DEEPENING AND SEISMIC REHABILITATION OF THE TERMINAL 5 WHARF, PORT OF SEATTLE

D.D. Lindquist1, M.P. Whelan2, and G.E. Horvitz3

ABSTRACT

In the era of post-panamax container vessels, ports are increasing their berthing depths to accommodate larger ships. This paper presents the results of geotechnical engineering analyses related to deepening three berths along the Terminal 5 Wharf, in the Port of Seattle, Washington. The Port of Seattle hired KPFF Consulting Engineers and Hart Crowser to investigate deepening the North and Middle Berths by 10 feet and the South Berth by 5 feet. The intent of the analyses was to determine what, if any, structural support was necessary to maintain the static and seismic stability of the under pier slope and to prevent increased deflections of the wharf structure in a seismic event. The stability was evaluated using a limit equilibrium slope stability program (UTEXAS3) and a 2-dimensional finite difference program (FLAC) to calculate displacements and loads on the structure before and after dredging. Soldier piles, sheet piles, and “pinch piles” under the wharf to densify the potentially liquefiable soils were modeled in these analyses. Based on the results of these analyses, sheet piles were used for berths being deepened by 10 feet, and a wall of closely spaced soldier piles was installed for areas deepened by 5 feet. The potential for pinch piles to improve wharf performance in a seismic event was also studied, on a preliminary basis. Keywords: Numerical modeling, dredging, case study, slope stability, pinch piles.

INTRODUCTION

The purpose of this work was to evaluate the feasibility of deepening the wharf at Terminal 5 and to provide the Port of Seattle with recommended method(s) for avoiding adverse impacts to the existing wharf structure while deepening the berths. Prior to this work, the mudline elevation at Terminal 5 was -40 feet (except for the northern 600 feet of the North berth). The Port of Seattle wished to deepen the entire 2,700-foot length of the Terminal 5 Wharf as follows: North Berth (1,000-foot length) deepened as much as 10 feet, to elevation -50 feet; Middle Berth (1,000-foot length) deepened by 10 feet, to elevation -50 feet; and South Berth (740-foot length) deepened by 5 feet, to elevation -45 feet.

The layout of these berths and the location of geotechnical explorations are shown on Figure 1. The intent of the analyses was to determine what, if any, type of structural support would be necessary to maintain the existing static and seismic stability of the under pier slope and the wharf structure while accomplishing the desired deepening.

1 Lindquist, D.D., Staff Geotechnical Engineer, Hart Crowser, Inc., 1910 Fairview Avenue East, Seattle, WA 98102 USA, 206-324-9530 (phone), 206-328-5581 (fax), e-mail: ddl@hartcrowser.com 2 Whelan, M.P., Senior Project Geotechnical Engineer, Hart Crowser, Inc., 1910 Fairview Avenue East, Seattle, WA 98102 USA, 206-324-9530 (phone), 206-328-5581 (fax), e-mail: mpw@hartcrowser.com 3 Horvitz, G.E., Senior Principal Geotechnical Engineer, Hart Crowser, Inc., 1910 Fairview Avenue East, Seattle, WA 98102 USA, 206-324-9530 (phone), 206-328-5581 (fax), e-mail: geh@hartcrowser.com

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Figure 1 – Site and Exploration Plan

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SEISMIC BASIS OF DESIGN AND LIQUEFACTION POTENTIAL

Two levels of seismic events were analyzed in this study: The Operating Level Event, which is a nominal “100-year” event, with a 50 percent probability of exceedence

in 50 years. (Statistically, this equates to a 72-year return interval.) Such a seismic event was analyzed with a 6.5 magnitude and a 0.20 g peak horizontal acceleration.

A Contingency Level Event, which is a nominal “500-year” event, with a 10 percent probability of exceedence in 50 years. (Statistically, this equates to a 475-year return interval.) Such a seismic event was analyzed with a 7.5 magnitude and a 0.34 g peak horizontal acceleration.

