Are we approaching the universal shield?

1. Classification of the trenchless construction method
In the following application case, two main process groups are differentiated in connection with the trenchless construction method [1]:

• Trenchless installation of cables and pipes by jacking
• Tunneling and gallery heading method

If the tunneling and gallery heading method is applied, the installed cavity is saved and/or excavated depending, amongst others, on the method technology, the rock mass stability and the intended use of in-cast concrete, sprayed concrete or prefabricated segments (tubbing segments). In the following, this process group, which is mainly applied in the construction of traffic tunnels, is not described in detail.

The term "trenchless installation of cables and pipes by jacking" - referred to as "trenchless installation" in the following - is understood to describe "the underground installation of lines by means of pulling in, pushing in, pressing in or ramming in into a cavity (borehole) made in the soil by means of boring or drilling" [1]. It comprises the underground installation of man-accessible or non-man-accessible supply and disposal lines ("utilities") in the form of single installation of cables for electricity, communication etc. and pipelines for natural gas, water, sewage, district heating, oil etc. as well as utility ducts and utility tunnels [2] with arbitrary diameters or dimensions for multiple installation.

In the accessible nominal size range, pipe jacking is predominantly applied (Figure 1). For this process, jacking pipes are driven through the subsoil from a starting manhole to a target manhole by means of a jacking station, i.e. a main jacking station and intermediate jacking stations. The jacking in a straight or curved line path is facilitated by a steerable shield machine which is situated in front of the first pipe and which is subject of the following presentations [1].

At the moment, jacking pipes with a circular cross section are almost exclusively applied for pipe jacking. The upper limit of the pipe diameters to be applied is determined by the economic aspects of the pipe production and jacking technology, the statutory restrictions for the transportation of the pipes as well as the pipe weight. The transportation limits for pipes in general restrict the external diameter to about 5.4 m and the weight to 50 t [3] if §70 “Ausnahmen“ (Exceptions) of the StVZO Straßenverkehrs-Zulassungs-Ordnung (Road Traffic Licensing Regulations) [4] as well as §29 “Übermäßige Straßenbenutzung“ (Excessive Use of Roads) para. 3 of the STVO Straßenverkehrs-Ordnung (Road Traffic Regulations) [5] is taken into account.

2. Classification of shield machines
In pipe jacking processes, the shield machine has the following functions:

• to protect the personnel,
• to create the necessary cavity in order to press in the pipe string with a minimum of soil deformations and the least skin friction possible,
• to secure the cavity until the jacking pipes definitely bear all loads and forces,
• to protect the working face against collapsing soil or rock as well as groundwater, and
• to steer the jacking along the designed line and gradient in compliance with the permissible deviations [1].

There are numerous shield machines on the market that differ from each other especially with regard to the respective type of earth and water pressure support and to the type of rock excavation at the working face that they provide. In this connection, a superordinate division is made between two main shield types [1]:

• Open shield machines do not have a pressure-tight separation between the working face and the pipe string behind it. They are characterised by a relatively good accessibility to the working face and a machine technology that is easy to manage.
• Closed shield machines are equipped with a pressure bulkhead (partition bulkhead) between the working face and the shield tail in order to create artificial pressure. They are used as compressed air shields, fluid shields or earth pressure balance shields depending on the applied support medium.

Depending on the excavation method at the working face, the shield machines are divided as follows [6, 7]:

• Shield machines with partial excavation
• Shield machines with full-face excavation

Figure 2 shows the possible combinations with the different methods for supporting the working face and their corresponding names.

