Ten Years of SR 520 Bridge Project

Ten Years of SR 520 Bridge: West Connection Bridge (WCB)
April 14, 2017

This is the fourth in a series of articles discussing different projects associated with the SR 520 Bridge project for the Washington State Department of Transportation (WSDOT). View all posted articles here.

The West Connection Bridge (WCB) routes eastbound traffic from the existing West Approach Bridge to the new floating bridge. In a future phase, 10 of the 19 WCB drilled shaft foundations will be reused to support the proposed West Approach Bridge – South (WABS). The WCB is a vital link to keep traffic flowing during the SR 520 Bridge Replacement project.

The WCB is a 1,335-foot-long, 65- to 160-foot-wide, 11 pier, conventional bridge structure, extending entirely over water. Lake water depths vary from about 10 to 35 feet along the alignment. The eastern end of the bridge connects to Pier 36 which was constructed by the floating bridge design-builder. The WCB drilled shaft foundations were designed to not only support the loads during the life of the WCB, but also be forward-compatible with the future WABS design-builder design.

Between 2009 and 2012, Shannon & Wilson, as part of HDR’s General Engineering Consultant (GEC) team, coordinated and then observed:

• Drilling and sampling 23 overwater borings,

• Installation of triple-stacked vibrating wire piezometers in 3 boreholes, and

• 1 overwater shear wave velocity test. 

The stacked vibrating wire piezometer cables were bundled together, laid across the lake bottom over to the existing West Approach Bridge where a solar panel and readout box were mounted. The vibrating wire data was transmitted via cellular technology to WSDOT and then forwarded to Shannon & Wilson for evaluation. The instruments indicated the presence of a pressurized, confined aquifer with groundwater readings about 5 feet above lake level.

WCB Piers 25 through 30 widened the existing West Approach Bridge by adding 8-foot-diameter drilled shafts adjacent to the existing bridge pile caps. The Pier 25 through 30 drilled shafts are within 7 to 11 feet horizontally from driven pile foundations supporting the existing West Approach Bridge.  The shafts also extend approximately 27 to 42 feet below the driven pile foundations. To protect the existing pile foundations, the shafts have permanent casing installed to about 15 feet below the existing pile tips.

Where the WCB is an independent structure, Piers 31 through 34 are founded on 10-foot-diameter drilled shafts, and Pier 35 is on 12-foot-diameter shafts.

Slurry head, along with temporary and/or permanent casing, were used to address the groundwater pressure and maintain drilled shaft integrity during construction.

As the geotechnical engineer of record, Shannon & Wilson completed 100% design for the Design-Bid-Build WCB project, submitting a geotechnical report and a Summary of Geotechnical Conditions as well as helping prepare geotechnical- and environmental-related specifications. We also developed a geotechnical instrumentation plan to monitor construction-induced vibrations and deformations of the adjacent West Approach Bridge and buildings located on shore near the work zone. During construction, Shannon & Wilson reviewed Contractor submittals and provided WSDOT construction support in matters related to drilled shaft foundations and geotechnical instrumentation.

West Connection Bridge

Ten
Years of SR 520 Bridge: Eastside Transit and HOV

October 19, 2016

This is the third in a series of articles discussing different projects associated with the SR 520 Bridge project for the Washington State Department of Transportation (WSDOT). View all posted articles here.

The Eastside portion of the SR 520 Bridge project successfully relieved traffic congestion by improving transit facilities, service, and HOV operations.  In addition, landscaped “lids”, typically 500 feet in length, were constructed to reconnect the communities on either side of the highway.

The Eastside project is about 2.5 miles long, extending from Evergreen Point Road in Medina, WA, just shy of Lake Washington, to 108th Avenue NE in Bellevue, WA.

Between 2006 and 2010, Shannon & Wilson, as part of HDR’s General Engineering Consultant (GEC) team, coordinated and then observed:

  • Drilling and sampling almost 300 borings,
  • Installation, development, and slug testing of 98 observation wells,
  • Installation of 38 vibrating wire piezometers,
  • 6 pressuremeter tests, and
  • 7 shear wave velocity tests.

Soil and groundwater conditions included landslide/landslide prone areas, a glacially-overridden lake deposit with sheared/diced/disturbed zones, and unconfined, confined, and perched groundwater aquifers.

