The provision of a temporary stabilisation of poor ground by artificially freezing contained groundwater (including porewater) requires expertise that is in limited supply around the world.
The method is recognised as being expensive but what really matters is the relative costs to that of the whole tunnelling project, and how much might be wasted on other unsuccessful consolidation methods. A well-planned groundfreezing operation using brine circulation (in preference to large-scale use of liquid gas) might save more expenditure on, for example, large grouting or dewatering programmes. Natural salts such as calcium chloride are used in the brine so that there are no residual environmental problems, as long as the freeze tube holes are secured.
One source of groundfreezing expertise is the FreezeWall division of geotechnical specialist Moretrench, in the US. The company has been involved in many particularly difficult projects in which a high groundwater flow across the zone requiring treatment has made formation of a secure frozen enclosure uneven and potentially incomplete, unless additional measures had been taken. Recent projects have included the Boston ‘Big Dig’ Interstate 90 highway project, in which FreezeWall created a frozen zone beneath the existing Amtrak multi-track railroad to allow the passing of jacked large-section tunnel structures. 2000 freeze pipes were required.
Grout assistance
In difficult circumstances, the formation of a secure enclosure of frozen ground can be aided by working with permeation grouting. This is likely to be the case where groundwater flow may be deterring the formation of a complete groundfreeze barrier. A secure, if temporary, grout seal can reduce or greatly reduce the water flow so that the groundfreeze operation has a better chance of working. Conversely grouting alone may not work without groundfreezing.
A careful assessment of the ground conditions, including grain size, porosity, structures and water pressure, is necessary to select the method most likely to succeed. This can have a major economic effect on the whole tunnelling project, not only in selecting the cheapest ground improvement method, but also in preventing delays. With this ground investigation data the ground treatment specialist can plan and design its procedures. As listed by ECO Grouting Specialists, of Canada, this will include a calculation of the grout-hole spacings, grout selection, any custom-made formulation of chemical and cementitious grouts, set times, grout pipe details including sleeves, and borehole preparation.
Developments in the use of ‘chemical’ or solution grouts, especially water-reactive types, have greatly improved the efficiency of permeation grouting in fine-grained ground or small fissures that cannot be penetrated by cementitious grouts, consolidating the ground structurally as well as deterring water flow. The properties of these grouts tend to be more amenable to control than traditional types, and different formulations can be devised to produce the required properties. More recent formulations have little environmental concerns associated with them it is claimed, so long as the correct materials are selected for the application conditions.
Another approach to high groundwater flows with potential structural problems is a revival in the use of hot bitumen grouting as practiced by ECO Grouting. Amongst other projects, this was successfully used to stem a 4000l/s flow from fractured marble in the Yung-Chung railway tunnel in Taiwan, as well as stabilising the geological formation.
In addition to general consolidation of potentially unstable ground, grouting may be used in different formulations to form a structural canopy or ‘umbrella’ over the excavation. This may be in the form of compensation grouting in which the pressure and amount of grout injected is calculated to compensate for any induced subsidence that may affect surface structures above or adjacent to the tunnel, and controlled according to comprehensive monitoring of ground movements.
Canopies & umbrellas
Various technologies and terms have been used for the formation of a canopy or near-horizontal reinforcement element over the crown and in advance of a soft or mixed ground tunnel excavation, including forepoling, forepiling and pipe roofing.
In some methods the elements are combined with grouting to consolidate the ground itself and link the main reinforcement elements. Most recently reinforced plastics grouting tubes, such as those produced by Sireg, act as reinforcement elements as well as conduits for grout injection. As these are cuttable they can be used across the edge of, or inside the tunnel section as well as the face itself. The Sireg perforated pipes are also equipped with valves at regular intervals to aid the selection of the zone to be grouted as, for example, where the holes are drilled from a separate site.
Where steel tubes in the crown are used they can be equipped with non-return valves for grout injection. Trevi calls its version RPUM or Reinforced Protective Umbrella Method and employs its own Trevitub or Springval elements. Rotary percussive drilling is used to introduce Trevitub forepiling with non-return valves in the tubes into glacial or coarse soils, which may include boulders. In the Springval method the Atlas Copco-developed ODEX undercut drilling method is used to introduce casing into the ground. This special casing has valves included in the wall that do not protrude and therefore do not get damaged during insertion.