These systematics are based on a contribution published for the first time in [6] and [8] in 1997 as a common recommendation by the German Society for Trenchless Technology e.V. (DAUB), the Austrian Society for Geomechanics (ÖGG), Research Society for the Traffic and Road System and the Section for Mining of the Swiss Association of Engineers and Architects (FGU). This contribution was transferred to pipe jacking by Stein, D. [1] in 2003 and extended by detailed shield systematics in tabular form (see Table 9-2 on page 391 in [1]) [9] as well as by different special forms that are also in international use. The shield type SM-T5 “Shield machines with partial excavation and earth pressure supported working face” (Figure 2), which, under the name of hilco-Varioschild, has successfully been used in pipe jacking processes for several years now, was included as one of these special forms. It is the focus of the following observations.

3. Planned method change by conversion
Despite numerous efforts of the machine manufacturers, it has not been possible yet to develop a universal shield machine that is in the position to carry out a pipe jacking process taking into account all possible geological and hydro-geological conditions, all limiting jacking conditions (e.g. jacking length, nominal size of pipe, cover etc.) as well as all economic, ecological, logistic and safety-related aspects.

Every shield type presented in Figure 2 has technological application limits. However, these can be shifted or extended thanks to further developments of the excavation devices and mining techniques as well as the use of chemical additives in order to condition the soil or the supporting suspension and especially thanks to the possibility to make in-situ modifications of the machines. In this regard, the following variation possibilities are generally available [1]:

• Variation of excavation machine or excavation device and/or
• Variation of the principle for working face support (method change)

One differentiates between planned and unplanned method changes by rebuilding or conversion. In all cases, it is necessary to provide access for executing the required modifications of the machines [10]. The rebuilding is generally carried out in a manhole, the conversion is realised in the underground jacking line.

The so-called “Mixschild” [11] is an attempt at a universal solution of full-face excavation. This concept is meant to make it possible to realise all combinations of fluid, earth pressure, compressed air and open shields (see Figure 2) by a planned method change (conversion/rebuilding) [12]. However, Mixschild has up to now only permitted a combination of only one each of the variations presented in Figure 2 [13].

Shield machines with partial excavation
„hilco-Varioschild“ is an approach to realise the universal shield with partial excavation. (Figure 3) [14].

“hilco-Varioschild“ (see Figures 3 and 4) is a cowl shield with partial excavation which is generally operated in the mode SM-T5 (working face with earth pressure support).

In the present case, the partial excavation is characterised by a partial loosening of the soil/rock mass at the working face by a hydraulically moveable spiral conveyor/auger and/or a boom cutting flap situated at the crown of the front shield segment (Figure 3). The cutting head of the very solid conveyor is equipped with hard metal inserts and round shank cutter bits along the spirals. When the working face is excavated, the conveyor can be pulled back in compliance with the geological and hydro-geological limiting conditions until it is in front of or possibly in the pressure-keeping spiral conveyor guide pipe that has been lead through the dividing wall in order to regulate the supporting pressure. Its diameter is e.g. 2,000 mm with an external diameter of the shield of 3,120 mm [14]. It is able to break the cobbles and boulders that have entered the excavation chamber between the soil entry opening and the spiral to a conveyable size. Cobbles or boulders and fragments with an edge length of up to 600 mm can be discharged through the spiral conveyor guide pipe and the pressure-keeping transfer lock by means of the spiral conveyor/auger without any obstructions. The thrust unit that is integrated in the shield machine can produce a max. jacking force of 12,000 kN with a cylinder stroke of 1.50 m. By the use of this thrust unit, segmental lining can be carried out, if desired [14].

hilco-Varioschild is operated in the mode SM-T5 as a modified blind shield, which is the simplest type of the earth pressure shields [1] . It is pushed into the in-situ subsoil by means of the cutting edge of the shield. The subsoil is possibly displaced to a minor degree and/or is pushed into the excavation chamber that is open to the fore in front of the dividing wall and compressed. The pressure-keeping conveyor that is moveable in the direction of the jacking for the excavation of the working face conveys the soil out of the excavation chamber according to the jacking progress. At the end of the spiral conveyor guide pipe there is a piston-operated transfer lock and a discharge pipe through which the spoil of a self-propelling conveying truck ready to be landfilled is transmitted for transport to the starting manhole (Figure 4) by the jacked pipe string. The discharged amount of spoil for the mass balance is determined by means of crane scales when the truck is emptied [14].

