Geotechnical Design: Deep Water Pontoon Mooring Anchors

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1 1077 Geotechnical Design: Deep Water Pontoon Mooring Anchors Ben Upsall 1, Garry Horvitz 1, Bob Riley 2, Tripp Howard 2, Kimball Olsen 3, and James R. Struthers 4 1 Hart Crowser, Inc., 1700 Westlake Avenue N., Suite 200, Seattle, WA, 98109, USA, (206) , ben.upsall@hartcrowser.com, garry.horvitz@hartcrowser.com. 2 KPFF Consulting Engineers, 101 Stewart Street, Suite 400, Seattle, WA, 98101, USA, (206) , briley@kpffspd.com, thoward@kpffspd.com. 3 Kiewit Engineering Co., East Fremont Drive, Centennial, CO, 80112, USA, kimball.olsen@kiewit.com. 4 Washington State Department of Transportation, 1655 South 2 nd Avenue, Tumwater, WA, 98512, USA, struthj@wsdot.wa.gov. ABSTRACT Anchoring floating structures in deep water can be challenging because of varying water depths, soil types, uses, and environmental loads. Three different anchor types, drilled shaft, gravity, and fluke anchors were used to meet these challenges for the new SR520 Evergreen Point Floating Bridge and Landings project in Seattle, Washington. The anchors were designed to resist the horizontal and vertical components of the maximum resultant anchor cable load of 570 tons under static and seismic conditions. The anchors are being proven through exhaustive full-scale field load tests. Verification tests are being loaded to a maximum of 570 tons, performance tests to 425 tons, and proof tests on all remaining production anchors to 300 tons. All anchors tested to date, including all performance and verification tests for all three anchor types, have met all (creep and total displacement) testing requirements. INTRODUCTION This paper presents a case history of the geotechnical challenges of designing and testing large-scale submarine anchors that will permanently moor the floating concrete pontoons for the SR520 Evergreen Point Floating Bridge and Landings Replacement Project in Seattle, Washington. The new floating bridge will replace an existing floating bridge and improve a major traffic corridor between Seattle and Bellevue, connecting the north-south I-405 and I- 5 highways. The new bridge will consist of 23 primary concrete pontoons and as many as 84 supplemental stability pontoons at maximum future capacity. At approximately 1.5 miles long, it will be the longest floating bridge in the world upon completion.

2 1078 Much like a floating dock, the floating bridge will be subjected to wind, wave, current, potential vessel impact, and seismic seiche forces. In order to maintain the floating bridge alignment, the loads on the pontoons must be resisted by anchors set into the sediments at the bottom of Lake Washington. Each pontoon will be held in place by at least two 3-1/8 inch structural strand cables attached to anchors installed in water ranging from 30 to 210 feet deep. The design of the anchorage system for the new bridge was complicated by: Variable subsurface conditions ranging from very dense glacially overridden soils to thick deposits of very soft diatomaceous sediments with unique engineering properties; Limited strength testing of the soft diatomaceous sediments from which to develop passive resistance values for anchor design. Very demanding lateral load requirements consisting of a 570-ton load applied to each anchor s low-angle cable connection; and Testing requirements that were difficult both to measure with the specified accuracy and to verify in deep water. SITE AND SUBSURFACE CONDITIONS Site. The site is located on Lake Washington spanning from just north of the Madison Park neighborhood of Seattle on the western side to Medina on the eastern side (Figure 1). An existing bridge currently spans this stretch of Lake Washington about two hundred feet to the south of the proposed bridge alignment. The existing bridge consists of two lanes in each direction and is a major east-west lifeline connecting north-south highways (I-5 and I-405) on either end. Figure 1. Vicinity Map Lake Washington is a long north-south glacially formed lake feature, carved out by the advance and retreat of several different glaciations. It is almost 18 miles long, as much as 4 miles wide, and up to about 214 feet deep. The lake bottom profile (Figure 2) along the proposed bridge alignment resembles the cross-section of a bath tub with a deep flat bottom and steep walls on the east and west sides.

