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|Federal Highway Administration > Publications > Public Roads > Vol. 73 · No. 1 > Reconstructing Snoqualmie Pass|
Publication Number: FHWA-HRT-09-005
Reconstructing Snoqualmie Pass
by Amanda Sullivan and Amy Danberg
WSDOT takes on current challenges and anticipates new ones in a project to improve the I-90 corridor over the Cascades.
Interstate 90 (I-90), which traverses the northern United States from Seattle, WA, to Boston, MA, is the main east-west transportation corridor across Washington State. The highway connects Puget Sound's deep-water ports, large population centers, and retail and service businesses with agricultural and recreational areas in eastern Washington. The highway facilitates uninterrupted movement of people and freight, which is essential to the State's economic vitality, over the Cascade Mountains via Snoqualmie Pass.
Before 1930, Snoqualmie Pass would close for approximately 6 months out of the year due to severe weather and unsafe driving conditions. Since the 1930s, the Washington State Department of Transportation (WSDOT) began a concerted effort to keep the pass open year-round. Despite multiple improvement projects to the interstate, avalanches, rock slides, and extreme weather continued to require closing the pass for an average of 120 hours per year.
In 2005, recognizing the growing need for a safe and efficient roadway, as well as year-round travel, the Washington State Legislature secured funding for a reconstruction project known as the I-90 Snoqualmie Pass East Project (I-90 Project). Through the project, WSDOT will improve the safety and reliability of a 15-mile (24-kilometer) stretch of I-90 east of Snoqualmie Pass. Specifically, the department will widen the existing four-lane interstate to six lanes to account for recent and predicted increases in traffic; fix structural deficiencies, such as replacing worn-out pavement and straightening sharp curves; and reconnect habitats across I-90 to minimize wildlife-vehicle collisions. Other critical components of the project include reducing the risk of rock and debris falling onto the interstate from unstable slopes, reducing avalanche risks to the traveling public, and minimizing road closures required for avalanche control work.
To put the wheels in motion, the Washington State Legislature provided $387 million as part of the State's 2005 Transportation Partnership Program. The underlying source of funding was a new voter-approved increase in the gas tax of 9.5 cents per gallon. The legislature designated supplemental funding to cover escalating raw material costs and inflation over the last 4 years, bringing total project funding to $595 million (as of May 2009).
The funding enables design and construction of a 5-mile (8-kilometer) project from the community of Hyak to Keechelus Dam, and leaves an unfunded 10-mile (16-kilometer) section from the dam to the town of Easton. Construction of the Hyak to Keechelus Dam project begins in summer 2009 with the building of a long-term detour bridge and excavation of materials from the Keechelus Lake reservoir, which mitigates for the construction's future impact on water storage.
"The groundbreaking for the Hyak to Keechelus Dam project is exciting for WSDOT and our project partners," says I-90 Project Director Randy Giles. "The milestone represents 10 years of partnerships, planning, studies, and designing for a project that achieves the delicate balance of integrating transportation objectives with ecological needs."
Identifying a Preferred Alternative
The topography, geology, weather, ecological connectivity commitments, and traffic constraints in the I-90 Project corridor presented a variety of engineering challenges. The topography of the Central Cascades is mountainous peaks and valleys. For the first 6 miles (9.7 kilometers) of the project area, I-90 runs along a narrow corridor between steep mountain slopes and the shores of Keechelus Lake, a deep agricultural reservoir. The slopes contain volcanic bedrock at varying depths that are subject to deep fissures and cracks with weakened slip planes from which rock could break off and slide. Combined with high annual precipitation and freeze-thaw conditions, the slopes are susceptible to landslides, debris flows, and avalanches.
The last major road construction on I-90 Snoqualmie Pass began in the 1950s, when President Dwight D. Eisenhower signed the Federal-Aid Highway Act of 1956, which started the construction of interstate highways. Construction was completed in the 1970s. Since then the State's transportation needs and economy have changed.
