History is being made, and BASF plays a small but significant part in it. Seattle’s SR 99 tunnel, a two-mile-long tunnel being built underneath its downtown area to replace the city’s State Route 99 Alaska Way Viaduct, is one step closer to reality. While it’s been a long road, “Bertha,” the largest tunnel boring machine (TBM) at the time that it was built, has been digging beneath the city, and reached the end of its journey in early April. It took nearly four years after Bertha embarked on the job to break through on the opposite end of the tunnel, excavating, approximately 28 dump-truck loads of soil every 25 minutes during the height of its operation.
The proximity of Seattle’s buildings and roadways near the SR 99 tunnel required a precise logistics plan.
The Emerald City’s project to replace the aging infrastructure—a major North/South connection through its downtown area—has been a work in progress for nine years. The 60-year-old structure has been suffering from decades of wear and tear, along with the effects of population growth and congestion. As a solution to alleviate some of the traffic, the city decided to replace the viaduct with a double-deck highway tunnel along Seattle’s waterfront.
While many might know about this five-story-tall tunnel undertaking, few are aware that BASF contributed several primary products, along with tunneling expertise and logistics. BASF’s solutions are well-suited for Earth Pressure Balance tunneling, to not only seal the TBM from incoming water and soil, but also to condition the soils for efficient excavation.
As with all massive projects, there were many technical challenges. To overcome one such challenge, BASF initiated a University Cooperation and Research program with the Colorado School of Mines for the testing of chemically enhanced soils to replicate conditions found in the Seattle environment. This procedure helped BASF to better understand the difficult tunneling conditions of this project, and customize the best solutions.
Below are some of the BASF Master Builders Solutions product technologies that contributed to help move the tunneling to a successful completion:
Soil conditioners—Earth Pressure Balance tunneling requires the correct use of soil conditioners to reduce cutterhead torque and increase advance rates (the TBM moving forward in the tunnel). The more efficiently it advances, the quicker the project can be completed.
Tail sealant greases—They are used for sealing against water, soil and annulus grout ingress into the TBM.
Annulus grout admixtures—When the segmental lining, made of precast concrete segments, is built inside the TBM, the space between the ground and the segments is called the annulus gap. BASF provided retarders used in the grouting process to affect the setting time of the grout.
Polymer injection systems—Injection grouting near the end of the tunnel drive was used for ground consolidation and water control to prevent water ingress into the tunnel and at the portal.
Precast concrete segments admixtures—High-performance concrete additives were used to ensure durability and efficient production of the precast concrete segments, such as the MasterGlenium superplasticizer, MasterFinish finish-enhancing admixture and MasterLife, silica fume mineral admixture.
“We’re very excited to have been part of the world’s largest diameter tunnel,” said Jim Lindsay, Regional Business Segment Manager, Construction Chemicals N.A., BASF.
“The sheer magnitude of the project was a technical and logistical challenge, and BASF’s specialized tunneling expertise and logistical infrastructure made it possible to effectively meet the challenges of this landmark project.”
It will still be two years before the highway opens, as crews need to complete the double-decker lanes and other infrastructure inside the tunnel.
The article was written by Anna Spiewak and posted on the BASF website.