Bed deposits compositional and structural variations

Qualitative analysis of photographic and video images for all the tests show clearly that the segregation behaviours within the bed deposits display different compositional and structural features and transitions with depth. A schematic representation of the compositional and structural transitions observed within the bed deposits is shown schematically in Figure 5.4. At the initiation of the settling phase in all the tests, all mixtures were uniformly distributed, mixed slurries (Figure 5.4a), and they settled to form distinct bed deposits with vertical changes in composition and structure. In general, deposit regions (or segments) with four distinct and different composition and structure have been identified for all the segregated bed deposits. These are presented as Segment Type I – IV in Figure 5.4b, with corresponding brief descriptions of each observed segment given. It is interesting to observe that some experimental runs exhibit all the four segment types during the settling phase while others show only some regions/segments to be present. It is also noted that the time of formation of the different segments differs between sand-clay mixtures. The number and nature of segments exhibited in each deposit and the corresponding time of formation are controlled largely by the parametric conditions in each run (e.g. solid volumetric concentration of mixture, sand-clay composition and pore fluid salinity).

In all tests, after the initiation of hindered settling phase and prior to the onset of primary consolidation stage, near-vertical drainage paths (i.e. dewatering channels), were formed within the predominantly clay suspensions in Segment Type III and IV. Pore fluid was seen being expelled through these dewatering channels (∼ ≤ 3.0 mm in diameter). After the onset of primary consolidation phase, they later became smaller or even disappeared completely. It was also observed that these dewatering channels not only allowed the upward expulsion of water, but sand grains were also shown to settle through the dewatering channels; an observation also recorded by Merckelbach (2000). When the dewatering channels have significantly diminished or disappeared (analogous to the clay

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matrix reaching the gelling concentration, e.g. Winterwerp and van Kesteren, 2004), the remaining sand grains were trapped within the clay matrix, thereby contributing to the development of sand clusters or patches (e.g. Segment Type II and III).

Figure 5.4 Schematic representation of the compositional and structural features observed in the sand-clay bed deposits over the range of parametric conditions tested (a) at the start of the test (b) of the final bed deposit formed

Figures 5.5 & 5.6 present time-lapsed images of the developing bed deposits for all the mixtures tested to investigate closer, the structure and composition of the segregated sand-clay regions/segments that occurred within these deposits in line with schematic representation in Figure 5.4. For the majority of the experimental runs, it is apparent that a proportion of the clay fraction is trapped in the sand-dominated base layer; while some sand particles become trapped as clusters in the upper clay-dominated layer during the bed formation process, resulting in segment types discussed above (Figure 5.4).

(a) Start of Test

(b)

During & after Test

El eva ti on, z ( mm) IV III II I

Purely clay matrix with no visible sand grains

Predominantly clay rich matrix with visible sand grains

Clay dominated matrix with trapped sand patches or clusters. The sizes of the clusters vary from mixture to mixture

Base sand dominated matrix with/without clay particles

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Figure 5.5 Time-lapsed images showing segregated deposit formation at elapsed times t shown for run (a) SET-EX1 (85s:15c) (b) SET-EX2 (85s:15c) and (c) SET-EX3 (75s:25c) (see Table 5.1); Elapsed times of (i)→(iv) = 30, 180, 360,1440 minutes respectively.

(a)

(b)

(c)

(i) (ii) (iii) (iv)

(i) (ii) (iii) (iv)

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Figure 5.6 Time-lapsed images showing segregated deposit formation at elapsed times t shown for runs with 65s:35c mixtures (a) SET -EX4 (b) SET-EX5 (c) SET-EX6 and (d) SET-EX7 (see Table 5.1); Elapsed times of (i)→(iv) = 30, 180, 360,1440 minutes respectively.