Today, daily traffic on Snoqualmie Pass averages about 27,000 vehicles, typically 22,400 passenger vehicles and 4,600 freight vehicles. Traffic volumes can rise to more than 58,000 vehicles on weekends and holidays. According to WSDOT, travel across Snoqualmie Pass is growing at an annual rate of 2.1 percent, with 51,000 vehicles projected to use I-90 daily by 2028.
Identifying the preferred alternative for the roadway design and alignment involved collaboration with resource agencies, technical investigations, engineering reviews, and public participation. Since 1999, WSDOT and the Federal Highway Administration (FHWA) have worked with dozens of government agencies and nongovernmental organizations to develop and consider a range of potential solutions to meet project needs and mitigate impacts on motorists, the economy, and the environment.
WSDOT created a multiagency interdisciplinary team as an advisory group to recommend a preferred alternative that incorporated relevant science and the concerns of various stakeholders. That team led to creation of a mitigation development team--a technical advisory subcommittee of hydrologists, biologists, and engineers--to identify sensitive environmental areas and develop criteria for investments in ecological connectivity. In addition to these teams, WSDOT formed technical committees to assist in permitting and planning final designs for wetlands mitigation, wildlife monitoring, and stormwater treatment.
"Although we've accomplished remarkable permitting milestones due to our stellar design team and partnerships with permitting agencies, keeping up with the forthcoming construction phases of the project will be a challenge," says Mark Reynolds, I-90 Project permitting coordinator at WSDOT. "To maintain our successful permitting process, our design and permitting agencies will need to identify challenges early, communicate solutions to those challenges, and deliver on our commitments."
WSDOT formed partnerships with university researchers and conservation groups to help establish citizen awareness, initiate wildlife monitoring, and target land acquisitions. The agency also formed relationships with transportation-related organizations, associations, and businesses to gain insight into the requirements of interstate users. The Washington State Good Roads & Transportation Association and local and national freight companies, for example, identified concerns regarding delivery disruptions during construction. WSDOT listened to these concerns and committed to keeping two lanes of travel open in each direction through the construction zone during peak travel times to limit traffic delays over I-90 Snoqualmie Pass. WSDOT also committed to providing freight organizations with construction information in advance, such as e-mail notices and other tools, so they can plan their delivery times accordingly.
WSDOT and FHWA released a draft environmental impact statement (EIS) in 2005, highlighting six design alternatives for the I-90 Project. The agencies held public hearings during the 45-day comment period and received comments from more than 3,300 individuals, groups, and agencies.
In 2006, WSDOT began multiyear preconstruction activities, such as design, geotechnical drilling, ground water monitoring, surveying, right-of-way acquisition, final analysis under the National Environmental Policy Act (NEPA), and permitting. WSDOT also developed its Wildlife Monitoring Plan in cooperation with the Wildlife Monitoring Technical Committee and the Western Transportation Institute at Montana State University.
WSDOT and FHWA released the final EIS identifying the preferred alternative in August 2008. The preferred alternative, the Keechelus Lake alignment, follows the existing roadway alignment because this approach involves less construction risk, costs less than the tunnel alternatives, and will result in fewer impacts on the environment. In October 2008, FHWA issued a record of decision (ROD) based on the final EIS.
"The ROD was a significant milestone representing many years of working with communities and building a coalition of environmental interests, business groups, and project advocates around a plan to improve Snoqualmie Pass so that it is safer for drivers and more dependable for our economy," says WSDOT South Central Regional Administrator Don Whitehouse. "The overwhelming support for this project shows the importance I-90 plays in connecting our State."
