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Slurry Walls for Permanent Lateral Resistance in Zones of High Seismicity

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Slurry Walls for Permanent Lateral Resistance in Zones of High Seismicity Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 10NCEE Anchorage, Alaska SLURRY WALLS FOR PERMANENT LATERAL RESISTANCE IN ZONES OF HIGH SEISMICITY Sitotaw Y. Fantaye, P.E.1, Lisa Papandrea, P.E.2, and Jesse Richins, P.E., G.E.3 ABSTRACT Slurry wall construction is commonly used for excavation support, particularly in areas where a hydraulic barrier is needed. However, in areas of high seismicity where significant lateral loads must be resisted, slurry walls are typically used for temporary excavation support only. Interior systems, such as shear walls, moment frames, cross bracing, or a combination of these systems, are typically added for permanent lateral support and load distribution. This paper presents a case study in which an innovative engineered shear connector was used to provide in-plane shear capacity between slurry wall joints for a successfully constructed buttress wall at the Vehicle Security Center, World Trade Center, New York. The challenging site was constrained with limited space and required the excavation support wall to resist typical earth and hydrostatic pressures as well as the additional lateral loads of an adjacent historic highrise. This paper will detail the design of the wall, numerical modeling of wall performance and stresses, and construction challenges. In addition, a hypothetical design of a structural slurry wall in a zone of high seismicity was evaluated as an example of applying this technology in areas with high seismic loads. Future applications of this technology should be considered in areas of high seismicity to integrate slurry wall excavation support into the permanent lateral support system of deep excavations to avoid costly secondary support systems. 1 Senior Associate, Mueser Rutledge Consulting Engineers, New York, NY. [email protected] 2 Structural Engineer, Mueser Rutledge Consulting Engineers, New York, NY. [email protected] 3 Supervising Engineer, Mueser Rutledge Consulting Engineers, New York, NY. [email protected] Fantaye, Sitotaw, Y., Papandrea, Lisa, Richins, Jesse. Slurry Walls for Permanent Lateral Resistance in Zones of High Seismicity. Proceedings of the 10th National Conference on Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014. Slurry Walls for Permanent Lateral Resistance in Zones of High Seismicity Sitotaw Y. Fantaye, P.E. 1, Lisa Papandrea, P.E.2 and Jesse Richins, P.E., G.E.3 ABSTRACT Slurry wall construction is commonly used for excavation support, particularly in areas where a hydraulic barrier is needed. However, in areas of high seismicity where significant lateral loads must be resisted, slurry walls are typically used for temporary excavation support only. Interior systems, such as shear walls, moment frames, cross bracing, or a combination of these systems, are typically added for permanent lateral support and load distribution. This paper presents a case study in which an innovative engineered shear connector was used to provide in-plane shear capacity between slurry wall joints for a successfully constructed buttress wall at the Vehicle Security Center, World Trade Center, New York. The challenging site was constrained with limited space and required the excavation support wall to resist typical earth and hydrostatic pressures as well as the additional lateral loads of an adjacent historic highrise. This paper will detail the design of the wall, numerical modeling of wall performance and stresses, and construction challenges. In addition, a hypothetical design of a structural slurry wall in a zone of high seismicity was evaluated as an example of applying this technology in areas with high seismic loads. Future applications of this technology should be considered in areas of high seismicity to integrate slurry wall excavation support into the permanent lateral support system of deep excavations to avoid costly secondary support systems. Introduction Slurry wall construction is a technique for building reinforced concrete walls below the ground surface. Slurry walls are often the most economical method of building deep structural walls, particularly when a hydraulic barrier is needed to cutoff groundwater. Construction typically proceeds by excavating a slurry trench using either a clamshell excavator or a hydromill trench cutter. The trench is excavated one panel at a time and each panel is typically 1.5 to 3 ft wide, 20 to 30 ft long, and as deep as is needed to reach rock or some other suitable low permeability stiff layer. Trench excavations are kept open by using weighted slurry. After final depth is excavated, a rebar cage is lowered into the slurry trench. Concrete is placed through tremie techniques and slurry is removed from the excavation at the top of the trench (typically recycled into another trench). After completing a slurry panel, the excavator moves on to excavate additional panels. A temporary or permanent end-stop is installed at the ends of completed panels to allow the construction of an