Beacon Hill Tunnel
- Location: Seattle, WA
- Owner: Sound Transit
- General Contractor: Obayashi Corporation
- Technique(s) Utilized: Diaphragm Walls, Vibro Stone Columns, Jet Grouting
- Subsurface Conditions: Silt, Clay, Till, Sand, Gravel
- Approximate Key Quantities: Diaphragm Wall - 54,142 square feet, Jet Grout Columns - 357, Jet Grout Column Diameter - 5 feet, Jet Grout Column Depth - 65 feet, Shaft Diameter - 30-52 feet, Shaft Depth - 60-181 feet
Background
This project involved the construction of two shafts totaling 54,142 square feet for a new light rail tunnel station. The main access shaft, constructed with diaphragm walls, had a diameter of 52 feet and reached a depth of 181 feet. A 30-foot diameter ventilation shaft was also constructed with diaphragm walls at the site.
Both shafts were excavated using the Hydrofraise 4000 equipped with the Enpafraise, a real-time verticality monitoring device. The Enpafraise records all excavation parameters such as verticality, rotation of the Hydrofraise by means of a gyroscope, and hydraulic motor pressure. The excavation parameters can be downloaded to a computer. Production reports and 3D drawings showing all recorded excavation paths were prepared from this information.
Jet grout columns were installed to reduce ground loss and water infiltration at various locations where the tunnel alignment intersected the vertical access shafts and other miscellaneous structures. A total of 357 jet grout columns were installed to depths of 65 feet.
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