The Open face Shield (OFS) tunnel-boring machine is used to provide initial ground support when tunnelling is in soft ground, typically clays & Silts. It is fitted with fore pole Blades that, under unstable ground conditions can be advanced into the uncut ground, in front of the OFS to support the arch and the face.
HERRENKEHT GmbH designed the Athens Metro OFS named “DAFNI”, for excavation of rock with maximum unconfined compressive strength (UCS) of 120 Mpa.
The OFS is operating under atmospheric pressure and does not require a closed system for pressure balance at the tunnel face. The arch of the ground is supported by the skin of the Shield, ensuring the excavation of soil, the segmental tunnel lining ring erection under safe conditions and the better control of ground settlement.
The OFS consisted of 2 main sections:
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Front Shield: including the Fore pole blades over 180° arc, in the top section, road header, two excavating shovels, two telescopic drilling machines, face rams, twin control cabins and a screw conveyor. |
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Rear Shield and tail skin: including segment erector, grout injection points, tail seal brushes and de - watering equipment. |
The OFS can be dismantled in place, and the machine parts can be removed in pieces through the constructed tunnel.
The drive of the OFS main equipment is electric hydraulic with the following Characteristics:
| Over all length of the OFS |
12.68 m |
| Total Installed Power |
4.000 kW |
| Main Power Supply |
20 kV |
The OFS front section is equipped with a telescopic Road header boom, with 83 picks (arranged in a spiral) and two telescopic loading shovels.
The Road header and the excavator shovels are controlled during the excavation, through two control identical cabins. The Road header operator can select individually the extension of the seven fore poling plates, starting from the tunnel crown for face protection.
The over cutting profile is computer controlled and the excavation cycle time for a rock ground condition (UCS 70 Mpa), is about two hours. Safety interlocks exist to prevent Road header operation from the drilling machines operation and the fore pole plates during OFS advance. The excavated over cutting profile (cutting edge of over cut up to 50 mm) by the road header boom is determined in relation to the present position of the Shield, the outer diameter of the segments and the requirements imposed by the alignment of the Design tunnel Axis (DTA).
Each tunnel shove excavation is 1.5m long, for placing a segmental tunnel ring and is performed by two half shoves 750mm long.
The Road header, telescopic boom has an axial movement of cutting profile 1100mm, in front of the hood. The road header can reach the lower part profile by retracting the screw conveyor and using the seven fore pole plate’s extension to support the face and the crown. The rotation of the Road header is in one direction (clock wise).
Water or foam spray and de-dusting system are used to reduce dust during the excavation mode.
The Fore pole blades consists of seven fore poling plates (stroke of 1100 mm up to 1700 mm) and the seven extending breasting plates, arranged in the upper part of the OFS front section cutting edge.
The fore poling plates No.3, 4 & 5 (Crown) can be extended up to 1.7m, the No. 2 & 6 up to 1.4m and the No. 1 & 7 up to 1.1m.
The maximum operating pressure for both extension and retraction movement is 250 bar. The fore pole advance force range from 5 up to 127 tonnes and retraction force from 7 up to 64 tonnes. During the OFS excavation time, the fore pole plates operation thrust force is 50 bars (25tones).
By extending forward the seven fore pole plates, after the excavation, the OFS provides a sufficient stability in the tunnel crown and support almost of the non-cohesive ground. In soft ground conditions the fore poles used to trim the profile of the ground.
Each fore pole plate carries a breasting plate (operating pressure of 250 bar or 100 tonnes) with four (4) holes 120mm diameter to allow ground anchoring when plates are fully open and to provide active mechanical support to the corresponding tunnel face.
Ripper teeth are placed on the lower part of the front shield skin.
To prevent the possibility of rolling phenomenon the front shield is adjusted by all thrust jacks cylinders, hydraulically create forces, which lead to correction of the shield, position during tunneling.
The lower half of the shield is fitted with two face rams (Elephant feet, 2300mm stroke, holding pressure 300bar).
The excavated material is guided to the screw conveyor hopper by the telescopic excavator shovels (extension up to 2000mm).
The excavator shoves are mounted on each side of the Road header.
The screw conveyor fixed to the front and rear shield, discharge the excavated material to the crusher through the primary conveyor belt. The screw conveyor can be retracted and extended and makes possible liberation of the screw if any excavated material squeezes it. The screw conveyor consists of conveyor helix-telescopic station and planetary gearing.
The crusher is installed between the primary and secondary belt conveyor.
The maximum product size after crushing with twin roll sizes is not exceeds the 200 x 200mm.
The amount of the excavated material for a shove of 1.5 meter is approx. 192m3 with a squeezing factor of 1,8.
The excavated material passes, through the screw conveyor to the primary conveyor belt (7m long, 1m wide), located at the level of the tunnel axis and then to the crusher machine.
The second belt is conveyor 29m long, 1.2m wide and the third conveyor belt is 38 m long, 1.2m wide and is located on the structures of the back-up gantries and continues the removal of the excavated materials.
The third conveyor dumps the material to the shuttle conveyor (30 m long, 1.2 m wide), which moves parallel to the tunnel axis and fills the empty main wagon skips without moving them.
The conveyor belts are designed for a capacity of 950 t/h (750 m3/h).
The train side tipping spoil cars (Munhlhauser , 6 cars , 30 m3 capacity each ) are parked between the OFS Back-up gantries , dispose the material through the dumping wall , installed at the site area.
Three gripper pads are fitted to the rear shield body for stabilising the OFS Shield, during segment erection in hard rock conditions.
The gripper pads can be extended up to 150mm with operating pressure 300 bars.
The 28 hydraulic thrust rams are positioned in a way (arranged in 14 x 2 pieces) that they can support the prefabricated concrete tunnel segments used during erection.
A total thrust of 5,600 tons (Power 2 x 160kw) is available from the thrust rams, to advance the Shield by pushing against the pre-fabricated segmental tunnel ring.
Two rows of wire brush tail seals are fitted to the rear shield supplied by grease, excluding the ground water and the primary grout from re-entering at the Rear Shield.
The Tail Shield skin thickness is 70 mm.
The greasing system is controlled automatically by means of pressure sensors.
Two emergency pumps 35KW (400volt) are installed at the segment erection area to evacuate the water.
The TBM control room placed at back up 1st gantry is about 25m backwards from the excavated face.
The operator controls the front and rear shield to keep them on line and level.
The acceptable tolerance of the segmental tunnel lining from the Design Tunnel Axis (DTA) is 80mm.
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