Location

Arlington, Virginia

Session Start Date

8-11-2008

Session End Date

8-16-2008

Abstract

Expansions at the Washington-Dulles International Airport since 1999 have required extensive vertical, open-cut rock excavations in Triassic age siltstone bedrock. These excavations have extended to depths of up to approximately 65 ft (20 m) adjacent to existing infrastructure for construction of new below-ground stations for the new Automated People Mover (APM) light rail system. The selection of design support pressures for the rock excavations was an important decision, balancing the projects’ risks and construction costs. At the center of this issue was the development of a geotechnical model of the rock mass and its primary failure mechanism. Thus, a comprehensive subsurface characterization was required. The rock mass characterization included observation and mapping of excavation faces, detailed logging of rock cores, use of optical and acoustic televiewer, testing of discontinuity samples for shear strength evaluation, groundwater monitoring, and inclinometer monitoring of supported faces. The televiewer data, combined with site observations, allowed for a more complete understanding of the engineering characteristics of the bedding plane and joint discontinuities within the siltstone rock mass. Based on the pattern of the predominant discontinuities, it was concluded that bedding planes dipping into the excavation at approximately 30 degrees intersecting near-vertical joints would present the greatest risk for rock cut failures. Extensive laboratory testing and field inspections at a variety of exposed cuts with varying bedding plane and joint orientations suggested that the potential for a large slide along a bedding plane was relatively low. This conclusion was based on observations of discontinuous clay seams of limited number, the first- and second-order roughness of joint and bedding plane surfaces, and the limited persistence of joint and bedding plane discontinuities. Previous design lateral pressures for permanent station walls had been based on an assumed potential failure model of a large, excavation-scale block failure. However, using the recent characterization data, the rock mass failure mechanism of a local joint- and bedding-controlled sliding block mechanism was considered more appropriate. The resulting design lateral pressure necessary to support a rock face using this mechanism and the shear strength of discontinuities and intact rock was significantly lower than the initial design values. Construction-phase observations and monitoring, which included detailed field mapping, automated instrumentation monitoring, and groundwater monitoring, have verified the rock characterization and design assumptions. The reduction in design pressures for the permanent below-grade walls for the APM station structures resulted in major cost savings for the projects now in design and construction. Based on the scale of future expansion plans at Dulles, the projected total cost savings resulting from the reduced design lateral rock pressures will be considerable.

Department(s)

Civil, Architectural and Environmental Engineering

Appears In

International Conference on Case Histories in Geotechnical Engineering

Meeting Name

Sixth Conference

Publisher

Missouri University of Science and Technology

Publication Date

8-11-2008

Document Version

Final Version

Rights

© 2008 Missouri University of Science and Technology, All rights reserved.

Document Type

Article - Conference proceedings

File Type

text

Language

English

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Aug 11th, 12:00 AM Aug 16th, 12:00 AM

Support of Rock Cuts at Washington-Dulles International Airport

Arlington, Virginia

Expansions at the Washington-Dulles International Airport since 1999 have required extensive vertical, open-cut rock excavations in Triassic age siltstone bedrock. These excavations have extended to depths of up to approximately 65 ft (20 m) adjacent to existing infrastructure for construction of new below-ground stations for the new Automated People Mover (APM) light rail system. The selection of design support pressures for the rock excavations was an important decision, balancing the projects’ risks and construction costs. At the center of this issue was the development of a geotechnical model of the rock mass and its primary failure mechanism. Thus, a comprehensive subsurface characterization was required. The rock mass characterization included observation and mapping of excavation faces, detailed logging of rock cores, use of optical and acoustic televiewer, testing of discontinuity samples for shear strength evaluation, groundwater monitoring, and inclinometer monitoring of supported faces. The televiewer data, combined with site observations, allowed for a more complete understanding of the engineering characteristics of the bedding plane and joint discontinuities within the siltstone rock mass. Based on the pattern of the predominant discontinuities, it was concluded that bedding planes dipping into the excavation at approximately 30 degrees intersecting near-vertical joints would present the greatest risk for rock cut failures. Extensive laboratory testing and field inspections at a variety of exposed cuts with varying bedding plane and joint orientations suggested that the potential for a large slide along a bedding plane was relatively low. This conclusion was based on observations of discontinuous clay seams of limited number, the first- and second-order roughness of joint and bedding plane surfaces, and the limited persistence of joint and bedding plane discontinuities. Previous design lateral pressures for permanent station walls had been based on an assumed potential failure model of a large, excavation-scale block failure. However, using the recent characterization data, the rock mass failure mechanism of a local joint- and bedding-controlled sliding block mechanism was considered more appropriate. The resulting design lateral pressure necessary to support a rock face using this mechanism and the shear strength of discontinuities and intact rock was significantly lower than the initial design values. Construction-phase observations and monitoring, which included detailed field mapping, automated instrumentation monitoring, and groundwater monitoring, have verified the rock characterization and design assumptions. The reduction in design pressures for the permanent below-grade walls for the APM station structures resulted in major cost savings for the projects now in design and construction. Based on the scale of future expansion plans at Dulles, the projected total cost savings resulting from the reduced design lateral rock pressures will be considerable.