blasting operations. With the cooperation of Weeminuche Construction Authority, Yenter Construction (the blasting subcontractor), and the local community, all shots were scheduled for early morning just after sunrise to minimise the impact to the commuting public.

Construction of the intake structure and the HDPE fish by-pass pipeline was in the winter 2003/2004 and spring of 2004 and required excavation through the river bank. Two cofferdams were constructed to isolate the excavation from the river. Construction of the pumping plant began in the summer of 2004.

As of March 2006, a total of 11,010m3 of concrete has been placed in the pumping plant and appurtenant structures. The concrete superstructure of the plant is scheduled to be completed in late 2007. Installation of the pumping units and major electrical equipment will begin once the installation of the roof is completed in late 2007. Installation of the pumping units, and associated electrical and mechanical equipment, is scheduled to be completed in late 2008, in preparation for the filling of the reservoir.

Environmental and cultural mitigation

The Animas-La Plata project has been the subject of substantial public interest and environmental review since its authorisation in 1968. The project planning has been delayed and the project downsized several times due to environmental and National Environmental Policy Act (NEPA) issues.

In July 2000, USBR – in cooperation with the Environmental Protection Agency (EPA), the Ute Mountain Ute tribe, and the Southern Ute Indian tribe – completed the Final Supplemental Environmental Impact Statement (FSEIS) for the Animas-La Plata project. According to the FSEIS, construction of the Ridges Basin dam, drop structures in Basin Creek, and the filling of the reservoir will result in the permanent loss of 54.2ha of emergent wetland/riparian habitats. The construction of the dam, appurtenant features, and the filling of the reservoir will further impact 1092 to 1173.6ha of vegetation and wildlife habitat combined. Additionally, Ridges Basin has been determined a District for its eligibility to the National Register of Historic Places due to the archeological and historic resources found within the basin. The construction of Ridges Basin dam, appurtenant structures, and the filling of the reservoir will impact approximately 80 to 90 cultural resource sites. USBR is committed to mitigating for this.

The environmental mitigation work consists primarily of wetlands/riparian mitigation and development of upland areas to mitigate for the loss of wetland/riparian and wildlife habitat. USBR coordinated potential mitigation opportunities to replace the functions and values of the wetland/riparian habitat that will be impacted by the Animas-La Plata project with specialists representing the EPA, the US Fish and Wildlife Service and the Colorado Division of Wildlife (CDOW). To ensure that the environmental and cultural resource activities do not impact construction activities, the environmental and cultural mitigation activities have been given a funding priority within the project budget.

In addition, the FSEIS committed USBR to minimise the environmental impacts of a number of construction related activities related to noise, dust, traffic, sediment control, and other similar impacts. USBR has assigned a staff of individuals to oversee the construction activities to ensure compliance with our environmental commitments.

Author Info:

The author is Barry Longwell, PE, Deputy Construction Engineer, U.S. Bureau of Reclamation, Durango, Colorado.

Portions of this article were originally published in the proceedings of the 25th Annual United States Society on dams (USSD) Conference, ‘Technologies to Enhance dam Safety and the Environment’, Salt lake City, Utah, June 6-10, 2005.

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Figure 1 Figure 1

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