AKNPSDENA_Preliminary_Design Narrative.pdf

AK NPS DENA 10(49), Polychrome Area Improvements Design Narrative: 30% Design Milestone

Design Narrative Contract No. 69056721D000007, Task Order No. 69056722F000017

Design Milestone: 30%

DRAF

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Table of Contents Introduction

Major Revisions from Previous Submittal

Survey and Staking

Environmental

Traffic Data

Geotechnical

Previous Geotechnical Reports and Submissions

Geology and Project Site Conditions

West Abutment Subsurface Conditions

East Abutment Subsurface Conditions

Climate

Geotechnical Design Parameters

Bridge Abutment Design

Permafrost

Perlite Wall

Rock Cut Slope

Rockfall Hazard

Disposal of Cut Material

Erosion and Sediment Control

Roadway Design

Bridge Design

Feasibility of Launched Single Span Bridge

Alternative Bridge Construction Methods

Arch Structure

Truss Structure

Progressive Cantilever Construction (PCC)

Truss Geometry

Truss Elements

Element Sizing

Structural Member Transportation

Floor System

Abutments ................................................................................................................................. 27 DRAF

Project Standards

Temporary Traffic Control

Landscaping

West Rock Cut Turn Area

Deep Patch Removal

Thermosiphon Area

Vegetation

Right-of-Way

Construction Schedule

Construction Cost Estimate

Appendix A: Environmental Commitment Summary

Appendix B: Construction Costs – Basis of Estimate

Appendix C: Design Standards

Appendix D: Plans and Cross Sections

Introduction The Western Federal Lands Federal Highway Administration (FHWA) is intending to complete the design of the Polychrome Area Improvements project (Project).

The Partner Agencies consists of FHWA and Denali National Park & Preserve (DENA).

The proposed project consists of construction of a bridge over the active portion of the Pretty Rocks Landslide. The rock mass below the bridge abutments may require reinforcement to ensure the global stability of the bridge abutments will meet minimum acceptable standards. Rock cuts are anticipated to facilitate construction of the bridge and the associated roadway realignment. A cut-side retaining structure and drainage elements is anticipated to mitigate the Perlite Landslide at the eastern bridge approach.

The project will be advertised as a Construction Management/General Contractor (CM/GC) procurement with a construction contractor assisting in design and a construction contract. The project is currently slated to be complete by Spring of 2025.

The FHWA is providing assistance to the Denali National Park (DENA) and the Alaska Region (AKR) of the National Park Service (NPS) to improve the resilience of the existing Denali Park Road through Polychrome Pass in Denali Borough, Alaska. Landslide movement at the Pretty Rocks Landslide is impacting approximately 350 feet of the Denali Park Road at about MP 45.3.

Road subsidence at this location, because of the continued landslide movement of the Pretty Rocks Landslide, has increased to such a rate that it has become difficult to maintain the roadway through the summer season by Park maintenance crews. In addition to the Pretty Rocks Landslide, the Perlite Landslide, located directly east of the Pretty Rocks Landslide and above the Denali Park Road, poses a danger to the roadway.

The Park Road is the primary access into the Park and is 92 miles long, ending in Kantishna to the west. Landslides, including but not limited to the Pretty Rocks and Perlite Landslides at MP 45.3 and the Bear Cave Landslide at MP 44.8, have begun to more quickly deteriorate portions of the Denali Park Road along Polychrome Pass.

In August of 2021 DENA closed the Park Road west of the East Fork of the Toklat River Bridge because the rate of land sliding movement increased to a point where it was no longer safe or feasible to maintain the Road through the Pretty Rocks Landslide. This closure occurred several weeks prior to the typical winter shutdown.

Major Revisions from Previous Submittal N/A

Survey and Staking

N/A DRAF

Environmental Environmental commitments are based on the measures set forth in the Environmental Assessment (EA) issued by the National Park Service (NSP) and adopted by the Federal Highway Administration (FHWA), and the subsequent Finding of No Significant Impact (FONSI) issued by the FHWA for the Polychrome Area Improvements. Commitments were determined through a process of public outreach, consultation with stakeholders and agency coordination. Factors taken into consideration for the development of the environmental commitments include social and cultural (including socioeconomic, visual resources, noise and soundscapes, and historic and archaeological resources), wildlife, wilderness areas, and wetlands and vegetation. The necessary permits will be secured by a third-party contractor, with oversight from FHWA and NPS.

The Park Road currently provides the only access into DENA, with views of the Toklat River, Wonder Lake, and Kantinshna. Polychrome Overlook is the nearest overlook to the Pretty Rocks area and the project area is partially visible from there. Measures to limit visual and audio impacts include limiting the number of visitors into the park during construction, natural contouring, placement of materials in locations that minimize contrast, and the use of natural materials that blend with the existing natural surroundings.

Potential impacts to birds would be in compliance with the Migratory Bird Treaty Act, and a permit for nest disturbance from increased road activities associated with construction was obtained from the U.S. Fish & Wildlife Service on account of nesting golden eagles in the immediate vicinity.

Impacts to other wildlife include the temporary displacement of small and large mammals, including Dall sheep, bear, and caribou, and not anticipated to be long term.