3 1079 Soil. The subsurface conditions at the site were based on over 50 geotechnical explorations and hundreds of associated laboratory tests. In general, the steep sidewalls of the profile consist of very dense glacially overconsolidated sediments and the deep flat lake bottom consists of very soft diatomaceous silt (with unique engineering properties) and unconsolidated lacustrine sediments. Figure 2. Generalized Lake Bottom Profile ANCHOR TYPES, SELECTION, AND INSTALLATION Three different anchor designs were proposed to provide permanent moorage for the floating concrete pontoons: drilled shaft, gravity, and fluke. Bathymetric, geologic, and functional constraints provided the framework for selecting the type of anchor to be used at each location along the bridge alignment. Drilled Shaft Anchors. A total of five ten-foot-diameter drilled shaft anchors, ranging from 72 to 86 feet long, were used at the eastern- and western-most reaches of the project. These drilled shaft anchors are located in relatively shallow water (25 to 40 feet deep) in locations where the very dense glacially overconsolidated sediments were present near the mudline. The drilled shaft design, where only about 6 feet of shaft was exposed above the mudline, was well suited to these locations because sufficient clearance for deep-draft vessels was a project requirement. The drilled shaft anchors were installed through a steel casing embedded approximately 10 feet into the mudline. The excavations were performed under a synthetic slurry head to maintain an open hole and to minimize sloughing of the sidewall soils beneath the bottom of the casing. Gravity Anchors. A total of eight gravity anchors, 40 feet by 40 feet in plan and 23 feet tall, loosely resembling a reinforced concrete box without a lid (Figure 3), were designed for the steep side slopes of the lake bottom profile where the very dense glacially overconsolidated sediments were present near the mudline. The deeper water provided sufficient clearance for deep-draft vessels above the anchors which are exposed as much as 25 feet above the mudline.

4 1080 Figure 3. Basic Geometry of Gravity Anchor The gravity anchor design calls for first dredging a horizontal pad into the side slope of the lake bottom profile followed by placing a 2-foot thick shoulder ballast pad over the subgrade soils. The anchors would then be flooded and placed on the prepared gravel pad and finally filled with approximately 1,700 tons of ballast rock. Fluke Anchors. A total of 45 fluke anchors, which resemble a vertical plate anchor made of reinforced concrete (Figure 4), are used in the deep soft diatomaceous sediments on the lake bottom. The fluke anchors are 35 feet wide and 26 feet tall and were installed vertically into the soft lake bottom sediments. Figure 4. Basic Geometry of Fluke Anchor The fluke anchors were each lowered into place with a crane and installed into the soft lake bottom sediments using gravity along with the aid of an external water jetting frame. The area in front (towards the pontoon) of each anchor was then ballasted in a specific geometry with 1,250 tons of ballast rock to improve lateral resistance.

5 1081 ANCHOR DESIGN Due to the complex loading conditions, environmental constraints, soil-structure interaction, and testing complications, the design of each anchor type was completed in close cooperation between the project s geotechnical engineers, structural engineers, and contractors. The result is three vastly different anchor designs specifically tailored to meet the requirements for anchor function in difficult site conditions and the desire for minimizing the complexity of construction and testing. Drilled Shaft Anchors. The geotechnical design of the laterally loaded drilled shaft anchors had to consider both the lateral and vertical components of the resultant cable load (570 tons) in addition to seismic slope stability. The vertical component was easily resisted by the weight of the reinforced concrete shaft itself. The lateral component was evaluated using a p-y based lateral pile analysis method. Static and seismic slope stability was evaluated using limit equilibrium slope stability methods. Analyses for three different soil conditions were performed: static, pseudostatic, and liquefied. Based on the results of the stability analyses: The slopes into which all five drilled shafts will be installed appear to be stable during static soil conditions; The slopes into which three of the drilled shafts will be installed were found to have insufficient factors of safety against sliding during the pseudostatic (design level seismic) event; and The slopes into which two of the drilled shafts will be installed were found to have insufficient factors of safety against sliding with liquefied conditions. The drilled shafts that had insufficient factors of safety against sliding during the design level seismic event were further evaluated using probabilistic seismic slope displacement estimate methods (Bray and Travasaru, 2007) and were predicted to move on the order of 1 to 5 feet during the seismic event. These seismic slope displacements were used in conjunction with the limit equilibrium slope stability analyses to develop additional lateral loading profiles on the affected drilled shafts. One of the three affected shafts was capable of resisting the additional lateral loads due to seismic slope displacements without damaging the shaft largely because the failure surface was relatively shallow (about 10 feet below mudline). The other two drilled shafts were predicted to be intersected by a deep failure surface about 60 feet below the mudline. The loads created by the predicted slope movements were found to be too large to be reasonably resisted and these two drilled shafts would likely be irreparably damaged following a design level seismic event. The owner confirmed, due to redundancy in the system, that the loss of these two drilled shaft anchors during the seismic event would not likely jeopardize the overall performance of the bridge and decided to accept the risk of anchor failure and the long-term operations and maintenance responsibility if the drilled shafts needed to be replaced.