The liquefaction potential at the site was analyzed for both of the design level earthquakes. Site soils consist of loose to dense sand and silty sand with occasional layers of soft to stiff sandy silt and clayey silt, as presented in the generalized subsurface profile on Figure 2. Many of the primarily granular soils (sands) will undergo fairly widespread liquefaction in the 500-year seismic event, particularly below the North Berth, where over 50 percent of the Standard Penetration Test (SPT) values obtained in the borings are low enough to imply susceptibility to liquefaction. Use of the standard analytical method, after Seed et al. (1985) and updated at the NCEER Workshop (1996), predicted liquefaction to depths of more than 80 feet below existing mudline. To the south, the liquefying zones appeared to become smaller and more widely spaced. Throughout the wharf length, in the 100-year seismic event, only scattered zones of liquefaction are expected, based on the described analysis.

ANALYSIS TECHNIQUES

The stability of the pier was evaluated using two methods: the slope stability program UTEXAS3, and the two-dimensional finite difference program FLAC (Fast Lagrangian Analysis of Continua). The UTEXAS3 slope stability computer program uses the method of slices and Spencer’s limit equilibrium procedure to determine a factor of safety for a given potential failure geometry. The program can take an initial estimate of the failure surface and search for the “critical” surface (i.e., the surface that has the lowest factor of safety). The FLAC program provides information on slope displacements, which control the future serviceability of the site. FLAC is a two-dimensional explicit finite difference program that simulates the behavior of earth materials and structural members (beams, piles, and cables), which can be inserted into the model with their corresponding soil/structure interaction properties. The use of the FLAC program for analyzing deflections of wharf structures in a seismic event, similar to this project, was previously documented for a new container wharf at the Port of Long Beach, California (Johnson et al., 1998). FLAC was used to assess displacements after deepening and to analyze the effectiveness of various structural upgrade options. The typical FLAC grid and selected locations where displacements were compared are shown on Figure 3. These locations were selected to be representative of the soil and structure in general, and were used to allow relative comparison of slope and structure displacements under various conditions.

RESULTS OF ANALYSES – CONDITIONS PRIOR TO DREDGING

Slope Stability Analyses. Slope stability was analyzed for slope conditions prior to dredging using UTEXAS3 and the geometry shown on Figure 4. The analyses indicated that under conditions prior to dredging, the slope had a

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Figure 2 – Generalized Subsurface Profile

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factor of safety of 1.2 to 1.3 under static loading (assuming a friction angle of 32 degrees and low tide, which is the critical case for slope stability). This factor of safety applies to potential failure surfaces that are several feet deep or more. The factor of safety was lower for shallow, surficial ‘raveling’ types of soil movement, although in reality this is partially mitigated by the presence of riprap on the slope, which was not explicitly modeled in our analysis. A design-level seismic event was modeled using both pseudo-static and residual strength techniques. In pseudo-static slope stability analyses, a horizontal acceleration (typically half the design level acceleration) is applied to the soil to account for seismic loading. Under pseudo-static conditions, the factor of safety for the existing slope dropped to approximately 0.9 to 1.0, depending on whether the 500-year or 100-year seismic event was modeled. In residual strength slope stability analyses, the strength of liquefiable soils is lowered to their residual (liquefied) shear strength, which is usually much lower than their static shear strengths. A single liquefied layer was assumed between elevations -40 and -65 feet. This 25-foot thickness was similar to the total liquefying thickness determined from our borings, and represents a condition midway between the 100-year and 500-year seismic events. Residual strength was determined using the procedure developed by Seed and Harder (1990) based on SPT blow counts observed in the explorations. This resulted in a residual strength of approximately 220 psf. The residual strength analysis yielded factors of safety well below 1.0 for the pre-dredging scenario. Failure surface geometries were similar to those shown on Figure 4. Both the pseudo-static and residual strength analysis, therefore, indicate the existing wharf structure will have relatively low seismic stability, and thus a potential for significant deformations. Deformation Analyses. A section of the wharf near Bent 74 (Pier Station 21+00) was modeled using FLAC. This section is typical of the 400-foot-long deepened section of the North Berth, and has a bulkhead relieving platform and under pier slopes of 1.5H:1V. Seismic loading conditions were modeled using pseudo-static conditions and residual strength conditions, as was done for the slope stability analyses. As was indicated by the slope stability analyses, the deformations based on pseudo-static conditions yield substantially higher stability levels (i.e., lower deflections) than those for the residual strength conditions. Under the critical residual strength conditions, the wharf had displacements as shown in the first row of Table 1. Figure 5 illustrates the structure and post-liquefaction displacement vectors for seismic conditions prior to dredging.

