A detailed physical inspection of the sewer system makes it possible to go beyond a simple hydraulic study. It is an unavoidable step for anyone wishing to optimise the functioning of his equipment and investment on maintenance.
A poorly maintained sewer system can result in pollution of the water table and overloading of a treatment station. “Sewer systems were built to last. However, they are out of sight and maintenance is often neglected”, explains Jean-Michel Bergue of the civil engineering mission at the French Ministry of Equipment, Housing, Transport and Tourism. However, these assets are subjected to a considerable amount of strain from road traffic, subsidence around the sewer casing, tree roots, etc. The system eventually wears out. It is therefore vital to conduct regular surveillance and so be able to act rapidly in the event of a major dysfunction (or if possible even before).
In-depth monitoring is particularly useful when the network is dense. This is performed by taking regular measurements. The results obtained in the field are then carefully analysed and lead to a diagnosis of the state of the system. On the basis of this diagnosis, decisions can be taken on whether to increase surveillance or carry out any specific maintenance operations. Maintenance can therefore be adapted to the real needs of the system.
Up until quite recently, hydraulic testing was often the only approach used to determine
the state of the network.
Hydraulic diagnosis
In many small public authorities, monitoring of the inflow and outflow is still commonly employed. The aim is to detect extraneous or seepage waters, which enter through a break in the system due to a crack, crushing, a faulty connection, etc., and overload the treatment works. The study is conducted both in dry weather and wet weather.
When evidence of extraneous water penetration is obtained, the fault must then be located before repairs can be carried out. This is no easy task! To track down the source of anomalies, the system needs to be divided into zones. At this stage, colour tests can be called for to visualise the input of clear water or smoke trials to test whether the various parts of the system are watertight. As a last resort, a visual inspection, if the conduit can be visited, or else a video inspection can be requested to back up the diagnosis. Once the source of the problem has been localised, the necessary corrective maintenance can be undertaken.
Edacére is a French firm specialising in network diagnostics (drinking water and sewer systems). “Our studies are conducted in several stages and may be preceded by the development of a sewerage master plan,” explains A. Ruche.
Initially, the firm carries out a reconnaissance and investigation phase. This consists of collecting and interpreting the documents needed for the preliminary analysis: water utilisation, treatment plant assessment and cost-effectiveness, industrial pollution tax, layout, etc. Visiting the system and its equipment provides further input on the state of the installations. Enquiries made among local industry and the general population help to establish the use made of the system. The second stage consists of taking measurements:
* measurement of flow and pollution, detection and preliminary localisation of extraneous waters through a night-time inspection,
* use of special methods to try to locate the source of inputs: fluorescein tests, smoke trials, video inspection,
* study of the receiving water and the impact.
All these results are then brought together in a comprehensive report and the results of experiments are analysed. This allows modelling of the flow within the system and provides information on the management and treatment of stormwater, leading to an improved definition of the treatment plant, and clarifies the situation regarding the impact of discharges on the receiving water. As they point out at Edacére, “these studies can be used to develop sewerage master plans presenting costed and prioritised pluriannual work programmes.”
Water boards, wishing sewer system operators to pay greater attention to upkeep, encourage such hydraulic diagnostic studies. Furthermore, all the necessary tools are now available to allow preventive actions to be taken and potential problems to be detected even earlier. The physical deterioration of the network can be monitored step by step. The regular check on the various installations allows a precise diagnosis to be made. This approach means that action can be taken before any major problems occur. There is thus more time for careful decision making on the most appropriate repair technique, which may save the community money since the situation no longer needs to be treated under emergency conditions.
Monitoring physical deterioration
At the Environmental Services and Sanitation Directorate for Val de Marne (DSEA 94), near Paris: “Regular surveillance of the physical structure of the network and its immediate surroundings highlights weaknesses and allows problem sections to be more closely controlled.” Increased surveillance implies monitoring effluent collection installations. It also increases the effectiveness of the diagnosis. This is the method chosen by most managers of large systems, who prefer to carry out preventive maintenance so as to ensure the health of the system.
As they state at the Val de Marne General Council, “networks should ideally be inspected once a year, but this is an unrealistic target because there are far too many structures to control.” Sewermen make a visual inspection during their rounds, but do not always have the necessary training to detect physical deterioration of the conduits. Furthermore, there is still the problem of controlling all the parts of the network that cannot be visited.
In Val de Marne, the sections of the departmental network to be inspected are therefore selected on the basis of different criteria relating to:
- • previous diagnostic results,
- • the location of the structure: beneath a roadway or close to trees (problem of roots), deep or close to the surface, etc.,
- • problems reported by local councils and field operators.
This approach requires an excellent knowledge of the network layout (including regular surveying and updating of plans), the geology of the area, and its history. A study of the urban and industrial environment in which the network is located is particularly useful in helping to determine the likely weak points. These can then be the first to be controlled.
For system assessment there is no one universal tool. All the existing tools can be used in conjunction. This was the conclusion reached in Rerau Report No. 1, a study on examination and diagnosis of accessible networks, conducted by the Ministry of Equipment and various local authorities and completed on 8 January 1998.
