Development of Seismic Design Approach for Freestanding Freight Railroad Embankment Comprised of Lightweight Cellular Concrete

Recent advances in research, laboratory testing and field evaluations of lightweight cellular concrete have led to an increased understanding about its application as a geomaterial. Recently, lightweight cellular concrete has been used to construct a 40-foot high by 50-foot wide freestanding railroad embankment with vertical sidewalls near Colton, California. The embankment and flyover structures are about 7,000 feet long and consist of 220,000 cubic yards of lightweight cellular concrete. The embankment was designed to support 3 simultaneous Cooper E-80 freight railroad live loads and seismic loading from a 2500-year return period earthquake event. In order to provide an earthquake resilient material, cellular concrete was selected because of its relatively low density (25 to 37 pounds per cubic foot) and high compressive strength (140 to 425 pounds per square inch), when compared with traditional backfill materials. This alternative also provided a reduced embankment footprint and corresponding dead load, which reduced foundation settlement and possible inertial interaction with nearby utilities and infrastructure. Comprehensive seismic design guidance for lightweight cellular concrete embankments has not been fully developed in the U.S, but a rational approach has been developed for freestanding geofoam embankments. A similar approach was incorporated in the design process of the Colton, California embankment. This paper discusses the design process including: (1) selection and development of spectrumcompatible time histories for both horizontal and vertical components of strong ground motion; (2) development of design and evaluation methodologies; (3) detailed numerical evaluation using finite element [QUAKE/W] and finite difference techniques [FLAC 2D]; (4) assessment of the load-deformation characteristics of the embankment system under seismic ground motion and (5) assessment of benefit of shear keys and ground improvement on limiting basal sliding during a seismic event.