This paper demonstrates the use of numerical modeling as an essential tool in an integrated rock excavation support design. This concept is implemented and presented for the tunnel/chamber designs of the Narragansett Bay Commission Combined Sewer Overflow Facility Project in Providence, Rhode Island, which is situated in a complex Pennsylvanian aged sedimentary rock with widely variable lithology and structure. Both the numerical method and empirical method or expert system used in the design process are shown to be complementary with each other, with their own advantages and disadvantages.


Design approaches for support of underground excavations in rock can be generally categorized into three more commonly used methods: empirical method using rock mass classification systems, analytical method, and observational method. Contractual procedures in the United States generally preclude the observational method as a viable design method. The New Austrian Tunneling Method (Rabcewicz 1965) is the pioneer of this approach. The empirical method is an expert system that relates experience encountered at previous projects to the conditions anticipated at a proposed site using the rock mass classification systems as correlation tools. Analytical method, on the other hand, uses mathematical models to provide solutions for stresses, strains, displacements, or stability conditions around underground excavations. This method ranges from explicit closed-form solutions for the most simple or idealized projects, to computer-aided numerical models capable of handling the most complicated project conditions and geometries. Designers frequently incorporate several techniques into a design process as a means of checks and balances, a design philosophy that has also been suggested by Bieniawski (1984).

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