Estimating relative density from shallow depth CPTs in normally consolidated and overconsolidated siliceous sand

Abstract

Current interpretation of relative density (<i>D<sub>r</sub></i>) based on CPT end resistance (<i>q<sub>c</sub></i>) data in sand relies on empirical expressions established from calibration chamber studies for which a deep failure penetration mechanism is attained. These expressions are mainly based on stress normalised <i>q<sub>c</sub></i> data. Previous studies have highlighted the importance of performing the normalisation procedure with respect to the mean effective stress (<i>p'</i>) in overconsolidated (OC) sand deposits instead of the vertical effective stress (<i>σ'<sub>v</sub></i>) that has been used in normally consolidated (NC) deposits. Due to a large variation in the coefficient of earth pressure at rest (<i>K<sub>0</sub></i>), on which <i>p'</i> depends, in the uppermost (3-5) m of OC sand, a recent study has demonstrated a rigorous unified approach for estimating a varying <i>K<sub>0</sub></i> with depth. However, the surficial shallow failure penetration effects are not accounted for, and consequently, interpretation of the upper (0.5-1.5) m of sand is still erroneous. This depth range is very important for low stress geotechnical applications such as offshore flowlines and mudmats. With a reinterpretation of previously published data, a new global model is presented that enables estimation of <i>D<sub>r</sub></i> from shallow CPTs in siliceous sand by taking into account both the shallow failure penetration effects (important in the uppermost 0.5-1.5 m) as well as the varying <i>K<sub>0</sub></i> with depth for OC sand (important in the uppermost 3-5 m).