Modified Terzhagi’s Equation for Modulus of Subgrade Reaction

Abstract

The structural design of substructure requires the modulus of a subgrade reaction of the underlying stratum at the soil-structure interface. Its magnitude is estimated by plate load tests at the site. The present study reviewed the magnitudes of the modulus of subgrade reactions obtained from plate load tests, elastic continuum solutions, and a soil-structure interaction analysis of a 2-km tunnel resting on weak sandstones and siltstones. The review showed that the structural adequacy of the substructure using the modulus of subgrade reaction obtained from the Terzaghi equation for plate load tests would not be satisfactory. A modified Terzaghi’s equation is presented so that the magnitude of the modulus of subgrade reaction obtained is of the same order of the magnitude as that obtained from the soil-structure interaction analysis and elastic continuum solutions. The proposed modified Terzaghi’s equation is applied to the plate load test results of two separate projects elsewhere to confirm its validity.