The large scale projects shown in fig 1 are either being undertaken or are planned for the near future in the Netherlands.

The tunnel Heinenoord. The shield tunnelling for this pilot project has already been successfully finished and a lot of valuable experience was gained (1). Furthermore it was proved that the use of shield tunnelling technology is possible in the soft soils in the Netherlands.

The crossing of the Westerschelde as a road tunnel.

A high speed railway line for passengers, which connects Amsterdam and Paris, with the tunnel Groene Haard, a Metro-line in Amsterdam.

The Betuwe-line is a new double-track railway route for the transportation of goods between the Rotterdam harbour and the German / Dutch border. The planned route can be seen in.

Three tunnels with two tubes and with an excavation diameter of about 10m using shield tunnelling technology have been built. These are: the 1.8km long Botlektunnel which crosses the ‘Oude Maas’; the 4.2km long Sophiatunnel, which passes under a polder region, two rivers and a motorway; and a third 1.4km tunnel near Arnheim which crosses the river Pannerdensch.

The Botlektunnel (2,3) is a good example of the conditions the engineers had to face. Fig 3 shows a longitudinal section of the tunnel and fig 4 is a cross section of the tunnel showing the segmental lining. The subsoil conditions especially at the shafts, were charactised by very soft, very low-bearing but Holocene ground, typical in the Netherlands, consisting of clay and peat. The Pleistocene sand layers which dominate the middle zone, are more favourable for tunnelling. The Botlektunnel is surrounded by a complex infra structure and it was important that this was not interfered with in any way. This included the parallel running motorway tunnel of the A15, the crossing waterway ‘Oude Maas’ as well as the highly sensitive cables and pipelines of the related petrochemical plants.

The construction of the two tubes of the Botlektunnel faced very difficult challenges. The first was the sheer size of the excavation, 10m. Second was the presence of groundwater with a pressure head of up to about 3.0bar. Third, the tunnels went through soil with a low load bearing capacity. Fourth, the cables and lines the tunnel went under were very sensitive to any settlement. Fifth, the potential drift of tunnels into highly sensitive adjacent complexes such as refinery plants, freight railway lines and the Botlek bridge.

And finally, there was the possibility of running up against obstacles in the soil such as steel remains of mooring posts, cables for telecommunications or reinforced concrete foundation piles.

The following measures were undertaken. Soil was improved on a 125m length in the eastern section to guarantee the stability and density of the tunnel tubes. A holding device was constructed to limit the settlements of cables and lines.

The river bed was ballasted in the area of ‘Oude Maas’ in order to be able to prevent water ingress. To ensure that the shields could be installed in safety, injection facilities were carried out in the face area in order to guarantee blow-out safety as well as to avoid shields sinking in the soft ground. A preliminary exploration of obstacles in the soil was undertaken. The mass balance was supervised. The contract was awarded in October 1997, and shield tunnelling started in March1999.

Shield machine

After discussion, an EPB shield machine, delivered by Herrenknecht, was commissioned.

The Botlektunnel was a pilot project similar in construction to the second Heinenoordtunnel. The organisation of the project was co-ordinated by the Centrum Ondergronds Bouwen (COB). The possibility of redesigning the EPB into a hydro shield had to be included in the plan. Corresponding to the EPB-mode (7), the characteristic EPB features – excavation chamber, foam conditioning, earth pressure cells, screw conveyor and conveyor belt – were incorporated in the Botlek machine.

The redesign into a hydro shield was made possible by the following specifications which were integrated into the machine at construction stage: two-chamber-system, ie excavation chamber when using it as an EPB-shield or a chamber with an air pressure cushion divided by a submerged wall when using it as a hydro shield; eccentric positioning of the screw conveyor; axle displaced arrangement of the stone crusher in front of a screen and suction pipe; submerged wall gate for sealing the submerged wall then working in the slurry mode.

The necessary openings in the submerged wall and the pressure bulkhead were included in the construction. The openings for the EPB-mode were closed afterwards. The EPB method requires the cutting wheel to be formed as an almost closed cutterhead which provides a high degree of face stability. The central area is kept open to prevent of clogging and wearing and is equipped with a centre cutter that is designed for both directions. The muck buckets reach up to the centre. The tools, predominantly cutting knives, are installed on both sides of the inlet silt. A rim increases the stability of the cutterhead and is accordingly armoured against wear.

Two hydraulically extendable copycutters guarantee the required overcut. Foam injectors are integrated into the front part of the cutterhead in order to guarantee a constant supply with foam. For inspections, a bentonite filter cake is possible.

Drilling and injection installations

While the shield was being designed, it was realised that need for a grouted zone required a change to the design of the shield area right behind the cutter head. This was to make room for the necessary drilling and injection installations.

The peripheral driving concept allows to have a drilling rig in the central area to produce a grout zone in front of the cutterhead. By turning the drilling rig the entire radius of the cutterhead is covered and by a gradual turning the entire space in front of the shield can be treated. In this way, the safety of the staff when entering the excavation chamber, can be improved significantly and the machine is much more versatile.

Geotechnical investigation

Before tunnelling could begin, an acoustic reflection measurement system was used to pinpoint obstacles and discontinuities of the ground. The acoustic signals are reflected by transmitters, which are installed on the cutting wheel. Receivers, also located on the cutting wheel, receive the sent out signal by the sender as well as the echo signal.

The seismic variety of data is evaluated automatically and then presented on the shield driver’s monitor. The first experiences under practical conditions during shield tunnelling in Duisburg and Hamburg produced valuable information. Faults and geological changes of formation appear in a coloured format.

Mucking

A further special characteristic of this tunnelling machine is the mucking system.

Due to the earth pressure of 3.6bar, the high groundwater level and the sandy outcropping formations, transport by screw conveyor could not always be guaranteed. Therefore the contractors decided to install piston pumps as well. The ability to change to the operating mode with piston pumps guarantees a continuous transport.

This increases the operating reliability. The material, that is either conveyed by the conveyor belt or the piston pump, is transferred into the slurry box near the machine where it is diluted with water and then pumped to the surface. Although the hydraulic transport technique is unusual, if offers certain economic advantages. It should be pointed out that in recent years there has been an increase in the use of ‘integral’ machines which display classic methodical characteristics.

The flexibility and adaptability of today’s machines mean that they can have an expanded range of applications (5,6).

The experiences during construction will demonstrate in how far an orderly EPB-shield tunnelling with economically acceptable wearing can be guaranteed with these measures. The possibility to change over to an operation as hydro shield certainly builds a reassuring buffer.

Related Files
Cross section of the tunnel with segmental lining
Tunnelling projects in the Netherlands
Longitudinal section of the Botlektunnel
EPB -shield in Botlektunnel