Sewer systems are constantly or intermittently subject to differing physical, chemical, bio-chemical and biological stresses.
- Type and duration of use
- External influences such as ground at the construction site, traffic loads, etc.,
If the actual condition reaches the damage limit, or even the zero line (failure), it must be raised at least to the nominal condition by means of suitable damage repair measures.
The line of the curve in (Image 2.2-1) shows a possible sequence of wear during the time of utilisation. It is determined by inspection and depends, on the one hand, on the installation itself and, on the other hand, on the stressing and the maintenance condition of the structure.
The condition of a structure or individual parts determined at a given point in time.
The determined (required) condition of a structure or individual parts.
Nominal condition deviation:
The non-agreement between the actual condition and the nominal condition of a unit being reviewed at a given point in time.
Actual condition deviation:
The non-agreement between the actual conditions of a unit being reviewed at different points in time.
Depletion of the wear reserve resulting from physical, chemical, biological and/or bio-chemical influences.
Reserve for the possible fulfilment of its function under defined conditions, which a unit being reviewed inherently possesses on the basis of manufacture, or as a result of damage elimination.
In the sense of rehabilitation, damage is the condition that is inherent or to be expected in view of the impermissible impairment of functionality.
Condition of sewers and channels in view of their constructive substance.
The depletion of the wear reserve, and thus the time when damage occurs, can be delayed by means of maintenance measures (Abschnitt 3.1), which also pre-suppose a corresponding inspection programme (Abschnitt 4.1). A general prevention of damage, however, is not possible [Renke84].
Generally, it can be assumed that sewer systems have a long life and, as a rule, are subject only to a slow worsening of their condition. The following list of write-offs that apply to Germany [LAWA93a] do not take the technical standard at the time of laying of the sewers into account, nor the influence of materials, nor the possible alterations of the loads on the respective pipelines within the period of the utilisation.
Sewers 50-80 (100) years
Sewer systems manholes 50 years
Pressure pipes and inverted siphon pipes 28 - 50 years
Storm water overflow installations and surface water tanks
Construction part (40) 50 - 70 years
Machine part 5 - 20 years
Pump and lifting installations
Construction part 25 - 40 years
Machine part 8 - 20 years (e.g. Archimedes pumps)
In view of the age structure of the sewer systems in Germany (Abschnitt 1.1), it must be assumed that the technical utility period has been exceeded today for a part of it and that it no longer fully measures up to the requirements of the times. This applies especially to the pipe joints with the seals utilised earlier (Abschnitt 1.7.1).
It can generally be determined that damage is caused, on the one hand, by exceeding the normative utility period and, on the other hand, also by the different external or internal influencing stresses during operation already mentioned above. Besides this, there are so-called starting damages, which can arise in a time frame of up to several years later and are attributable mainly to faults in workmanship and/or material errors [Gale81b] [Jones84a] [Leste79] [Li77] [WPCF85] [Haend88] [Ramme87] [Stein93a] [Stein94b] [Lidst94] [Lidst96].
The types of stresses that cause wear and that can lead to damage are extremely numerous so that under certain circumstances one and the same damage figure can be the result of completely different causes.
The damage can either be limited to a small area or can be spread over a wider region of a section of a sewer or network region.
The knowledge of the damage, its manifestation, causes and consequences is one of the most important conditions for successful planning and structuring of maintenance and rehabilitation measures and here especially for the selection of suitable processes for damage elimination or rehabilitation.
In the following, the main damage groups that occur in sewer systems and which are mentioned in [ATVM143-1:1989] are:
- Leaks (Abschnitt 2.2.1) ;
- Flow obstacles (Abschnitt 2.3.1) ;
- Positional deviation (Abschnitt 2.4.1) ;
- Mechanical wear (Abschnitt 2.5) ;
- Corrosion (Abschnitt 2.6.1) ;
- Deformation (Abschnitt 2.7.1) ;
- Cracks, pipe break, collapse (Abschnitt 2.8) ;
and will be dealt with in more detail, whereby no claim is made for completeness as regards the causes of damage and their consequences.
Information on damage distribution and extent in the sewer systems of the Federal Republic of Germany is contained in (Abschnitt 2.9).