The replacement of sewers, especially with the open cut method, is made much more difficult in inner urban areas by the existence of other supply pipes (Abschnitt 1.11) (Image 5.4.5-1). They often present a substantial hurdle in carrying out construction work, which is reflected in the costs. In narrow streets the obstructions can go so far that, in the course of sewer replacement, all the other lines in the cross section of the street also have to be replaced (Image 5.4.5-1) (Image 5.4.5-2).
The results of this are once again only conventional supply and drain lines that are laid separately under the ground and are thus inaccessible from outside in the street cross section and this can lead to numerous problems such as [Girna70] [Stein97d] :
- Most of the underground conduits form networks, i.e. they are only fully functional as units. Malfunctions at one point can have effects on whole regions.
- The individual installations are subject to their own particular planning and routing rules. As they are laid in parallel as well as laterally, points of contact inevitably occur. Furthermore individual installations have mutual disadvantageous influences on others (e.g. electricity cables - telecommunicationcables - gas piping - district heating) so that special attention must be paid to this in the planning. The space requirements of the individual underground installations vary greatly. In places where large structures must be accommodated, the place for the remaining underground installations is greatly reduced. Thus careful arrangement is necessary.
- All underground installations are complicated engineering structures. Later alterations such as repairs, replacement or additions as well as the measures taken for maintenance and inspection are mostly possible only with a great deal of effort. Every change requires impairment of the traffic space.
In recognition of these problems and the great advantages of accessible utility tunnels, which will be dealt with further below, it is recommended to include this latter in the investigations as a serious variant in the case of sewer replacement measures.
This is an long enclosed non-accessible structure for accessibly laying supply and disposal lines consisting of utility tunnel line as well as access, erection, ventilating, branch and jointing installations (Image 5.4.5-3) (Image 5.4.5-4).
The idea of the accessible and collective laying of pipes in tunnels or a structure is not new. In Paris the sewers have served since the 19th Century at the same time as supports for the water, compressed air and cable lines (Image 5.4.5-5) [Frühl10] [Hobre90]. These are accommodated in the upper "also the free portion above rain flooding part level" [Hobre90] of the basically man-accessible cross sections. The building connections were placed in a so-called access tunnel so that excavations in laying or rehabilitation were not necessary (Image 5.4.5-6).
The first accessible pipe utility tunnels were installed in the 19th Century in London (1869) and Hamburg [Hobre90] [Roepe93] (Image 5.4.5-7) (Image 5.4.5-8). The reason was given by Frühling in the year 1910 [Frühl10] "There are special conditions in the English capital because not only is traffic increasing constantly but also the supply networks are in the hands of different bodies who have received - each for themselves alone - the rights to them from Parliament also for tearing up the road for their purposes without official permission. Installations that remove the traffic disturbance caused thereby are therefore in evidence in London more than elsewhere."
Much use was made of utility tunnels in the former GDR and the Soviet Union where several hundred kilometres of standardized structures were installed. More recently, mention should be made of large projects in Taipei and Prague where a start has been made to replace the piping infrastructure of whole parts of the towns with a utility tunnel system. In Zurich one of the most modern installations of this type was taken into operation in 1991 (Image 5.4.5-9) (Image 5.4.5-10) (Image 5.4.5-11). In West Germany, utility tunnels have up to now been used primarily in the private sector. As examples one should note, among others, the universities of Bochum and Cologne and the university clinic in Heidelberg but also in sewage treatment plants, fair grounds etc.
- In the course of the construction, all the lines, sewers and resources in the road cross section are replaced in an environmentally friendly manner.
- The requirement of easy maintenance is realised for all lines so that damage or malfunctions of individual systems also from outside can be recognised in good time and future high resulting costs can be saved. These costs concern not only those for repair of the damage but also especially for the inspection that in the past has often been neglected and now will have a much higher priority (Abschnitt 4.2).
