If the material is failing in situ, without any excavation, then either the strength is incorrect or the stress field is incorrect.
If it is the case that the stress field is incorrect, the following documents may provide some guidance about specifying in situ stress:
Sheory, P.R. (1994). A Theory for In Situ Stresses in Isotropic and Transversely Isotropic Rock. Int. J. Rock Mech. Min. Sci. & Geomech. 31(1)23-34
Stacey, T.R. and Wesseloo, J. (1998). In situ stresses in mining areas in South Africa. Journal of the South African Institute of Mining and Metallurgy. November/December: 365-368
Stacey, T.R., Xianbin, Y., Armstrong, R., and Keyter, G.J. (2003). New slope stability considerations for deep open pit mines. Journal of the South African Institute of Mining and Metallurgy. July/August: 373-390
Grov, E. (2006). The importance of in-situ rock stress in design and construction of sub-surface opening. International Symposium on Utilization of underground space in urban areas. 6-7 November, Sharm El-Sheikh, Egypt.