The SEATTLE PINE STREET TUNNEL By Bruce Erickson, P.E., S.E. f you were to drive your car today along Pine Street in downtown Seattle, you would notice one lane blocked off for construction equipment; and, you might see construction materials stacked in a couple of adjacent lots. These sights would indicate some sort of construction activity, but wouldn’t give you any idea of the ® magnitudeI of construction taking place directly below you. The driving surface below your car appearing to be concrete pavement, but in actuality would be precast concrete planks supported on steel beams spanning across the road. Below these planks and beams, Pine Street Tunnel, which The Challenges will be part of a future light rail transit line, is being con- Construction of the Pine Street Tunnel presented a series of challenges for structed. This new 900-foot long tunnel connects to the Sound Transit, the agency responsible for developing the light rail system. existing Downtown Seattle Transit Tunnel, constructed in These challenges included: the early 1980’s, that up until now has carried bus traf- • The requirement that the tunnel meet strict Sound Transit seismic criteria, fic. Together, the two tunnels will function as part of a which included the need for the tunnel to survive a 2500-year earthquake. light rail train system that will allow riders to travel from Copyright• A City of Seattle requirement that street vehicle and pedestrian traffic be the Seattle-Tacoma International Airport in the South to maintained at all times during construction. downtown Seattle in the North. • The need to maintain service in buried street utilities during construction. These included sewer, water, gas, electrical, telecommunications and steam lines. Structural Systems • The need to protect adjacent buildings from damage due to potential The tunnel structure consists of convention- ground settlements. ally reinforced concrete slabs and walls. Roof and bottom slab thicknesses range from 4.5 Design Criteria to 6 feet, and wall thicknesses vary from 2.5 to 3.5 feet. Tunnel interior dimensions vary The 1997 Uniform Building Code (UBC), from 30 to 96 feet wide by 16 feet tall. Most which was the building code for the City of of the tunnel is one level, but one portion of Seattle at the time design began, was selected the tunnel has an upper level for ventilation by Sound Transit as the building code for the fans and traction power electrical equipment. tunnel. The tunnel was designed to resist static Maximum depth from street level to bottom pressures due to self-weight, soil, hydrostatic, of tunnel ranges from about 40 to 70 feet. adjacent building surcharge and traffic weight. The temporary shoring system for the tun- Seattle is located in an area of high-seismic- magazineity, so the tunnel also needed to be designed nel excavation consists of steel wide flange sol- dier piles in drilled shafts, with timber lagging for seismically-induced ground motions. The S T R UBC’sU seismic provisions C are oriented Ttowards U R E between the soldier piles. The soldier piles along the two long sides of the excavation building behavior, and are not appropriate for are braced against each other using cross-lot tunnel design. Therefore, the Sound Transit braces constructed of 24- to 36-inch diameter Design Criteria Manual contains seismic de- pipe sections. The brace loads are distributed sign criteria developed specifically for buried to the soldier piles with steel wide flange wales. tunnels. The general approach of these crite- The soldier piles at the bulkhead wall were re- ria is to check the tunnel for imposed racking, strained by tieback anchors, since there was no vertical seismic, axial elongation and curva- nearby opposing wall against which to brace ture deformations imposed by motions of the at this location. surrounding soil during the earthquake. The performance standard is that the tunnel be able to survive a 150-year earthquake without significant structural dam- age, and a 2500-year earth- quake without collapse. The design procedure consists of calculating tunnel structure deformations based on the stiffness of the tunnel and the surrounding soil, then sizing the concrete thicknesses and reinforcing quantities so that strains in the reinforcing steel are kept below limiting values. Tunnel excavation underway continued on next page STRUCTURE magazine 35 December 2006 perimeter of the future tunnel, which rough- they installed steel frameworks ly corresponded to the locations of the street beneath the utilities and con- curbs. Then, traffic was temporarily confined nected those frameworks to to the south side of the street while a traffic the underside of the deck with deck was constructed on the north side of the hangers and braces. Once all