Uncertainty correlation factor Nkt : influence of sample disturbance

An investigation is carried out in order to verify whether sample disturbance could influence the strength of soil samples and therefore influences the derivation of the empirical correlation factor Nkt. Based on an indicative analysis (Deltares, 2017) it is suggested that sample disturbance might be an important factor which causes the uncertainty of Nkt.The preliminary conclusions from the 2017 research are verified by performing in situ field vane tests (FVT), which measure peak shear strength and residual shear strength. In the laboratory, also the peak shear strength on intact samples and the remoulded strength on reconstituted samples are determined. The analysis shows that the results of the triaxial tests on intact samples generally agree well with the FVT results. Sample disturbance seem however to be an issue as several samples undergo substantial volume changes due to sampling and extrusion of the samples. As these volume changes are very variable per sample, and the undrained shear strength ratios from triaxial tests on intact samples compare well with the undrained shear strength ratios from the FVT, this may indicate that the effect of sample disturbance on the undrained shear strength might be very small or the effect of sample disturbance cannot be distinguished from the natural variability of the soil. Based on the findings in this study the indicative analysis of the contributing factors of the uncertainty of Nkt (Deltares, 2017) have to be adjusted. As the contribution of sample disturbance to the uncertainty of Nkt will be smaller than expected, other factors have to be more important. A number of factors which play a role on the derivation of Nkt and which are uncertain are identified in this research.

Dit is een rapport van Kennis voor Keringen (Rijkswaterstaat/Deltares)
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