Influence of the initial soil stress state on the cone factor

Abstract

The uncertainties in undrained macro-stability analysis of dikes lead to overdimensioned and inefficient designs. The undrained shear strength (su¬), which describes the strength of clay and peat layers, is important information for the macro-stability analysis of a dike. The cone factor (Nc) gives a linear correlation between undrained shear strength (su¬) of the soil and the cone resistance (qc) from a cone penetration test (CPT)¬. In practice the uncertainty on this correlation, given by the variation coefficient, is high, especially it overestimates the su for high values of qc. As a result, a large reduction factor (RF) is taken into account between the expected value of su and the characteristic value of su used in calculations.
In literature the cone factor (Nc) is assumed to depend on the soil stiffness, the cone roughness and the initial soil stress state (∆). Including the initial soil stress state, or more specifically the horizontal soil stresses (σ’h0), in the cone factor determination can lead to a reduction of the variation coefficient. Based on literature and a numerical analysis it is found that including the initial soil stress state in the determination of Nc can lead to a difference of 10% in cone factor. However, the difference is less than 10% for most measures due to the relatively similar stress conditions over a large part of the cross section of a dike. In addition, a theoretical application of the initial soil stress state (Δ) shows that the overestimation of su for high values of qc cannot be solved by including it as found in literature.
A case study on the Hollandse IJsseldijk project has been conducted to back up the theoretical approach with experimental measurements. As a conclusion it is found that the variation coefficient is reduced slightly taking into account the initial horizontal soil stresses, resulting in a decrease of the reduction factor (RF) of 2-3%. Taking this small improvement and remaining unknowns in the calculation into account, it is concluded that including a measure of the in-situ soil stress state (Δ) in the determination of the cone factor (Nc) does not result in a significant reduction of the present uncertainties in dike assessment with existing approaches.