Master plan for a sustainable supply and disposal infrastructure of mega cities - Part II

Nov 09, 2006

This is the second part of the technical report series “Master plan for a sustainable supply and disposal infrastructure of mega cities”. It is exclusively about utility tunnels with special emphasis on structural design and commercial aspects.

The Man-Accessible Utility Tunnel

In recognition of the linkage between line networks and changing user structures, there is a need for sustainable systems that are able to react smoothly or by small adaptations to changing and new requirements whose time, locality, scope and duration cannot be foreseen.
In order to meet this challenge in the future, also in the sense of sustainable development, e.g. to fulfil the needs of the present without limiting the possibilities of the future generations and to force them to build new networks, there is a need for modern, e.g. adaptable and easy-to-service line networks.
A system that meets these needs to a large extent and at the same time avoids the problems of the conventional single installation of supply and discharge lines is the man-accessible utility tunnel that will be the focus of the following discussion.
The man-accessible utility tunnel is an enclosed, longitudinal accessible structural installation for accessible installation of supply and discharge pipes consisting of pipe stretches as well as access, erection, ventilation, branch and connection structures. It represents the ideal variant for the multiple installation of lines which is understood as the joint installation of several lines (sequentially or at the same time) also of different types in the same line and gradient.
Structural Design Fundamentals

Characteristic features of utility tunnels, as already mentioned, are the installation of all possible supply and discharge lines into a structure situated underground whose traversability or accessibility is possible for personnel without elaborate equipment (Figure 1). These features make demands on the design of this type of structure that impinge on safety and operational matters of the structure as well as construction and installation points of view of the line networks. Further specific requirements emerge from the special conditions of operation. Thus, the following requirements must be taken into account in the structural design of a man-accessible utility tunnel [1, 2, 3]:
  • Sufficient size of the cross section for inclusion of the planned lines taking into account the number and sizes of the individual lines, reserve or surfaces for extensions.
  • Accessibility by adherence to minimum dimensions for the gangway and access for various purposes (including control, execution of maintenance works);
  • Consideration for the mutual compatibility conditions of the individual pipes and cables in the utility tunnel for prevention of pipe damage or reduction of quality of network parameters (e.g. increase in temperature for drinking water);
  • Ensuring installation-specific requirements of the individual pipes and cables (including gradient conditions especially for gravity piping in the sewer network, high and low points for ventilating and venting and drainage, support possibilities);
  • Ensuring the operation of the individual networks (including expansion requirements for heating pipes, heat insulation of district heating pipes, drinking water temperature in the water supply, branches and connections in the network);
  • Ensuring the operational capability of the man-accessible utility tunnel (including climatic conditions, protection against unauthorized entry, safety systems such as gas warning installations, drainage installations, lighting installations, fire warning and protection systems);
  • Construction and erection requirements (e.g. sealed and strong structure of the shell, installation possibilities for the different pipes and cables, support design, connection and transfer points from the gangway to the surface network, erection openings).
Cross sections

