invert levels calc

Calculation of Invert Levels

© Copyright FENC

Table of Contents

Calculation of Invert Levels............................................................3





Calculation of Invert Levels© Copyright FENC

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 theproposed connection, take a reading of the invert level in eachinspection chamber and note the difference in levels.

Measure the distance between the inspection chambers andmake a note of the distance to each inspection chamber from theproposed connection.

Calculate the invert level of the sewer at the point where you areto 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.


Thus the fall from chamber (B) to the proposed connection is 0.6 metres.

NOTE:


Since the staff reading at chamber (B) = 1.6 metres, the depth to invert of the

sewer at the proposed connection is:

1.6 metres + 0.6 metres = 2.2 metres

WHICH IS DEPTH AT (B) + FALL

Longitudinal Section of 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.

NOTE: 0.5 metres in 18 metres = 1 in 36


invert levels calc

Documents