Tenth U.S. National Conference on Earthquake EngineeringFrontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska 10NCEE

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. sfantaye@mrce.com 2 Structural Engineer, Mueser Rutledge Consulting Engineers, New York, NY. lpapandrea@mrce.com 3 Supervising Engineer, Mueser Rutledge Consulting Engineers, New York, NY. jrichins@mrce.com 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. sfantaye@mrce.com 2 Structural Engineer, Mueser Rutledge Consulting Engineers, New York, NY. lpapandrea@mrce.com 3 Supervising Engineer, Mueser Rutledge Consulting Engineers, New York, NY. jrichins@mrce.com 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 stems 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. However, as a T-beam in bending, the construction panel joints in the buttress needed to resist in-plane shear forces (internal forces) acting as one. To minimize the number of panel joints in the buttress, the buttress was constructed as a combination of a T-panel, with its flange along the alignment of the slurry wall, and an additional panel, connected to the stem of the T-panel, to provide the required buttress length. This provided the most practical and cost effective solution. However, since the required width of the buttresses was wider than a conventional slurry wall panel, these buttresses needed to be composed of at least two individual slurry wall panels. Hence, a method to connect the individual panels capable of transmitting significant in-plane shear forces had to be developed. To minimize the number of shear connections between the panels forming the buttress wall, each buttress wall consisted of a T-panel (primary panel) and a wide single-bite panel (follow-up panel).

Typically, slurry walls are constructed by excavating a series of individual panels with the use of some type of permanent or removable end-stop between panels. Whether the end-stop is removed prior to pouring the adjacent panel or it is left in-place, a cold joint exists between consecutive panels. Some proprietary end-stops are designed such that they form a shear key and when the end-stop is removed a waterstop is left in place creating a watertight joint between panels. Nonetheless, slurry walls constructed by such methods possess very limited in plane shear transfer capacity at these joints because the frictional shear resistance provided by the joint is limited. For this reason, slurry walls are not often used as shear walls in high seismicity zones or if they are subject to in-plane lateral loading. In buttress walls where in plane shear capacity is required at the joint between panels, conventional slurry wall construction method is not practical without first addressing the required transfer of the in-plane shear. This challenge was overcome on this project by the use of a permanent joint shear connector.

Engineered Shear Connector

The engineered shear connector, Figure 3, consisted of two parts:

A box-out comprised of plates welded to the flanges of an I-beam which house hooked reinforcing bars welded to the web. It includes a lift gate and a bottom plate to prevent the area from being filled with concrete when the primary panel is tremie poured and;

Hooked reinforcing welded to the web of the I-beam that project into the excavated slurry filled primary panel, for splicing with the reinforcing cage of the primary panel.

The box-out was placed at the end of the slurry filled trench; it was filled with water to

counteract buoyancy during installation. For ease of splicing, the ends of the horizontal reinforcing bars of the panel were crimped (slightly bent inwards) so that the panel reinforcing cage would fit between the columns of the hooked reinforcing bars that were attached to the I-beam. When the buttress wall was excavated after the T-panel cured, the lift gate not only was used to protect the bars for future splicing but also acted as the vertical guide during the buttress

panel trench excavation. Once the buttress panel was excavated to the desired tip elevation, the lift gate was lifted and the reinforcing cage of the buttress panel was lowered into place creating a spliced connection capable of transmitting the in-plane shear. Figure 3. Engineered shear connector details

Slurry Wall Construction in Areas of High Seismicity Due to the limited in-plane shear capacity of typical slurry wall construction (described above), slurry walls are not typically expected to contribute to a structures lateral resistance system. For this reason, in areas with high seismicity, slurry walls are often used as temporary excavation support systems for building structures, with deep basements and in challenging geologic formations, with shallow groundwater table. The engineered slurry wall connector we developed makes it possible to transmit large in-plane shear loads through a slurry wall panel joint, taking advantage of the inherent lateral resistance provided by a continuous slurry wall and potentially reducing the amount of interior structures needed to resist lateral load. In essence, once continuity through panel joints is developed, the slurry wall acts much like a cast-in-place shear wall. As an example application of this technology, we evaluated a hypothetical building constructed in an area with seismicity matching the Market District of San Francisco, California. The hypothetical building was a 20 story reinforced concrete high-rise with a five-level basement and a 100 ft x 100 ft footprint. The subsurface conditions at this hypothetical site are presented

