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Last edited 19 Feb 2019
In a pipe, sewer, channel or tunnel, the invert is the lowest point upon which water can flow on the inside. If circular in shape, the invert is the lowest point on the inner circle if looking at a section of the construction. The invert may be considered the 'floor level' of the vessel in question.
In a tunnel constructed by drill and blast or by NATM (New Austrian Tunnelling Method), the invert will vary according to the stage of the construction: the invert will always be the lowest point inside that has been excavated at that point in time. This will become lower as the excavation proceeds, with the final invert level being that of the complete tunnel.
Also, a tunnel being constructed using NATM (sometimes called 'sequential excavation') may have two or three varying inverts as excavation of the centre and sides of the tunnel may be at different stages.
An invert will typically be laid to a gradient, particularly if it is to convey liquids, such as in a pipe or sewer. The level of the gradient will therefore vary according to the position of the invert along the flow.
The invert level is usually given as a distance to some fixed point. In tunnel construction, invert level may be cited as 10m above tunnel datum (ATD) where the tunnel datum (based on an ordnance datum) is set at a particular level for the entire project. Alternatively, a survey of a property for a preliminary design may record the varying invert levels of the drainage system beneath the building.
In contrast, the ‘crown’ of the tunnel or pipe is that point located on the top external surface that is directly above the invert. One way to calculate the crown position is by adding (or subtracting) the internal diameter and the material’s thickness to (or from) the invert level.
For example, if a 10m-internal diameter tunnel of circular-section with uniform 500mm thickness and an invert level of 16m below ground level, the crown level would be established by subtracting from the invert level the internal diameter (10m) plus subtracting the material thickness, which is equivalent to: 16m – 10m – 0.5m = 5.5m.
So, the crown datum level is at 5.5m below ground.
Sometimes it may be necessary to reverse this calculation when the crown is known and the invert level is not known.
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