6 1082 Gravity Anchors. The geotechnical design of the laterally loaded gravity anchors had to consider both the lateral and vertical components of the resultant cable load (570 tons) in addition to seismic slope stability. The vertical component was easily resisted by the weight of the concrete anchor filled with 1,700 tons of ballast rock. The lateral component was evaluated by modeling the anchor as a frictional sliding block. The primary challenges to the lateral capacity evaluation were determining the frictional characteristics of the in-situ soil beneath the anchor. A level pad was excavated on the steep side slope of the lake using a clam-shell bucket. The clamshell was assumed to produce a hummocky surface, which resulted in some uncertainty when estimating how much soil would directly contact the anchor base. These challenges were circumvented by adding a minimum two-foot-thick gravel leveling pad between the clam-shell excavated surface and the bottom of the anchor. This provided an easily verified interface friction value as well as a level pad providing more uniform anchor contact. The pad also reduced the concerns that the anchors could exhibit long-term creep behavior under constant loading conditions. One additional component to the lateral capacity design was the inclusion of teeth or corrugations on the base of the concrete box (Figure 5) running perpendicular to the anchor cable direction. These corrugations allowed for the use of the internal friction angle of the gravel itself rather than just the interface friction angle between gravel and concrete by forcing the failure surface down into the gravel pad. Figure 5. Gravity Anchor Corrugation Static and seismic slope stability was evaluated using limit equilibrium slope stability methods. Analyses for three different soil conditions were performed: static, pseudostatic, and liquefied. Based on the results of the stability analyses: The slopes onto which the eight gravity anchors will be set appear to be stable during static soil conditions; The slopes onto which seven of the gravity anchors will be set were found to have insufficient factors of safety against sliding during the pseudostatic (design level seismic) event; and

7 1083 The slopes onto which all eight gravity anchors will be set appear to be stable during liquefied soil conditions. The gravity anchors that had insufficient factors of safety against sliding during pseudostatic soil conditions were further evaluated using probabilistic seismic slope displacement estimate methods (Bray and Travasaru, 2007). Two separate cases were evaluated for each affected anchor: 1) the critical anchor case which was defined as the critical failure surface that extends beneath the anchor leveling pad, and 2) the critical impact case which was defined as the critical failure surface located upslope of the gravity anchor that could potentially impact the side of the anchor during seismic slope displacement. Seismic slope displacements for the affected gravity anchors were predicted to be on the order of 2 to 4 feet and 0 to 4 feet for the critical anchor and critical impact cases, respectively. Movements of less than 4 feet for the critical anchor case fell within the project requirement that the anchors not move more than 10 feet from their original location as a result of the design level seismic event. Movements of less than 4 feet for the critical impact case were mitigated by excavating additional material upslope of the anchors and providing a minimum 5-foot buffer between the upslope gravity anchor walls and the toe of the soil slope above the anchor. Fluke Anchors. The geotechnical design of the laterally loaded fluke anchors had to consider both the lateral and vertical components of the resultant cable load (570 tons); however, due to the relatively flat lake bottom profile, seismic slope stability was not a concern. The vertical capacity design component was easily resisted by the skin friction along the perimeter of the fluke anchor and the weight of the anchor itself. The more complex lateral capacity design was performed using lateral earth pressure theory and a deformation based modeling program (FLAC). Two main stress cases were evaluated representing total and effective stress soil conditions. The total stress condition represented short duration (anchor test and seismic) loading while the effective stress condition represented the long duration (service) loading. The first challenge to the lateral capacity evaluation was determining the engineering characteristics of the in-situ diatomaceous silt present at the bottom of the lake. The diatomaceous soil was extremely difficult to sample and test. The significant concentrations of diatoms made the estimation of several common engineering properties difficult. Of these properties, those of which can be obtained or correlated from highly disturbed samples (such as water content and atterberg limits) were rendered irrelevant for typical soil types in the region due to the nature of the silt. The useful data included buoyant unit weight (average of about 12.5 pounds per cubic foot), diatom content percentage (ranging from about 0 to 60 percent), four sets of CU triaxial test results, and four load-displacement curves (Figure 6) from tests completed on fluke anchors installed to moor the existing bridge. The average strength parameters deduced from the CU triaxial tests for the total stress case included an internal friction angle of about 9 degrees and a cohesion value of

