Building New State Route 99 through Seattle - page 2 of 3 >>>
With tunnelling well advanced, work is progressing on completing the lining and early stages of fitting out.
Bertha now climbing
After steadily descending over the first part of the tunnel drive, Bertha has now begun climbing toward her eventual exit point near the Space Needle.
Interestingly, the lowest spot in the tunnel is not its deepest point underground. Due to the city’s topography, the top of the tunnel will be more than 200 feet below the surface near Virginia Street.
Over the next several weeks, specialized crews will complete routine cutterhead maintenance at the front end of Bertha, the SR 99 tunneling machine. To safely do this, crews must stabilize the ground in front of the machine.
They do this by injecting a type of clay, known as bentonite, into the front end of the machine. This creates a seal that prevents water and soil from entering – and air from escaping – their work space.
Next, crews over-pressurize the space by introducing compressed air, which pushes against the bentonite to counteract the ground and water pressure at the front end of the machine. This newly created “hyperbaric” work space has pressure levels that are higher than regular atmospheric pressure, similar to conditions found in an underwater dive.
Preparations for this work are underway now that Bertha has stopped for planned maintenance beneath Spring Street. Crews have made their first “hyperbaric intervention”. Seven 5--member crews worked around the clock to perform maintenance in the space behind the cutterhead.
Each crew member must spend several minutes in a special chamber to prepare for the greater pressures they will experience while working in hyperbaric conditions. The amount of time that crews can safely work in these conditions varies depending on the pressure of the hyperbaric work space.
In previous interventions on this project, crews were able to spend up to an hour in these conditions before decompressing and returning to the surface.
The video above, provided by Ballard Marine Construction, the firm responsible for completing this work on the tunnel project, shows crews at work behind Bertha’s cutterhead during an earlier planned maintenance.
The duration of the maintenance stop will depend on the extent of the work that’s needed. STP’s previous maintenance stop near Yesler Way lasted approximately six weeks.
Not Without Its Problems
Any contractor will tell you that tunnelling is an inexact science. No matter how many ground studies are done, their is always the unexpected.
One incident was when the TBM struck a steel pipe that had been installed as a well casing for an exploratory well drilled as part of the planning. This caused a long delay as the contractors had to excavate a 37 m (120 ft) vertical shaft to reach the cutterhead so that repairs could be made.
No sooner was this problem solved when 23 days later on January 14, 2016, a 9.1 m (30-ft) wide sinkhole developed on the ground in front of the machine. This brought a halt to drilling until the contractors demonstrate through a root cause analysis that the machine could continue safely. Contractors filled the hole with 190 m³ (250 yd³) of concrete.
[Photographs and videos Copyright WSDOT © 2016 unless otherwise indicated.)