street. The beam ends located at the curb were the utilities were suspended supported on the soldier piles, while the beam from the deck, the contractor ends located along the centerline of the street was able to begin mass excava- were temporarily supported on concrete foot- tion. ® ings. Next, the traffic was moved onto the traf- fic deck on the north side of the street while the Protecting Adjacent traffic deck was installed on the south side of Buildings the street. Once decking was installed on both sides of the street, the steel beams were spliced The tunnel excavation would at the street centerline, the temporary footings be close to existing buildings, were removed and the contractor began install- and excessive deflections of ing supports for utilites that were to be sus- the shoring system might al- pended from the underside of the deck. low ground settlements that Copyright could damage the buildings. Sound Transit and the City of Seattle were particularly con- cerned about the historic Para- mount Theater, which would be only about 10 feet away from the tunnel at the closest point. Because of its relatively Tunnel excavation underway brittle brick construction, it Keeping Traffic Flowing would be particularly vulner- able to damage from ground This process of digging a tunnel below a settlements. Other adjacent street, while maintaining traffic, was aptly de- Tunnel excavation completed buildings included two hotels scribed by one engineer as “removing the cake Maintaining Utility Service (both constructed within the without disturbing the frosting.” The relatively last 10 years) and a 1960’s era shallow depth of the tunnel and various other Existing utilities buried below the street in- highrise apartment building. constraints precluded boring or mining the cluded sewer, water, gas, electrical, telecommuni- Shoring system deflections tunnel without disturbing the street, so a “cov- cations and steam lines. These services needed to are influenced as much by soil er-and-cut” process was chosen. This consistedmagazine be maintained throughout construction. Some characteristics and installation of installing a deck over the street to support of the utilities were re-routed around the con- procedures as they are by the traffic,S then mining theT soil from underneathR structionU site prior toC traffic deck installation.T U R E shoring structural stiffness. the deck to create the excavation for the tunnel. Utilities that could not be re-routed needed to be Accordingly, specifications and The traffic deck consisted of precast concrete suspended from the underside of the traffic deck Looking up through drawings contained a variety deck opening planks supported on steel beams. The first step during construction. As the contractor began of provisions to insure that was the installation of soldier piles around the mining soil from underneath the traffic deck, shoring would be installed in a careful and con- trolled manner. These included requirements that cross braces be installed before excavation proceeded more than two feet below brace level, and that braces be preloaded with jacks to 50% of the design strut load. Another key element in the strategy for pre- venting damage to adjacent buildings was an extensive monitoring program. The monitor- ing system included the following elements: • Soldier piles were monitored for hori- zontal and vertical movement using optical surveying equipment. Selected piles were installed with inclinometers, which provide a graph of the pile deflect- ed shape along its entire height. • Adjacent streets and buildings were monitored for vertical and horizontal movements using optical survey- ing equipment. • Loads in selected Tunnel base slab under construction STRUCTURE magazine 36 December 2006 struts were moni- tored using strain gages mounted on the struts. • Existing cracks in nearby buildings were monitored with crack gages. This monitoring pro- ® gram gave the team the ability to take remedial action if observed move- ments or forces at a par- ticular location became excessive at any time Completed tunnel segment during construction. The Results PROJECT CREDITS Not surprisingly, the Copyright Owner: Sound Transit stringent seismic criteria Prime Design Consultant for Final Design: KPFF Consulting Engineers — particularly the im- posed racking deforma- Geotechnical Consultant and Instrumentation Designer: Shannon & Wilson, Inc. tions — resulted in the Construction Manager: URS Corporation need for heavy reinforc- ing steel in the tunnel. Prime Contractor: Balfour Beatty Construction Inc. Reinforcing steel sizes Prime Design Consultant for Preliminary Design: Puget Sound Transit Consultants were typically governed by racking requirements at interior and exterior Bruce Erickson,