- The utility tunnel offers a secure protection against static loading. Danger to the environment, e.g. due to damaged sewers in the form of exfiltrating sewage is excluded as is the stability of the neighbouring structures and especially the sewers themselves due to infiltrating groundwater.
- Damage to pipes caused by frost, settling, point and line loads, flushing, external corrosion, construction at neighbouring or crossing lines, etc. is prevented.
- Trees and plantings in inner urban streets have enough space again without damaging water and drain pipes or being themselves damaged by their laying, operation and maintenance (Abschnitt 1.12). Living comfort and city ambience is positively influenced in this way.
- The utility tunnel is a resource-saving enclosure solution, preventing media losses and reducing substance and energy use for all the construction and maintenance measures during its useful lifetime.
- With the creation of the utility tunnel by the trenchless method in a second horizon below the existing piping, they can stay in use until the completion of the new system. In this way the supply and drainage bottlenecks can be minimised.
- Humanising the workplace (better access) as well as the independence of the maintenance works on the system from weather conditions and/or vegetation periods.
- With future-oriented it is possible to design for problem-free installation of further, today as yet unknown, supply and disposal lines or the adaptation of existing networks to increasing or changing requirements within the scope of the user development and the ecological restructuring of a city without influence on groundwater, vegetation and local climate.
This last-named advantage of the accessible utility tunnel will be very important in the future as, besides the repair of damage or rehabilitation of existing defective or over-aged pipes and piping systems, it is conceivable that a much more important problem region will open up for the inner-urban infrastructure. This is the result of the constant change and development of a society, whose requirements of the community supply and drainage lines are changing.
- The development of glass fibre technology with optical fibres that replace the electrical conductors for data transmission;
- The lifting of the monopolies of the TELECOMS, which will permit other companies to offer telecommunication services;
- Future developments of data transmission (Internet, digital television, etc.) whose effect on their cabling measures cannot yet be foreseen;
- The alteration of the materials for gas piping towards plastics with the necessity of removing old cast iron piping from the ground;
- The changeover from the strongly emissing oil and coal fired individual heating systems towards building heating relying on piped-in energy systems, e.g. gas or district heating;
- The changing over of large industrial areas to residential or commercial use, which will be combined with different capacity requirements on the piping-bound infrastructure;
- The draining-off or utilisation of rainwater whereby the discharge cross section of the sewers will be insufficiently hydraulically utilised;
- Creation of separated water circulating systems for drinking and utility water as only about 2 % of the water usage in the private sector is used for drinking and cooking and over 30 % is utilised for the flushing of WCs [WBGU97].
- The changed behaviour of the user, e.g. saving of electrical energy or drinking water;
- Transport of piece goods through tubes in order to relieve above-ground traffic ways; as well as
- Exhausting and removal of contaminated air from inner-urban pathways [Stein97j].
It is difficult or impossible to react to these and other developments that are not yet foreseen with the current practice of pipe laying as short-term access to the piping is not possible and cannot be financed.
In recognition of the interconnections between the piping networks and changing user structures, the requirement is for sustainable systems, which fulfil the demands of the present without limiting the possibilities of future generations and constantly forcing them to build new networks. They must make it possible for the supply and disposal companies to:
- adapt their networks quickly and cheaply to the requirements of their customers;
- react easily to new developments and
- integrate new pipelines into the total system.
The possibility of transferring the concept of the utility tunnel that fulfils all these requirements ideally has been negated for the replacement of piping in the inner urban regions especially because of feared substantial technical, administrative and legal difficulties. There is also the idea current that a utility tunnel will never bring an economic return on the high additional financial expenditures that would be incurred in the building of an additional structural envelope.
Technical problems in building and operation of accessible utility tunnels can be taken as solved according to [Stein97d] [Stein97c] [Klemm97] [Girna68b] [GSTT6]. Numerous installations have been privately economically operated for years without serious difficulties; important cases of damage are not known.