Construction under the first contract of the Hyak to Keechelus Dam project began in summer 2009. In October 2009, WSDOT will advertise the next contract to begin replacing the old lanes and adding a new lane in each direction, rebuilding bridges, and extending chain-up/off areas along the first 2 miles (3.2 kilometers) of the project. (Chain-up/off areas are safe areas for motorists to pull off the interstate to put on and take off tire chains. WSDOT often requires motorists to put chains on their vehicles for maximum traction.) Construction on this phase will begin in summer 2010. By fall 2010, WSDOT will advertise the third contract, which continues adding new lanes in each direction, replacing the snowshed (a concrete shed used to provide permanent protection from avalanches and other falling debris to travelers passing through Snoqualmie Pass), addressing unstable slopes, building new bridges, and constructing new chain-up/off areas on the project's next 2 miles (3.2 kilometers). That contract is scheduled to begin construction in 2011.
To move design plans forward, WSDOT gathered geotechnical data from extensive drilling operations on the mountain slopes and in the lakebed. Findings indicated that certain areas of the project contained stable rock and favorable sediment, while others contained soft, fragile rock and liquefiable soil. Based on this information, WSDOT design engineers had to create plans for improving ground conditions for foundations, elevating the road profile to accommodate unstable slopes, and stabilizing rock slope cuts before and during construction.
"Liquefiable soil conditions result in global instability of structure foundations," says Scott Golbek, a WSDOT engineer with the I-90 Project. "In order to achieve acceptable seismic conditions for the foundations, WSDOT first has to improve the liquefaction-susceptible soils beneath the approach fills and abutments of the multispan Gold Creek bridges."
Soil improvements are slated to begin in 2010. Geotechnical engineers propose to improve liquefiable soils with compaction grouting. A very viscous (low-mobility), aggregate grout will be pumped in stages, starting up to 50 feet (15 meters) deep in places and working toward the ground surface. The grout will form a column of bulbs, which displace and densify the surrounding soils. The grout columns will be constructed on a grid pattern designed to suit load, soil, and seismic performance. Once complete, the grout columns will reduce foundation settlement, mitigate liquefaction potential, improve shear resistance, and increase bearing capacity.
WSDOT's investments in preliminary geotechnical work "will reduce the risk of encountering unexpected rock and soil conditions, which could lead to cost overruns during the construction phase," Golbek says.
Widening in a Narrow Corridor
A major purpose of the I-90 Project is to increase capacity by adding a lane in each direction. To realign and widen the interstate to six lanes, WSDOT engineers and contractors had to consider the effects of making additional rock cuts into zones of previously unstable fracture planes or weak rock present along the mountainous corridor.
"Rock conditions in volcanic terrains are highly variable and defy accurate characterization," says Norm Norrish, principal of Wyllie & Norrish Rock Engineers Inc., an engineering contractor involved in the project. "The new alignment of the interstate, particularly where slopes attain vertical heights in excess of 120 feet [36 meters], could be in jeopardy if faults; flow boundaries; ash layers; weak rock masses; thick, marginally stable soils; or other unfavorable geologic conditions are present."
To identify terrain conditions and design the realignment, WSDOT, the contractor, and geologists conducted state-of-the-art field investigations that included geotechnical drilling, downhole surveys, structural geologic mapping, and installation of instrumentation to measure ground water pressures and slope displacements. The team coupled data from these activities with laboratory testing, input the information into detailed slope stability analyses, and used historical performance of slopes to "calibrate" new designs wherever possible.
Field investigation findings identified a series of linear depressions across a troublesome slope aptly named Slide Curve. The fissure was nearly 4 feet (1.2 meters) across and more than 30 feet (9 meters) deep in places, indicating displacement of a large block of bedrock. Potential instability of these localized zones of poor quality rock mass required WSDOT design engineers to raise the westbound roadway grade up to 50 feet (15 meters) above the previously designed grade. This alteration will provide the dual benefits of reducing the size and extent required to cut slopes, while buttressing the existing, marginally stable slopes.
WSDOT and its consultants have identified a number of other potentially unstable slopes along the project corridor that will require stabilization as the rock cuts are excavated. Crews will stabilize these slopes with grouted steel bars designed according to the structural geology and height of each rock cut.
Operating Efficiently And Reliably
The project also seeks to reduce avalanche risks to the traveling public and minimize road closures required for avalanche control work. WSDOT will begin working toward this objective in summer 2011 with construction of a 1,100-foot (335-meter)-long concrete snowshed and avalanche fencing.
