Depth of cover
Depth of cover is designated in EN 1610 [DINEN1610a] as the "vertical distance from the top of the pipe barrel to the surface".
The depth of the cellar bottom to be drained normally defines the minimum depth of cover of the combined water and wastewater sewers.
Taking into account the required minimum gradients of 1.0 % to 2.0 % for the laterals and pipes whose course follows the access path under all the supply lines, and the depth of the cellar inlets including drain traps, (valid to [Busch65] for the individual installation types) guide values are shown in (Table 1.5-1) for cover depth.
The average depth of cover for sewers in urban drainage areas is usually found to be 3.0 to 4.0 m [ATV95b].
(Image 1.5-1) shows that the depth of cover mentioned above can vary greatly, depending on the topography. It contains the smallest and greatest values of the depth of cover as well as calculated average values for every nominal size of the sewers laid in the years 1974 to 1983 in the City of Düsseldorf.
In regions with larger height differences, one must reckon with a smaller distribution of the depth of cover.
In Zurich for instance, the regulation cove depth, due to an agreement with the other sewage operators, is 2.70 m. The terrain conditions generally permit this value to be adhered to and thus there is little deviation upwards or downwards [Zäsch86].
The minimum depth of cover for surface water drains of a separate system results from the frost-free position of the drains to 1.5 m due to being laid below gas and water pipes and with connections of rainfall downpipes to the sewers from 1.5 to 1.7 m.
Therefore, in a separate system, the wastewater sewers are situated below the surface water sewers. For reasons of economy, when both these piping systems are laid at the same time it has been the normal practice up to now to dig a pipe trench with a stepped bottom (Image 1.5-2) whose width is smaller than the sum of the trench widths of separately dug trenches. Only seldom are the pipelines arranged directly above each other (Image 1.5-3) (Image 1.5-4).