Many cross-sectional shapes are possible for man-accessible utility tunnels (Figure 2):
  • Circular cross sections (e.g. full circles, circular segments)
  • Limited curved cross sections (e.g. arched profile)
  • Quadratic and rectangular cross sections (the latter in lateral or vertical form)
  • Combined cross section (e.g. semicircular arch).
In the choice of the best cross section, the interactions between the design-structural aspects and the construction and operational as well as the commercial aspects must be considered. With a view to a good arrangement of the supply lines and their spatial requirements, it is better if the utility tunnels usually have a rectangular shape.
The upper shapes can be used for single as well as multiple-chamber utility tunnels (Figure 3). The multi-chamber utility tunnels are created by arranging them longitudinally, laterally or vertically or partially inside each other of differently sized or shaped cross sections.
Decisions for the choice of the multi-chamber cross sections are, among others:
  • Prevention of mutual influences of lines,
  • Damage limitation in the case of a catastrophe,
  • Insufficient space in the underground space,
  • Structural reasons.
Mutual influences of lines occur mostly due to temperature and electrical fields. Drinking water in particular can be affected in quality by heat-emitting lines (district heating, high voltage). These types of influences can be minimised when these media are installed in different chambers thus avoiding costly and voluminous insulation being applied to the lines. The influencing of telecommunication lines by high voltage cables can also be excluded by installing them in different chambers.
Damage limitation in case of catastrophes can be achieved by the multi-chamber arrangement. This is particularly important in the cases of damage by fire, explosion or flooding. On the one hand, the consequences of the damage are usually limited to the affected chamber and, on the other hand, the separation of the media reduces the probability of damage as, for instance, sources of sparks (high voltage cables) can be installed separately from gas pipes.
By grouping the cross section of the utility tunnel into separate chambers, the underground space available in limiting conditions such as already existing structures could be better utilized. If, for example the extension in width of the structure cannot be achieved in narrow streets, then a vertical arrangement of chambers can still permit the accommodation of all lines. The division into several chambers can also have advantages from a structural point of view. Thus, the centre support of the reinforced concrete roof reduces the required structural thickness and permits a more economical dimensioning of the shell. By means of a multiple indeterminate structural carrier construction, multi-chamber cross sections are in a better position to withstand unforeseen external loadings (e.g. earthquakes, settling). A counterargument to the design of a multi-chamber utility tunnel is the fact that the construction and operation of the utility tunnel is more expensive, and branches, crossings as well as laterals are difficult to install due to the separation of the chambers. The increase in the construction costs of the utility tunnel is due to the complicated geometry of the multi-chamber cross section when compared to a single-chamber cross section and because of the larger proportion of unused space (several gangways) as well as the increase in equipment for the chambers. Also the costs of operation of the utility tunnels are increased with multi-chamber cross sections as the effort for traversal, lighting and ventilation for each chamber must be considered.
In the region of branches and crossings, adherence to separate chambers is only possible at different levels and thus with increased financial cost and effort. The line path of laterals is more complicated as they must either divert around neighbouring chambers or be installed in casing pipes.
Dimension of Cross Sections and Arrangement of the Lines

The clear dimensions of a utility tunnel are determined by the following:
  • Accessibility,
  • Number and sizes of the lines to be accommodated and crossing the tunnel including the need for reserve space in accordance with the expected development,
  • Safety and operational conditioned spacing between the lines,
  • Construction and erection requirements for spacing of the lines,
  • Dimensions of valves and fittings.
In order to ensure accessibility, utility tunnels must have a clear walking height ³ 1800 mm. The width of the gangway should usually be 800 mm but at least 700 mm. If the utility tunnel contains pipes with a diameter > 500 mm, then the width (b) of the gangway must be calculated as follows:

b = max. installed DN/ID + 200 mm.
According to [6], the width of the gangway should not be less than a minimum of 1000 mm in order to ensure good pipe transport. A height of 2000 mm for the gangway is usually sufficient. If, however, laterals are installed, then it is possible that these will cross above the gangway in the case of which a height of ≥ 2.20 m is necessary. This is also required for the installation of lights. However, the clear height above the gangway should not be less than 1.90 m.
The most important factors for the dimensioning and designing of the cross section of the utility tunnel are the number and sizes of the lines to be installed.
In every case, good usage of the available room should be aimed at for reasons of cost effectiveness. Naturally, when determining the utility tunnel sizes, a future-oriented point of view as well as developments and needs in the supply and discharge should be forecast and taken into consideration.
Commercial Aspects

