This chapter has extensively described results of an experimental study exploring the potential of non-intrusive electrical resistivity techniques to characterize the spatial and temporal variation in sediment bed structure and composition, resulting from the differential settling of cohesive clay and non-cohesive sand mixtures. Various associated technicalities have been identified and finally, continuous bulk density profiles and an expression for porosity profiles have been successfully obtained for these sand-clay mixtures with ERMT. In addition, the time series profiles of these physically relevant properties (e.g. bulk density, porosity and material ratio) obtained from formation factor F have been demonstrated to allow quantitative analyses of different settling conditions, segregation/stratification mechanisms and structural densities of the resulting bed deposits from sand-mud mixtures.
The bulk density profiles obtained for the different sand-clay mixtures in the current work demonstrate, to a large extent, the influence of the non-cohesive sand fraction, in particular, on the spatial and temporal variation of the resulting composition and structure of mixed sediment bed deposits (e.g. Torfs et al. 1996; Manning et al., 2010; Xu et al., 2012; etc.). Generally, considerable care should be taken when comparing results from different devices for characterisation of bed deposits as variation in results could be technically linked to various factors such as scale of deployment, environmental conditions, etc., nevertheless, the bulk density values obtained from the current study are broadly in agreement with values obtained by other authors utilizing other non-invasive techniques for similar sand-mud compositions. For example, Torfs et al. (1996) recorded bulk densities ranging from 1.0 to 1.4 g cm-3 for single shot experiments on Scheldt mud
with sand additions of 0%, 5%, 10% and 20%. Similarly, Been and Sills (1981) obtained bulk densities ranging from 1.02 to 1.53 g cm-3 for various consolidating soft soils, (see
Table 4-1).
Table 4-1 Comparison of ER with other common non-invasive techniques
Author Bulk density
ranges (g cm-3) Technique Used Sediment Compositions Accuracy (+/-)
Current study 1.2 – 2.0 ERMT 0-75% Sand 0.025-0.04 g cm-3
Torfs, et al. (1996) 1.0 – 1.4 Gamma-ray (MAST- G6M, 1992) 0-32% Sand 0.01 g cm-3
Been and Sills (1981)
1.02 – 1.53 X-ray 75µm sieved Silt + 30%Clay
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Therefore, in spite of identifying some technical issues that must be given consideration when using EMRT, such as electrode polarization, temperature effects, electrode configuration and CEC of clay (or surface conduction effects), the results presented in this chapter have established that, ERMT is an extremely useful non-invasive characterization methodology to study sand-mud mixture sedimentation and bed formation processes, and, provided a suitable calibration is carried out, it is anticipated that this methodology can be deployed on variety of samples both at laboratory and field scales.
Finally, based on the findings of the current work, the following general relationship is proposed between the normalized bulk density of the sediment bed deposit and corresponding formation factor F value:
𝒃𝒖𝒍𝒌
𝒑 = 𝑎. 𝐹
𝑏 (4-4)
Also, to capture the relationship between porosity, 𝜑 and formation factor F, the following equation is equally proposed:
𝜑 = 𝑎̂𝑒− 𝑏̂.𝐹 (4-5)
The coefficients in Equations (4-4) and (4-5) are experimentally-derived [e.g. the coefficients in Equations (4-2) and (4-3) respectively, have been derived for sand-clay mixtures tested in this study]. The require condition for the proposed relationships is such that bulk/p and 𝜑 1 as F 1. As an extension of the application of ERMT in settling
column experiment, use of this technique in quasi-field environment has been demonstrated by adapting it to a benthic annular flume to study bed entrainment and erosion processes (see chapter 6 for further details).
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CHAPTER FIVE
Experimental Results: Sand-Clay Suspension Settling Experiment (ES-2)
“There are three principal means of acquiring knowledge…observation of nature, reflection and experimentation…”
—Denis Diderot
5.1 Introduction
The experimental findings from chapter 4 have served to demonstrate the ERMT to be suitable for the non-intrusive characterisation of spatial and temporal changes in mixed (sand-clay) sediment bed deposits forming as a consequence of sedimentation (i.e. settling and deposition) and subsequent consolidation behaviour for mixed sediment slurries in settling column tests. This will help improve fundamental understanding of the dynamic behaviour of mixed sediment beds within estuaries.
The current chapter details a parametric study on the spatial and temporal variations in sediment bed structure and composition resulting from the differential settling of cohesive clay and non-cohesive sand mixtures for a range of different mixture compositions, initial mixture mass concentrations and ambient fluid salinities (see Table 5-1). The main objectives of these experiments are as follows:
To study the influence of these parametric conditions on the spatial and temporal variation of sediment bed layer composition and structure (i.e. mixed or segregated) resulting from the differential settling of mixed sediments. This is investigated in terms of settling and consolidation rates, depth dependent bulk densities, and bed porosities.
To provide a significant dataset on sand-clay sedimentation processes, over a wide range of initial mixture concentrations and compositions required to (i) investigate further the parametric dependence of mixed and segregated bed deposit formation and (ii) test the polydisperse hindered settling formulation proposed by Cuthbertson et al. (2008) in terms of its predictive capabilities for the generation of these mixed and segregated bed deposits.
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To derive appropriate parametric criteria to describe the conditions under which well mixed or segregated bed layers will form in mixed sedimentary environments.
To achieve these objectives, an experimental programme and methods have been designed, as described in sections 3.6 (pg. 74) and 3.7 (pg. 78) of Chapter 3, a summary of which is provided in the following section.