Washington-Dulles International Airport (Virginia, USA) has been expanding its capacity since 1999. This expansion has required extensive vertical, open-cut rock excavations to depths of up to approximately 20 m adjacent to existing infrastructure for construction of new below-ground stations for the new Automated People Mover (APM) passenger transportation system. As the construction could not impact the airport operations, the selection of support pressures for the rock excavations had to balance the projects’ risks and construction costs. Site investigation identified that bedding dipping at approximately 30° would be the primary hazard to the vertical rock cuts. Extensive laboratory testing and field observations suggested that the potential for a large slide along a bedding plane was relatively low. Therefore, design lateral pressures for permanent station walls were based on a potential failure model of local wedge/block failure. The reduction in lateral support requirements for the permanent below-grade walls for the APM station structures resulted in significant cost savings to the project. Detailed field mapping combined with automated instrumentation were successfully used to confirm the design assumptions.


Estimating the support pressure for a rock cut depends on the assumed failure mode and that failure mode is a function of the rock mass and its geological history and the method of excavation. At one extreme the rock mass can be a homogeneous intact block whereby the designer is only concerned with elastic deformations and for such situations no support is required. At the other end of the spectrum the rock mass may be so heavily jointed that the rock mass behaves as an assembly of discrete blocks. For this situation the failure mode may be described as an active wedge and the support must be designed to stabilize this wedge.

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