Survey on the Condition of Road Gully Systems in Germany (Part 2)

2 Cleaning of Road Gully Systems

The subject matters of the following considerations are both an analysis and an evaluation of the hitherto applied cleaning practice of road gullies. The key aspects of the survey referred to the responsibility for cleaning, cleaning methods, number of cleanings, and cleaning costs that accrued.

2.1 Solid Retention of SB and SS

Road gullies fulfil the function of taking up the road drain and discharging it. In the process, they retain gross solids that were introduced, as for example sand and gravel, grit applied in winter, leaves, cigarette ends, and other waste. The SB type, road gully with floor discharge (Straßenabläufe mit Bodenauslauf, auch Trockensystem oder Straßenablauf für Trockenschlamm), retains solids by using a bucket. In case of the SS type, road gully with sludge space (Straßenabläufe mit Schlammraum, auch Nasssystem oder Straßenablauf für Nassschlamm), the sludge space serves as a settling basin. In Figure 1, the road gully SB is contrasted with the SS to compare the average amount of retained solids. Here we can observe that the amount of retained solids regarding the SS is twice as big as with the SB.

Mineral entries of solids come from different sources. The surface water drainage through road gullies is considered as main entry from soil surfaces. Almost half of all solids drained through combined sewers and almost 100% of all solids drained through surface water sewers of the separate system come from this source. Depending on their physical properties and the hydraulic conditions, solids can result in deposits in the sewer network.

As analyses and practical experience have shown [2,3], the hitherto applied road gullies that are standardised in DIN 4052 [1] fulfil the function of retaining solids only very inadequately. Their efficiency in this regard is very limited and is furthermore significantly influenced by the number of cleaning intervals that are exercised. Cleanings of the road gullies that are not exercised early enough before approaching the performance limits repeal their already low function of preliminary purification, so that henceforth, the road drainage arrives at the sewer system in an untreated form.

This statement is also confirmed by the results of the survey regarding the accruing clearing material in the process of cleaning drain and sewer systems.

Depending on the local authority, the amount of clearing material accruing in the sewer networks varies between < 100 t/year and > 5000 t/year (see Figure 2). The average amount of clearing material in Germany adds up to about 900 t/year, relating to 1 km sewer about 0.77 t/year km. In the survey, 43% of all local authorities could not give full particulars regarding this point.

2.1.1 Process of Cleaning Road Gullies

Although road gully systems are part of drain and sewer systems, from an administrative and proprietary point of view, they do not belong to the actual sewer network operator, but they are assigned to the relevant road authority as a general rule.

In many towns, the relevant road authorities pass along the responsibility for operation and maintenance of road gullies to the sewer network operators. There, a sewer operating division that is responsible for the sewer system takes over these tasks. In some cases, these services are also passed over to private companies. In principle, financing is assured through general budget funds of the Road Construction Offices.

In 48% of all cases, cleaning of road gullies is carried out by the drainage department of the respective local authority, in 27% by the relevant road authority itself, and in 25% by private drainage companies (see Figure 3).

2.1.2 Number of Cleanings of SB and SS

The question concerning the number of cleanings of SB and SS was answered by 94% of the local authorities. Merely 6% of the participating local authorities could not produce any data in this respect.

In the participating local authorities, the cleaning of both SB and SS is carried out on average twice a year (see Figure 4 and Figure 5).

As far as the SB is concerned, cleaning of the bucket is carried out manually to 54%, and to 46% per suction vehicle (see Figure 6a). In case of the SS, the suction vehicle is used to 97%. A secondary cleaning that is carried out manually is infrequent. (see Figure 6b).

The distribution of cleaning costs of SB and SS differs considerably. The cleaning of a SB amounts to 5.50 €/piece on average. In comparison, the cleaning costs of a SS are about 130% higher and add up to an average of 12.60 €/piece.

Thus, the cleaning costs that accrue for different road gullies all over Germany amount to ca. 251 million €.

3 Inspection of Road Gullies

This battery of questions dealt with inspection procedures and inspection techniques as well as the existent damage potential of road gully systems.

3.1 Inspection Scheme

Addressing the issue, whether road gullies with their connection sewers are included in the sewer inspection scheme, only about 20% of the questioned local authorities answered "yes" (see Figure 7). As a consequence of this result, we can record that only 1/5 of the participating local authorities are aware of the significance of their road gully systems and keep informed about their current constructional state on a regular basis.

As far as the public sewer system, sewers and manholes are concerned, it is a different matter. According to the DWA [4], inspections that bring to light current states are carried out in about 80% of the local authorities. However, the actual state of laterals is recorded in 9.7% of the local authorities only. The latter is mainly to be ascribed to the fact that these drains are privately owned and are normally not operated by local authorities (see Table 1).

