HJC黄金城

Soils can experience consolidation through several mechanism over their geological history including mechanical unloading due to erosion, glaciation, changes to groundwater elevations or others. For geotechnical engineers the prevailing stress regime of soils is of great importance to predict expected subsoil settlements. Depending on the soil history, these conditions persist and affect the propagation of seismic waves, especially the two different types of shear waves SV and SH (vertically and horizontally polarized shear waves). Shear waves are sensitive to changes in dynamic soil parameters such as the stiffness of soils. The directional characteristics of the shear waves is useful for evalsuating the soil stress history in terms of the over-consolidation ratio OCR (Ku and Mayne, 2014): OCR= σ' / σ' or over-consolidation difference (OCD= σ' - σ' ). The OCR is defined as the ratio between the maximum overburden stress that the soil has ever experienced σ' (i.e. with the ice covering it) and the current overburden stress σ' (i.e. without the ice covering). A soil is considered to be over-consolidated (OCR>1) if the current effective vertical stress is greater than the maximum stress it has experienced in the past, resulting in a higher shear strength and stiffness than would be expected on the basis of its current effective stress state.

OCR affects the shear strength, compressibility and permeability of a soil and is therefore an important parameter in geotechnical engineering. OCD is strongly correlated to the paired stiffness ratio (G0,HH /G0,VH.) as measured by geophysics. Mackens et al. (2017) used the relationship after Ku and Mayne (2014) to calculate the over-consolidation difference with an assumption of transverse isotropy along the vertical axis (v s, VH =v s, HV ).

References:

Mackens, S.; Hocine, Y.; Werban, U. (2017). Direct-push Based Seismic Crosshole Testing for Geotechnical Engineering Applications, Journal of Environmental and Engineering Geophysics 2017; 22 (3): 291–297. doi: 10.2113/JEEG22.3.29

Ku, T., and Mayne, P.W. (2014). Stress history profiling in soils using OCD, G0 anisotropy relationship: Proceedings of the Institution of Cicil Engineers,Geotechnical Engineering, 167, 476-490

Roesler, S. (1979). Anisotropic shear modulus due to stress anisotropy: Journal of Geotechnical Engineering Division, 105, 871–880