(b)

(i) (ii) (iii) (iv)

(c)

(i) (ii) (iii) (iv)

(d)

(i) (ii) (iii) (iv)

(a)

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Comparison between Figures 5.5b & 5.6c, show the temporal development of strongly segregated bed deposits for the 85s:15c mixture (run SET-EX2, Table 5.1) and the transitionally segregated deposit arising from the 65s:35c mixture (i.e. run SET-EX6 (Table 5.1), respectively. These images again highlight the rapid formation of sand- dominated layer (i.e. Segment Type I, Figure 5.4) at the base of the column (i.e. z = ∼140 mm thick and ∼100 mm thick at t = 30 mins for SET-EX2 and SET-EX6, respectively). The layer in SET-EX6 is characterised by near-vertical banding indicative of the development of clay-pore water “dewatering” channels during this initial settlement phase. However, this is not visible in the thicker sand-dominated bottom layer of SET- EX2. For both mixtures, this bottom layer (Figure 5.4) appears to be overlain by a thicker clay-dominated layers containing discrete sand patches and grains [i.e. z =140∼200 mm and 110∼260 mm after t = 180 mins for SET-EX2 (Figure 5.5b) and SET-EX6 (Figure 5.6c)]. The discrete sand patches appear to be large in SET-EX6 and smaller in SET-EX2. With increasing duration (i.e. for t = 360→1440 mins), this layer (i.e. Segment Type II) is shown to slightly compact, as indicated by the downward displacement of the trapped sand patches (Figures 5.5b & 5.6c), although, this is less obvious in SET-EX2 than observed in SET-EX6 due mainly to (i) the lower volumetric clay concentration in run SET-EX2, compared to SET-EX6, and/or (ii) the formation of a sharp segregational interface between the incompressible base sand layer and overlying clay-rich deposit.

The presence of discrete sand patches within the clay-dominated layers of the two mixtures is particularly interesting when considering the time at which trapping occurs. The significant difference in the mixture composition for SET-EX2 and SET-EX6 (see Table 5-1) appears to indicate that a proportion of the sand fraction becomes trapped at elapsed times t ≥ 150 mins (see also Appendices 5-1 and 5.2b), most probably, as the clay concentration Øscl reaches the gelling concentration. Above segment type II layer, the

remainder of the overlying clay suspension deposits to form a (relatively) sand-free surface layer (i.e. Segment Type IV) within the bed deposit (i.e. z > 200 mm and 260 mm for SET-EX2 and SET-EX6 respectively; Figures 5.5b & 5.6c).

A similar comparison is carried out on runs within parametric classification Group-D in Table 5.2 (i.e. runs SET-EX4 to -EX7), to identify any significant influence that the pore fluid salinity may have on the temporal and spatial development of the resulting bed deposit composition and structure in particular in the presence of high % clay content (i.e. 35%). From the images in Figure 5.6, the development of transitional segregated deposits

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is shown for all the mixtures considered in classification Group-D (Table 5.2). Indeed, the temporal and spatial evolution of the bed deposits are similar to the descriptions given above for the bed deposit formation from the 65s:35c mixture (i.e. SET-EX6). This is obvious as they all have the following parametric conditions in common: initial mixture concentration and sand and clay contents (Table 5.1). In summary, these runs appear to have similar segment types (Figure 5.4) of approximately the same vertical extent (see Figure 5.6), with each final bed deposit having all the four distinct segment types as defined in Figure 5.4. In other words, salinity has little or no influence on the formation and structure of the final segregated (sand-clay) bed deposits in comparison to the strong influence of initial mixture concentration and fractional sand and clay concentration. However, further analysis of a large number of their photographic and video images, indicates that the appearance of segment type-II is delayed in mixture with the highest salinity concentration i.e. SET-EX7 (40 ppt) [at t = 150 mins; Appendix 5-2c], compared to SET-EX4 (0 ppt) [at t = 60 mins; Figures 5.2]. Similar trend was equally observed in SET-EX2 (30 ppt) [Appendix 5-1] and SET-EX1 (15 ppt) [Figure 5.1a], where Segment Type II appeared at t = 60 mins and 30 mins respectively. Further analysis on the parametric influence of pore fluid salinity on for example differential settling behaviour of sediment mixtures shall be discussed later in the chapter.