The literature already contains individual commercial comparisons between the single installation of lines in the form of directly buried installation and multi-installation in man-accessible utility tunnels which have led to the conclusion that man-accessible utility tunnels are the expensive installation alternative for pipe-referenced infrastructure systems. As these cost comparisons are usually limited to the time period of original installation, i.e. the initial investments, the alternative of the utility tunnel usually must be worse than the single installation of various line networks and lead to an incorrect assessment.
Such a commercial comparison does not mirror the real relationship as it does not view the overall life and utility duration of the investment and all the associated incomings and outgoings over this time period. Thus, for instance, with a utility tunnel all future road openings and earth works are prevented which are otherwise necessary for the rehabilitation, adaptation or new installation of underground lines. For this work, double the costs per meter of line must be calculated. In a real cost comparison between the single installation underground and multi-installation in a man-accessible utility tunnel, these costs must be considered by a multiple of the costs of direct installation. However, it is very difficult to make general statements on the use or residual utility duration of installations buried in the ground for an urban infrastructure. There are many individual factors that play a role, such as age, definition of purpose and material of the pipeline, quality of installation, type of soil, method of bedding and local conditions as well as operating and servicing management.
The same applies to all measures of inspection and maintenance, which, because of the direct accessibility to the line in the utility tunnel and the possibility to react quickly to determined damage can be carried out with little expense.
In order to realistically present the economic advantages of the utility tunnel over the whole lifetime of the investment of approximately 150 years, the commercial comparison should be carried out according to the attributable cost-value point of view. This method takes into account the above-mentioned dynamic effects and thus permits a realistic comparison of the alternatives. The attributable cost-value point of view, however, makes it necessary to estimate all future payments and finance-mathematic parameters (average actual interest rate and average price increase rate) in connection with every alternative. These vary naturally with the aims that are being followed and the applicable external limiting conditions (e.g. number of lines networks that are to be integrated into a utility tunnel and which will then lead to correspondingly fewer earthworks).
Figure 4, for example, shows the development of such defined project costs for the renewal of the line infrastructure in a road section in a city in North Rhine Westphalia (NRW). The procedure used for this commercial analysis was worked out especially for this purpose within the framework of the already-mentioned research project. A detailed description can be found in [7].
Figure 4 reveals that the project sequence between the direct installation of the lines and the construction of a utility tunnel with installation of the lines changes with reference to time. The basis for the calculation was detailed site-specific calculations of the investment and running costs. Taking into account and considering all cost-influencing factors as well as financial mathematical weighting, the example shows cost – utility – time periods that are possible in the region of the rehabilitation cycles of the individual lines.
As a conclusion it can be shown that, depending on the local conditions of the above-mentioned renewal proposal in NRW, the installation in a utility tunnel is far above all the expected parameters as the clear dominating installation alternative of the commercial comparison. The research result is even more in favour of the installation in the utility tunnel when the assumption is made in the commercial comparison that
  • operated and abandoned directly installed line networks compete for space in installation zones in the pavement area and increasingly have to be removed from the road area for future new installations thus usually causing high direct costs;
  • besides the replacement of old line-referenced infrastructure, often also later additions, the replacement of existing or the installation of completely new lines systems such as transport piping for goods or waste are necessary and these are possible in the utility tunnel with significantly lower investment;
  • in the course of more stringent environmental responsibility laws, the inspection and servicing needs such as the shortening of inspection cycles for sewers, rise and these incur less costs in a utility tunnel or fall away entirely because they take place within the scope of an optical check;
  • especially in the field of telecommunication, additional network operators can enter the supply market and contribute, because of the planned reserve space in man-accessible utility tunnels to a reduction of the empty costs, thus the unused capacity of the utility tunnel.
Also in the example given in Figure 4, as in all the commercial comparisons carried out so far, the external effects that permit the results for the utility tunnel to be reflected even more positively were not taken into account. The external effects of a project are the advantages and disadvantages that are not subject to the market mechanisms and are thus under-compensated, i.e. remain without repayment.
In the case under consideration, these external or indirect costs arise as follows:
  • Influence on retail trade
  • Traffic deviation and hindrance
  • Noise and pollutant emissions
  • Reduction of the remaining life of road surfaces and the loss of value associated with it
  • Damage to growth
  • Further negative interruptions with reference to the specific limiting conditions of the construction site.
There are at present no concrete data determinations for quantifying and monetarizing these indirect costs and statistic processing for subjective malfunction findings of the effects of construction site measures. However, efforts are being made all over the world to take into account in the future the indirect costs in the comparison and the assessment of pipe installation measures. The first comprehensive attempts to quantify and monetarize the indirect costs involved in the new installation of lines in development areas were developed in Germany and described in detail in [9, 10].
An investigation ordered by the government of Great Britain in 1985 [11] for approximately 3 million construction sites in roads mentions a figure of 35 million pounds Sterling as the indirect costs to the motorists. Not last because of this investigation, there resulted in 1991 the promulgation of the “New Roads and Street Work Act” that regulates the coordination of construction sites in traffic spaces and opened up a discussion on fees for the use of road space [12].
The results obtained from the research proposal showed that utility tunnels can be usefully used where their many advantages have their maximum effect. From a current point of view, such cases are [13]:
a) Re-designing the city centre and renewal of individual or all directly buried line systems in the inner-urban area (city centre and region of compact structures)

In order to maintain functional safety while adhering to or increasing the capacity. Already the partial renewal of deeply buried lines (especially sewers) can lead to grave interventions into the road cross section and can justify a premature renewal of all lines with re-routing into a utility tunnel. The same applies to the case of increases, new installations or the design of networks.

