As part of the widening of U.S.-30 between McCammon and Lava Hot Springs in southeast Idaho, a new bridge is being constructed over a crossing of the Marsh Valley Canal and the Union Pacific Railroad tracks. This new, three-span, plate steel girder structure will replace an existing bridge built in the mid to late 1940s. Because of the large skew angle at which the highway crosses the railroad tracks, and to leave room for a future additional set of tracks, the required length of the center span of the new bridge is 320 feet, making it the longest center span in the state. The total length of the bridge is 613 feet. Foundations for the new bridge consist of drilled shafts socketed into basalt bedrock underlying the bridge site at relatively shallow depth.
The existing west approach fill consisted of an earthen embankment about 600 to 700 feet long and as high as 45 feet. The embankment was constructed mostly of compacted clay or silt fill with steep side slopes inclined typically at about 1.5:1(horizontal to vertical). Clearance requirements over the railroad and the deep girders required to support the long center span of the bridge required raising the grade of the existing roadway and the approach embankments about 6 to 13 feet at the west and east abutments, respectively, in addition to widening the embankment to the north. The Portneuf River flows beneath the west approach fill through three, 10-foot-diameter corrugated metal culverts. Because of wetlands and right-of-way constraints, an early project requirement was that the base of the embankment could not be widened significantly beyond its pre-existing footprint. As a result, a tall wall would be required to retain the widened approach fill along its north side.
A complicating factor affecting design and construction was the requirement to maintain traffic onsite throughout construction of the new bridge and widening of the approach fill. This required staged construction, building the new bridge and widening and raising the roadway in halves, and meant that the existing embankment could not be removed as part of the construction.
Site and subsurface conditions
The Portneuf River meanders extensively in a generally westerly direction between steep hills and mountains that border the valley on the north and south. U.S. 30 crossed the Portneuf River valley at the site location on a combination of embankment fill and the old Topaz Bridge. The total length of the crossing is about 1,000 feet. The highway crosses the valley at a skewed angle, so the total length of the crossing is greater than the actual width of the valley, which is about 600 feet.
Geotechnical explorations revealed that while the bedrock is relatively shallow at the location of the bridge itself, a deep buried river channel filled with a heterogeneous mixture of very loose or soft sediments underlies the west approach fill. The presence of the old buried channel is consistent with the meandering nature of the river. The borings indicate that the sides of the buried channel are steep, and are underlain by basalt bedrock. A near-vertical exposure of the bedrock is visible at the ground surface at one edge of the buried channel near the west end of the west approach fill. Standard penetration test values (blows per foot) in the sediments in the buried channel ranged from zero (the sampler sank under the static weight of the hammer) to values in the teens or twenties with isolated higher values, but were typically in the low to mid single digits.
The 45- to 50-foot height of the embankment is too high to be retained by a conventional concrete cantilever or gravity retaining wall. Consequently, a mechanically stabilized earth (MSE) wall was initially considered to retain the embankment. However, stability and bearing capacity analyses indicated that the soft and loose sediments in the buried river channel did not have sufficient strength to support the wall without some type of special treatment or ground improvement. To make room for the necessary reinforcement for an MSE wall and to provide access for installation of ground improvement, it would have been necessary to remove large portions of the existing embankment. To maintain the roadway at the top of the existing embankment, removing such a large portion of the fill would have required a large, temporary, top-down type of wall, such as a soil nail wall, extending the length and height of the existing approach fill.
Faced with these difficulties, several alternative solutions were considered and compared, including revisiting the possibility of conventional sloping embankment fills. However, the wetlands, right-of-way constraints, and hydraulic issues related to extending the existing culverts precluded a full-height sloping embankment to the north. Other alternates considered included the MSE wall with ground improvement consisting of either stone columns or deep soil mixing, building a bridge for the full length of the widened fill, or use of lightweight fill consisting of block-molded expanded polystyrene lightweight fill (EPS-block fill), commonly referred to as geofoam.
A consideration relative to the use of EPS fill was the height of the required wall. Based on a review of projects using this material, it appeared that the maximum height of EPS fill used previously in the United States was about 30 feet. Information that higher EPS fills had been used in other countries, particularly Japan, was found, but there was a concern that constructing higher fills than previously used in the United States would be considered experimental and might require special approvals that could delay the project.
Based on discussions with the Idaho Transportation Department (ITD), it was determined that the footprint of the west approach fill could be widened a small distance to the north to allow a small sloping earthen embankment fill to support a required access road near the base of the wall. This created the possibility of another alternate for constructing the widened fill, consisting partially of conventional earth fill and partially of EPS-block fill.
