The Effect of Sewage Effluent on the Permeability of Soil\
Aims and objectives
Most ancient structures collapse, mainly due to lack of knowledge on permeability. Properties such as permeability help an engineer, to determine the suitability of soil for some engineering works.
Hence, modern civil engineering works allows for site investigation, which includes the determination of the permeability relative to the soil which would be used for the work.
The main aims of this research are.
- To determine the maximum dry density (MDD) of soil, thus increasing its shear strength and bearing capacity.
- To determine the rate of seepage of water through a given soil sample.
- To determine how permeability of soil is affected by sewage effluent of different levels of pollutal strength.
The properties of soils depend to a large degree on the amounts of pore water they contain. In addition, the state of stress in the pore water has a pronounced effect on soil action. The part played by the water is thus seen to be an important one.
The facility with which water is able to travel through the pores also has much significance in many types of engineering problems. This property of soil, commonly called the permeability, is one of the most important of soil properties.
The pore of most soil is so small that flow of water through them is laminar. However, in very coarse soils, the flow of many some times be turbulent. The pore channels of a soil the mass are so narrow and tortuous, so irregular in cross-section, and so complex in their interconnection that an analysis of the flow through individual pores is not possible.
However, in engineering problems involving seepages through soils, it is not the flow through individual pores that is of interest. Instead, the flow desire is the combined flow through all pore of an element of volume, which is sufficiently large to give a typical representation of the soil mass as a whole.
The law flow through soil is named after Darcy who demonstrated experimentally that the rate of flow is proportional to the gradient. Darcy found out that the rate of flow is related to the hydraulic gradient according to the equation,
Where, Q=Rate of flow.
A= the total cross-section area of the soil mass.
K= the constant of proportionality, called the permeability of the soil.
In any random cross- section through a soil sample, this total area A will consist of area within solids As and void area Av. Hence, the void ratio may be expressed as ratio Av/As.
But, from equation (1), Q =V=Ki …………. (2.2)
Since the equation of equation (2.2) is equal to Q divided by the total area(not the void area through which the liquid is actually flowing), it should be noted that V must be a superficial velocity.
Permeability is taken into account for pumping ground water, spacing well point for de-watering foundation site for excavation, retention of water in reservoirs, design of damns and reservoirs.
Factors Affecting Permeability of Soils
It is a known fact that the rate at which water flow through soil varies from sample to sample. Permeability among other factors is affected in the following ways.
Effect of grain size on permeability.
Experiment demonstrated by poiseuille, shows that the average velocity through a capillary tube is proportional to the square of the diameter of the tube. Therefore, it is reasonable to expect that the seepage velocity through a given soil and the permeability of the soil are proportional to the square of the average for dimension. Since the item having the greatest effect on sizes of pores in a given soil is the grain size, it may be concluded that permeability varies approximately as the square of the grain size.
Effects of properties of the fluid
In soil mechanics, there will be little occasion to study any pore fluid except water, therefore, when permeability is mention in soil engineering, it is understood that water permeability is referred to.
The Darcy’s equation and others indicate that the permeability is directly proportional to the unit weight w and inversely proportional to the viscosity µ vary considerably with temperature. The effect of fluid properties on the value of the permeability when other factors are constant is thus given by, It has common practice in soil laboratories to test for permeability at the most convenient temperature and reduce to results for a standard temperature, commonly, by the used of equation (2.3) above…
The research on the effect of sewage effluent on the permeability of soil was a very rigorous one. Was therefore, carried out in several stages.
Collection of sample
Two different samples were collected for this research. The samples are the soil, the test specimen and the sewage effluent, which serves as the pollutant.
The soil sample was collected by excavating up to depth of one meter below the ground surface, in a farmland located at the junior staff quarters.
This resulted in the collection of a salty clay sample. This exercise was facilitated by the use of hoe and shovel.
The collected soil sample was then conveyed to the civil engineering soil laboratory, using a sack. It was then sprayed properly for air-drying, where it was left for up to two weeks.
The pollutant, sewage effluent, was collected from the disposal site of the University of Nigeria, Nsukka treatment plant. This collection was done on 7th August, 2012 by some minutes past 8 ‘o ‘clock. Am. It was done by the use of 5 liters keg, and was conveyed to the laboratory, immediately.
RESULTS AND DISCUSSION
The BOD value of sewage
Following the procedure described in chapter three for the determination of the BOD value of sewage, the following result were obtained as the sewage-water ratio is varied.
CONCLUSION AND RECOMMENDATION
The major aim of this research is to know how pollution affects the rate of seepage of water through soil sample collected one meter below ground level in a farmland at the junior staff quarters.
Generally, the permeability of any soil sample depends on the void ratio of the soil, since water passes through only the voids present in a soil sample. The effect of pollution on permeability of soil should depend on the effect of the pollution on the void ratio of the sample.
From the research carried out, the values of the permeability obtained, and the shape of the graph showed that the permeability of the soil sample is hindered by pollution. This believe is as a result of the sewage effluent containing some microscopic solid particles. These particles move in between the soil particles and plug up pore passages. At the same time, the rate at which water passes through these pores is reduced.
Out of the experience I have gained in the course of my research, I recommend the following for those who may carry out such research in later days.
- The test is a very vital one in civil engineering practice: it will help civil engineers to know the suitability of a soil sample for some engineering works. it will also help engineer to use some alternative sources of material in period of scarcity. For instance, sewage effluent of very weak pollutal strength can be used for water in mixing concrete.
- The research should be carried out using pollution of higher pollutal strength. This will help in the easier identification of the result of the research. From the research I carried out, it was not very easy to identify the relationship between the sewage effluent and the permeability of soil sample because of the low pollutal strength of the sewage.
- The test should be carried out using samples collected from different areas. The research carried out was on sandy clay soil. This result may not be achieved on loam soil, or sand. Hence, more tests should be carried out on other types of soil, collected from different areas to ensure that the result is satisfactory.
- The test should be carried out using the constant head permeameter. This is to ensure that the variable head apparatus used has no influence on the permeability of the soil sample with respect to the sewage effluent. ·
- The research should be carried out using other phase of the pollution. Sewage effluent used for my test is a liquid form of pollution, powdered or gaseous form of pollution may used to carry out the same research to ensure similar results.
- The research should be carried out leaving the polluted sample for some varying times to know if would be changes in the result with time.
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