Plymouth Tunnel

Double track light rail tunnel , Purple Line, Silver Spring, Maryland


Maryland Department of Transportation/ Maryland Transit Administration
Maryland, USA


Purple Line Partners (Fluor – Traylor JV)

Consultant for SEM/ NATM: Beton- und Monierbau USA, Inc.:
(100 % subsidiary of BeMo Tunnelling GmbH, Austria)
2 operating SEM/ NATM Superintendents during tunnel excavation (top heading and bench/ invert) on site for 10 months plus 1 superintendent on site during execution of shotcrete inner lining (6 months)

Project location

Silver Spring

Start of construction


End of construction


Project total value

approx. 1 million €


Top heading: Mixed face conditions consisting of sandy material on top and rock in the bottom;
Bench/ invert: weathered to compact rock in different stages

Technical Data

Double track light rail tunnel as part of the Maryland Purple Line along the Northeast periphery of Washington DC

• Tunnel
Length: (311 m) 1,020 feet
• SEM/ NATM top heading excavation followed by bench/ invert
• Liebherr LH950 tunnel excavator; Potenza shotcrete robots, E2C drill rig
• 2 pipe umbrellas (appr 40 pipes each)
Length: (15,25 m) à 50 feet
• Systematic spiling over whole length of tunnel
• Challenging mixed face conditions require use of typical soft ground tunneling methods; excavation by tunnel excavator with bucket-, road header- and hammer attachments.
• Parts ov invert had to be excavated using drill & blast method
• Application of wet shotcrete, batched with on site batching plant (Wiggert)

Further project related information

• The construction site is situated in a densely populated
urban area
• Know-how transfer of the New Austrian Tunnelling Method (NATM)