We completed 60% design for:

  • 10 fish passage structures
  • 12 stormwater facilities
  • 5 major intersections/interchanges with lids or bridges
  • Multiple pedestrian facilities
  • Nearly 100 cut and/or fill retaining walls ranging in height from a few feet to over 70 feet

We also completed four Phase 2 Environmental Site Assessments (ESAs) and one supplement Phase 2 ESA.

Shannon & Wilson conducted phased corridor-wide seismic studies for 1,000-year ground motions.  First, we performed a literature review of published research on seismic sources, directivity, and basin effects that had not yet been incorporated into the 2009 AASHTO ground motions maps, and provided preliminary design ground motions.  Second, we performed a site-specific probabilistic seismic hazard analysis (PSHA), incorporating the research and recently published ground motion attenuation relationships.  From the PSHA results, we developed design response spectra along the project alignment.  Additional seismic studies were completed as the overall SR 520 project progressed.

When WSDOT decided to use a Design-Build (DB) delivery system, we prepared a Geotechnical Baseline Report (GBR), Chapter 2.6 of the Request for Proposal (RFP) document, a Geotechnical Data Report (GDR), a seismic criteria memo, and provided review and comment on DB questions throughout procurement.  We also provided preliminary engineering reports for the various design elements listed above.  Throughout DB design and construction, Shannon & Wilson provided submittal review and support, working alongside WSDOT’s geotechnical representative.

The bulk of our Eastside exploration and preliminary design effort was completed in a couple years’ time.  Shannon & Wilson worked collaboratively with WSDOT to keep on schedule while juggling multiple tasks.  As the project evolved over several months, we were moving the geotechnical design forward to 60% at the same we were preparing DB RFP documents.  By utilizing our combined geologic, hydrogeologic, environmental science, and GIS specialties, we were able to assist the GEC team and WSDOT in applying for and reducing stormwater treatment requirements along portions of the Eastside alignment based on historic peat areas and their impact on infiltration.

East Side Evergreen Point Road Lid

East Side 84th Ave Lid

Ten Years of SR 520 Bridge: Pontoon Casting Facility

September 21, 2016

This is the second in a series of articles discussing different projects associated with the SR 520 Bridge project for the Washington State Department of Transportation (WSDOT). View all posted articles here.

This $367 million design-build project was a major contributor to the overall SR 520 project. Built at the Aberdeen Log Yard site in Grays Harbor County, Washington, it involved the construction of a 200 feet by 900 feet by 25 feet deep pontoon casting facility.  The casting basin was used to construct 33 concrete pontoons needed for the new SR 520 floating bridge on Lake Washington. Shannon & Wilson was part of the Kiewit-General Joint Venture, a team that also included HNTB and KPFF.  Shannon & Wilson provided geotechnical design and construction services for the project including:

  • Strong ground motion time history development for the 1,000-year return period.
  • Driven 18- and 24-inch steel pipe pile recommendations including axial and lateral resistance and pile drivability for crane gantry, basin slab, and gate structure pile foundations.
  • Two-dimensional numerical modeling included dynamic, non-linear, effective stress, and soil-structure interaction simulations.
  • Numerical groundwater modeling to evaluate dewatering well spacing, depth, and configurations that are required to lower groundwater levels 20 feet.
  • Settlement estimates of the surrounding ground surface as a result of the groundwater lowering.
  • Temporary and permanent sideslope recommendations for the basin excavation.
  • Pavement recommendations to support construction equipment traffic, including a large gantry crane that transported precast slabs that were used in the pontoon construction.
  • A geotechnical instrumentation program to monitor groundwater levels below the basin slab during flooding and unwatering cycles, groundwater levels in the surrounding areas around the basin, and settlement of the adjacent ground surface and existing facilities.






Ten Years of SR 520 Bridge:
Floating Bridge & Landing

August 25, 2016

This is the first in a series of articles discussing different projects associated with the SR 520 Bridge project for the Washington State Department of Transportation (WSDOT). View all posted articles here.

The SR 520 FB&L project incorporates the world’s longest floating bridge.  It’s true, you will find it in the Guinness World Records book.  The floating portion of the bridge is about 1.4 miles long, with an overall project length of approximately 1.7 miles.

The 1960s era floating bridge deck sat directly on top of its concrete pontoons and was only a few feet above lake level.  During high, sustained winds, waves would crash across the bridge deck making it rather dangerous (and exciting) to drive across.  At times WSDOT had to close the floating bridge to traffic because the waves and wind were too high.  The old bridge carried two lanes of traffic in each direction, but did not offer shoulders or a pedestrian/bicycle crossing.  Not only did the lack of shoulders mean that a car breakdown or wreck would cause a traffic backup, but WSDOT had to close the bridge for maintenance tasks because their vehicles and personnel had to be in the traffic lanes.