The Park Road is itself a historic site and a contributing element to the Mount McKinley National Park Road Historic District. Mitigation measures to avoid and minimize impacts to the historic district and cultural landscape (including visual effects that could be detrimental to the historic character of the sites) were developed in consultation with the State Historic Preservation Officer (SHPO), NPS, and additional public consulting parties. The resulting Memorandum of Understanding (MOA) was developed in the spring of 2022 and signed by NPS, SHPO, Ahtna Incorporated, Doyon Limited, Tanana Chiefs Conference, and the Denali Borough. Mitigation measures include conducting oral history interviews, the production of a video about the history of the park, collection and curation of surface artifacts in the project area, and cultural resource monitoring for project activities.

The Environmental Commitment Summary is included as Appendix A.

Traffic Data The Park Road is a low-volume rural roadway with less than 200 vehicles per day, and less than 100 vehicles per day at the project site. Traffic is controlled by seasonal limit of 17,600 vehicles past Mile 15. Traffic consists primarily of visitor buses with a small mix of Park Service vehicles and maintenance and construction traffic. The roadway is open to public traffic for a short period in the fall and is limited by lottery permit to 400 vehicles per day. DRAF

Design vehicles for determining roadway horizontal and vertical geometry include a truck and trailer (WB-67) and a truck with a lowboy trailer (Rogers CR50). These vehicles are referred to in the roadway design criteria in the Project Standards.

Geotechnical The geotechnical components of the project include the following:

Foundations of the bridge spanning Pretty Rocks Landslide.

A rock cut slope on the western side of the Pretty Rocks Landslide.

Rockfall protections of the roadway in the zones where engineered rock cuts are required for the project.

Scaling of rock cut slopes.

Mitigation of the Perlite Landslide at the east side of Pretty Rocks Landslide via design of a wall.

Mitigation of permafrost thaw at the eastern bridge abutment.

Previous Geotechnical Reports and Submissions

Western Federal Lands Highway Department (2020, March 10). Geotechnical Memorandum 10-20 (WFLHD GM 10-20): Pretty Rocks Landslide Rockfall Analyses, Pretty Rocks Landslide Repair AK NPS DENA 10(50), WFLHD Geotechnical Section.

Western Federal Lands Highway Department (2020, March 23). Geotechnical Memorandum 03-20 Revised (WFLHD GM 03-20): Pretty Rocks Landslide Bridge Feasibility and Constructability, Pretty Rocks Landslide Repair AK NPS DENA 10(50), WFLHD Geotechnical Section.

Western Federal Lands Highway Department (2020, April 20). Geotechnical Memorandum 14-20 (WFLHD GM 14-20): Pretty Rocks Landslide Earthwork Feasibility and Constructability, Pretty Rocks Landslide Repair AK NPS DENA 10(50), WFLHD Geotechnical Section.

Western Federal Lands Highway Department (2020, August). Geotechnical Report No. 11- 20 (WFLHD GR 11-20): Pretty Rocks Landslide 2018-2019 Geotechnical Investigation and Conceptual Design Alternatives Report AK NPS DENA 10(45), WFLHD Geotechnical Section.

BGC Engineering USA Inc. (2020, August 20). Geotechnical Report 05-20, AK NPS DENA 10(49), Geotechnical Summary Report of Existing Conditions. Prepared for Western Federal Lands Highway Division, Federal Highway Administration, FHWA.

BGC Engineering USA Inc. (2021, May 19). Geotechnical Memorandum 14-21, AK NPS DENA 10(49), Polychrome Area Improvements – Preliminary Geotechnical Memorandum. Prepared for Western Federal Lands Highway Division, Federal Highway Administration, FHWA. DRAF

BGC Engineering USA Inc. (2022, January 13). Geotechnical Report 01-22, AK NPS DENA 10(49), Polychrome Area Improvements – Geotechnical Data Report – Draft.

Prepared for Western Federal Lands Highway Division, Federal Highway Administration, FHWA.

BGC (BGC Engineering, Inc.). (2022 March 29). Geotechnical Report 01-22 AK NPS DENA 10(49): Polychrome Area Improvements Geotechnical Data Report FINAL_Rev 1.

Western Federal Lands Highway Division; Federal Highway Administration.

These reports provide the basis for and, where in apparent conflict, take precedence over the summary discussion in this geotechnical section and throughout this document.

Geology and Project Site Conditions

The Pretty Rocks Landslide is located near an area known as Polychrome Pass in Denali National Park and Preserve in the Central Alaska Range. The landslide is on a south-facing slope above a primary tributary of the East Fork Toklat River. The Denali Park Road (the road) traverses the mid slope of a mountainside as it passes through the landslide at an elevation of approximately 3,620 feet. The length of the Pretty Rocks Landslide is approximately 1,535 feet and the width ranges from approximately 350 feet at the Denali Park Road, approximately 245 feet at the mid slope, and approximately 635 feet at the toe area. The landslide elevation ranges from a scarp elevation of about 3,700 feet to a toe elevation of 3,240 feet. A smaller slide, known as the Perlite Landslide, intersects the road east of the Pretty Rocks Landslide and, to-date, has required maintenance activities including the removal of debris after episodes of material movement. Historically, this landslide activity has been primarily above the road.

