Abstract: The mechanical characterization of sandy cobble mixtures is challenging due to their heterogeneous lithology, loose structure, and large particle size. Obtaining reliable strength and stiffness parameters through conventional laboratory and in-situ tests is both technically difficult and costly, often leading to significant uncertainty in engineering design. In this study, strength and modulus parameters of sandy cobble soils were back-calculated using a plastic-hardening constitutive model in FLAC3D, based on plate load tests performed to failure. The results were compared with those obtained using the Mohr–Coulomb model. Numerical back-analysis indicates that the plastic-hardening model better captures the stiffness-degradation behavior characteristic of sandy cobble materials. The inferred internal friction angle is notably high, typically ranging from 50° to 60°, and the elastic modulus is also substantial, on the order of 200~300 MPa—significantly exceeding the parameter values commonly adopted in engineering design. It is therefore recommended that design practice appropriately increase the strength and stiffness parameters for sandy cobble soils, and that numerical back-analysis be used as an effective supplementary approach for parameter determination.

Key words: Sandy Cobble, Back Analysis, Numerical Simulation, Modulus Parameter, Bearing Capacity, Plate Load Test