Friday, 31 January 2014

What is the Shear strength of that shit you drop in that pit

Synthetic sludge is a mixture of top soil and kaolin clay which when mixed with water in appropriate quantities; it emulates the behavior of faecal sludge.

Why use synthetic sludge
It is quite disgusting to use faecal sludge for experiments as many tests need to be carried and it is also difficult to handle faecal sludge in a clean manner without spillage.

Why carry synthetic sludge experiment
In engineering it better to establish the forces you are working against before making any decisions. So in the synthetic sludge experiment we are trying establishing the shear strength of sludge and establish the shear stress range over which a particular pump can perform. The available pumps we have are; the Gulper, Rammer, Nibbler, Modular gulper and the Auger

One of the modest ways of establishing uniform composition of sludge is through a slump cone test. The slump cone test also aids figure out the behavior of sludge of different compositions
A slump cone is a cone with a standard bottom diameter of 200 mm and top diameter of 100 mm with a height of 300 mm.
The slump cone experiment involves filling the slump cone with synthetic sludge and measuring the slump.

Figure 1: Measuring components of the synthetic sludge
Figure 2: Mixing the components with a quantified amount of water
A slump cone is placed on a level ground, filled with sludge ensuring that all voids are filled and then the cone gently removed. The collapse of the sludge called slump is measured. The slump is the distance from the initial level of the sludge when in the cone to the point where it collapsed.

Figure 3: The cone of sludge after removing the slump cone

Figure 4: Measuring the slump
Some equations have been modeled which predict the behavior of feacal sludge and these equations also apply to synthetic sludge. Each value of water content corresponds to particular shear strength. A graph of shear strength against water content shows a linear relationship. From this relationship, synthetic sludge with specified shear strength can be mixed since approximate water content can be got from the plot
Table 1 : Plot of shear strength against water content

From the chart the shear strength of sludge at any water content can be established or the water content at a given shear strength can be got.
During this experiment synthetic sludge was taken to contain 85% top soil and 15% Kaolin clay by dry mass and then varying the amounts of water to obtain specific shear strength.
It is at this point that three barrels of synthetic sludge of shear strength 2kpa, 500Pa and 100Pa were mixed for testing the pumping ability of the available pumps

Figure 5: Lowering a barrel containing synthetic sludge with specific shear strength into a pit for testing the available pumps

Figure 6: Testing of the Rammer

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