Polychrome Pass is located in the Teklanika Formation which is mechanically influenced by varying degrees of weathering, strength properties, bedrock structure and permafrost. In general, layers of rhyolite and perlite dip to the east with intrusions of basalt present on the western abutment side. The intrusion of basalt hydrothermically altered and completely weathered a layer of rhyolite that now forms the Pretty Rocks Landslide.

The Pretty Rocks Landslide is a rock glacier where debris from a source zone falls onto a lower slope angle region. Material release from the source zone is likely related to ground ice conditions and permafrost degradation within the completely weathered high plastic clay. The middle and lower section of the slide consist of permafrost material which continues to creep over time. The slide deformation has accelerated exponentially from the first movement noted in the 1980s.

West Abutment Subsurface Conditions

At the planned west abutment location, the rock is generally basalt with highly plastic clay infill in joints and clay seams present within the existing western rock cut slope above the roadway. The presence of permafrost was confirmed via ground temperature measurements in BGC-PR21-06 in the ridgeline above the roadway. Permafrost was not observed in boreholes drilled to explore subsurface conditions below the road elevation. DRAF

East Abutment Subsurface Conditions

The eastern abutment is located on a layer of rhyolite overlying the completely weathered rhyolite, which has a highly plastic clay consistency. Ground ice observations and ground temperature measurements in PR19-07 and PR21-01 confirmed the presence of permafrost at the eastern abutment. The ground ice, approximately 1-foot thick, is present between the rhyolite and the completely weathered high plastic clay.

Climate

The project design considers climate change. The base climate used in the analyses are CMIP5 or CMIP6 global climate models using RCP8.5 or SSP5-8.5, respectively, or equivalent. Thermal modeling at the east abutment shows degradation of the permafrost at depth under this climate model by 2100 (BGC, 29 March 2022).

Geotechnical Design Parameters

The geotechnical design parameters and profiles have been selected based on current and expected future site conditions and are informed by the results of the geotechnical investigations, instrumentation data, geological understanding and engineering experience.

Bridge Abutment Design

The abutment foundations consist of micropiles which have a ground bond strength of 10 kips per square foot (ksf) of bond area. The structural design (discussed in a different section) has considered nineteen (19) micropiles oriented in two rows with vertical piles in the front row and back row piles battered in both directions at each abutment. Given the possible change or retrogression of the scarp in front of the abutments on either side, the micropiles closest to the scarps have a vertical orientation.

Additional work is needed to determine:

the internal, external and global stability to meet 1.5 FoS for static conditions and FoS of

1.1. for pseudo-static conditions, including kinematic analysis of discontinuities, in the following directions:

o Along centerline, o Abutment fall line, and o Unfavorable jointing directions seismic hazard, permafrost degradation, and initial and long-term settlement of the abutment area.

Continued evacuation of the material in front of the east abutment is a concern for the design. This is being addressed in four ways. First, thermosiphons, which are discussed in the Permafrost section, are being used to mitigate potential warming and thawing of permafrost that would accelerate the evacuation of material from in front of the abutment. Second, an additional row of DRAF battered micropiles is being included to ‘knit together’ the rock strata by crossing nearly perpendicular to the rock structure and extending out beyond the bridge girders for aiding in global stability. Third, an apron of patterned soil nails and sculpted shotcrete facing is being included beneath the bridge deck and in front of the abutment. And, fourth, a sleeve is being cast into the precast abutment sections that will allow for a future installation of ground anchors or other foundation elements, should the effects of the changing climate and continued slide movement be greater than anticipated.

Monitoring of the site post construction should include routine visual and topographic survey of the ground and survey of the bridge to assess potential future changes. Recommendations for maintenance and thresholds for corrective action will be created.

Permafrost was not detected on the west abutment, and the geologic structure is less adverse, so no thermosiphons are planned. The other measures of battered micropiles, soil nails and cast sleeves for potential future installations are included for the stability of the west abutment.

Adaptive designs, such as the cast sleeves for potential future anchor installations, will be considered for future climate conditions as the design is developed. Design parameters will be re-evaluated after 2022 site investigation has been completed.

Permafrost

Approximately 85 feet below existing ground surface at the east abutment is an approximately 1-foot layer of ice-rich clay. A three dimensional (3D) thermal model forms the basis of these recommendations (BGC, 2022, March 29). The thermal design is based on keeping the ice-rich clay material at a low enough ground temperature to maintain the majority of the clay’s moisture content in a frozen state at approximately 28°F.

Approximately 24 thermosyphons are recommended at the east abutment as shown in the drawings. These thermosiphons should consist of 3-inch diameter steel pipes, pressurized with CO2 and a condenser section area of approximately 170 square feet at a minimum. Drill holes for installation will be vertical for the evaporators and backfilled with grout or another thermally conductive material. The working (heat removal section) section of the thermosyphons is recommended between 3,570 feet and 3,520 feet elevation. All the condensers are planned to be located in a group on the outboard side of the eastern bridge abutment. A sloped pipe section will connect the evaporators and their condensers; there should be a 2% down slope from the condenser to the evaporator. Thermosiphon effectiveness should be monitored routinely via thermal cameras in the winter months or heat flux sensors.