Five natural avalanche chutes regularly funnel snow onto the roadway in the project corridor. The existing snowshed, built in 1950, only protects the westbound lanes from the two most active of the five chutes, leaving portions of the interstate vulnerable to snow from natural avalanches and avalanche control measures. WSDOT must undertake extensive winter maintenance to keep this portion of the pass open.
The new snowshed will protect the entire width of interstate from four of the avalanche chutes with an enlarged ditch providing a catchment area for the remaining minor chute. Avalanche fencing, like that used extensively in the Alps, will protect the interstate from a smaller avalanche zone located at Slide Curve.
"The closures on I-90 Snoqualmie Pass can vary from year to year but average over 60 hours per year within the project area alone," says Giles. "When the snowshed is complete, we hope to reduce those closures by 70 percent."
WSDOT will construct the snowshed using standard bridge design techniques. A reinforced concrete wall on the mountainside and a pier cap over columns on the lakeside will support a concrete roof over precast girders.
"In addition to designing the snowshed around the area's unfavorable soil and rock slope conditions, weather limitations, and construction access, maintaining existing traffic levels during the construction phase will be challenging," Golbek says.
The Central Cascades experience extreme temperature fluctuations, severe seasonal rains, and heavy snowfalls that limit the construction season to about 7 months a year. WSDOT estimates that building the snowshed will require five seasons to complete, and engineers have devised a detailed construction phasing plan that enables crews to build the new snowshed around the existing one to protect the interstate. In the fourth year, WSDOT will remove the old snowshed and complete the new snowshed's roof.
Another variable in construction phasing is Keechelus Lake's water level, which varies by some 70 feet (21.3 meters) depending on irrigation needs and precipitation. Therefore, construction activities such as installing vertical anchors and setting precast girders will follow the receding lake water levels downslope throughout the summer and fall of each construction year. Keechelus Lake is one of several irrigation reservoirs in a system where water rights are overallocated, so WSDOT must be careful not to affect lake storage levels. WSDOT has a no-net-loss commitment with the U.S. Department of the Interior's Bureau of Reclamation, for which WSDOT agreed to excavate materials from the lake to allow for expansion of I-90. The agency will recycle the excavated materials, mostly gravel and soil, back into various phases of the project.
Staging construction and keeping traffic moving through the narrow project work zone is another challenge. To mitigate effects on the traveling public, WSDOT has committed to keep two lanes open in each direction during peak travel times, with single-lane closures during offpeak times. The department will employ an extensive communications program to inform the public about construction, using its Web site, traffic cameras, highway advisory radio, variable message signs, and phone hotline. Further, WSDOT created a detailed traffic management plan to establish detours for the public and emergency vehicles during construction.
Improving ecological connectivity (defined as continuity between neighboring habitats and ecosystems and their flora and fauna) and preserving habitat also are critical components of the I-90 Project. The large areas of protected Federal, State, and conservation lands north and south of the project corridor support a broad range of habitats. I-90 itself severely limits wildlife movement and forms a physical barrier between upstream and downstream aquatic environments. Culverts and narrow bridges limit movement of aquatic species, and in many cases the highway embankment has filled in habitat that once made up channels, floodplains, and associated wetlands. In addition, wildlife that cross the interstate at grade represent a safety concern to the traveling public.
To establish ecological objectives during the NEPA and State Environmental Policy Act scoping phase of the project, WSDOT partnered with the U.S. Forest Service and conservation groups. These entities already were working to acquire conservation land through land exchanges and purchasing much of the remaining private property along the I-90 corridor to preserve the delicate ecosystems of the Central Cascades.
To ensure that the I-90 Project's objectives align with the actions of the other groups, the project's mitigation development team worked with WSDOT design engineers and environmental planners to advise the interdisciplinary team of locations along the project corridor that were suitable for investments in infrastructure and restoration activities. The mitigation development team's strategy considered landscape-, watershed-, and habitat-specific variables to identify connectivity emphasis areas. The teams used these recommendations to develop a comprehensive list of connectivity objectives and performance standards for evaluation of design options.