Table 1- Seismically Induced Displacements Calculated by the FLAC Model (under Residual Strength Conditions)

Model Total Displacement at Various Locations in Inches Location (1)* (2)* (3)* (4)* (5)* (6)* Conditions Prior to Dredging 44 49 48 57 41 39 Dredged with AZ-48 Sheet Pile 38 47 46 58 40 39 Dredged with AZ-48 Sheet Pile and Pinch Piles

24 25 26 41 23 23

* These locations are shown on Figure 3.

RESULTS OF ANALYSES – STRUCTURAL UPGRADE OPTIONS

Dredging to elevation -50 feet at the North Berth required the removal of as much as 10 feet of soil from the toe of the slope. It was determined that the factor of safety for deep-seated failures under static conditions would drop to less than 0.8 if dredging were done without any structural upgrade.

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Figure 3 – Grid used in the FLAC Analyses

Figure 4–Stability Results: Conditions prior to Dredging

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Figure 5–Post-liquefaction Displacement Results: Conditions prior to Dredging

Therefore, the relative value of a variety of structural upgrade possibilities was compared, as measured by their ability to maintain the existing level of stability of the slope-structure system under both static and seismic conditions. The design philosophy adopted at this stage was that the structure would be returned to its current stability levels, and would not be upgraded beyond its condition prior to dredging, recognizing that the current structure exhibits marginal stability with substantial expected deformations under design-level seismic conditions. The structural upgrades that were assessed include: Installation of a sheet pile wall at the toe of the slope; Installation of soldier piles in the toe, between the existing fender piles; and Installation of pinch piles below the deck.

Assessment of Sheet Piles (North and Middle Berths). A structural upgrade at the toe of the slope would essentially serve to replace the passive resistance provided by the soil mass that was dredged. Modeling the dredged slope under seismic conditions using UTEXAS3 indicated that a force ranging from about 8,000 to 10,000 pounds per linear foot would be needed to achieve seismic factors of safety that are equivalent to current conditions (see Figure 6). This agrees with the passive force provided by the removed soil wedge, as determined by Rankine theory. It was assumed that this force would be applied at one-third of the wall’s retained height, given a roughly triangular pressure distribution behind the wall. A sheet pile wall could provide a resisting force of this magnitude. Available sections of sheet pile included: AZ-18, with a section modulus of 33.5 cubic inches/linear foot; PZ-38, with a section modulus of 46.8 cubic inches/linear foot; AZ-48, with a section modulus of 89.3 cubic inches/linear foot; and A composite section of sheet piles and H-piles.

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Figure 6–Stability Results: Conditions after Dredging with Sheet Pile Wall Use of the AZ-18 and PZ-38 sheet pile sections produced computed displacements larger than those of the pre-dredging conditions, so a stiffer section, AZ-48, was selected. Analyses using the AZ-48 section produced the computed displacements presented in the second row of Table 1. These are, on average, 4 percent less than those expected for conditions prior to dredging. Figure 7 illustrates the structure and post-liquefaction displacement vectors for the conditions following installation of an AZ-48 sheet pile wall and dredging to elevation -50 feet. The sheet pile wall section was designed to resist the worst case, or residual strength, scenario. The FLAC analyses indicated that the maximum bending moment of the AZ-48 section would be reached for some part of its length. The bending that is occurring in the section and the total amount of deflection reflect this. Evaluation of the input stress-strain characteristics of the sheet pile steel led to the conclusion that the section would undergo plastic yielding, but would not break from the model loads. Assessment of Soldier Piles (South Berth). The South Berth differs from the North and Middle Berths in that the dredging elevation is shallower (to -45 feet rather than -50 feet), and the constructed geometry of the wharf is different. While the relieving platform that exists throughout the Middle Berth and much of the North Berth does not exist for a portion of the South Berth, slope stability analyses were run to determine if this condition would be offset by the stabilizing influence of the shallower slope, better soil conditions, and lesser dredging depth at the South Berth. The analyses indicate that the overall static and seismic stability of the South Berth is better than that of the Middle and North Berths, primarily because of the lesser dredging depth.