adjacent panel. The end result of slurry wall 1 Senior Associate, Mueser Rutledge Consulting Engineers, New York, NY. [email protected] 2 Structural Engineer, Mueser Rutledge Consulting Engineers, New York, NY. [email protected] 3 Supervising Engineer, Mueser Rutledge Consulting Engineers, New York, NY. [email protected] Fantaye, Sitotaw, Y., Papandrea, Lisa, Richins, Jesse. Slurry Walls for Permanent Lateral Resistance in Zones of High Seismicity. Proceedings of the 10th National Conference on Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014. construction is a continuous reinforced-concrete wall that is suitable for resisting perpendicular lateral loads (such as horizontal earth pressure) and transmitting vertical loads from the structure. However, since steel reinforcing is not continuous between slurry wall panels, the in-plane shear capacity of the wall is typically negligible. For this reason, in zones of high seismicity, where very large shear loads are transmitted into basement walls, slurry walls are usually used as temporary excavation support structures, with interior cast-in-place shear walls or other structures used to resist lateral loads in the permanent structure. As part of developing the buttress walls to support excavations at the Vehicle Security Center (VSC) at the World Trade Center in New York City, an engineered shear connector between slurry walls panels was developed and successfully implemented. This new technology could be used in high seismicity zones to implement slurry walls as part of the permanent lateral resistance system of structures, effectively reducing the need for costly interior lateral resistance systems along the perimeter of basement walls. Slurry Wall Construction at the Vehicle Security Center The VSC, located on an approximately 200 ft (60.96m) by 400 ft (121.92 m) parcel of land bounded by Liberty Street on the north (original World Trade Center bathtub slurry wall), Cedar Street on the south, Greenwich Street on the east and West Street on the west is vital to the function of the World Trade Center (WTC), Figure 1. When completed, it will serve all buildings within the WTC. This multi-level state of the art vehicle screening and parking facility required excavation ranging between 60 ft (18.3m) and 100 ft (30.48m) below grade. Figure 1. Site Plan of Vehicle Security Center One of the most difficult challenges in the design and construction of the VSC excavation support and permanent perimeter basement walls was the slurry wall adjacent to 90 West Street, a 20-story historic masonry building supported on timber piles. This wall is located along the southern alignment of the VSC between West and Washington Streets. Installing tieback anchors beneath the timber pile supported building was not permitted. After considering several wall support alternatives , widely spaced buttress walls, with one level of raker supports to the top of the buttress walls (see Figure 2) was selected as the most efficient and appropriate support. Figure 2. Typical Buttress Wall Section with Engineered Shear Connector In conditions where tieback anchors cannot be used to laterally support a slurry wall, alternative lateral support systems include: cross lot struts, rakers, closely spaced buttress walls, or a combination of these elements. However, these conventional support systems could not be used at the VSC for various reasons. Cross lot struts require forces from each side to generally be balanced; this was not possible because the World Trade Center to the north had previously been excavated and there was no other structure along the north to provide a reaction for the forces from the south. In addition, the span between the north and south walls was more than 200 ft (60.96m). This would have required very heavy, large diameter temporary struts spaced frequently and or the addition of strut bracing systems. These struts would have interfered with the construction of the VSC structure. Rakers were also ruled out because of the complexity of their installation due to the depth of excavation being 60 ft (18.3m) to final subgrade, length and size of rakers and the associated risk with unacceptable wall movement inherent in long raker installation. Frequently spaced buttress walls were also not practical because the buttresses projecting into the VSC would have rendered a significant amount of space unusable. A unique solution was required: widely spaced buttress walls were selected and designed such that they became part of the final garage floor support system. In essence, a buttress support system works as an upright “T-beam” cantilevering from a fixed point at its base to support lateral forces applied on its flange. These lateral forces consist of earth, water and building surcharge pressures. The stem’s length of the “T-beam”, buttress, is dictated by the spacing of the “T-beam”. With increased tributary area longer buttress walls are required to resist the lateral pressures. The required buttress length for the selected spacing on this project was larger than a typical panel width because the buttresses were widely spaced; it needed to consist of multiple panels.
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