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Table of Contents Calculation of Invert Levels............................................................ 3 Calculation of Invert Levels............................................................ 5 Calculation of Invert Levels............................................................ 6 Calculation of Invert Levels............................................................ 7 Calculation of Invert Levels............................................................ 8
Calculation of Invert Levels Before any pipes are laid, it is necessary to know the invert level of the drain or sewer to which the connection is to be made and to relate this to the ground level at which pipelaying is to begin. The invert level is arrived at by one of three methods. The local authority may be able to provide the information. By calculation. By excavating vertically down to expose the drain or sewer. If the local authority is unable to provide the necessary information and it would not be convenient to excavate – then you must establish the invert level by calculation.
Example If you need to make a connection to the main sewer, then you will need to carry out the following procedure. Remove the inspection chamber cover on either side of the proposed connection, take a reading of the invert level in each inspection chamber and note the difference in levels. Measure the distance between the inspection chambers and make a note of the distance to each inspection chamber from the proposed connection. Calculate the invert level of the sewer at the point where you are to make your connection.
The drawings above show details of the existing sewer and the proposed drain connection – if the distance between the inspection chambers A and B is 30 metres and the difference in invert levels at these chambers is 1 metre – then it follows that the existing sewer is laid to a fall of 1 : 30.
You now need to calculate the fall from inspection chamber (B) to our new connection.
Now, the ground level staff reading at the proposed connection is 0.9 m so the depth of the invert level below ground level = 2.2 – 0.9 = 1.3 m.
Having found this, it is necessary to relate the measurement to the invert level of the new chamber at the dwelling, to find out if the fall of the new drain is satisfactory.
Assume that the invert level of the new chamber is 0.6 metres below ground level. Set up the engineer's level and take readings at (Y) and (X), these being 1.6 metres and 1.4 metres respectively.
Since the invert level at (X) is 0.6 metres below ground level, the fall from invert level at (X) to invert level at (Y) is:
2.7 metres – 2.2 metres = 0.5 metres
thus the fall is 1 : 36, which is satisfactory for a 100 mm drain.
