Tunnel Design and Modulus of Subgrade Reaction

Abstract

The modulus of subgrade reaction is not a unique fundamental soil property that can be readily measured, and its magnitude is an outcome from a soil-structure interaction. But it is required by structural engineers for the structural design of a tunnel. The adoption of an appropriate magnitude for the modulus of subgrade reaction is crucial to ensure that the structural design of the tunnel will be adequate. Some engineers inappropriately rely on the plate load test to determine the magnitude of the modulus of subgrade reaction of the underlying stratum for the structural design of the tunnel. This paper will discuss an appropriate approach to estimate the magnitude of the modulus of subgrade reaction of weak sandstones and siltstones underlying a 2-km tunnel. This tunnel was completed and extensive cracks were noted prior to the opening of the tunnel. This prompted an investigation that included a design review that identified the important issue of an appropriate method for determining the magnitude of the modulus of subgrade reaction for the tunnel design. Typical values for the moduli and Poisson’s ratios of weak sandstones and siltstones required for the soil-structure interaction analysis will be discussed based on a review of the existing literature and the results of plate load tests, pressuremeter tests, and unconfined compression strength tests of the soil investigation report for the tunnel project.