3.1.1 Inspection Procedure and Inspection Technique

Both the inspection procedures and the percentage of the inspection procedures employed to road gully systems in practice are shown in Figure 8Figure 8. Twelve local authorities did not supply information. Most of the local authorities (67%) check their road gully systems by visual inspections, as this is the simplest and most cost-effective method. However, using this method, only gross damages in the area of the road surface and in the upper part of the corpus can be detected, so that an explicit statement concerning the leak tightness of the road gully (SB) and the waterless corpus above the sludge space (SS) cannot be made.

About 50% of the local authorities could not give full particulars to the question concerning inspection procedures of connection sewers.

Connection sewers are inspected both via road gullies and public sewers (see Figure 9). 96% of the local authorities had positive experience in using TV-sewer cameras out of road gullies. The TV-satellite camera is mainly used out of sewers (86%). In some cases (14%), the inspection of connection sewers is carried out by reflection out of the sewer.

3.1.2 Leak Tightness Testing

Only 10% of the local authorities carry out leak tightness testing at road gullies for acceptance after road construction. Reasons for the extremely seldom appliance of leak tightness testing are a lack of practical experience, and special guidelines and regulations in the effective laws/ specifications, as well as rules and standards.

3.1.3 Kinds of Damages of Road Gullies

The survey showed that damages of road gullies that occur most often are both positional deviations of the top (30%) and congestions (26%) (see Figure 10). These two kinds of damages add up to 56% of the total damages to road gullies. Broken frames follow with a proportion of 15%. Leakages in the wall area (W) amount to 14%, and leakages in the bottom area (B) amount to 7%.

3.1.4 Documentation

A documentation of the damages to road gullies is not carried out by the bigger part of the questioned local authorities. Merely 35% document the actual state of road gullies (see Figure 11), mostly on the basis of handwritten protocols. A computer-assisted recording is only very rarely used.

This result means that the bigger part (65%) of the local authorities do not have documentations on the actual state of road gullies at hand. Consequently, an essential basis for optimal planning of remedial actions is missing.

3.1.5 Number of Defective Road Gullies

Figure 12 and Figure 13 show the distribution of defective road gullies SB and SS. Concerning this question, 32% of the local authorities could not give any particulars regarding the number of defective road gullies. The remaining local authorities make an average damage potential of 13% as far as SB is concerned, and 17% with road gullies of the type SS. The statistical spread of the number of damages is very wide-ranging there. It ranges from < 1% to a percentage of 55%. These data are not based on results of inspections that were carried out, but on a subjective assessment of network operators. Provided that only data of network operators, who inspected their road gullies at least at random, are taken into account, these show damage potential between 40% and 55%.

3.2 Rehabilitation of Road Gully Systems

In the fifth section, the questionnaire dealt with remedial actions of road gully systems. Questions regarding rehabilitation procedures, techniques, practised rehabilitation strategies, and accruing costs were included.

3.2.1 Reason for Launching Rehabilitation Action

The main reasons for launching rehabilitation action of road gullies were positional deviations of the top (43%) and broken frames (19%) (see Figure 14). This might relate to the fact that positional deviations on the road surface are recorded within the scope of sewer rehabilitation and maintenance and that they require urgent remedial action due to the risk of endangering traffic.

The main reasons for launching rehabilitation action of sewers are ingrown roots (20%) and congestions (17%), as well as pipe fractures and pipe collapses (19%) (see Figure 15). In these cases, there is immediate need for action (firefighting strategy) to recondition the functionality of the respective road gully system, and, where required, to remedy subsequent damages to the road surface.

3.2.2 Rehabilitation Procedures of Road Gullies and Connection Sewers

The rehabilitation of road gullies was carried out by repair (to 48%) and replacement (to 52%) (see Figure 16).

In the course of rehabilitation action of connection sewers, renovation (13%) took place in addition to repair (44%) and replacement (43%) (see Figure 17).

3.2.3 Repair of Road Gully Systems

The predominant reasons for repair measures of road gullies were positional deviations/settlement of the top (33%), followed by broken frames and leakages due to rust (24%), and bursting joints (14%). Ingrown roots, congestions, and other damages played a minor part with a percentage of less than 10% (see Figure 18).

The most frequent reasons for launching repair measures of connection sewers were pipe fractures (21%), ingrown roots (19%), and the formation of shatter cracks (16%). Altogether, these constitute a share of more than 50% (see Figure 19).

3.2.4 Replacement of Road Gully Systems

The most common reasons for launching replacement action of road gully systems are road renewals or road reconstructions (39%), sewer replacement (32%), and severe damages to road gullies (29%), as for example cracks or positional deviations (see Figure 20).

Concerning replacement measures of road gullies, there is a clear trend in favour of the SB type (66%). With 34%, the SS type is employed only half as often (see Figure 21). This is justified by saving of costs regarding material and installation in consequence of a low overall height.