8 1084 approximately 170 pounds per square foot (psf). The average strength parameters deduced from the CU triaxial tests for the effective stress case included an effective internal friction angle of approximately 33 degrees and an effective cohesion value of approximately 50 psf. Load Displacement Curves Displacement in Feet EN ES KN KS FLAC Horizontal Load in Kips Figure 6. Historic Fluke Anchor and FLAC Model Load-Displacement Curves The FLAC model was used to independently back-calculate the same engineering properties of the lake-bottom soils using the load-displacement curves from the existing bridge. The model geometry matched the existing fluke anchor sizes and the model was calibrated to the curves by adjusting friction angle, cohesion, and stiffness of the diatomaceous soil unit. Figure 6 presents the four original load-displacement curves and the resulting load-displacement curve from the calibrated FLAC model. The resulting soil parameters from the FLAC model for the total stress case were an internal friction angle of about 9 degrees and a cohesion value of approximately 160 psf. The resulting soil parameters for the effective stress case were an effective internal friction angle of about 29 degrees and a cohesion value of 0 psf. These values correlate well with the average strength parameters obtained from the limited CU triaxial tests that were available. Once the engineering properties for the soil model had been back-calculated, the proposed fluke anchor geometry and ballast rock quantity were optimized for the maximum design cable load of 570 tons. The estimation of the ballast rock quantity was difficult to determine as it was unknown how much of the ballast rock, which would likely be placed via tremie tube from a barge, would sink into the soft diatomaceous silt. Therefore, three cases of ballast rock geometry were evaluated so as to bracket the potential range of solutions.

9 1085 Case 1 assumed that the ballast rock pile remains intact on top of the mudline, Case 2 assumed that the ballast rock mixes with or displaces the soil to a depth equivalent to the height of the ballast rock pile in Case 1, and Case 3 assumed that the ballast rock mixes with and disperses through the soil to a depth equivalent to twice the height of the ballast rock pile in Case 1. The three potential ballast rock geometry cases were each evaluated for the two different stress condition cases using the FLAC model. The optimized design included 1,250 tons of ballast rock placed in a stepped configuration in front of the anchor (Figure 4). This ballast rock configuration satisfied the loading requirements for all 6 different geometry and stress condition case combinations. ANCHOR TESTING Because of the difficulty in determining the engineering properties of the soils and the fact that the design anchor load requirements are almost four times higher than those of the existing bridge anchors (570 tons versus about 125 tons), verification testing was required in the contract. An anchor testing barge was designed and outfitted specifically for this project. Every production anchor will be proof tested in addition to a comprehensive suite of performance and verification test requirements. During each test, the anchors will be subjected to load hold periods ranging from 1 to 24 hours in duration to confirm that the soils are not susceptible to long-term creep failure. The creep rates must be decreasing over successive log cycles of time and must not exceed threshold values in order for the anchor to be accepted by the owner. The anchors must also have less than 10 feet of total displacement during the test. A data acquisition and processing system was designed for this project to monitor the testing barge position (continuous determination of latitude, longitude, elevation, pitch, roll, yaw, and tilt of the barge). The system also collected the tensile load in the test cables, the take-up of the test cables as the loads were applied, and the wind speed. This information was all time-stamped as it was collected and the processing programs calculated movements of the barge that was presented in real-time. The system also used the cable load, barge position, and cable take-up measurements in conjunction with theoretical cable catenary calculations to evaluate the amount of anchor deformation that had occurred during the test. The shaft anchors displayed negligible creep movement during the load holds and exhibited less than half of an inch of total displacement. This behavior generally met the expectations for laterally loaded drilled shafts in very dense soils. The gravity anchors displayed negligible creep movement during the load holds and exhibited total displacements on the order of less than about 12 inches. Creep movement was not anticipated due to the granular nature of the gravel leveling pad. The total displacements can likely be explained as the setting of the concrete teeth or corrugations on the bottom of the anchors into the gravel leveling pad.

10 1086 The fluke anchors exhibited negligible creep movement during the load holds and exhibited total displacements of less than about 4 feet. Some creep behavior had been anticipated because of the slowly draining nature of the diatomaceous silt. Anchor displacement during the test was anticipated by the designers as movement was considered necessary to fully engage the strength of the uncompacted pile of ballast rock in front of the fluke anchor. CONCLUSIONS The geotechnical design and testing of these deep water mooring anchors was complicated by: Highly variable subsurface conditions and bathymetric profile; Limited information on the engineering properties of the lake-bottom soils; Very demanding lateral load requirements; and Stringent acceptance criteria for deformation and creep on anchors set and tested in deep water conditions. These complications were addressed and overcome by the following: Three different anchor designs were used to meet the bathymetric, geologic, and functional constraints of the project; Anchor scale and design ingenuity were used to meet the demanding lateral load requirements; and A time stamped data tracking and monitoring system was used to provide real-time anchor testing results with an appropriate level of resolution. All anchors tested to date, including all performance and verification tests for all three anchor types, have met all (creep and total displacement) testing requirements. REFERENCES Bray, J.D., and Travasarou, T. (2007). Simplified procedure for estimating earthquake induced deviatoric slope displacements. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, 133(4), Idriss, I.M., and Boulanger, R.W. (2008). Soil Liquefaction during Earthquakes. EERI, Oakland, California. Itasca, (2011). Fast Lagrangian Analysis of Continua (FLAC) 2-D. Version