Jet grouting
The creation of jet-grouted piles or other reinforcement and load-supporting elements in soft and mixed ground is a technique of increasing importance. Most commonly it is carried out from the surface, whether vertical to create walls or partial walls, or inclined such as to form a canopy over the tunnel excavation. It has also been used underground to form overlapping canopies of near horizontal columns of concrete mixed with the ground material.
Considerable care has to be taken to ensure that jet-grout columns overlap if a continuous wall is required including directional control and for the possibility of adjacent structural elements interfering with proper formation of the column by shadow effects. Economical planning of the optimum jet grout may be aided by various computerised procedures, many of which would not be possible manually (see Schat diagrams).
Other cut-offs
Diaphragm walls can be used on a temporary or permanent basis. A trench is excavated by special grab or hydrocutter-type head, usually with a supporting fluid such as bentonite mud. The dimensions of the wall will probably affect the type of equipment chosen. The walls are usually lateral along a tunnel route or linked in a continuous form, often rectangular, for structures such as station boxes. Reinforcement may be introduced to the excavation, and the void backfilled with concrete or other material. The resultant sub-surface wall can serve several functions, depending on its design, including being part of the underground structure, foundations for a surface structure, and prevention of inflow of ground material to the excavation and so stabilising the ground, and deterring groundwater movement.
Trenching grabs can be used in softer ground for depths down to 60m. Hydrocutter or rock-mill (hydrofraise) equipment such as Soilmec’s Hydromill can be used to break up harder layers and to greater depths (down to 100m). Such equipment is also likely to be more accurate for where construction tolerances are tight. The Hydromill uses two cutting wheels, and reverse circulation fluid flow by centrifugal pump is employed to lift the excavated material to the surface and the fluid recycled. The more mechanised diaphragm trench equipment now available lends itself to better instrumentation such as the Soilmec’s Drilling Mate System for the CTJet equipment for monitoring of all functions and giving potentially better control of the barrier being created.
Soilmec has developed a flight auger grab system including four or six vertical tubes, 5m high, to guide the excavation and store spoil. Each tube contains a continuous flight auger linked to a drive. Cutter teeth cover a larger diameter than the tube. Integrated chisels in the assembly can convert the initial excavation section of secant circles into a rectangle. This system is claimed to double the excavation rate of a simple grab system.
The underground transport hub in the reconstruction of New York’s World Trade Center, due for completion this month (July 2007), includes the formation of substantial diaphragm walls. The work was awarded to Trevi Group subsidiary TreviIcos in partnership with Kiewit Construction who created a strong diaphragm to both protect the underground excavation and form the towers’ foundations, together with the base of the planned memorial to 9/11. The total value of the TreviIcos/Kiewit contract is US$34M. The overall project design is being carried out by the Phoenix Constructors joint venture consisting of Fluor Enterprises, Slattery Skanska, Bovis Land Lease and Granite Construction. The wall will cover 80,000ft2 (7400m2) and range in depth from 70 to 120ft (21 to 37m) for a total length of 1200ft (366m), and will secure the existing structure that was damaged by the 9/11 attacks. Fifty rebar cages are being installed in the wall. After wall construction the interior space at the eastern end of the box or ‘bathtub’ structure will be excavated down to bedrock. An existing diaphragm wall, installed in 1967, will also be used at the western end of the site.
Another example of the use of a diaphragm wall for surface structure support as well as a retaining barrier in soft or unstable ground is the work for the Amsterdam North-South metro line under the Amsterdam Central Station listed building. The structure in this case, designed by Arcadis, comprises two rows of screwed steel piles with a jet-grouted ‘sandwich’ filing of the 2.5m-wide (max) gap. The piles are 457mm round and placed one metre apart centre-to-centre. The interaction between the steel piles and the jet grout provides the horizontal bearing capacity. Shear forces are transferred between the steel pilings and the grout by rings on the outside of the steel piles.
