Quality assessment of organic geotechnical samples

Abstract

For certain geotechnical structures, such as dikes and levees, the design quality is heavily reliant on the sampling quality. In addition, the extent of consequences on geotechnical design of using parameters from samples with a certain degree of disturbance in laboratory tests is unclear. While the available literature has been mainly focussed on disturbance experienced by clay samples, the same empirical findings are not always applicable to organic soils such as peat and organic clay. The global objective of this thesis was to gain an understanding of sampling disturbance in organic soils using both established and non-destructive experimental methods. In order to achieve this, the traditional Dutch Ackerman sampling technique was compared to that of a recently developed sampler, the Deltares Large Diameter Sampler (DLDS). Non-destructive methods and 1D consolidation parameters were used to quantify disturbance in organic soil samples.

The results from existing databases showed that organic clay has a clear linear trend between w and RR and peat showed no definitive correlation, although the peat sampled by the DLDS had significantly higher water contents than the peats from the Ackerman sampler. Moreover, 37% of organic clay and peat Ackerman samples had an ∆e/e0 below 7%. Meanwhile, all the DLDS results from the Deltares laboratory had an ∆e/e0 of less than 4%. The use of this parameter is still questionable for peats, as the higher e0 found in DLDS peats is the cause for low disturbance measurements. However, the ∆e/e0 index may be used in conjunction with other destructive and non-destructive disturbance parameters. Furthermore, the trend showed it becomes increasingly difficult to produce high quality samples with increasing in-situ vertical stress.

Using differences in shear wave velocity as a non-destructive disturbance determination technique, it was concluded that the increased shear wave velocity in Ackerman samples relative to the DLDS samples, although slight, was due to densification of the outermost volume of the sample being compressed by the sampling procedure. This effect was further observed in the other non-destructive method use in this investigation, CT scanning. Analysis of micro-CT scans showed that all specimens had higher greyvalue intensities at the perimeter than at the centre of the sample, however the observed effect was 7.4 times higher with the Ackerman specimens than with the DLDS specimens.

The main recommendation is to continue research on the effectiveness of using non-destructive methods to quantify sample disturbance in peats. For the sampling industry, it is advised to increase the diameter of the Ackerman sampling tubes to between 70 and 120 mm when sampling in peat. In addition, in light of the lesser disturbance caused by the DLDS, it is advised to deploy it in shallow peats instead of the Ackerman sampler with the current inner diameter of 67 mm. Furthermore, it is recommended to deploy both types of samplers side-by-side in sampling projects in order to diminish the effect of site-specific variables.