Results;
Lithologies;
Beach sand;
- Poorly sorted
- Angular grains and some rounded grains
- 50 % coarse quartz grains and 10 % muscovite mica – flaky tabular medium grains.
- 20 % platy medium shell fragments
- 20 % black sub rounded mineral
Desert sand;
- Grains all well rounded and well sorted.
- Mainly fine-medium grains
- 95 % quartz grains
- 5 % plant fragments
- Grains red coloured – desert varnish
Figure 1 shows the angles of repose of different sized sediments and the differences between a beach and a desert sand compared to the control. The control has followed the pattern I expected. The fine control (fine glass balls) has the highest angle of repose at 36.7° and then the medium at 35.8° and the coarse glass balls have the lowest angle of repose at 35.5°. This is because in a finer sediment the grains can pack more closely together and so the pore spaces are smaller. This results in more contact between the grains which means there is increased friction between the grains. This therefore means that the shear force has more friction to overcome to move the sediment. The coarse glass balls have a lower angle of repose because the pore spaces are larger and so there is less contact and therefore less friction for the shear force to overcome. The larger the grains the larger the force and the greater the tendency is to slide.
However, my results for the beach or the desert sand do not follow this pattern. In the beach sand the fine grained sample does have the highest angle of repose at 36.9°. This is because some of the shell fragments and the fine grains which means that the flat, platy, angular shell fragments will act as ‘shelves’ in the unconsolidated sediment and help stabilise it.
The combination of the close packing and the medium, angular shell fragments will increase the angle of repose due to the increase in friction and the shell fragments would naturally lie at an angle to the slope therefore stabilising it. The medium beach sand is much lower than the coarse or the fine at 34.4°. This is because that sample would have contained mostly shell as the shell was almost all of a medium size. This would mean that all the flat, angular shell fragments would lie flat on top of each other which would allow the fragments to easily slide over each other and their flat shape would reduce the shear strength of the sediment and therefore the shear force would have less counter forces to overcome
Another contributing factor is the lithology of the beach sand. The sand is polymineralic and contained many different minerals such as quartz and mica and some shell fragments. This would produce many different shaped grains depending on the mineral. For example, the muscovite mica has tabular crystals which would again act as shelves like the shell fragments. Lithology influences the shape the grains make and therefore the angle of repose.
The desert sand follows a different pattern to the beach sand. The coarsest sample has the highest angle of repose as shown in figure 1 at 36.8° then the fine sample at 36.3° and the medium at 35.9°. This isn’t what I expected to happen and I think the coarsest sample has the highest angle repose because the grains would have a greater tendency to stick to the slope due to the larger amount of gravity. There would therefore be more resistance to move.
The desert sand has more spherical grains and therefore the grains have a larger mass and so again there is more gravitational pull and more resistance for the shear force to overcome. I think the fine and the medium samples are similar are considerably less than the coarse sample at 36.3º(fine) and 35.9º(medium). This is because the high sphericity of the grains means that they cannot pack as closely together as a low sphericity sediment.
Also because they are finer they will have a smaller mass and will have a smaller amount of gravity acting upon them.
Figure 2 shows the percentage weight of each size of sediment for beach sand. I can see that in general the beach sand is a more coarse sand and that the most frequent grain size is 0.5 mm at 79.3 % and there is only 0.1 % of fine sediment at 0.063 mm. The graph shows there isn’t a great evenly spread distribution and this is not what I expected as a beach sand would typically be poorly sorted and polymineralic which it is. This is because the 79.3 % is mainly all shell fragments which were all the same size and these shell fragments make up a high amount of the sediment. The desert sand doesn’t follow this what I thought would happen either. Figure 3 shows that although the sediment is mainly made up of finer grains there is more of a range of grain sizes than the beach sand. For example, there were 2 relatively high percentages of grain sizes instead of one at 34.9 % for 0.25 mm and 52.4 % for 0.125 mm and the two coarsest sizes were purely plant material and not actually grains.
Figure 4 shows that the well sorted control has a higher angle of repose at 31º than the poorly sorted control at 30.3º. This is unusual. I think that this is because in the poorly sorted control the fine glass balls settled to the bottom of the petri dish instead of packing in between the coarser glass balls. This results in the coarser glass balls remaining in a layer on the top with large pore spaces.
The beach sand has a higher angle of repose in general at 36.8º than the desert sand at 35.6º. This is what I expected. See diagram 1 for this.
Figure 5 shows the grain size and angle of repose. This scattergraph tells me that there is no obvious visible relationship between grain size and angle of repose so I did not feel it appropriate to carry out a chi squared test or Spearman’s Rank.
In conclusion, I have found that the textural maturity of sediments does influence angle of repose. I did find my first hypothesis to be correct in that the fine sediment will have a higher angle of repose. This is true of the beach sand and the control but not of the desert sand. I have found my second hypothesis to be true in the actual sediments but not in the control and I have found that the beach sand does in general have a higher angle of repose.