The subject of face stabilisation is a bit of an anomaly when the very act of excavation destabilises the face. Perhaps it would be best to describe the subject as ground control. The objective is to provide permanent support to the planned sectional perimeter of the tunnel whilst advancing the face without uncontrolled ground movement. These actions are now widely recognised to include control of ground movement in advance of the face as well as of the face itself.
Naturally the degree of ground control required, and the methods selected to achieve the degree of ground control required will depend chiefly on the types of ground to be excavated and supported, including the pressure and volume of any groundwater. Other factors to be considered include whether the means of ground stabilisation constitute an obstruction to face advancement, whether the method can be included a part of the permanent tunnel support and the ever present matter of cost.
TBM assistance
If it is practical to use TBM with either slurry or earth pressure systems, these systems provide a means of comparatively rapid response to face pressures with the possibility of automatic control. This should counterbalance any otherwise uncontrolled ground and water pressures on the face, and probably in advance of the face. At the same time the shield of the TBM protects the excavation process and the support erection programming which, together with a likely grouting programme, provides both temporary and a large part of the permanent support.
However, TBM ground control tends to be not so successful in sections with widely varying ground types, or where extreme ground pressures can grip the machine’s shield. Sometimes methods more suited to open-face excavation, such as permeation groutng in front of the face, have to be employed from within the TBM. Many TBMs are now designed to leave enough free space within the shield machine allow grouting operations into the face or to form a ground improvement canopy around the passage of the shield. Some also have built in bore channels through the shield jacket. The Herrenknecht TBM used for the Botlek Tunnel in the Netherlands was modified in order to provide space or grouting equipment to form a soil improvement zone.
This short review concentrates on the methods now used to control ground at or near the tunnel face in open section. In poor ground these can allow the use of non-circular sections, the elimination of the need for staged partial excavation in favour of full section, and better progress on full sections. Some methods provide better ground control ahead of the face to ensure stability of the face as it reaches these controlled sections.
Most of the methods employed to improve face stability are not new in principle, but in many the sophistication of monitoring and control has increased rapidly in recent years together with the inclusion of the methods within a definite philosophy of tunnel support design. This includes the rapid installation of temporary support near the face, which may be later integrated into the permanent support. Whilst this forms part of the NATM system, the observational method of reacting to ground movement measurement usually means that full ground control is not achieved until some distance back from the face.
So, basic methods of stabilisation in open section include:
1) Rapid installation of tunnel perimeter support near the face including steel arches, rock-bolts, permanent lining segments, reinforce shotcrete among a wide range;
2) Permeation grouting in advance of and around the tunnel section;
3) ‘Canopy’ or ‘umbrella’ construction methods including filled precut methods, forepoling, pipe roofs, jetgrouted canopies;
4) Rockbolts or spiles inserted into the face, usually made of cuttable material;
5) Partial section excavation so that substantial tunnel perimeter support can be installed before excavating the full section;
6) Compressed air working to prevent groundwater from deteriorating the face structure;
7) Ground freezing with similar objectives to compressed air but also for temporary consolidation of weak, wet ground.
In recent years there has been increased recognition of the importance of controlling ground stress and consequent movements at the face and, in soft ground, in advance of the face. This function is not only important for safety and efficiency, but also for the security of the final structure and to minimise any danger to adjacent structures. Thus there has been a merging of the traditional concept for temporary support solely for enablement and safety, since good temporary or primary support will promote good permanent support, whether or not it forms part of it. The additional support materials, and improvements in materials technology, have been key factors in the achievement of more stable face conditions. In large tunnel sections this may be aided by various methods of staged excavation to reduce the amount of uncovered face under stress.
Staged excavation
Common practice today makes staged extraction synonymous with shotcreting of the walls, crown, invert and sometimes the face, for immediate flexible support. Obviously the use of traditional reinforcements such as steel-mesh and rock-bolts would make it much more difficult to excavate intermediate surfaces. The use of steel-fibre reinforcement in the shotcrete mix can ease this problem.
Partial excavation is not, however, restricted to tunnels of large section. In many smaller tunnels in the extension work for the Madrid Metro the ‘classical’ Madrid method or ‘Belgian method’ is used ( T&T I November 1998). In this case it involves hand excavation of 2m square advance galleries with timber supports to allow casting of a concrete roof. Mechanical excavation of wall galleries follows with concrete wall castings, and lastly the invert is excavated and concrete cast there. Not only has this method minimised settlements and made potential face movements more manageable, it has been used to recover at least face collapse.
Canopy methods
Construction of crown support in front of the face can take many forms, from the insertion of RSJs into large—diameter pre-drilled holes, as used in mining bad ground (forepoling) to horizontal jet grouting with sophisticated controls. Intermediate methods include rammed steel bar or pipe, or inserted into grout filled holes. Grout can also be injected through a self-boring or rammed grout lance to prevent the use of open standing holes.
