Economic efficiency

The large number of sewers that have been commissioned and constructed in Berlin permit definite statements to be made on the relative costs of the open trench and trenchless or microtunnelling modes of construction. The tender prices for the years 1997 and 1998 were taken as the basis for the preparation of the above example, which represents just one of a large number.

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With every sewer construction project, the relevant economic factors should be considered at the planning stage for both the open trench and trenchless modes of construction. Not least to be considered here are all the factors of influence likely to increase costs with in particular the open trench mode of construction. These include the possible requirement to divert existing lines in advance, the traffic light systems needed and the length of the construction period.

If the costs appear approximately the same, the market should be challenged to submit alternative bills of quantities so that the most economic method of construction can be seen and selected. Decisions to execute a project by combinations of open trench and trenchless modes of construction often result from such economic comparisons.

The three most important parameters determining the economic efficiency of micro-tunnelling are:

• Soil and ground water conditions - favourable conditions permit lower estimates
• Manhole to manhole lengths - long manhole to manhole lengths give savings in the cost intensive setup times
• Overall quantity of microtunnelling to be carried out - the costs of setting up and later dismantling the site are spread over the total project

The depth at which the sewer is to be laid is not of decisive importance with micro-tunnelling since it affects the price per metre merely through the costs for the start, target and inspection shafts.