Question

Soil A has a volume of 297.01 cm3, a wet weight of 498.12 grams, and an oven dry weight of 356.41 grams. It has a liquid limit of 50 and a plastic limit of 20. Soil A has a moist bar length 100 cm and an air-dry bar length of 80 cm.


Soil B has a volume of 297.01 cm3, a wet weight of 529.86 grams, and an oven dry weight of 445.52 grams. It has a liquid limit of 30 and a plastic limit of 22. Soil B has a moist bar length of 100 cm and an air-dry bar length of 95 cm.

For both soils, assume particle density equals 2.65 g/cm3 and density of water equals 1 g/cm3.

Consider soil A and soil B for probable locations for installation of a septic system. Based on your answers from parts 1, 2, and 3, what would be the pros and cons of using each soil as a potential site? What are some other considerations and limitations one should consider when installing a septic system? (7 points)

Answer 1

Soil A has high liquid limit which high compressibility and a high shrinkage.

A high-plasticity results in a low shear strength

soil B loses its shear strength and becomes liquid-like at a low water content than soil A.

Liquid and plastic limits are determined by relatively simple laboratory tests that provide information about the nature of cohesive soils. Engineers use these tests extensively for the correlation of several physical soil parameters as well as for soil identification. Casagrande (1932 cited in Das, 1998) studied the relationship of the plasticity index to the liquid limit for a wide variety of natural soils. On the basis of the test results, he proposed a plasticity chart as shown in Fig. 9.1.

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Figure 9.1. Plasticity chart after Casagrande.

The knowledge of Atterberg limits and plasticity is often used to determine the amount of silt and clay in a fine-grained soil, as plasticity generally increases with higher clay content. On the other hand, it gives important information on the mechanical performance of a soil used as foundation or construction material. A high liquid limit normally indicates a high compressibility and a high shrinkage/swelling potential. A high-plasticity index Ip generally results in a low shear strength. A low Ip means that a soil used as foundation will change significantly in consistency even with a small change in water content. However, the greater the plasticity, the greater is the shrinkage on drying.

Looking at the plasticity chart, we see that clays have a higher plasticity index as silts. This means that, in terms of consistency, silts react more sensitively to water content changes, or, in other words, clays can adsorb more water without losing too much of their consistency.

Soil	Liquid limit wL (%)	Plastic limit wp (%)	Plasticity Ip (%)
Silt, low plasticity	25–35	20–28	4–11
Silt, medium plasticity	35–50	22–23	7–20
Clay, low plasticity	25–35	15–22	7–16
Clay, medium plasticity	40–50	18–25	16–28
Clay, high plasticity	60–85	20–35	35–55

Soil A has medium plasticity which is more than soil B and less approprite for septic tank preparation.

Other considerations

1 – Septic system design

2 – Choosing the right septic tank or sewage treatment plant

Choosing the right septic tank or sewage treatment plant is a crucial step in the process. At this stage, you will already have decided whether you will be installing a soakaway or percolation area, or having a direct discharge.

3 – Installation

When installing your septic tank or wastewater treatment system, there are two options. The first option is to complete the installation works yourself. Many people choose the DIY septic tank installation route. There are cost savings when choosing this option, but it is important that all drainage works completed meet the building regulations. If you are any way unsure about doing a septic tank upgrade or new system installation yourself, then you should hire a professional.

Option 2 involves getting a professional septic tank installation company to complete the works for you. If you are doing this then it is always advisable to get two or three prices to make sure you are getting excellent value. If you are using a professional septic tank installer, then you should make sure they have experience installing sewage treatment plants, that they are fully insured and have references.

4 – Ventilation

Good ventilation is often something that is overlooked when installing a septic tank or sewage treatment plant. Bad ventilation can lead to odour issues onsite. It is very important that the ends of percolation trenches are vented, and that there is either a vent on the wastewater treatment plant or that there is a soil vent stack at the house. For further information on venting and preventing septic tank odours please check here.

5 – Maintenance and operation access

When installing your sewage treatment plant, consideration should be given to future maintenance. Any lids on the tank should not be covered over with soil. Access will be needed in the future for septic tank emptying and for inspection and checks that the system is running properly. The tank should be within 30m of a driveway, as septic tank emptying companies generally only have 30m long hoses for emptying tanks. If a tank is over 30m from a driveway your septic tank emptying costs may increase