The hydraulically moveable boom cutting flap (Figure 3) also serves as supporting flap for the mechanical regulation of the earth pressure in the excavation chamber. This is done by the impact of the soil discharge in the operating range of the spiral conveyor and by changing the adjusted angle of the flap. The earth pressure is measured in the excavation chamber by means of the restoring force of the hydraulic cylinder of the boom cutting flap. In order to improve the supporting effect of the excavated soil in the excavation chamber and its discharge by the spiral conveyor, conditioning suspensions (e.g. Bentonite suspension, foam) can be applied, if required. [14].

Based on the above-illustrated main type of hilco-Varioschild, the following method variations can be realised by conversion without the need of complex structural rebuildings of the machine within the shortest time possible (compare Figure 2) according to the manufacturer’s instructions [14]:

• SM-T1 (working face with natural support) ↔ SM-T2 (working face with partial support)
• SM-T1 (working face with natural support) ↔ SM-T3 (working face with compressed air support)
• SM-T1 (working face with natural support) ↔ SM-T5 (working face with earth pressure support)
• SM-T2 (working face with partial support) ↔ SM-T3 (working face with compressed air support)
• SM-T2 (working face with partial support) ↔ SM-T5 (working face with earth pressure support)
• SM-T3 (working face with compressed air support) ↔ SM-T5 (working face with earth pressure support)

For the operation by compressed air (SM-T3) in subsoil that leads into groundwater, the excavation chamber with the spiral conveyor guide pipe is solely pressurised by compressed air to the transfer lock so that the operation personnel can work in the pipe string under atmospheric conditions. The working face can be viewed either by the machine operator underground through a bulletproof glass window installed in the dividing wall or, during a remote-controlled operation of the steering container, above ground via a video camera.

The removal of obstacles that may be necessary at the working face can be carried out in groundwater under compressed air. For this purpose, a combined lock for personnel and material is installed between the pressure ring and the last jacking pipe. After the concerned area is pressurised by compressed air according to the compressed air ordinance [15], it is possible to open the access hatch in the dividing wall, to hydraulically run up the boom cutting flap and thus to remove the obstacle by hand.

hilco-Varioschild is applied for the underground installation of sewers in the nominal size range of pipes of 1600 £ DN/ID £ 3000. In consideration of the possibilities of the above-mentioned method changes, the manufacturer [14] indicates the following application fields for subsoil with and without groundwater:

• Loose soil: all classes L according to DIN 18319 [16] with the additional classes S 1 to S 4 (cobbles and boulders with a mass percentage of more than 30% and a max. height of up to 600 mm)
• Rock: rock with unaxial compressive strengths of max. 5 N/mm^² or 5 Mpa, respectively (class FZ 1/FD 1 according to DIN 18319 [16])

Jacking lengths of up to 1.000 m have been realised with nominal sizes of pipes of up to DN/ID 3000 [14].

4. Summary
For political and ecological reasons, the trenchless construction method has become an essential part of the installation of underground pipeline networks in our cities.

In this context, there are permanent efforts to develop universal shield machines for full or partial cutting excavation that can excavate the respective underground cavity independent of their geological and hydro-geological limiting conditions.

hilco-Varioschild represents one step towards a universal shield for partial cutting excavation. It is applied in pipe jacking processes and can be converted to almost all shield types known except for type SM-T4 within the shortest time and without complex structural rebuildings of the machine. Thus, the well-known shield systematics of DAUB, ÖGG and FGU can be extended by the type „shield machines with partial excavation and working face with earth pressure support (SM-T5)“ (Figure 2).

Literature
[1] Stein, D.: Trenchless Technology for Installation of Cables and Pipelines. Private publisher Prof. Dr.-Ing. Stein & Partner GmbH (www.stein.de), ISBN 3-00-014955-4. Bochum, 2005.
 