Another reason for the SB type being employed twice as often compared to the SS type can be seen in the present frequency distribution of road gully types (see Part 1 of the article). In the course of replacing defective road gullies, the participating local authorities most times see no reason to change the existing road gully type. Regarding the question, whether a replacement of road gullies is also used for a general change of the road gully type, 48% of the respondents made no statement. Whereas a general change of the road gully type is out of the question for 84% of the responding local authorities, only 12% deem it wise to change an SB to a SS, and 4% consider a change of SS to SB suggestive (see Figure 22).Hence it can be derived that factors, as for example hydraulic capability or retention of solids of the different types of road gullies, play a part in planning measures of replacement only rarely. In fact, the principle of keeping the existent system applies here.

3.2.5 Number and Costs of Repair Measures/Replacement of Road Gullies

The number of repairs and replacements of road gullies launched strongly depends both on the size of the local authority and the number of the installed road gullies. Per local authority and year, an average of 182 road gullies is repaired, and an average of 105 is renewed (see Figure 23). The minimum number of repair measures carried out at road gullies adds up to 5, and the maximum number to 1,600 per year. Replacement sum up to a minimum of 2 and a maximum of 350 road gullies per year.

Regarding the costs of repair and replacement of road gullies, about 20% of the questioned local authorities could not give particulars. The replacement of a road gully represents the most cost-intensive recovery procedure with a maximum of 1,500 € and an average of 1,000 € (see Figure 24). In comparison, the repair of a road gully is cheaper and amounts to an average of about 450 €. Thus, compared to a repair, the costs of replacement are more than twice as high.

3.3 Summary

The article discussed the survey that was carried out nationwide with the objective target to obtain, for the first time, data and information on the actual state, construction, maintenance, operation, as well as rehabilitation of road gullies installed in Germany, to be able to make well-founded statements concerning the present rate of damage, as well as the resulting investment needs for maintaining road gullies and their connection sewers. Based on the results of the survey, where 50 local authorities took a share, 15.2 million road gully systems exist in Germany. The road gully type that is installed most in the participating local authorities is the road gully with floor drain (SB) with a percentage of 61%. More often than not it is made of concrete and has an average age of 34 years. The average length of an installed connection sewer amounts to 5.0 m.

Independent of the road gully type, the maintenance of road gully systems is mostly limited to cleaning, which is carried out on average twice a year in the participating local authorities. Road gully systems are included in sewer inspection schemes only rarely (ca. 20%). Leak tightness testing is carried out in 10% of the participating local authorities for final acceptance after road construction work.

According to specifications of network operators, an average of 15% of road gullies show defects. Translated to the whole of Germany, this means that about 2.2 million road gullies are defective. There, about 1.2 million of the SB type and 1.0 million of the SS type are in need of rehabilitation. About 95,000 defective road gullies are subject to rehabilitation per year. The relation between realised repair and replacement measures in practice is 48% (repair) vs. 52% (replacement). The average costs for repair sum up to approx. 450 €/road gully, and approx. 1,000 €/road gully for replacement. The overall outcome of this is that, related to the above estimated 2.2 million defective road gullies, a total need for rehabilitation of 1.63 billion €. Due to unavailable data, these costs do not include expenses for the rehabilitation of defective sewers.

The results of the survey confirm the necessity to include road gully systems in maintenance schemes of drain and sewer networks. If this does not happen, there is the risk that, after completion of rehabilitation measures in drain and sewer networks and drain systems, the already known technical, ecological, and economical damages to defective drains, sewers, and manholes shift to consequences of damage for defect road gully systems. For this reason, all current and future efforts that are made at high expense to improve the state of drain and sewer systems are often led ad absurdum.

4 Literature

[1] DIN 4052 (2006): Betonteile und Eimer für Straßenabläufe (Concrete elements and buckets for road gullies); Teil 1: Bauart und Einbau (Part 1: General requirements and installation); Teil 2: Zusammenstellungen und Bezeichnungen (Part 2: Summary and designations); Teil 3: Betonteile (Part 3: Concrete elements); Teil 4: Eimer (Part 4: Buckets), Publishing House: Beuth Verlag, Berlin.

[2] Stein, R., Cakmak, H., Dettmar, J. (2005): Untersuchungen von bestehenden Straßenabläufen bezüglich ihrer Leistungsfähigkeit und Realisierung von technischen Möglichkeiten zur Verbesserung des Feststoffrückhaltevermögens, Forschungsbericht im Auftrag des MUNLV NRW, Bochum/Aachen.

[3] Stein, R. (2008): Auswirkungen optimierter Straßenabläufe auf Feststoffeinträge in Kanalisationen, Dissertation, RWTH-Aachen.

[4] Berger, C.; Lohaus, J.: Zustand der Kanalisation in Deutschland – Ergebnisse der DWA-Umfrage 2004, Hennef (2004).