Face reinforcement
The use of cuttable elements is well established, at least as far as the use of grouted wooden dowels is concerned. However the introduction of GRP elements has extended the possibilities for structural performance. GRP elements have found particular applications in partial excavation methods including pilot tunnels, as well as advance reinforcement of ground in front of the face, as well as the crown.
FiReP (fibre-reinforced polymer) products from Rockbolt Systems include profiled rod rockbolts produced by a pultrusion process with a fibre content of 80%. The long strands of glass fibre are embedded in either a polyester or epoxy resin matrix to give high tensile strength. The shaped profile and coating is claimed to protect against mechanical damage and chemical attack. FiReP self-drilling bolts, as other self-drilling anchors, have particular value in loose or structurally disturbed rock. They can be drilled into rock of hardness up to 60MPa and are suitable for use with resin or cementitious grouts. As with other FiReP elements they are easily cut for resumed excavation and are corrosion resistant.
Durglass FL elements from Sireg can transfer its resistance values to the ground aided by a quartz sand treatment to the surface. The use of flat or Y-shaped elements increases the contact surface of the elements with injected grout, and can reduce bonded lengths to less than a metre. Flat sections can be rolled for site installation in length longer than 50m.
MAI Systems SDAs (self-drilling anchors) from Atlas Copco were used in face ground stabilisation during driving of the Mitholz section of the Lötschberg Tunnel is particularly difficult zones. As with other self-drilling anchors, the supporting element is installed at the same time as the hole is drilled, with simultaneous grouting through the hollow-bar element if required, so the drill hole does not have to be self-supporting in poor ground.
And other bolts
Systematic bolting of the crown and walls lends itself to mechanisation, which, if well planned, can increase the efficiency of the whole tunnelling operation.
In very poor ground the bolt will need to be fully bonded to the whole of the hole, thus helping to bond the ground together. If the tunnel can be tied back to a zone of stable ground by rock bolts of suitable length, the element can be anchored at the rear end and placed in tension to secure the ground as part of a pattern.
Atlas Copco’s Swellex range is particularly suited to mechanised installation, requiring only pressurised water to secure it. In common with some other rock bolts, it can be installed by Atlas Copco Rocket Boomer blasthole drilling rigs with suitable attachments, including testing instrumentation. To install Swellex bolts a Swellex chuck is attached to the COP drill hammer, the bolt located with the faceplate in the drill-steel support of the rig and inserted the full length into the hole using the drill feed, then the Swellex inflated with an on-board hydraulic pump. For particularly long bolting requirements, suitable lengths of Swellex can be connected to form almost any total length.
Multiple methods
In complex tasks, whether due to the design and purpose of the excavation, or because of varied difficulties in the surrounding ground, multiple skills are necessary to provide optimum solutions. The experience available from contractors who, whilst being ground support specialists, do not rely on one or two techniques, could well be valuable in this context.
One example of such a situation is the extension work on the Naples Metro Line 1. Working in consortium with SGF Inc, the Trevi Group has been contracted to work on the Stazione Garibaldi and the Brin access shaft for TBM access. The rock encountered in Naples can alternate between volcanic tuff, pozzulano, and lapilli, with often unmapped cavities, and high water table in low-lying areas. Stations can be as deep as 47m. The techniques employed by Trevi include drilled forepoling pipes, diaphragm walls, jet grouting, underground anchors, cementitious and chemical grouting, and groundfreezing.
Installation of crown support by the RPUM (Reinforced Protective Umbrella Method) on the Bologna-Florence rail project using a Soilmec 605-D7 twin-boom rig Installation of crown support by the RPUM Example of a Monte Carlo simulation diagram for the placement of the jet-grout columns in plan (from Schat et al, RETC 2007, Toronto) Example of a Monte Carlo simulation diagram The Soilmec Hydromill The Soilmec Hydromill Diaphragm wall construction by Trevi for the Boston CA/T ‘Big Dig’ project Diaphragm wall construction by Trevi Mechanised systematic rockbolting with a Sandvik Tamrock Robolt rig Mechanised systematic rockbolting Atlas Copco Roofex, MAI, and Swellex bolts Atlas Copco Roofex, MAI, and Swellex bolts