Cleaning of road gully systems

This section of the survey included questions about the responsibility of cleaning, cleaning methods, the number of cleanings, and the accrued costs of cleaning.

Solid retention of SB and SS

The function of road gullies is to collect drainage from road run-off and discharge it. This process also involves collecting waste such as sand and gravel, grit, leaves and cigarette butts. Road gullies with floor discharge, known as the SB type, retain solids by using a bucket. Road gullies with sludge space, known as the SS type, have a sludge space that serves as a settling basin. Figure 1 shows that SS types retain twice as much solids as SB types.

The systems standardised in DIN 4,052 have a limited efficiency when it comes to retaining solids. Furthermore road gullies that are not cleaned before reaching their performance limits reduce their already low function of preliminary purification.

Depending on the local authority, the amount of clearing material accruing in the sewer networks varies between less than 100 tonnes per year and more than 5,000 tonnes per year (see Figure 2).

Process of cleaning road gullies

Although road gully systems are part of drain and sewer systems, they generally do not belong to the sewer network and are assigned to the relevant road authority.

In many towns, the relevant road authorities delegate the responsibility for operation and maintenance of road gullies to the sewer network operators or private companies, where a sewer operating division is responsible. Financing is assured through general budget funds of the road construction offices.

The cleaning of road gullies is carried out by the drainage department of the respective local authority in 48 per cent of cases, the relevant road authority in 27 per cent of cases or by private drainage companies in 25 per cent of cases (see Figure 3).

Number of cleanings of SB and SS

For the participating local authorities, the cleaning of both SB and SS occurs every two years on average (see Figures 4 and 5).

With SB types, cleaning of the bucket is carried out manually 54 per cent of the time and via suction vehicles in 46 per cent of cases (see Figure 6.a), this is compared with SS types where cleaning is undertaken via suction vehicle in 97 per cent of cases. It is uncommon that a manual cleaning is carried out on SS systems (see Figure 6b).

The distribution of cleaning costs of SB and SS differs considerably, with the cleaning of an SB averaging at €5.50 per piece and the cleaning costs of a SS averaging at €12.60 per piece. The cleaning costs accrued for different road gullies all over Germany amount to approximately €251 million.

Inspection of road gullies

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

Inspection scheme

Approximately 20 per cent of local authorities confirmed that road gullies and their connection sewers were included in the sewer inspection scheme (see Figure 7). Consequently, we can record that only one fifth of the participating local authorities are aware of the significance of their road gully systems and keep informed about their current state on a regular basis.

The same cannot be said of the public sewer system. According to the DWA [4], inspections of sewers and manholes are carried out in about 80 per cent of the local authorities. However, the actual state of laterals is only recorded in 9.7 per cent of the local authorities. This can be ascribed to the fact that laterals are privately owned and are not usually operated by local authorities.

Inspection procedure and technique

Figure 8 shows the inspection procedures used on road gully systems. The majority of the local authorities (67 per cent) check their road gully systems by visual inspections, however this method only allows the detection of gross damages in the area of the road surface and in the upper part of the corpus, and an explicit statement concerning the leak tightness of the road gully (SB) and the waterless corpus above the sludge space (SS) cannot be made.

Connection sewers are inspected both via road gullies and public sewers (see Figure 9). The TV-satellite camera is mainly used out of sewers (86 per cent). In 14 per cent of cases, the inspection of connection sewers is carried out by reflection out of the sewer.

Leak tightness testing

Only ten per cent of the local authorities carry out leak tightness testing at road gullies after road construction. The lack of leak tightness testing has been attributed to a lack of experience, and a lack of special guidelines, rules and standards.

Types of damages of road gullies

As shown in Figure 10, the most common form of road gully damage is positional deviations of the top (30 per cent) and congestions (26 per cent). Broken frames are the next most common type of damage (15 per cent), followed by leakages in the wall area (14 per cent), and leakages in the bottom area (7 per cent).

Documentation

Only 35 per cent of local authorities document the actual state of road gullies (see Figure 11).

This result means that the majority (65 per cent) of the local authorities do not have records of the state of road gullies, which are important for future planning.

Number of defective road gullies

Figures 12 and 13 show the distribution of defective road gullies SB and SS. Of the 68 per cent of the local authorities who responded in full to this question, 13 per cent make an average damage potential for SB and 17 per cent for SS. The statistical spread of the number of damages is very wide-ranging there, ranging from less than 1 per cent to 55 per cent. This data is not based on results of inspections that were carried out, but on a subjective assessment by network operators. Provided that only data of network operators is taken into account, it shows damage potential of between 40 per cent and 55 per cent.

Rehabilitation of road gully systems

Questions in the final section of the survey dealt with rehabilitation procedures, techniques, practised rehabilitation strategies and accruing costs.

Reason for launching rehabilitation action

The main reasons for launching rehabilitation action of road gullies were positional deviations of the top (43 per cent) and broken frames (19 per cent) (see Figure 14). This may 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 per cent) and congestions (17 per cent), as well as pipe fractures and collapses (19 per cent) (see Figure 15).

Rehabilitation procedures of road gullies and connection sewers

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

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

Repair of road gully systems

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

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

Replacement of road gully systems

The most common reasons for launching replacement action of road gully systems are road renewals or road reconstructions (39 per cent), sewer replacement (32 per cent), and severe damages to road gullies (29 per cent), 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 per cent). The SS type is employed only half as often.

Number and costs

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. On average, 182 road gullies are repaired and 105 road gullies are renewed annually per local authority.

The survey showed that the costs of replacement are more than twice as high for road gullies than for repair. 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. In comparison, the repair of a road gully is cheaper and amounts to an average of about €450.

Summary

Based on the results of the survey, 15.2 million road gully systems exist in Germany, with an average of 15 per cent of these showing defects. This translates to 2.2 million defective road gullies across Germany. Based on the rehabilitation costs for SB and SS road gullies, it would cost €1.63 billion to repair the 2.2 million defective road gullies.

The results of the survey confirm that the maintenance of road gully systems in drain and sewer networks is a necessity, otherwise the already known technical, ecological and economical defects of sewer systems will shift their damage consequences to gully systems.