Besides the laying of sewers by the open cut method of construction, the trenchless or underground method of construction has been used since earlier times in special cases such as greater cover depths, in confined or heavily used streets and when crossing railway tracks or water courses.
Today the trenchless method of construction is becoming more important due to increasing political-ecological pressures and because the open cut method is often associated with
- Creating noise, vibration and emission nuisances from construction operations and traffic diversions;
- Influences on neighbouring structures and plants, e.g. through water retention measures;
- Increase in energy usage as well as business turnover and working time losses due to traffic diversions;
- Safety risks for occupants;
- Increase in the uses of resources; as well as
- Increase in the utilisation of dumping places.
In the past years attempts have been made to minimise the above-mentioned disadvantages, for instance by means of mechanised trench construction methods or so-called point construction sites and to take the increasing environmental awareness of the population into account. Despite the advances achieved, however, none of the solutions that have been found meet all the requirements.
One way out is the trenchless or underground method of construction. In the building of accessible piping, it has proved itself for many years with its high level of technology.
The oldest trenchless method of construction is the gallery technique (heading). The dispersal of the external loads is achieved either by means of a wooden support (Image 1.7.3-1) or by means of steel support arches (Image 1.7.3-2) with a lining of wooden boards or steel sheets. Depending on the subsoil, the lining is hammered in in advance or pressed in or, depending on the tunnelling advance, installed at intervals afterwards.
For safety and protection, the sewer was built of masonry or built by the in-situ concrete method (Image 1.7.3-3) (Video 1.7.3-1) whereby the safety risk was confined to the construction site. The smallest cross sections built were those of 60 cm width and 90 cm height [Büsin12]. This method is still used today by individual municipalities for laying new, or replacing sewers (Abschnitt 5.4).
With construction elements left in the ground, especially wooden ones, there remains the danger for the pipe of the formation of cavities due to rotting, together with changes in the bedding conditions and crack formation, etc.
Since approximately the turn of the Twentieth Century, there have arisen new alternatives to gallery techniques (heading) for the accessible nominal sizes range:
- Shield tunnelling and
- Pipe jacking
Brunel's patent application 180 years ago provides the definition of a shield as "a steel coating is pushed into the yielding ground with presses or spindles. In order to protect this coat, the ground is removed in front of it. The rear extension of the coat, also called the shield tail, overlaps the already erected tunnel cladding, so that under its covering, the tunnel casing can be installed in short sections in accordance with the advance (Animation 1.7.3-1)."
As regards the lining type, one differentiates between segments with elements of cast iron, cast steel, steel, concrete, reinforced concrete (Image 1.7.3-4) and reinforced steel fibre concrete in combination with annulus grouting of cement mortar.
In the case under consideration, the casing only provides temporary security for the space excavated. In order to take up the forces resulting from the internal pressure in sewer operation, a final casing of prefabricated parts, in-situ concrete or masonry is applied.
Pipe jacking of accessible cross sections has developed since the start of the 20th century in Germany into the most important trenchless method of construction in the communal region. Starting from a construction shaft, product pipes are installed with the aid of hydraulic jacks up to a target shaft (Image 1.7.3-5). At the same time, earth is removed from the face through the advancing pipes.
In contrast to the gallery technique (heading) and shield tunnelling with lining, in pipe jacking, the pipe takes on the double role of support against the surrounding earth on the one hand and the finished construction on the other.
In this method of construction, the ground on the outside of the pipe cross section is hardly influenced so that very good bedding conditions are obtained for the product pipes.
In the beginning, the following two variant application methods were used for laying non-man-accessible sewers:
- Construction of accessible lines in accordance with the process mentioned above; installation of the smaller product pipeline in the main line and subsequent annulus grouting (Abschnitt 5.4).
- Creation of a non-man-accessible cavity by means of steel pipes with the aid of a non-steerable operating method; installation of the product pipe and filling the annular space [Stein82c].
Since about 1982, the microtunelling for the building of sewers with non-man-accessible cross-sections (Image 1.7.3-6) using the jacking pipes described in (Abschnitt 1.7.1) has been available in the Federal Republic of Germany.
The processes mentioned above have been dealt with in detail in the literature so that a further discussion can be dispensed with here [Maidl84b] [ATV95a] [Stein89g] [Stein84e] [Stein83a] [Stein84c] [Stein86b] [GSTT4] [Stein96a] [Stein84d] [Stein84g] [Stein85e].