Precast Solutions - November/December 2008 - (Page 16) were produced for the railway tunnel structure that stretches 1,332 feet in length. Seventy-five percent of the precast pieces are on a skew to accommodate the curved track. The twin-leaf C-Series BEBO arch will include keystone headwalls and wingwalls. For more than 60 years, planners have been discussing construction of the additional north-south transportation corridor in Spokane to relieve traffic congestion. With funding from the Federal Highway Administration (FHWA) and WSDOT, the U.S. 395 Corridor Project is a $3.3 billion effort to address increasing congestion and improve north-bound and south-bound traffic through Spokane. The second largest city in Washington state, metropolitan Spokane is home to nearly 500,000 people. The tunnel structure, which is part of a fully funded $28 million project, was completed in September. WSDOT and BNSF. With typical bridge solutions off the table, WSDOT contacted CONTECH Construction Products Inc. through HDR Engineering, the project engineers, to discuss the potential for a precast concrete arch solution at the site. Together, BNSF, CONTECH and WSDOT considered the precast arch option and its ability to meet the site conditions. Engineering the project was certainly challenging with three entities involved, each with its own standards and criteria. Because of the deep sand strata at the site (sandy soils do not compact well), settlement was a concern when the project team considered various structural solutions. However, the BEBO Arch System is exceptionally tolerant of settlement – up to 9 inches at the middle of the tunnel. A differential settlement of up to 3 inches between the strip footings requires the system to be relatively WSDOT LOOKS BEYOND insensitive to soil settlement. In the tunnel’s longitudinal direction, differential settlement of up to three-fourths inch in 15 feet can be accommodated by the openings or gaps between the precast concrete arches. Because the BEBO Arch System is a proprietary product, WSDOT was required to write a justification letter to federal transportation officials. By 2007, BNSF approved construction of a BEBO Arch System, realizing that not many suppliers could meet the specific dimensions required at the site and also comply with inhouse requirements. CONVENTIONAL SOLUTIONS While selecting the best solution for the project, conditions at the Spokane project site presented WSDOT with several challenges. Geometric restrictions, soil conditions and BNSF’s safety requirements significantly expanded the project‘s scope and cost. The crossing required more than 20 feet of fill. The skew angle required a bridge that was exceptionally long for this amount of fill. Adding to these issues were the poor foundation properties of the soil. Up to 9 inches of settlement were expected because of deep, sandy soils. In 2004, the project design team began investigating alternative bridge solutions that are not generally considered on Washington state’s highways. During the evaluation of various bridge systems and design options, HDR Engineering, WSDOT and BNSF considered the following alternatives: • Conventional steel or concrete beam bridge • Steel plate arch (two versions considered) • Precast concrete arch Conventional bridges were ruled out primarily because of price, required span length and lack of adequate rightof-way for future expansion. BEBO ARCH SYSTEM SOLUTION PROVIDES The BEBO Arch System is a combination of precast concrete arch elements, headwalls, wingwalls and cast-inplace footings. Available in spans from 12 feet to 102 feet, the BEBO Arch System provides the largest precast concrete arch spans in the world. This precast system offers a cost- and time-effective solution for a wide range of applications, including: short and medium span bridges; tunnels and mine entrances; underpasses; pedestrian access; highways; and environmentally sensitive sites. BEBO arches are available in various shapes from elliptical to circular cross sections to fit site-specific needs. SEEKING COMMON GROUND FOR A The complete precast concrete system meets standard AASHTO and AASHTO Load and Resistance Factor Design (LRFD) specifications. A cast-in-place concrete foundation provides the footings for the precast arches. DIFFICULT SITE Selecting an acceptable alternative, however, was a process that required extensive meetings between 16 PRECAST SOLUTIONS | NOVEMBER/DECEMBER 2008
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