Ground temperature monitoring is planned in the region of the east abutment thermosiphons to monitor the ground temperatures of the clay permafrost at depth. Thermistors with varied spacing focused on the ground temperatures within the working region of the thermosiphons. Details and locations will be planned with the design. DRAF

Perlite Wall

At the Perlite landslide, a Mechanically Stabilized Earth (MSE) retaining wall faced with rock filled gabion baskets and a timber façade, and using welded wire fabric as reinforcement, in general accordance with WFLHD Design Detail 253 is planned. The geometry of the wall includes a facing slope of 1H:6V to a height of approximately 14 ft above finish grade at toe of wall. A backfill slope is planned from the top of the wall at a 10H:1V slope to a maximum elevation of approximately 3,647 feet elevation. This relatively gentle slope will form a platform at the elevation of the top of the wall and it will allow for the accumulation of slide material.

The buttressing of the slope and the storage of slide debris are two ways that the perlite wall will mitigate the hazard at the Perlite slide. At some future time, it is likely that slide debris will have filled the platform at the top of the wall and will form a slope angle of approximately 1.5H:1V.

This apron of slide debris will, in itself, buttress the higher portion of the slope, though it will be only marginally stable. As a result, it is expected that some slide debris will overtop the wall periodically and will need to be cleared from the ditch. The ditch in front of the wall has added width to catch debris and lessen the chance any debris reaches the travel lanes.

It is planned to use high porosity cobble material in the gabion baskets (gabion rock) to facilitate air convection within the pore space during the winter months, increasing the heat removal from the in-situ material and possibly preserving the presence of permafrost within the Perlite subsurface materials. These materials will be free draining but may require layers of graded material or geosynthetic separation fabric or geogrid to prevent infilling of the pore space over time. Gabion baskets will be turned endwise from typical construction or built two rows thick to increase the volume of air circulation within the structure. Behind the gabions, backfill will be specified for strength properties, and the wall will be designed for a presumed future case where slide debris forms a 1.5H:1V surcharge. Standard drainage details will be included behind the reinforced backfill near the bottom of the wall.

The gabion rock may be sourced from the Park’s existing material; however, material need to be sourced outside the Park to meet the specifications.

Wall types considered but ruled out include sheet pile, soldier pile, timber walls, and reinforced concrete retaining walls.

Rock Cut Slope

A rock cut slope is planned at the western side of Pretty Rocks Landslide. The slope is planned at 0.5H:1V. A kinematic review of the available stereonet data has been completed.

Construction should be completed from the top down. Based on current understanding, blasting is likely required for construction of the rock cut. Geologic conditions within the rock cut area will be determined in the 2022 geotechnical investigation.

A benched slope was considered but ruled out due to concerns maintenance could not be completed without connection to the roadway. DRAF

Rockfall Hazard

Rockfall catchment criteria specified by FHWA previously for this project required a rockfall catchment area design for rockfall impact of 99% and rockfall rollout of 75% at the west abutment.

These requirements are achievable for 0.5H:1V slopes up to 80 feet tall, with a 19 feet wide ditch, per the Rockfall Catchment Area Design Guide (Oregon Department of Transportation, 2001);

however, slopes on the west rock cut reach a maximum of 110 feet tall. Slopes greater than 80 feet tall are not covered in Rockfall Catchment Area Design Guide, so “impact” and “rollout” values for 90 feet, 100 feet, and 110 feet tall slopes were preliminarily estimated by extrapolating the published slope heights. Values for the 99% Impact Ditch Width, which controls the ditch width design, were confirmed using a calibrated model using the RocScience RocFall modeling program.

Results of the analysis are shown in Table 1.

Table 1. Ditch width requirements per the Rockfall Catchment Area Design Guide (Oregon Department of Transportation, 2001).

0.5H:1V Slope Height (ft)

99% Impact Ditch Width

(ft)

90% Impact Ditch Width

(ft)

75% Rollout Ditch Width

(ft) 40 6 4 9

50 11 8 11

60 15 11 13

70 17 12 14

80 19 13 14

901 221,2 151 161

1001 241,2 171 161

1101 261,2 181 171

Notes:

1. Slopes greater than 80 feet tall are not covered in the Rockfall Catchment Area Design Guide. Impact and rollout values for 90 feet, 100 feet, and 110 feet tall slopes were estimated by extrapolation.

2. Values for 99% impact ditch width were confirmed using a calibrated RocScience RocFall model.

The analysis shows that to meet the 99% impact criteria for a 110 feet tall slope, the ditch would have to be approximately 26 feet wide. DENA has indicated in the past that they are concerned with this the type of impact that such a large ditch represents, the use of mesh to lessen effective fall heights and the use of barriers to limit rollout, thus the 30% design is based on a transitioning ditch from 12 to 19 feet that meets the past criteria (99% impact, 75% rollout) for most of its surface area and a lesser criterion (90% impact, 75% rollout) for the portion of the slope that is higher than 80 feet tall (and the ditch is 19 feet). The appropriate criteria and the mitigation of risk from rockfall will be addressed further at subsequent design development stages.

