GeoCongress 2012 © ASCE 2012 1720 Development of Seismic Design Approach for Freestanding Freight Railroad Embankment Comprised of Lightweight Cellular Concrete J. Anderson1, Dr. S. Bartlett2, N. Dickerson3, and P. Poepsel4 1Geotechnical Engineer, HDR Engineering, Inc., 8404 Indian Hills Drive, Omaha, NE, 68114; PH (402) 399-1000; email: [email protected] 2Associate Professor, Department of Civil and Environmental Engineering, University of Utah, 201 Presidents Circle, Salt Lake City, UT 84112; PH (801) 587-7726; email: [email protected] 3Structural Engineer, HDR Engineering, Inc., 8404 Indian Hills Drive, Omaha, NE, 68114; PH (402) 399-1000; email: [email protected] 4Senior Geotechnical Engineer, HDR Engineering, Inc., 8404 Indian Hills Drive, Omaha, NE, 68114; PH (402) 399-1000; email: [email protected] ABSTRACT: 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 Downloaded from ascelibrary.org by Steven Bartlett on 09/25/18. Copyright ASCE. For personal use only; all rights reserved. incorporated in the design process of the Colton, California embankment. This paper discusses the design process including: (1) selection and development of spectrum- compatible 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. GeoCongress 2012 GeoCongress 2012 © ASCE 2012 1721 OVERVIEW Union Pacific Railroad (UP) has a shared at-grade crossing with BNSF Railway in the City of Colton, San Bernardino County, California. The crossing of these tracks is referred to as the “Colton Crossing”. This project provides grade separation for these two heavy traffic lines and increases the efficiency of traffic flow in this area. This grade separation, or “flyover”, structure consists of a cellular concrete embankment and a series of bridges along the alignment of the UP tracks. The total length of the Colton Crossing Flyover Retaining Structure is about 7,000 feet with an embankment width of about 50 feet and a maximum height of about 40 feet. See Figure 1 below. Embankment section. The flyover embankment structure supports two railroad tracks and a maintenance access road, which was planned with lateral clearance for a possible future third track. The embankment section consists of, in descending order: (1) 8.5-foot wide concrete ties with ballasted track section [12 inches ballast/18 inches subballast], (2) 3-foot thick upper layer of Class IV cellular concrete, (3) variable thickness of Class II cellular concrete, (4) 2.5-foot thick Class IV layer of cellular concrete with a 4-foot deep shear key embedded in the foundation soils (at higher embankment sections), and (5) vibro-replacement stone columns approximately 15 ft deep in the foundation soils. Cellular concrete was selected for the embankment based upon its low density and relatively high compressive and shear strength when compared to earthen fill. Use of lightweight material reduces the bearing pressure and the static and dynamic lateral earth pressures in the bridge abutment areas. A typical section of the proposed embankment structure with ground Downloaded from ascelibrary.org by Steven Bartlett on 09/25/18. Copyright ASCE. For personal use only; all rights reserved. improvements is shown in Figure 1. Cellular concrete can also be Figure 1. Typical flyover embankment section. pumped long distances for placement within a small footprint for the on-site equipment, which is critical in a tight construction site between Interstate-10 and active railroad tracks. Cellular concrete. Cellular concrete is an engineered, low density material having a homogeneous cell structure formed by the addition of prepared foam or by the generation of gas within the fresh cementitious mixture. It is usually cast in densities GeoCongress 2012 GeoCongress 2012 © ASCE 2012 1722 ranging from about 20 to 120 pounds per cubic foot (pcf). The air cells created by the preformed foam may account for up to 80% of the total volume (Fouad, 2006). Based upon research by Sandia National Laboratories (Sandia, 2004), the tensile to compression strength ratio is approximately 10% and the stress-strain relationship of unreinforced cellular concrete is similar to that of conventional concrete. Also, based upon available technical literature (Cellular Concrete LLC, 2011), the wet cast densities for Class II and Class IV cellular concrete are about 30 and 42 pounds per cubic foot (pcf), respectively; and the dry densities are about 25 and 37 pcf, respectively. The corresponding as-cast compressive strengths for these densities are about 40 and 120 pounds per square inch (psi), respectively; and the 28-day compressive strengths are about 140 and 412 psi, respectively. Ground Improvement and Shear Key. Vibro-replacement stone columns were selected as the ground improvement method at the site to increase the relative density and shear strength of the subgrade and shallow foundation soils, and improve the overall stability of the embankment structure. The vibro-replacement technique utilizes a small mobile rig to insert a vibrating probe and construct stone columns of depths up to 30 feet below the ground surface. In addition to the ground improvement, a shallow shear key consisting of light-cellular concrete was bedded in the foundation soils to improve the sliding resistance of the embankment during the design basis earthquake (Figure 1). The influence of these improvements on the overall stability of the embankment structure will be discussed subsequently. SITE CHARACTERIZATION Existing subsurface conditions. Geotechnical investigations for this site were performed by C.H.J. Inc. (C.H.J.) during the summer of 2010. The conditions at this site consist of [top to bottom]: (1) thin layer of loose, silty sandy fill [1 to 9 feet thick], (2) loose to medium dense silty sand [5 to 25 feet thick], (3) medium dense silty sand [0 to 35 feet thick], (4) dense to very dense silty sand [thickness unknown, bottom of boreholes] (C.H.J., 2011). Groundwater was located between 117 and 123 feet below the ground surface as measured in the boreholes. Shear wave velocity measurements were attempted at the site via the seismic cone penetrometer (SCPT). However, such measurements were not presented in the geotechnical report because of the excessive background noise caused by traffic on the adjacent freeway. The probabilistic seismic hazard analysis (PHSA) assumed that the site classified as Site Class D (stiff soil profile) with an estimated average shear wave velocity of 270 m/s in the upper 100 feet (C.H.J., 2011). Seismic Hazard. The site is not located in a mapped fault rupture zone (C.H.J., 2011); however due to the presence of several nearby, active faults the expected Downloaded from ascelibrary.org by Steven Bartlett on 09/25/18. Copyright ASCE. For personal use only; all rights reserved. ground motion at this site is large. The project geotechnical report provided acceleration response spectra for the three probabilistic design basis earthquakes (Figure 2). The Level 1, 2 and 3 events represent spectral accelerations having average return periods of 72, 475 and 2475 years, respectively. [Note: the calculated vertical design accelerations at this site are considerably higher than would be encountered in most other areas of California. For example, vertical Peak Ground Acceleration (PGAv) is on the order of 90% of the corresponding horizontal value based on published attenuation relationships (Campbell-Bozorgnia, 2008)]. GeoCongress 2012 GeoCongress 2012 © ASCE 2012 1723 The controlling fault at this site 2.5 for the Level 3 event is the San Level 1 (H) Level 2 (H) Jacinto fault zone (SJFZ) based 2 Level 3 (H) on the seismic deaggregations of Level 1 (V) Level 2 (V) PGA and the 0.2 sec spectral 1.5 acceleration values. Depending Level 3 (V) on the fault rupture scenario, the 1 controlling earthquake is approximately