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QUESTION 11
(a) You are called to a residence with a blocked foul water drain. You discover the property has a
conventional septic tank and effluent disposal system. The septic tank has recently been cleaned
out. You note the water level is up high in the square junction. You plunge the square junction at
the septic tank and the level remains above the inlet drain level.
Give TWO possible causes of the blockage in the foul water drain.
1
2
(2 marks)
(b) While installing a septic tank at a house under construction you discover during the installation that
the water table is high. State what action you would take to ensure that the installation remains as
laid.
(1 mark)
(c) Explain what is meant by the term dewatering.
(1 mark)
(d) Name TWO ways of dewatering excavations.
1
2
(1 mark)
(e) What is the purpose of well pointing?
(1 mark)
(f) Explain the following terms:
1 Datum level
2 Invert level
3 Water table
4 Porosity of soil
5 Compaction
6 Bedding
(3 marks)
Total 9 marks
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Question 11
a) 1. The disposal system is blocked.
2. disposal area is insufficient.
b) To ensure that the installation remains as
laid, is to leave the pipeline and tank full of water to prevent flotation.
c) Removing water from a trench or excavation.
d) 1. Well-point dewatering
2. via a Diaphragm pump
3. via a centrifugal pump
4. Automatic sump pump
5. Diverting the water.
e) To find the ground water level. This method can also be used with a pump to lower the ground water level in the vicinity of a trench, creating a reverse flow from the trench to the well points.
f) 1. A fixed point from which levels and distances may be taken. The datum will usually be fixed in relation to a bench mark.
2. The level of the lowest point of the internal surface of a pipe or channel at any cross-section and related to the datum level.
3. The highest seasonal surface of groundwater expressed as a distance below ground level.
4. The density of a soil (required information when installing disposal fields or soakage holes)