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Figure 7–Post-Liquefaction Displacement Results: Conditions after Dredging with Sheet Pile Wall Since the South Berth is being dredged to a shallower elevation, maintaining its stability level after dredging requires a less robust structural addition. Slope stability analyses indicate that an equivalent wall force of approximately 2,000 pounds per lineal foot is sufficient to replace the passive force lost by the dredging. This would not require the use of interlocking AZ-48 sheet piling. An appropriate option, instead, would be to use HP14x73 soldier piles at the toe of the slope with a spacing of 3 to 4 feet on center. Assessment of Pinch Piles. Pinch piles, an array of timber piles installed into the slope, were used in the 1998 construction of the northernmost 400 feet of the North Berth to increase the slope’s stability in a seismic event. Pinch piles were also assessed for deepening at the rest of the North Berth. Pinch piles (sometimes referred to as “compaction piles”) benefit the stability of the slope in three distinct and substantial ways: They increase the overall structural stiffness of the soil mass because they are stronger and more resistant to

shear than the soil; They increase the soil density by volumetric displacement of the soil as the piles are installed; and They densify soils dynamically by vibration as they are installed.

Figure 8 illustrates the slope stability modeling of pinch piles, which assumed 7 to 8 pinch piles up to 50 feet in length, spaced 2 to 3 rows per bent. Under pseudo-static conditions, the presence of the pinch piles increases the factor of safety by approximately 0.1. For the more critical residual strength case, the factor of safety is increased by about 0.2, to a value of 0.5 to 0.6. These numbers indicate that while the pinch piles have a beneficial effect on slope and structural stability, they are not necessarily sufficient to make the slope fully stable in a design-level seismic event if wide-spread liquefaction occurs.

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Figure 8–Stability Results: Conditions after Dredging with Sheet Pile Wall and Pinch Piles Additional deformation analyses were performed to include the beneficial effects of soil densification resulting from both volumetric displacement and vibratory action. The effects of volumetric displacement alone, for pinch piles installed on a 5-foot spacing, was calculated to densify the soil to a residual strength of approximately 300 psf. Additional soil densification from pile driving vibrations, was assumed to raise the residual strength further, to 500 psf. This appears to be a conservative assumption, since Fang (1991) cites relative densities of 75 to 80 percent as being reachable by pinch piles installed on 4- to 5-foot centers, and this relative density range is nearly enough to prevent liquefaction altogether. Nonetheless, even with this potentially conservative selection of residual strength, the deflections of the wharf structure in a seismic event are reduced by one-half to one-third. This is shown on the bottom row in Table 1. Figure 9 illustrates the structure and post-liquefaction displacement vectors for this case. The pinch piles would have to be installed below the existing deck, since removing and replacing the deck would be cost-prohibitive. For an existing wharf, it was assumed that the piles could be installed at an angle of up to 45 degrees, enabling the driving hammer to be held outside of the pierhead line while the piles are driven under the deck. (Local contractors have indicated an ability to drive the piles at this angle of batter.)

REVIEW OF CONSTRUCTION

The deepening of the North Berth was completed in the Spring of 1999. AZ-48 sheet piling was installed along the southern 400 feet of the North Berth. This section of the paper discusses the construction, observations of constructability, and wharf deflection during the construction process.

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Figure 9–Post-liquefaction Displacement Results: Conditions after Dredging with Sheet Pile Wall and Pinch Piles