The rockfall catchment ditch on the west side of the bridge varies in width from 12 to 19 feet as follows:

107+80 to 107+90 – 12 ft ditch 107+90 to 108+10 – Transition from 12 ft to 19 ft 108+10 to 109+40 – 19 ft ditch DRAF

109+40 to 109+60 – Transition from 19 ft to 12 ft 109+60 to POE 112+90 – 12 ft ditch

Rock scaling will be required in the regions identified on the plans. The identified areas will include the newly exposed face of the western rock cut and approximately 40 above existing roadway elevation at the eastern abutment region. Scaling will be light with the goal of removing large boulders and material perceived to pose an eminent threat to fall. Note degradation of the permafrost within the western ridge may result in higher rockfall rates the first few years following construction as a new thermal equilibrium forms. . The appearance of the rock cut consistent with the natural slopes should result.

Given the unknowns associated with the rock jointing and condition of the rock within the western rock cut, rock reinforcement in the form of rock bolts has been included in the project quantities.

The expectation is that controlled blasting techniques and scaling will be the primary means of constructing a stable slope, but non-tensioned rock bolts (dowels) may be required in certain locations. An estimated quantity of 50 20-feet rock bolts has been included.

Disposal of Cut Material

Spoil material from the rock cut, not used in temporary construction support or at the end of the project, is planned for disposal on the Pretty Rocks Landslide in the regions defined on the plans to the depths noted in Table 2. The final material placement configuration should have a greater quantity of material in Zone 3 compared to Zone 1 and be graded to allow drainage. The side slopes of the material should not exceed 2H:1V. Grading to match the surrounding topography may be needed.

Table 2. Spoil Disposal Zones and Fill Thicknesses.

Zone Number Maximum Fill Depth (ft)

Zone 1 35

Zone 2 20

Zone 3 60

The spoil disposal Zone 3 has been updated to remove regions of identified wetlands from the regions of allowable spoil disposal area. If possible, spoil placement should avoid the wetlands identified in the Environmental Assessment documentation.

Erosion and Sediment Control Roadway drainage is collected in roadside ditches and conveyed through culverts to downstream outfalls. This project shortens the length of roadway to be maintained temporarily during DRAF construction and permanently by constructing a bridge and abandoning the roadway over the slide area. Erosion and sediment controls will primarily include ditch controls to manage and direct runoff away from the slide area and protections at each outfall location.

Coverage under the Clean Water Act Section 402 National Pollution Discharge Elimination System permit will be required, necessitating the development of a Storm Water Pollution Prevention Plan (SWPPP). Both FHWA and the contractor are, by definition, "operators" under the permit and, therefore will each need to obtain permit coverage. FHWA and the contractor will partner in the development of the SWPPP. The contractor will be responsible for the day-to-day operational control over activities in the field, including those being performed by subcontractors.

These activities include installing, maintaining, inspecting, and correcting all erosion and sediment controls and keeping the SWPPP current.

Permanent soil stabilization will be in the form of rock blankets using rock material generated from the project.

Roadway Design The proposed roadway realignment has been developed to connect the existing road to the proposed bridge while minimizing rock cuts and walls. Design criteria have been selected to match the existing road features to the extent feasible with today’s road standards. Proposed roadway dimensions include 9-foot lanes and a 3% cross slope. The roadway cross slope will transition to match the bridge cross slope of 2%.

At the west bridge approach, the proposed roadway has a 12-foot interior ditch with 0.5:1 rock cut slope. The total excavation from the rock cut is anticipated to be 25,000 cubic yards. The exterior slope will match into the existing roadway slope. Portions of the existing roadway will be rehabilitated. At the east bridge approach, the proposed roadway has a 6-foot interior ditch with a bin wall to stabilize the Perlite slide area. Similar to the western approach, the exterior slope will match into the existing roadway slope.

Roadway design criteria can be found in the attached Highway Design Standards form.

Drainage design will be further developed for the next design milestone.

Bridge Design

Feasibility of Launched Single Span Bridge

An evaluation of the feasibility of a launched single span bridge was performed per section I.C of the project scope of work. Launching of a bridge structure can be used to refer to several construction methods.

“Incremental Launching”, is a method where a portion of the superstructure is constructed on the ground behind the abutment in sections which are then pushed with hydraulic jacks so that it cantilevers longitudinally from the abutment until it reaches a second support. In essence the bridge DRAF superstructure is extruded from behind the abutment in small increments. The superstructure must be long enough to have enough weight behind the abutment to counteract the weight of the cantilevering portion of the superstructure. This requires enough available construction staging area behind each abutment to build the counterweighting portion of the bridge.

Figure 1: Incremental Girder Launching

For incremental launching to be viable, the bridge must have multiple spans and a constant vertical profile grade and horizontal (tangent or constant curvature) alignment. It is common to build incrementally launched bridges using temporary intermediate piers between final piers and abutments to effectively make the bridge multi-span during construction. However, with the Pretty Rocks landslide occupying the area between the abutments, intermediate piers would be too risky.

Therefore, traditional incremental launching is not considered feasible for the single span Polychrome bridge.