There is no universal inspection tool
“The first inspection tool”, explains Jean-Michel Bergue, “is a visual inspection. This highly efficient technique allows a systematic visit to be made. It identifies the zones that are in the poorest condition and which will have to be reinspected with another, more precise tool.” In conduits that are accessible, the operation is carried out by maintenance personnel. It is accompanied by surveying and mapping of the network. For the remaining, non-accessible conduits, it is done using a video camera mounted on a mobile chassis.
Once this systematic inspection has been completed, an analysis of the first results helps to pinpoint those zones where a more detailed survey needs to be undertaken. This requires a more discerning method in order to identify the problem. In any operation of this kind, the cost/information quality ratio should always be borne in mind.
For accessible sectors, the inspector chooses from among the available methods according to the nature of the problem. The choice includes geophysical radar, ultrasound, automatic examination (MAC method), electrical methods, etc. He may also request an installation to be equipped with crack measurement apparatus to monitor the development of a crack. As they explain at Val de Marne General Council, “the aim of this more discerning exploration is to find the source of deterioration, whether it is the environment, effluent or the construction method used.”
For non-accessible sections, the choice is far more limited. The techniques available for large systems have to be miniaturised for use in small systems. Some methods, such as radar or the MAC method, are currently being adapted. However, as yet, very few devices are operational. Radar is on the way, but at the moment inclinometers and template style empirical tools are used. “The problem is one of small system maintenance”, points out Jean-Michel Bergue, “operators are far more sensitive to the problem of breakdowns in large systems, because they are more expensive to refurbish. They are consequently less concerned about developing instruments for controlling non-accessible systems.”
Going beyond visual inspection
The most widely used inspection method is geophysical radar and is commonly employed in public works. This highly practical tool explores beyond the structure to examine the environment in which the network is located. This is possible in all but a few exceptional cases, such as marl. “In such cases, radar is unable to see anything.” Geophysical radar works by emitting a very short beam of electromagnetic waves,
Video examination: improving image processing
Video examination consists of filming the interior of a non-accessible conduit with an automated video camera. Recordings can be made over several hundred meters and, as a result, provide a large number of images. A large amount of information is thus recovered, but is often poorly exploited. All too often only the information on problem areas is exploited and the remainder is ignored. “A quite remarkable wealth of information is obtained and yet is under-exploited,” notes Jean-Michel Bergue.
Computer capabilities and advances in storage systems (optical disks) should make it possible to develop a system of non-stop data processing from one inspection to the next. With this approach, different generations of images taken during successive inspections could be compared, making it a simple matter to pinpoint faults and observe their evolution over time.
which, after being reflected by the various irregularities underground (defects, pipe/pipe casing interface, gaps, pipe casing/ground, etc.), returns to the emitter. The time taken by the waves to bounce back can be used to calculate the distance from the target. While very precise, the method is difficult to apply. The qualifications and competence of the operator are of paramount importance. Regulating the device and using the appropriate frequencies for the antenna are the key parameters. The operator needs to know how to optimise them depending on the circumstances of the control being carried out.
To test the devices and their operators, Seine-Saint-Denis Département (near Paris) has created a reference wall, composed of four different materials and including test faults, such as gaps, decoys, etc.
The structure has been tested (on a voluntary basis) by a dozen devices and several different firms. The results are highly instructive.
The importance of having a skilled operator is clearly demonstrated: a tool incorrectly used gives unsatisfactory results. “And this is even more apparent when it comes to data processing,” notes Jean-Michel Bergue. “Radar shows only discontinuities. Interference and static have to be subtracted in order to reveal the defect.” As a general rule, radar images are interpreted only after a preliminary calibration procedure has been carried out. This is usually done by core drilling. The operator is thus informed on the environment and the geology.
In a similar manner, the Val de Marne General Council has installed an experimental section of piping to test devices intended for use in non-accessible networks. The installation was used to test the first radar devices for small-diameter networks.
Other investigation methods, such as MAC, the electrical method or mechanical impedance (stress analysis) can supplement and confirm the results obtained with the previous methods. They are less widely used, however.
Armed with all these experimental data, it is now possible to make a diagnosis.
Making a precise diagnosis
To be able to make the diagnosis, the engineering and design office has the inspection report on the sections that were visited (possibly with images showing deterioration) or video images for non-accessible sections.
A preliminary diagnosis, including a number of hypotheses, will already have been made. These can now be tested by analyzing the results of supplementary examinations of problem zones. The final diagnosis leads to a detailed examination of the network pathology. In its report, the engineering and design office lists the weak points that need to be monitored and the problems that need to be dealt with as a priority. However, the role of the engineering and design office does not end there. Based on the final diagnosis, it is up to the office to choose the right solution to refurbish the system. This approach implies a commitment to carry out the future work. Jean-Michel Bergue considers that: “to obtain an impartial diagnosis, it is essential to call in an independent body, capable of summarising the different examination data, so as to reach a reliable diagnosis. However, on the French market, there is a desperate shortage of independent engineering and design offices specializing in examination and diagnosis. And in the absence of a reliable diagnosis it is not possible to decide on the best method of refurbishment.”