Sheet Pile Installation. Although a sheet pile wall was selected as the best option for supporting the North Berth, there was some remaining concern about the constructability of this system. In particular, there was some question about how well the sheet piling would penetrate through riprap on the slope. If the sheet pile was unable to penetrate, then a series of soldier piles would be considered instead. Therefore, prior to sheet pile installation at the North Berth, a test installation program was conducted by KPFF and Manson Construction in October 1998. The pertinent conclusions from the program were: Sheet piles had no trouble penetrating the near-surface riprap layer, with and without driving shoes; A vibratory hammer was sufficient for sheet pile driving; The driven sheet piling was pulled out after the test, and exhibited no signs of damage; and Surficial sloughing, about 1-foot-deep, was observed on the slopes after the test sheet pile driving program was

complete. Similarly, little trouble was experienced installing AZ-48 sheet piling along the North and Middle Berths when construction and dredging occurred in January and February of 1999. The contractor reported very little resistance to driving sheet piling, even though a (nominal) 5-foot-thick riprap “key” was expected at the base of the slopes, based on original slope designs. It is possible that this riprap “key” was no longer in existence, or was less extensive than anticipated. It is unknown whether this is the result of its original construction, or whether riprap was removed by maintenance dredging activities over the years. Soldier Pile Installation. The contractor reported little resistance to driving of the soldier piles. However, quality assurance/quality control was very difficult because soldier piles do not interlock and they were being driven approximately 45 feet below water. Because of uneven spacing of the soldier piles, the contractor was required to go back through and drive an additional number of piles into the largest gaps between soldier piles. Given the difficulties encountered in soldier pile installation, in retrospect an interlocking retaining system (i.e., sheet piles) would have been preferable. Pinch Pile Installation Test Program (Planned for 2000). The next step in the Port of Seattle’s upgrade of the Terminal 5 wharf will be to revisit the use of pinch piles for improving the seismic performance of the North Berth

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slopes. As mentioned previously, the northernmost 400 feet of the North Berth, constructed in 1998, includes an array of pinch piling for this purpose. The Port is considering installing pinch piles in the southern 600 feet of this berth as well. As was mentioned previously, the challenge of this plan is that the pinch piles would need to be installed below an existing pier deck. The Port of Seattle is currently considering a pinch pile installation test program in which the constructability of this option can be tested. A likely form of this program would be to install an array of pinch piles below a 100- to 200-foot-long stretch of wharf. One aspect of this program also might be to advance geotechnical borings in the slope, before and after the pinch piling is installed, to get a quantitative measure of the amount of soil densification that the pinch piling causes. This in turn would lead to a “reality check” on the ability of pinch piles to hinder liquefaction in a seismic event.

CONCLUSIONS

The recent deepening of the Terminal 5 wharf at the Port of Seattle utilized both traditional and state-of-the-art engineering and analytical techniques, to provide deeper berthing drafts for the Port while not adversely affecting stability in an area where seismic stability is a primary concern. Two-dimensional limit equilibrium analyses (UTEXAS3) were conducted in tandem with finite difference displacement modeling (FLAC) to assess the performance of the wharf under several pre- and post-deepening scenarios. The combination of these two techniques yielded consistent results and appears to be a very powerful technique for assessing performance of structures such as these under various structural upgrades. The selected rehabilitation techniques (i.e., high-modulus sheet pile wall for berths being deepened 10 feet, and closely spaced soldier piles for berths being deepened 5 feet) are intended to replace the loss of passive resistance from dredging. As such, modeling indicates that with these structural upgrades in place, the performance of the deepened berths in a seismic event will theoretically match the performance before the deepening took place. Construction and deepening were recently accomplished and generally went smoothly. The lack of resistance to sheet pile and soldier pile installation was somewhat surprising and may reflect the absence of a full riprap “key” at the base of the slope. Maintaining a consistent spacing between adjacent soldier piles turned out to be difficult and in the future sheet piles may be the preferred option. It is important to note that the slope upgrade measures described herein were not designed to improve seismic performance, but rather to maintain existing stability levels. The Port of Seattle recognizes that some risk of damage does continue to exist at these facilities, should a design-level seismic event occur. It appears that additional improvement could be achieved by adding pinch piling to the under pier slope, although further study (i.e., a test program of pinch pile installation) is currently under consideration, to quantify the degree of geotechnical improvement and to assess the constructability of such an option.

ACKNOWLEDGEMENTS

The authors of this paper would like to recognize the critical roles played in this project by Gary Wallinder and George England at the Port of Seattle, as well as the engineering efforts of Bill Conley, Rick Johnson, and Kamyar Nikzad at KPFF Consulting Engineers in Seattle, Washington.

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Minneapolis, USA, Vol. I to IV.

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