[2] Stein, D.: Der begehbare Leitungsgang. ISBN 3-433-01263-6, Berlin: Ernst & Sohn, 2002.

[3] Hähnlen, V.: Einsatz, Fertigung und Verlegung großformatiger Stahlbetonrohre. 3R international 31 (1992), issue 3, pp. 128-137.

[4] Straßenverkehrs-Zulassungs-Ordnung (StVZO) 28 September 1988 (BGBI. I p. 1793), last amended by the regulation of 03 August 2000 (BGBI. I p. 1273).

[5] Straßenverkehrs-Ordnung (StVO) in the version of the effective date of 16/05/2006, last amended by the 16th regulation to modify the Straßenverkehrs-Ordnung of 11 May 2006 (Bundesgesetzblatt volume 2006 Part I No. 23 p. 1160, issued in Bonn on 15 May 2006).

[6] Deutscher Ausschuss für unterirdisches Bauen e.V. (DAUB), Österreichische Gesellschaft für Geomechanik, Forschungsgesellschaft für das Verkehrs- und Straßenwesen, FGU Fachgruppe für Untertagbau Schweizerischer Ingenieur- und Architekten-Verein: Empfehlungen zur Auswahl und Bewertung von Tunnelvortriebsmaschinen. Taschenbuch für den Tunnelbau 1998 (22nd edition), Essen: Glückauf GmbH, 1997, pp. 257-321.

[7] Maidl, B., Herrenknecht, M., Anheuser, L.: Mechanised Shield Tunneling. Berlin: Ernst & Sohn, 1996.

[8] Deutscher Ausschuss für unterirdisches Bauen (DAUB), Österreichische Gesellschaft für Geomechanik (ÖGG) und Arbeitsgruppe Tunnelbau der Forschungsgesellschaft für das Verkehrs- und Straßenwesen, FGU Fachgruppe für Untertagbau Schweizerischer Ingenieur- und Architekten-Verein: Empfehlungen zur Auswahl und Bewertung von Tunnelvortriebsmaschinen. Tunnel (1997), issue 5, p. 20.

[9] Stein, D.: Hindernisortung und beseitigung beim hydraulischen Rohrvortrieb. Taschenbuch für den Tunnelbau 1985, Essen: Glückauf GmbH, 1984, pp. 330-356.

[10] Deutscher Ausschuss für unterirdisches Bauen (DAUB), Österreichische Gesellschaft für Geomechanik (ÖGG) und Arbeitsgruppe Tunnelbau der Österreichischen Forschungsgemeinschaft Straße und Verkehr, Fachgruppe für Untertagebau (FGU), Schweizerischer Ingenieur- und Architektenverein (SIA): Empfehlung für Konstruktion und Betrieb von Schildmaschinen. Taschenbuch für den Tunnelbau 2001, Essen: Glückauf GmbH, 2000, pp. 256-288.

[11] Company information Herrenknecht AG, Schwanau.

[12] Herrenknecht, M.: Die Entwicklung der Mixschilde. Tiefbau (1994), issue 11, pp. 674-685.

[13] Herrenknecht, M., Bäppler, K.: Einsatz von Mixschilden – Asien, Australien, Europa. Taschenbuch für den Tunnelbau 1999 (volume 23 ), Essen: Glückauf GmbH, 1998, pp. 307-336.

[14] Company information hilco Tunnelvortriebstechnik GmbH, Bitburg-Masholder.

[15] Verordnung über Arbeiten in Druckluft (Druckluftverordnung). 4 October 1972 BGBl I p.1909, amended on 19 June 1997 BGBl I p.1384, last amended on 21 June 2005, BGBl I p. 1666.

[16] DIN 18319: Contract procedures for building works (VOB) - Part C: General technical specifications for building works; Pipe drilling works (12.2000).

This article was first published in Trenchless Australasia.