An adaptation of the Incremental Launching method was considered. It requires at least two girders stable for lateral torsional buckling (box girders) for the bridge cross section. As shown in Figure 2, the method would require ½ of the first girder to be temporarily spliced to ½ of the second girder making a complete span length section. An additional counterweight would be attached to the trailing end of each girder. Cranes located at each abutment would be required to handle the individual fabricated sections delivered to the site. These would be constructed behind each abutment and launched to mid-span where the two (2) girder sections would be spliced together.

The counter-balancing tail sections (Girder 2) would be detached, lifted to the top of the first girder and rolled out to midspan. While being supported by Girder 1, the two (2) sections would be spliced together to complete Girder 2. Finally, the cranes at each abutment would lift Girder 2 from Girder 1 and lower it to its final position on the bearings. The deck sections would then be placed to complete the bridge. DRAF

Figure 2: Potential Girder Launching Scheme at Pretty Rocks Slide Bridge Site

For the proposed span range of 400 feet to 450 feet, the girder depth would be 16 feet to 18 feet which would require fabricating the sections with a full-length longitudinal web field splice. It would also require a minimum of 250 feet behind each abutment for assembling the sections for launching. This will be a challenge at the west abutment, where it would require substantial rock excavation to create the space. These issues make the method infeasible.

When girders are launched for a single span structure it is often referred to a process as “Girder Passing” and involves two cranes and either a winch or other piece of equipment capable of pulling or pushing the girder longitudinally. Girders are delivered to the back of one abutment. The two cranes are situated behind each abutment. Each has the capacity to lift ½ of the girder weight at a lifting radius equal to ½ the span length. The first crane is used to lift the leading end of the girder while it is pulled or pushed out into the span. As the girder moves, the crane swings and booms DRAF out until the girder gets to mid-span, where the second crane’s hook is waiting to connect and lift the load. After load transfer the second crane swings and booms in while the girder continues to be pulled or pushed into its final position. The limiting factor for this method is span length and crane lifting capacity. This method has been used successfully for spans in the 150-foot range and is not feasible for use on the Polychrome bridge where the span will be in the 400 feet to 450 feet range.

Alternative Bridge Construction Methods

While the topography of the site is like a steep canyon, there is no ability to use supports within the landslide zone, thus limiting the available span arrangement to a single span. The 400-foot to 475-foot length of the clear span severely limits the available structure types and construction methods. The subarctic climate and remote location favor the use of prefabricated steel for the material. As already mentioned, steel plate girders would require extensive earthwork and very large cranes. Cable-stayed bridges have not typically been used below spans of 650 feet and are also almost exclusively at least 3-span bridges. For the required span range, arches and trusses are the most logical solutions. Both use structural elements that are relatively small dimensionally, making them easier to transport to the site and require smaller cranes for lifting.

Arch Structure

Most often in canyon like settings, a deck arch is the preferred option. Typical arch proportions use a rise to span ratio of 0.25, putting the base of the arch approximately 100 feet below the roadway level. At that elevation the foundations would be well within the landslide limits, rendering deck arches impractical. A through arch can either resist the arch thrust in the foundations or through a tie girder.

Arch construction often uses temporary towers and cable stays to support the arch ribs as they are built progressively from the main supports. Because the arch is a slender member primarily loaded axially in its final condition, many cable stays are required to reduce the bending stresses developed during construction as shown in Figure 3.

Figure 3: Conventional Arch Bridge Erection

As seen in Figure 3, the height of the tower structure is about ½ of the arch span length (225 feet) above the spring line. The temporary towers and cable stays represent a large material use and cost, but they are disposable items. Fabricated steel arch rib sections must be erected with conventional cranes or with a highline crane system. Conventional cranes would need a reach of DRAF over 250 feet and are likely impractical to be shipped and assembled at the site, particularly behind the west abutment. Highline crane systems would also have to be taller than the temporary towers to facilitate erection of the cable stays.

A unique arch erection method was also considered where the arch halves were constructed at the abutments in a vertical orientation, then rotated into final position as shown in the following figures.

Figure 4: Arch Rib Erection by Lowering Into Place

Although this is shown for a concrete deck arch, the method could also be used for a through steel arch. A single crane is required at each abutment to lift arch sections into the vertical position.

Lowering of the arch uses a single backstay cable system. The single cable is necessary for the lowering process, and this would require a much larger arch rib section to resist the larger bending stresses that will occur. The height of the crane boom would be approximately 250 feet and may be impractical for the site.

During investigation of various arch construction schemes, it became apparent that there were many obstacles to overcome, and attention was then focused on a truss solution.

Truss Structure

The bridge type and construction method that was selected to progress design is a truss built in a progressive cantilever fashion. The final product is a through truss, in a Warren Truss configuration, spanning approximately 475 feet and carrying a single lane of traffic on a steel deck.

This is built by constructing the truss from each end, progressing out from the abutments. DRAF

Figure 5: Proposed Pretty Rocks Slide Truss Bridge

Progressive Cantilever Construction (PCC)

The progressive cantilever method is ideal for a situation where crane access is limited, length behind either end of the crossing is limited, and only one span is required to be built - all conditions that exist here. No large cranes are required, nor are lengthy launching pits, nor even large, prefabricated pieces that might be difficult to deliver to or assemble on site. Instead, the bridge is built one element at a time, from its leading edge.

