This is Brenda, the 21-foot-wide tunnel boring machine that will be eating through soil and boulders under Capitol Hill to create the light rail extension from downtown. In a couple weeks, Brenda will start chewing the first tunnel, which is supposed to take four to six months; then they'll bring Brenda's head back to this site and she'll chew another tunnel alongside the first, which will take another four to six months. (One of those tunnels is for goers, the other one for comers.) Brenda is named after the project manager's wife.
"The concept is to keep a full head of pressure" on Brenda while she works her way through the ground and water table and to "manage and monitor the amount of pressure applied to the face," said construction manager Rick Capka. That data will enable them to measure how much the soil is shifting and better prevent the problems that tunneling on Beacon Hill caused.
Brenda will take a snapshot of all the pressure around her every 10 seconds. As she proceeds through the dirt, operators will assemble pre-cast concrete segments behind her head, which will then become the permanent lining of the tunnel. Meanwhile, a conveyor belt extending out from behind her head will dump dirt onto trucks, which will ship the dirt out of the excavation site 24/7. Brenda's head is only 30 feet long but the rest of her body—conveyor belt, operator's cabin, etc—is 330 feet. Ten or 11 people will be inside Brenda at any given time.
Glen Frank, quality control manager, describes her as a "very simple submarine, a water-tight system." When she punches through the "retrieval shaft" down by the Paramount Theater, she'll be 150 feet lower in elevation than where she is today. Here's video taken during today's tour. Members of the media get into a "man basket" and are lowered into the excavation site to get up close to Brenda's face.