Innovative Design for the Colton Flyover Grade Separation of UPRR and BNSF, Colton, CA

The Colton Crossing Design Team developed an innovative approach for grade separating the Union Pacific-BNSF quadruple diamond at-grade crossing at Colton, California. The site lies in a developed area bounded by Interstate 10 on the north and residential/commercial neighborhoods on the south. Due to the need to maintain operations on existing tracks during construction and close proximity to I-10, traditional soil embankment fill with side slopes was not feasible for most of the project limits. Since the grade separation required retaining walls as high as 40 feet to elevate UP tracks over BNSF, high bearing pressures and expensive retaining walls were expected. The retaining walls and fill were estimated to be about 40% of the total cost of the project. In addition, Colton lies within an active seismic zone and the nearby San Jacinto fault has the capability of producing very large ground motions in an earthquake. The weight of concrete retaining walls and soil fill would result in large inertial forces, high peak bearing stresses and the potential for catastrophic failure of the walls and foundation soils in a seismic event. A novel approach to the retaining walls and fill was developed by the design team which proposed the use of lightweight cellular concrete fill with precast concrete panel fascia walls to form a cellular concrete retaining structure. The design team developed design criteria since no AREMA or AASHTO guidance was available for this type of structure. The design team performed advanced service and seismic design utilizing finite element analyses as well as other traditional methods. The analyses verified that the expected performance and construction costs were reduced considerably. This innovative cellular concrete retaining structure has the following advantages: (1) lighter weight and stronger than compacted soil backfill; (2) built using small equipment and without importing a large volume of fill, thereby reducing impacts to the environment; (3) no lateral earth pressures and very small live load surcharge pressures on bridge abutments and side facing walls; (4) reduced seismic inertial effects due to the reduction in mass, as well as enhanced seismic stability due to the large block-like behavior of the retaining structure; (5) greatly reduced anticipated settlements, especially in combination with shallow ground improvement; and (6) cost savings of up to 30% compared to conventional cast-in-place concrete walls with tie rods and soil fill.