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Much resilient modulus research work has been completed recently, mostly as part of the Long Term Pavement Performance (LTPP) study. This work has led towards the adoption of the resilient modulus test procedure, T307-99, in the current release of the American Association of State Highway and Transportation Officials (AASHTO) Tests. Neither the test procedure nor the current design guideline addresses how to approach selecting a design resilient modulus value from the fifteen stress-dependent numbers generated by the laboratory test.
Cohesive and non-cohesive subgrade soils are generally nonlinear inelastic materials, thus their stiffness is dependent on the stress condition subjected to. Recognizing this, the project-specific case study discussed in this paper examines the stress-dependency of soils encountered through laboratory determination of resilient modulus.
This paper uses a recognized constitutive model and layered elastic methodology approach (iterative solution) to objectively interpret results from a laboratory test program and apply the results for input into the 1993 AASHTO Design Guide for Pavement Structures (or DarWIN).
Through the use of the stress-dependent constitutive model: Mr = K1(Sc)K2 (S3)K5 and simple 85th percentile statistics, the process created uses the predicted subgrade modulus values at an assumed stress state. The 85th percentile value is selected as input into the 1993 AASHTO structural number (SN) requirement. A conventional pavement system (layer type and thickness) is then developed using typical mechanistic properties of asphalt concrete and aggregate base, which satisfy the SN. The layered elastic model ELSYM5 is then used to approximate vertical and horizontal stresses at the top of the subgrade layer. These stresses are compared to those assumed values used in the initial iteration. Adjustments are made and iterations continue until the values used to predict subgrade modulus reasonably match the calculated stresses in the pavement system.
The final iterated value for resilient modulus is the design resilient modulus. It is recommended that this design value be used to calculate pavement thickness requirements for all pavement types under consideration for the project.
subgrade, resilient modulus, pavement design
President, Boudreau Engineering, Inc., Norcross, GA