Stage 1: Install engineered shear connector and tremie pour T-panel (primary Panel)

Stage 3: Lower buttress panel cage into trench to lap with shear connector

Stage 2: Excavate buttress panel (follow-up Panel), remove lift gate

in Figure 4. We assumed soil conditions with shallow groundwater at ten feet deep in a thick deposit of relatively loose, high permeability granular soils. We assumed that at 90 ft deep, a stiff overconsolidated clay layer with low permeability existed. This hypothetical situation would be ideal for slurry wall construction, as a deep slurry wall could be used to support the excavation and cutoff the shallow aquifer limiting the amount of groundwater pumped during dewatering and drawdown at adjacent sites. A perimeter 3 ft thick slurry wall was selected to dually function as the hypothetical buildings permanent foundation wall and as part of the buildings lateral seismic force resisting system. The total depth of the slurry wall was 100 ft, extending from ground surface to 10 ft into the stiff clay layer, with a final excavation depth of 50 ft for the basement levels. In our evaluation, we isolated one 100 ft long side of the slurry wall for a two-dimensional analysis.

Figure 4. Plan (left) and typical section (right) of hypothetical slurry wall. The site is located in an area of high seismicity, with expected Site Class B ground motion values of Ss = 1.5g and S1 = 0.67g (based on 2010 ASCE-7 mapped values). Base shear at the site was evaluated using a general code-based response spectra and using the Equivalent Lateral Force Procedure described in ASCE/SEI 7-10, with Site Class D ground motion modification factors. In calculating the in-plane shear forces induced by seismic forces on the slurry wall, we made two basic assumptions. First, half of the seismic force was resisted by the buildings lateral force resisting system located at the core, and twenty-five percent was resisted by perimeter slurry walls, located on each side of the core lying in the plane of the seismic force. Second, seismic forces from the basement levels were not included in this analysis, since these forces will load the slurry wall normal to its face and will generate negligible in-plane forces. Analysis of Hypothetical Slurry Wall The slurry wall was modeled as a two-dimensional structure consisting of discrete plate elements using RISA-2D, a structural finite-element analysis software. The applied loads and boundary conditions for the model are presented in Figure 5. External loads included twenty-five percent of the superstructures seismic lateral force, applied as a horizontal distributed load

at the top of the wall, and twenty-five percent of the seismic overturning moment, applied as a vertical distributed load at the top of the wall, assuming a linear elastic stress distribution. Gravity loads from the superstructures perimeter columns, assumed to distribute through a grade beam, were applied as a vertical distributed load at the top of the wall. Resistance to the applied loads by the soil was modeled in three ways:

The vertical resistance provided by the stiff clay bearing stratum was modeled as vertical one-way springs;

The lateral passive resistance of the soil at the end of the wall was modeled as horizontal

one-way springs, and;

The lateral frictional resistance provided by the soil-wall interface was modeled as horizontal one-way springs, acting in the opposite direction of the passive soil springs. We considered frictional resistance on both faces of the wall. On the outside face of the wall, we conservatively assumed that only the soil below subgrade (50 ft from ground surface) contributed to the frictional resistance.

Figure 5. Finite element plate model with soil springs and applied loads. The dimensions of each plate element are 5 ft x 5 ft; the total wall dimensions are 100 ft x 100 ft.