The new floating bridge deck is built on a bridge structure which sits on top of the new concrete pontoons – a bridge on top of a bridge.  The deck is about 22 feet above lake level so waves won’t be crashing across the roadway.  Maintenance trucks can access the pontoons from beneath the bridge structure so they do not interrupt traffic.  The new bridge includes shoulders and a protected pedestrian/bicycle path across the lake (soon to be connected to Seattle once the West Approach Bridge – North is completed in 2017).  The new floating bridge structure was designed for a larger storm event than its predecessor.

In addition to the floating concrete pontoons, the project also included four piers that were founded in the glacially overridden deposits: one overwater pier each to the west and east, along with two on-land piers to the east.  The east approach climbs a bluff up into Medina, where a Maintenance Facility Building was constructed into the hillside requiring an excavation height of around 65 feet.

Between 2006 and 2010, Shannon & Wilson, as part of HDR’s General Engineering Consultant (GEC) team, Shannon & Wilson observed:

  • Drilling and sampling of almost 60 borings,
  • Installation of 9 observation wells and 7 vibrating wire piezometers,

The majority of the borings were drilled in Lake Washington for the pontoon anchor design; these borings went through approximately 200 feet of water before soil sampling could begin.  Soil and groundwater conditions included thick lightweight, diatomaceous lakebed deposits and unconfined, confined, and perched groundwater aquifers.  Flowing artesian groundwater conditions were encountered near the Maintenance Facility Building, and there is an upward groundwater pressure within the lake because it is a regional discharge point for groundwater.

For the Floating Bridge and Landing Project, we completed preliminary design for:

  • 3 East Approach piers
  • Pier 36 (immediately west of the floating bridge)
  • Three types of floating bridge anchors – fluke, gravity, and drilled shaft
  • Maintenance Facility Building
  • Maintenance Facility Dock
  • Stormwater bioswale on top of the bluff
  • Driven pile foundations for two potential pontoon construction sites at the Ports of Olympia and Tacoma, WA

The fluke anchors, consisting of large rectangular vertical plates, were located in about 200 feet of water and founded in very soft, diatom-rich silt.  Gravity anchors, consisting of honeycomb precast concrete blocks filled with quarry spall/ballast rock, were located towards the east and west ends of the floating bridge, where the lake water level was deep enough to provide boat clearance overtop the gravity anchors.  Large diameter, laterally-loaded, drilled shaft anchors were located at and near to the east and west ends of the floating bridge where the lake water level was too shallow to accommodate gravity anchors.

WSDOT decided to use the Design-Build (DB) delivery system for this project.  To assist WSDOT, Shannon & Wilson prepared a Geotechnical Baseline Report (GBR), Chapter 2.6 of the Request for Proposal (RFP) document, a Geotechnical Data Report (GDR) and associated addenda, a seismic criteria memo, preliminary engineering reports to proof the conceptual design, and provided review and comment on DB questions throughout procurement.  Throughout DB design and construction, Shannon & Wilson, working alongside WSDOT’s geotechnical representative, provided submittal and anchor test results reviews and made periodic site visits.

Check out WSDOT’s drone video of the completed FB&L project here: https://www.youtube.com/watch?v=Nzd1G2ErB-I&feature=youtu.be


Ten Years of SR 520 Bridge Project

July 29, 2016

For the past ten years Shannon & Wilson has been providing geotechnical, geological, hydrogeological, environmental, seismic, and related GIS services on the SR 520 Bridge Replacement and HOV Project for the Washington State Department of Transportation (WSDOT).  This project, located between 108th Avenue NE in Bellevue, Washington, and I-5 in Seattle, traverses approximately 6.5 miles of diverse and challenging soil and groundwater conditions, and has involved an extensive variety of design elements. 

WSDOT divided the project up into several stages.  As the floating bridge portion of this project recently opened, we thought it was a great time to take a trip down memory lane reviewing our work on this historic project over the past decade.  In the coming months, we will be posting a series of articles highlighting different components of the project and the geotechnical breakdown by the numbers.  Be sure to follow us on social media or check back on our website for future editions.


To commemorate the one year anniversary of the completion of the SR 520 Floating Bridge, the Washington State Department of Transportation created a short documentary video, an online “booklet,” and a blog post. These are really great pieces that show how the project came to be.