Combined fill solution
The alternate selected for construction of the widened approach embankment included the smaller earthen embankment fill making up the lower approximately 15 feet of the fill, and EPS-block fill for the upper approximately 30 feet. Approximately 4-1/2 feet of granular fill, including the pavement section, placed on top of the geofoam complete the full required height of the embankment. A typical cross section of this concept is shown in Figure 1. The three following requirements or features were included in the design and construction of the fill.
1) Because of the soft sediments in the buried river channel, it was required that the small earthen embankment be constructed and allowed to settle prior to beginning construction of the overlying EPS-block fill. The required time for settlement was estimated to be about 80 days. Instrumentation was required to monitor settlement and stability of this fill to confirm when primary settlements were complete and when construction of the EPS-block fill could proceed. The contractor for the project was able to construct the small earthen embankment prior to the 2009-2010 winter shut down, so the fill was in place for a much longer period before placement of EPS-block fill began. Instruments indicated that as many as about 12 inches of settlement occurred under the approximately 15-foot-high earthen fill. Significant differential settlement also was observed in the roadway at the top of the existing embankment near what is projected to be the eastern edge of the buried river channel due to placement of the small earthen embankment.
2) It was required that the EPS-block fill be keyed into the existing embankment, with a minimum thickness of soil removed as part of the benching process. The primary purpose for this soil removal was to provide a near “net-zero” increase in the load or stress added after completion of settlement under the small earthen embankment. The soil removed was to offset partially the weight of the pavement section that would be placed later on top of the EPS-block fill. The requirement to keep traffic moving on the top of the existing embankment limited the amount of soil that could be removed from the existing slope, and prevented completely achieving a net-zero stress increase condition at all locations in the cross section. Several feet of surcharge fill were added to the top of the small earthen embankment fill to help achieve the net-zero condition under the outer (northern) portions of the EPS-block fill. Several feet of soil also will be removed from the top of the existing embankment during the second stage of construction to create a net-zero condition across that part of the roadway. Another reason for keying was to tie the EPS fill into the existing embankment and prevent creation of a continuous sloping plane of different material between the EPS and the underlying clay embankment fill.
3) To further limit the overall weight of the fill, a thin (2-inch-thick) shotcrete protective facing will be applied directly to the exposed exterior faces of the EPS-block fill. A separate heavy concrete wall supported on its own foundation has sometimes been used in the United States to provide protection for the EPS fill. At this site, the foundations to support the separate wall over the soft sediments may have required deep foundations.
Construction to date
As of this date, the northern half of the bridge and the widened northern portions of the EPS-block fill have been completed. The old Topaz Bridge was taken out of service on Nov. 3, 2010, and traffic was shifted onto the new bridge and the EPS fill. The old bridge has been removed so that construction may proceed on the southern half of the bridge and fill.
The EPS block portion of the new embankment is about 500 feet long, up to 30 feet tall, and will contain about 22,000 cubic yards of geofoam when complete. The EPS blocks are typically approximately 4 feet square in cross section, and range from 6 to 14 feet long as used in the fill. The blocks were molded as long as 24 feet, and were cut as necessary to provide the sizes needed to fit the placement plan for the fill. In accordance with typical design and construction with EPS blocks, alternating layers of blocks were placed with their axes oriented perpendicular to those of the adjacent layer. It also is required that the top layer of blocks be oriented perpendicular to the centerline of the road. This pattern helps tie the fill together and improves internal stability of the fill.
Large “tongs” that grip the EPS blocks without damaging them were used in final positioning of the blocks. The exposed EPS-block face will be covered with a shotcrete protective facing in the spring when weather permits. The project is scheduled for completion in the fall of 2011.
The bridge and civil design of the project were performed by J-U-B ENGINEERS Inc. of Boise, Idaho. Geotechnical engineering for the project was provided by the Boise office of Terracon Consultants Inc. John Horvath, Ph.D., P.E., of Manhattan College, and co-author of the NCHRP Web Document 65 (Project 24-11) “Geofoam Applications in the Design and Construction of Highway Embankments,” provided special consultation relative to the use of EPS materials. The bridge is being built by Idaho Construction Company of Kimberly, Idaho, and earthwork and construction of the EPS-block fill is being performed by Scarsella Brothers Inc. of Seattle. The EPS supplier and molder is Insulfoam LLC of Kent, Wash.
Clair A. Waite, P.E., is a senior geotechnical engineer with the Boise office of Terracon. Terracon, an employee-owned engineering consulting firm, provides geotechnical, environmental, construction materials and facilities services from more than 100 offices nationwide with more than 2,700 employees.
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