Last week we showed you the giant tunnel boring machine pieces arriving at Capitol Hill light rail station site. Thanks to an update from Sound Transit, we have a better idea when that TBM will start its journey to downtown before being hauled up and trucked back to Broadway to drill a second 21-foot-diameter tunnel. We also learn that about 30 days from now, one of the two machines that will drill tunnels up to Capitol Hill from Montlake will begin its journey. Here’s the brief from Sound Transit and an awesome photo of the University of Washington start-point for the twin tunnels:

(Image: Sound Transit)

Last month you were introduced to “Togo”, “Balto” and “Brenda” – the three tunnel boring machines that will dig the light rail tunnels between the University of Washington (UW) and downtown Seattle . “Togo” will be first out of the gate and is scheduled to begin tunneling at the UW site in late May. He will then be joined by “Balto” who will start his digging about one month after “Togo” starts. “Brenda”, the lone Capitol Hill machine, will make her debut this summer.

The journey for the two machines is documented in this paper on the Sound Transit U-Link project:

The twin tunnels running from the UWS to the southwill consist of 6.4-meter (21-foot) excavated diameterbored tunnels, of the general cross section shownin Figure 6. Based on the site constraints at UWS,the project is currently being planned assuming thattwo tunnel boring machines, working concurrently,will be required for U220. These twin drives willface the challenges along the way such as moderatelysteep grades and passing under sensitive areas. The vertical alignment of the tunnels beginswith a 4.5% downhill gradient out of the UWS, suchthat the tunnels will cross safely beneath the MontlakeCut. After crossing beneath the Cut by aboutone tunnel diameter, the drive turns upgradient, andcontinues for nearly 1.9 kilometers (1.2 miles) in along 4.1% uphill grade. Once near the top of the hill,the grade flattens out, and enters the CHS at a 0.5%grade. The tunnel alignment traverses beneath about250 homes, and several municipal structures, includinga historic Water Tower. Ground cover along thealignment ranges 4.6 to 91.5 meters (15 to 300 feet).The shallowest depth of ground cover is where thealignment passes beneath the base of the MontlakeCut with approximately 4.6-meters (15-feet) coverfrom the mudline to the crown of the tunnel. Stability analyses (based on the above description andresults of the geotechnical investigations) have indicatedthe side slopes of the cut will remain stableand that tunnel buoyancy will not be an issue.

Tunneling for the entire project — from UW to Capitol Hill and Capitol Hill to downtown — is expected to take about a year and a half.