When installing the utility tunnel in the trenchless method below the existing lines, they can remain in operation up to the completion of the new system.
b) New developments and re-designing industrial and business areas

With specific applications such as airports, exhibition centres, goods traffic centres, harbours, universities, medical centres, etc. which demand a high degree of safety of supply and/or a high degree of supply comfort or an adaptation to utility-referenced and changing line demands with little effort.
c) Re-designing the traffic installations

New planning of roads, changing the route and re-designing metropolitan rail systems present measures that can trigger the renewal of line networks. Utility tunnels should be provided in this connection at dense node points for a circular interception of crossing main routes and the provision for reserve space for later traffic structures. Utility tunnels can serve as crossing structures, for underpasses of bundled routes under heavily used main roads. Mutual constraint points of traffic and line roads, such as bridges, underpasses or rails or railway lines can be designed with the use of utility tunnels without interruptions. For the widening of streets, re-designing of rail installations and executing of main network improvements, they can be usefully employed for keeping the traffic lanes free from lines and for limited paths and open spaces or with tight time limitations of traffic construction measures.

The new construction of underground traffic installations can force the building of over- or under-crossings or deviating line paths at points of conflict if line rerouting is generally necessary. The integration in existing or planned building structures (e.g. underground garages, underground railroads, street tunnels) is also feasible.
d) New construction or re-designing dwelling areas

The utility tunnel can be used for main development sections for compact dwellings with corresponding requirements for supply and discharge safety.


[1] Drewniok, P.: Begehbare Sammelkanäle als effektives Erschließungsprinzip Entwicklung und Anwendung in der ehemaligen DDR. Dokumentation "Der begehbare Leitungsgang" Vol. 1: Beiträge zur Kanalisationstechnik (ed. D. Stein), Berlin: Analytica, 1991, 37-55.

[2] Komplexrichtlinie Sammelkanäle, Schriftenreihen der Bauforschung, Reihe Ingenieur- und Tiefbau (special edition 1), Bauakademie der DDR, Institut für Ingenieur- und Tiefbau, Berlin, 1976.

[3] SIA 205: Verlegung von unterirdischen Leitungen. Schweizerischer Ingenieur- und Architektenverein, issue 1984.

[4] visaplan GmbH, Bochum, Germany:

[5] Company information SAKA Sammelkanal- und Service GmbH, Berlin-Marzahn, Germany.

[6] Girnau, G.: Unterirdischer Städtebau. Düsseldorf: Ernst & Sohn, 1970.

[7] Köhler, T. Erneuerung urbaner Ver- und Entsorgungsinfrastruktur mit Hilfe begehbarer Leitungsgänge – eine ökonomische Bewertung. Dissertation Ruhr-Universität Bochum, Fakultät für Wirtschaftswissenschaften, Bochum, 1998.

[8] Stein, D.; Klemmer, P.; Tettinger, P. J.: Studie zur ökologischen Erneuerung innerstädtischer Ver- und Entsorgungsleitungen sowie zur Erschließung kontaminierter Industriebrachen mit Hilfe von begehbaren Leitungsgängen unter besonderer Berücksichtigung des bergmännischen Stollenvortriebs. Unpublished research report Ruhr-Universität Bochum (1997).

[9] Stein, D.: Instandhaltung von Kanalisationen. 3rd edition. Berlin: Ernst & Sohn, 1998.

[10] Grunwald, G.: Wirtschaftlichkeitsuntersuchungen bei Kanalsanierungen: Dissertation Ruhr-Universität Bochum 1996. Published in the paper series of the Institut für Kanalisationstechnik Ruhr-Universität Bochum; Report 97/3 (1997).

[11] Horne, M.: Roads and the Utilities. Department of transport, London, 1985.

[12] Ling, D.; Read, G.; Vickridge, I.: Gebührenerhebung für die Benutzung von Straßenraum. Tiefbau Ingenieurbau Straßenbau (TIS) (1993), H. 12.

[13] Stein, D.: Erneuerung innerstädtischer Ver- und Entsorgungsleitungen durch Leitungsgänge. In: D. Stein (ed.), Der begehbare Leitungsgang, Beiträge zur Kanalisationstechnik, Vol. I, Berlin: Analytika, 1990, 9-24.

To read Part I of this article series, please click here.
To read Part III of this article series, please click here.

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