The process begins with the construction of the abutments (the abutments discussed in greater detail later in this document). From the abutments, the first bay of the truss is built. This is connected and restrained translationally at the abutment and is restrained at its top by the attachment of tiebacks which are anchored into competent material (rock) behind the bridge.

This system maintains stability of the bridge under construction by restraining it at two points (the abutment at bottom and the tiebacks at top). It also allows for adjustments to the bridge geometry by pulling on the tiebacks to change the location of the tip of the cantilever.

Once this system is installed and the bridge construction has begun, the bridge can be continuously assembled on its leading edge. This extends the bridge further and further out to the center of the span. To facilitate piece delivery, each “stick” of the truss can be delivered and set individually in place. The construction continues from both ends of the bridge until they have reached the final bays at the center of the span.

Figure 6: Pretty Rocks Slide Truss Cantilever Erection Scheme

At this stage, final adjustments can be made to the positioning of the bridge through lengthening or shortening the tiebacks attached to the top of the truss. After this is complete, the closure pieces DRAF are put into place and connected across the gap between the two, cantilevered elements. The closure staging involves temporary or adjustable connections to allow for misalignments and resetting of bridge geometry to ensure that the connections can be made and that the final bridge structure has the desired shape and camber.

Once the closure elements have been placed and the final connections are installed, the required construction equipment can demobilize from the bridge and the tiebacks can be fully destressed and removed. Once this is done, the abutments can be released for horizontal restraint (required to resist the tieback forces) and the bridge will adjust to a simply supported condition.

At this point, the final elements can be added to the bridge (such as traffic barriers) and the bridge will be ready to begin its service life.

This type of truss erection was utilized for the construction of the 350-foot long Southbound SR 539 Bridge over the Nooksack River in Washington State in 2009.

Figure 7: Nooksack River Bridge Under Construction

Figure 8: Nooksack River Bridge Erection Detail

Truss Geometry

This is a straight alignment (no skews or curve) requiring only a single span with no approach spans. The geometry provided here is a result of the 30% design effort and may be changed and refined as the project continues into final design. The total clear length used for preliminary sizing and design of the truss is 475 feet. This is sufficient to span between two locations of more competent rock subsurface, and the length and location will reduce complexity in the roadway alignment (see the roadway documents for further discussion). The vertical alignment is anticipated to be a linear grade sloping down from east to west. The overall truss height is 50 feet, and the center-to-center distance between the truss lines is 24 feet.

Figure 9: Typical Section of the proposed bridge

Truss Elements

Given the loads and the required element lengths (distance between connection points) it was determined that the chord elements would need to be built up sections. They are assumed for preliminary design to be built up box sections of approximately 20-inches on a side. These would be fabricated from plate steel welded together at the corners. DRAF

Figure 10: Potential cross section of truss element

The diagonal (vertical) members also span a significant distance and at this point in the design are likewise assumed to be built up boxes. This assumption can be revisited in final design and the possibility of using rolled sections can be explored. The cross-frames between the two (2) trusses were not designed at this stage, but the use of either built up boxes or rolled sections will be examined in the final design phase.

Currently, traditional gusset plates using bolted connections are anticipated to join the truss elements together as well as the floor beams. Modern detailing practices will be referenced for final design of the truss elements and the connection of floor beams and framing elements.

Element Sizing

A preliminary design was completed on the proposed truss concept. This was done to check the feasibility of the design concept as well as develop initial loading values and create an estimate for element sizes and thus weights and costs. To do this, sizes and weights were estimated for the floor system, cross-frames, and miscellaneous dead loads (bridge rail). Estimated element sizes were checked for capacity against the anticipated design loads. The floor system is discussed in greater detail below; for loading purposes it was assumed to consist of a floor beam between the trusses, stringers supported between floor beams, and a steel deck on the stringers to directly carry the traffic loading. A detailing factor of 25% was added to the estimated member weights to account for gussets, bolts, stiffeners, etc., and as a design contingency.

Structural Member Transportation

To construct the truss, pieces will have to be delivered to the site, and then to the ends of the relevant, cantilever span. The dual challenge with this final step of the operation is the narrow and load-limited nature of the truss structure, combined with the fact that the west side of the bridge DRAF will not be continuously accessible through-out construction due to the connecting road being impacted by the rock glacier. One solution to the latter problem would be to deliver all the elements to the west side in a short duration, prior to construction. Then they could be moved from the abutment location to the end of the truss as needed, no longer having to consider the condition of the connecting road.

Another possible construction scenario is to have cranes placed on the advancing ends of the truss, capable of picking and setting the pieces. Those structural members would be delivered to the cantilever ends of the truss and then picked by the crane, put in position, and bolted into place.

The design sizing of the truss assumes this method will be used as it is the most conservative for determining construction loading (requiring a medium sized crane at the cantilever end of the truss along with some amount of construction loading for equipment and transport along the truss).

However, it does have some significant issues, including requiring the bulk delivery of pieces and the extremely narrow working space available between the two trusses.

The anticipated design method for member delivery is the use of a highline. This is a system of cables with a pick attachment that spans above and across the bridge area. It can be used to deliver pieces and equipment aerially. It can also be used to place the pieces in their final location and hold them there until they are bolted in place. See Figures 7 and 13.