Soil springs were modeled as simple linear elastic-plastic springs. Initial spring stiffness along the sides and toe of the wall were modeled as simplified springs developed for a vertically and radially homogenous soil system after Randolph and Wroth (1978). The resistance provided by the soil springs is limited to the ultimate strength of the soil or ultimate frictional resistance of the soil-wall interface. Therefore, analysis was an iterative process. After the initial execution, springs with reactions that exceeded their ultimate values were removed and replaced with an applied load corresponding to its ultimate strength. The process was repeated until the analysis resulted in no yielded soil springs. In the final analysis, only the upper third portion of soil-wall springs was mobilized for side friction, and moreover the mobilized springs were stressed below their yield point. As shown in Figure 5, the majority of bearing stratum springs yielded and were replaced with the limiting bearing force, however the resistance provided by soil and friction was sufficient in preventing bearing failure and overturning. The internal in-plane shear forces in the slurry wall are presented in Figure 7. Assuming the panel layout depicted in Figure 4, the shear diagram along the length of the critical slurry wall panel joint is also shown.

Figure 6: In-plane shear force contours, including shear diagram along critical panel joint.

In-plane vertical shear, kips per ft

Design of Shear Connector between Slurry Panels

The engineered shear connector successfully applied at the VSC can similarly be used for our hypothetical slurry wall to create continuity between panels and hence a functional shear wall. The shear connectors can be engineered using the principle of shear friction, which is addressed in Chapter 11 of the ACI design code. To follow through with our example, the ultimate design shear force, obtained from Figure 6, is 13 kips per foot. A shear connector consisting of a W36 beam reinforced with several rows of reinforcing bars welded to each side of the W36 web, can reasonably provide a design strength of twice the ultimate demand. In instances where bar congestion and development length requirements are an issue, larger diameter headed bars may be used.

Conclusion An innovative solution was provided at the World Trade Centers Vehicle Security Center for a structural connection between slurry wall panels that is capable of transmitting shear through the panel joint. The engineered shear connector can be designed to provide significant in-plane shear strength without forgoing constructability, as much emphasis was given to the construction of the system throughout the design process. The installation of the shear connector within the slurry wall is not unlike the installation of the panel end-stops used in traditional slurry wall construction. The hypothetical evaluation presented here illustrates that a slurry wall with engineered shear connectors can function effectively as a lateral force resisting system in an area of high seismicity. Incorporating the slurry foundation walls into a structures permanent lateral force resisting system can greatly reduce the size and cost of the remaining lateral force resisting components, without adding significantly to the cost of the slurry wall. This system should be considered in future construction in zones of high seismicity.

References 1. American Concrete Institute. Building Code Requirements for Structural Concrete (ACI 318-11). 2011.

2. ASCE-7. American Society of Civil Engineers/Structural Engineers Institute. Minimum Design Loads for Buildings and Other Structures. 2010.

3. Ashour, Mohammed, Norris, G., and Elfass, S. Analysis of Laterally Loaded Long or Intermediate Drilled Shafts of Small or Large Diameter in Layered Soil. CA04-0252. California Department of Transportation. Sacramento, CA. 2008.

4. Fantaye, Sitotaw Y. Pioneering Connector for Slurry Wall Joints Deep Foundations. Deep Foundations Institute. Hawthorne, New Jersey. July 2013.

5. Fantaye, S. Y., Sun, L., Law, T.C.M, and Poletto, R. Design and Performance of Buttress Walls Constructed by the Slurry Wall Method for the Vehicle Security Center, World Trade Center, NYC. Presented at Foundation Challenges in Urban Environments, ASCE Metropolitan Section/Geo-Institute Chapter. New York City. May 16, 2013.

6. Randolph M. F., and Wroth, C. P. Analysis of deformation of vertically loaded piles, Journal Geotechnical Engineering Division, American Society of Civil Engineers, Proceedings Paper 14262, Vol 104 (GT12). 1972.

7. RISA-2D Version 8.0. RISA Technologies, Foothill Ranch, California. 2007.