WSDOT worked with many agencies that manage land and resources in the project area through small, focused technical advisory teams that helped the department gather data on target species, habitat needs, and project constraints. WSDOT then used the data to help evaluate bridge and culvert designs that improve wildlife connections, stream channel migration, fish passage, and habitat. The department plans to begin building these improvements in spring 2010. The engineering team also is testing various prototypes of wildlife exclusionary fencing to develop a design that will withstand winter weather conditions and maintenance operations, and direct wildlife to crossing structures that enable them to pass safely under or over the highway.
"The I-90 Project's overarching wildlife connectivity objectives are to improve motorists' safety by minimizing the risk of wildlife-vehicle collisions, and to improve the ecological permeability of the highway for fish and wildlife," says Jason Smith, an environmental manager with the WSDOT South Central Region. "Objectives will be accomplished, in part, by installing crossing structures and fencing that will allow wildlife to safely cross over and under the interstate, and by constructing bridges and culverts that help restore hydrologic processes and fish passage through the interstate."
As WSDOT constructs new bridges and culverts, it will restore wetlands, stream channels, and riparian areas at the connectivity emphasis areas, which includes forested habitat, buffer improvements, and highway slope revegetation. WSDOT also acquired a 265-acre (170-hectare) property for habitat preservation in the Gold Creek valley that contains wetlands, riparian areas, and mature forest, including potential habitat for northern spotted owls, marbled murrelets (small birds), and bull trout, all listed for protection under the Federal Endangered Species Act. WSDOT committed to preserve this property in perpetuity.
To measure the effectiveness of the investments in ecological connectivity and evaluate fulfillment of the objectives, WSDOT partnered with the Forest Service, the Western Transportation Institute, and Central Washington University on a long-term, multiphased monitoring program that will yield scientific data regarding pre- and postconstruction wildlife activity.
Preconstruction wildlife monitoring objectives include quantifying existing rates at which various species cross the highway, assessing the rate of wildlife-vehicle collisions, and surveying the project area to evaluate species occupancy and distribution. Monitoring efforts have been underway since spring 2008 and include using remote cameras to assess wildlife use of existing culverts and underpasses, using snow tracking to document crossing rates, and employing noninvasive survey methods and live capture to evaluate the distribution of various target species.
"The program addresses not only large, wide-ranging species, but also lower mobility species such as amphibians, small mammals, and fish," says Smith. "Further, the monitoring program emphasizes a multitiered approach that will permit exploration of basic performance standards, as well as larger scale questions of wildlife connectivity. The systematic collection of pre- and postconstruction wildlife data will help with the design of highway improvements in later stages of the project and on projects occurring elsewhere."
The Future of I-90
The first 5 miles (8 kilometers) of the I-90 Project from Hyak to Keechelus Dam is slated for completion in 2015. WSDOT will continue roadway improvements on the remaining 10 miles (16.1 kilometers) of the project area from Keechelus Dam to Easton as funding becomes available. Construction of the remaining portion of the I-90 Project is estimated to cost between $700 million and $800 million, in 2009 dollars, and could require 7-15 years to complete.
In the meantime, WSDOT will work with its partners and use open communication with the public to ensure the continued viability of I-90 as a primary statewide and national corridor. "WSDOT, through the I-90 Project, will keep Washington moving by operating efficiently, adding capacity strategically, and managing demand," says Brian White, assistant regional administrator of project development for WSDOT's South Central Region.
Amanda Sullivan is an associate with public relations firm PRR and serves as a colocated communicator for WSDOT's I-90 Project. She has a bachelor's degree in public relations from Central Washington University.
Amy Danberg is a senior associate with PRR and serves as communications manager for the I-90 Project. She has a bachelor's degree in political science and a public relations certificate from the University of Washington.
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