The northern end of the Euerwang highspeed rail tunnel in Bavaria was constructed under the protection of a top cover due to the small depth of cover. The Hochtief/Universale Bau contractor joint venture also has to carry out an extensive grouting programme to reduce the amount of groundwater make throughout the tunnel. The ground comprises mainly fine sandstones and claystones.
Ground freezing
Often considered only as an emergency measure, ground freezing can be much more economical when planned in advance as part of the project. Thus freezing using brine refrigeration circuits is most often applied to shafts or tunnels near the surface. The use of liquid nitrogen freezing, due to its higher cost and speed of installation, is usually restricted to emergency situations such as when unexpected bad ground or water-bearing fissures are encountered. Ground freezing is frequently more successful than grouting programmes when dealing with high pressure or flows of groundwater.
The Scandinavian Rock Group discovered a water bearing glacial moraine gully during the construction of the Oslofjord subsea tunnel by probe drilling. Loose material contained water at the full 120m pressure head to the fjord. Grouting was ineffective and so freezing was chosen to stabilise the material before excavation chiefly because of the higher degree of safety and more controllable operation. Geofrost was subcontracted to design and carry out the ground freezing. 115 freeze pipes were drilled, of which 103 could be used. After freezing excavation by drill-and-blast proceeded successfully, using shotcrete mix with accelerator to line the ground and prevent falling stones as the ground thawed.
ADECO-RS
The chief ‘shop window’ from the ADECO-RS has been the construction of the highspeed rail tunnels in Italy for CAVET, including many instances of difficult ground. The method, simply put, depends on the control of ground movement in advance of the tunnel face, and recognises the paramount need to control the stresses in the delicate zone of tunnel construction at the face. The method is comprehensively described in the special supplement to the May 2000 issue of T&T I, ‘Design & Constructing Tunnel – ADECO-RS approach’.
The method of tunnel design and construction has been employed for in excess of 30km of successful tunnel building on the Bologna-Florence route through the Apennine mountain chain, with a total of 90km length planned.
Face stabilisation by the use of rockbolts and dowels has been practised for many years. It is preferable for the elements to be made of cuttable material to facilitate easier excavation by mechanical methods, but it is possible to use steel rockbolts. Wooden dowels and later, grp rock-bolts, usually fixed with cartridged resin mixes, have been used in mining and tunnelling since the 1960s. A more recent development is the use of glass-fibre spiles, and this has become a major feature of the ADECO-RS approach.
In the 860m long Tartaiguille tunnel on the French TGV MÈditerranÈe highspeed rail route, for example, the 180m2 section face, which had been halted by sever stress-strain conditions mainly in clay, was treated with fibreglass structural elements on ADECO-RS principles. These enabled excavation to continue and work was completed almost two months ahead of schedule.
Sireg’s Durglass FL glassfibre reinforced plastic structural elements combine the stiffening capacity with a facility for high pressure grout injection when installed in a pattern in the tunnel face. These properties are used to stabilise the face before disturbance by excavation, so aiding the loading placed on the ground and lining behind the face. The Durglass FL material is easily excavated by most plant. The system comprises glassfibre nails jointed to an injection pipe within a multiple unit pattern in the face. The injected grout binds the glass-fibre nails to the ground and the high tensional strength of the nails greatly improves the properties of the ground mass. Sireg coats the nails in quartz sand which gives high adherence to grout and ground. Since the loading on the permanent lining behind the face is reduced, it is claimed that substantial savings in concrete thickness can be made.
Durglass FL is available in a range of profiles and tensile strengths to suit various ground conditions. As they are light, and are available in coils when necessary, they are easily transported. Nails lengths up to 50m can be made. Apart from many tunnels in Italy, Durglas FL has been used for three tunnels on the Lyon-Marseille TGV, the TraversÈe de Toulon road tunnel, and metros in Rome, Athens and Lisbon. SNCF French Railways have approved the material.
Products of Sireg’s geotechnical division also include Durvinil S and Durvitech valve pipes for cement injection, Arapee aramid fibre tie rods and Technodrain smooth and corrugated sheaths for vertical drainage.
Exchem Mining & Construction supplies a complete range of rockbolts, resin capsules and drilling/bolting machines. The range of compressed air operated drilling/bolting machines is based on the Turbo Bolter. Recent tunnelling projects in the UK supplied by Exchem include the Stoke sewer and Bracon Hill rail tunnel for Severn Trent Water and Railtrack respectively. In 1999 thos supplied include the Connisborough and Porth Kerry tunnels for Railtrack and the Standedge canal tunnel (T&TI July). The contractor in each was Amco.
From the surface
Sometimes, when the tunnel is near the surface or to another excavation, it is possible to improve the face area ground by more efficient methods. Jet grouting has been used in such circumstances, such as for the West Bank Relief Interceptor sewer in Dallas, Texas. Specialist contractor Hayward Baker (Keller group) carried out a vertical jet-grout programme through the transition from rock to clay on the tunnel route. This allowed an open shield to proceed through mixed ground.