Figure 11: Highlines used to assemble the Hoover Dam Bridge

This solution solves both issues of access (not requiring the road to be traversable to deliver pieces) and the working space issue, as it delivers element from above, rather than turning and picking between the trusses. The highline could also be used in conjunction with smaller cranes on the bridge to facilitate placement of pieces and access for workers to attach members together. DRAF

It is anticipated that the highline towers and tie backs would be supported and anchored to rock with a combination of spread footings, micropiles, and rock anchors.

Floor System

Trusses traditionally use a panelized system where the floor system framing is supported on regularly spaced transverse floor beams. The deck may bear directly on the floor beams, or the floor beams may be crossed by longitudinal stringers which in turn support the deck. The latter option has advantages at this site as it can accommodate larger panel spacing and easily transportable prefabricated elements. Importantly, a framing system with longitudinal stringers is compatible with various types of lightweight prefabricated deck panels. An out-to-out deck width of 21’-2” is assumed, based on an 18’-0” clear roadway and two 1’-7” wide railings. Four lines of stringers are selected to keep stringer spacing modest (approximately 6’-7”) and improve compatibility with various deck systems.

The owner has expressed a preference for solid steel deck with friction coating. The following deck options provide a solid steel surface:

simple steel plate;

steel orthotropic deck; and steel sandwich plate.

Simple steel plate, as commonly used in ballasted railroad bridge construction, is deemed impractical because even with very closely spaced stringers a thick plate would be required, resulting in a heavy system. Steel orthotropic deck panels are deemed feasible, but they involve specialized design and fabrication that is typically warranted only where very light weight decks are critical, such as for movable spans or suspension bridges. Steel sandwich plate comprises a top and bottom solid steel plate with polyurethane core in between. Advantages of sandwich plate include light weight, thin (approximately 2-inches), narrow pieces that are easily trucked to the site and set into place with small equipment, bolted connections from panel to stringer and from panel to panel for continuity, and compatibility with metalizing for corrosion protection.

Figure 12: Steel Sandwich Plate Deck Panel

A prefabricated deck system is advantageous at this site to maximize shop fabrication in a controlled temperature environment, reduce the number of construction steps required on site, and generally accelerate the construction timeline. Other potentially viable prefabricated deck systems include fiber reinforced polymer (FRP) deck panels, extruded aluminum orthotropic deck, filled or partially filled steel grid, concrete planks, and steel corrugated deck with wearing surface. It is anticipated that the deck type will be further evaluated and refined as the project is developed, considering such factors as lead time, construction scheme and compatibility with equipment limitations, construction duration, cold weather construction factors, and cost.

Figure 13: Steel Sandwich Plate Deck, Dawson Bridge, Edmonton, Canada

Abutments

Since the site is remote, cast-in-place concrete is expensive and difficult to place with high quality.

Therefore, precast concrete abutments are being proposed. The footing can be delivered in segments that are sequentially placed and then post-tensioned together, with alignment ensured by matching keyways and even bearing ensured by adhesive bonding material between segments.

Keyway alignment could be controlled through match casting using the long line method (cast every other piece on a full-length form first, then cast the in-between pieces as a second pour).

When the pieces are placed in the field, a long set segmental adhesive would be used between prior to post-tensioning.

The abutments will be subject to very high lateral loads during construction since they will provide restraint for the truss against the erection stays that facilitate the progressive cantilever construction. This high lateral force will be resisted by a combination of footing friction, keyed and grouted into rock, and tensioned micropiles both vertical and battered. The precast footing segments will be constructed with voids that will be used as a template for installing the micropiles and then grouted to complete the structural connection between micropiles and abutment footing concrete.

During truss erection, temporary truss restraints will be bolted to the precast footing. After erection is complete, the temporary restraints will be removed and precast backwall and cheek wall segments will be placed and welded in place at steel embedments in the precast concrete elements. DRAF

Project Standards Standards

Geometric and Bridge Criteria Design Standard Functional Classification NPS Class IV - Primitive Park Road

Terrain Mountainous

ADT 2022 <100

ADT 2042 <100

Design Vehicle Rogers CR50 Gooseneck lowboy trailer

WB-67D

Design Speed (Minimum from Standard) 35 MPH

Design Speed (Chosen for Segment)* 20 MPH Traveled Way Width 18’

Shoulder Width 0’ Max. Longitudinal Grade 16%

Normal Cross Slope (crown) 3% Minimum Horizontal Curvature 100’

Max. Superelevation (e) 6% Stopping Sight Distance 200’

Design Loading Structural Capacity* HL-93 *Controlling Design Criteria

Temporary Traffic Control The roadway is currently closed at the slide and will remain closed during construction. The contractor will need to construct a temporary access road over the slide area to access the west bridge abutment and roadway. The Park would like access across the temporary access road during construction. The frequency of access needs to be determined and discussed with the contractor to understand what level of access, if any, can be provided by the contractor during each phase of construction.

Contractor traffic and access controls along the 43 miles of operational Park Road to the site will also need to be specified. The contractor will need to comply with load restrictions shown in the Special Contract Requirements.

Landscaping

West Rock Cut Turn Area

South of the…