The real obstacle to the wider use of utility tunnels can be seen in the structure of the public supply and discharge systems. The disposal of sewage is the task of the respective communities or their sewage departments; the supply is generally in the hands of various supply enterprises and is regulated by concession contracts. The financial interest of the supply and disposal organizations is fixated on their respective media so that a system such as a utility tunnel, which requires the cooperation of the enterprises, is generally not considered. Furthermore, they are reluctant to make provision for the necessary pre-financing, although ecological viewpoints over a long-term observation period have shown that the building of utility tunnels can be economical. This is also confirmed by the investigated model project in [Stein97c] [Klemm97] in the town of Herne in North-Rhine Westphalia (Germany). Whilst in the "Hauptstrasse" (main street) model project, the existing piping is to be rehabilitated or replaced, in the "Gewerbepark Hibernia" (Hibernia industrial estate); the area is to be newly enclosed in the course of a revitalizing according to an ecological point of view.
The (Image 5.4.5-12) and (Image 5.4.5-13) show the development of the accumulated project cost values for selected finance-mathematical variants. From this it can be seen that the project sequence between laying in the ground and the building of a utility tunnel with installation of the pipes changes with respect to time. The basis for the calculations was the detailed locality-specific calculations of the investment and the running costs with reference to piping and utility tunnel. Taking the general maintenance cycles and weighting all the cost-influencing factors as well as finance-mathematical factors there are shown the examples of cost - utility - time periods that are possible in the region of the rehabilitation cycles of the individual pipes. Here, indirect costs or reducing costs (e.g. preventable pipe damage, traffic impairment and similar indirect costs (Abschnitt 5.6.1)) that would make the economical comparison for the utility tunnel come out more positively, were not taken into account. These results should be cause for the supply and discharge enterprises to move away from the exclusive viewpoint of their own costs and to view the tasks of supply and discharge to the population as a community task that requires a close cooperation of the enterprises.
The above-mentioned example investigations have shown that utility tunnels are useful where their advantages become most obvious. Such areas of application, from today's point of view, [Stein90c] [Stein94a] are:
- Restructuring of city centres and supply-technically required replacement of individual or all line systems in the ground in the inner-urban region (city centre and regions of compact building areas)
For retaining functional security whilst maintaining or increasing capacity. Even the partial replacement of deep piping (especially sewers) can lead to a serious invasion into the road cross section and can justify a premature replacement of all piping with relocation into a utility tunnel. The same is applicable in the case of extension, new pipe laying or by restructuring the network. With the creation of the utility tunnel in the trenchless method below the existing piping, these can remain in use until the new system is in operation.
- New enclosure and restructuring of industrial and commercial areas
With specific utility requirements, e.g. airports, fair areas, goods traffic centres, harbour areas, universities, medical centres, etc. for which an extremely high discharge capability must be ensured and/or for whom discharge comfort or an adaptation to utility conditioned changing pipe requirements with little effort is required.
- Restructuring the installations for traffic
New planning of roads, changes of the traffic flow and restructuring in the rail region of urban short distance traffic media, present measures that can lead to the replacement of piping networks. Utility tunnels must be provided at heavily used node points for ring-shaped positioning of crossing main headers and the provision of space for future traffic structures. The crossing structures can be utility tunnels for passing under bundled terraces or heavily used main roads. Common bottlenecks for traffic and pipe terraces such as bridges, railway subways or rail installations can be provided for without interruptions by the use of utility tunnels. For road widening, restructuring of rail installations and breakthroughs for improving header networks they can be used for keeping the traffic lanes free of piping, for limited pathways and free surface areas or with traffic works with limited space availability. The new construction of underground traffic measures can force the construction of deviating utility tunnel lines that cross over or under at positions of conflict when piping deviations are generally required. In the same way, the integration of the utility tunnel in available or planned underground structures (e.g. underground garages, subways or road tunnels) can also be considered.
- New buildings or the renovation of residential areas
In a compact residential area with corresponding requirements on ensuring the supply and discharge capability, the utility tunnel can be used for enclosing header sections.