A comprehensive evaluation of strength and modulus parameters of a gravelly cobble deposit for deep excavation analysis

• Various laboratory and field tests of Taichung gravelly cobble are back analyzed.
• Strength and modulus parameters of the gravelly cobble deposit are evaluated.
• The strength parameters will decrease if the minor principal stress increases.
• The stress dependent stiffness modulus parameters are related to the grain size.
• The back analyzed parameters are valuable for numerical analysis of deep excavation.

This study presents results of numerical back analysis for estimation of strength and modulus parameters of gravelly cobble deposit through various laboratory and field tests. A non-linear and stress dependent stiffness soil model implemented in the finite element software was used to simulate four conventional triaxial compressive (CTS) tests, two pure shear triaxial (PS) tests, one field plate load (PL) test, one field direct shear (FDS) test, and one natural slope stability (NSL) analysis. The internal friction angle ϕ and stress dependent stiffness modulus E50ref back analyzed from those various tests were summarized and evaluated considering the load conditions and stress states. Results show that the strength parameter ϕ tends to decrease linearly with an increase of the minor principal stress σ3′ when they were plotted in a semi-logarithmic graph. The stiffness moduli evaluated from the back analysis, cyclic triaxial test, and downhole test are compared and discussed. In addition, results of numerical analysis of the recent excavation case history in Taichung gravelly cobble deposit are presented to verify the summarized parameters, which are selected with respect to the in-situ stress state.