According to EN 752-1 [DINEN752-1] rehabilitation is understood to be: All measures for restoring or upgrading the performance of existing drain and sewer systems.
With damaged sewers as well as structures of drain and sewer systems that cannot be taken out of operation, i.e. whose function cannot be purposely interrupted for an undefined period in the sense of DIN 31015 [DIN31051], rehabilitation must be undertaken when
- The wear reserve has been depleted to such an extent where a damage limit is reached that places the functionality in question (Image 5-1) ;
- Due to unplanned internal or coincidental external effects, a malfunction or a failure, i.e. an unplanned interruption of the functionality occurs.
If a defined point of damage is not repaired in good time, then it can lead to failure as well as having further grave effects, e.g. on the environment, the disposal safety, on traffic and to the economy on the whole.
In such cases the making good of the damage is very often carried out as a fire fighting strategy, that is, no proposal requests can be written for the corresponding immediate measures, which generally mean higher economical costs than in the normal case.
EN 752-5 [DINEN752-5b] provides the sequence diagram shown in (Image 5-1) for the processing, execution and control of the rehabilitation measures. It provides the immediate sequence of the working steps dealt with in the previous chapters
- Determining and evaluating the actual condition;
and assumes the completed discovery of the causes of the malfunction on the basis of the results of the hydraulic, environmentally relevant and structural investigations (Abschnitt 4.1).
The possible attempts at solutions can consist of hydraulic, environmentally relevant or structural aspects and achieve improvements in one or more of these fields [DINEN752-5b].
Hydraulic attempts at solutions are [DINEN752-5b] :
- Maximising the available flow capacity by means of:
- Removing the flow obstacles
- Reducing the flow in a sewer by means of:
- Deviating surface water into seepage installations or to porous surfaces;
- Utilisation of porous surface fastenings;
- Deviating the flows into another system;
- Laying of additional surface water drains;
- Reducing infiltration and the extraneous water intake.
- Damping the peak flow by means of:
- Utilisation of the existing storage capacity of the system (targeted discharge guidance);
- Utilisation of storage possibilities on the surface;
- Preparation of additional storage capacity (storage sewer or storage pond).
- Enlarging the flow capacity of the sewers by means of:
- Replacement with larger pipe cross section;
- Laying additional pipelines.
Environmentally relevant attempts at solutions can be [DINEN752-5b] :
- Reducing the contaminant inputs into the system;
- Reducing the planned contaminants input into the receiving waters by means of:
- Enlarging the flow into the sewage treatment plant;
- Improving the solids retention and the hydraulic capacity of the surface water relief structures;
- Sewer management (real time control).
- Reducing the effects by moving the inlet positions;
- Reducing the exfiltration, for instance by:
- Repairing the leaks;
- Leaktight lining;
- Replacing the pipeline.
Particular emphasis will be placed in the following on the structural solutions (also called structural rehabilitation). According to EN 752-5 [DINEN752-5b] they comprise:
- Protection of the sewer substance by means of suitable linings or inner coatings
- Rehabilitation of the sewer substance by means of:
The structural solutions for protecting the sewer substance are prophylactic measures that are applicable, for example, for the replacement or utility changes of sewers or sewer network regions. They are identical to the processes utilised in renovation and are therefore dealt with there (Abschnitt 5.3).
The processes belonging to the rehabilitation of the sewer substance for structural rehabilitation comprises several main groups of processes with numerous special processes (Image 5-2).
Within the scope of the evaluation of the possible solutions, the optimal solution must be selected taking into consideration the basic requirements (Abschnitt 5.1) and further aspects such as, for instance [DINEN752-5b] :
- Construction phases
A check should be made whether the solution can be spread over several phases. In doing so, the urgency of the various works, the utilities aimed at with the various phases as well as the cost savings by delaying to a later time should be considered.
- There-utilisation of materials
The possibility of the re-utilisation of the materials used in the rehabilitation, as well as the waste produced must be considered.
- Co-ordination with other infrastructure measures
The advantages of the co-ordination of the work with other infrastructure measures must be considered.
- Disturbances to the public
The disturbance of the inhabitants and the public by traffic obstructions, dust, noise and other sources of disturbance must be considered.
- Loading due to future maintenance
The future operating and maintenance costs of the rehabilitated system should be considered. The environmental effects of the disposal of the wastes from the maintenance must also be taken into account.
- Commercial evaluation
Costs and utility must be considered in order to determine whether additional utility of a solution when compared to another, e.g. duration of utility, is justified.
- Total costs
The total costs of the solution, together with the indirect costs (e.g.costs of disturbances of the public), must be taken into consideration. The total costs also include the costs of all provisional measures, the moving of other supply and drain pipes, and all planning and investigation costs [DINEN752-5b] (Abschnitt 5.7).
The selected rehabilitation solutions of a drain and sewer system must be documented in a single rehabilitation plan. This plan should contain:
- Detailed aims;
- Regulatory requirements or permissions including all rehabilitation time schedules;
- Work requirements;
- Proposed measures including costs and construction phases;
- Co-ordination with other construction measures or planned closing-off; as well as
- Effects on operation and maintenance [DINEN752-5b].
The supervision of the effectiveness of the rehabilitation and the updating of the rehabilitation plan including the recording (cadastre) as well as the hydraulic model are important [DINEN752-5b].