With increasing demands for tunnels in adverse environments, there are needs both for better methods of improving the ground and for ensuring that the effects of tunnelling on adjacent structures are minimised or eliminated. Ground consolidation enables new larger, and more complex underground excavations to be performed in urban areas in soft ground, and in severely faulted regions.
Environmental concerns place an extra burden on the tunnelling engineer trying to select the best method to consolidate the ground. The chemical interaction of materials with the natural environment is a popular concern, although grouting is sometimes banned on the frequently erroneous assumption that it might contaminate water supplies or otherwise damage the environment.
Dewatering
One of the simplest solutions for the temporary protection of shallower underground works is to dewater the ground by means of a series of wellpoints, but this method can only be applied in ground of suitable permeability. Material that is too dense does not allow water to be drawn off, while in highly permeable ground it may not be possible to install sufficient capacity to lower the water table sufficiently. In addition to keeping excavations relatively dry, successful groundwater lowering can consolidate material that would flow when wet and could provide better conditions for the use of other techniques like permeation grouting. A drawback is that the loss of groundwater can destabilise old foundations. Here, groundwater lowering has been combined with the installation of cut-off walls and reinjection of water as necessary according to monitoring instruments.
Grouting
Injection grouting used to consist of pumping cementitious slurry under crude pressure control through an array of bores drilled to intersect the estimated position of a fault, fissures or other water flow channel. The introduction of the tube á manchette device and other sleeved packers was an important advance in grouting technology. Grouting can now be carried out in stages from the same holes so that a cured grout wall can be built up gradually and with different materials, increasing the chances of success in cutting water flow and economising on materials.
Permeation grouting – filling voids by grout injection – is the simplest and probably the oldest underground ground improvement technique. The injected grout is intended to fill voids, fissures and other spaces between particles to consolidate the ground or to prevent water flow. It can be performed from the surface, from excavations adjacent to current tunnelling or from the new tunnel excavation itself. In the last case it can be difficult to integrate grout hole drilling with mechanised shield tunnelling. Garschol1 has commented on the need to plan for any drilling requirements before tunnelling begins and on the possibility of leaving untreated areas even when grout hole drill rigs are integrated with a TBM (Fig 1).
The finer the grout particles or the lower its viscosity, the easier it will be to fill smaller spaces and to inject greater distances. AMEC states that the practical lower limits of ground permeability to receive an initial grout injection are 5 x 10-4m/s for normal cement grouts; 1 x 10m
Cost is an important factor in the choice of grouting material, since, according to AMEC figures, a resorcinol-formaldehyde grout can cost 25 times more than a weak cement-bentonite grout and seven times more than a normal cement grout.
Suppliers of micro cement grouts such as MBT‘s Rheocem range offer a mid-way stage. Four products have particle sizes down to 20µm and all are used with the admixture Rheobuild 2000 PF promotes proper wetting of the cement particles and prevents flocculation before injection. It also promotes water retention stability for consistency during grout penetration and allows the water content to be reduced for a given suspension velocity.
CarboTech‘s high penetration chemical grouts and associated equipment are available as a complete system. CarboPur NK 2-component, low-viscosity grout, with its long flowing phase, can be used to penetrate ground with up to 20% silt. It has high swell potential to provide early load bearing properties. CarboPur WFA and WT compounds have been used to prevent water flows of up to 120m3/h ingress. CarboPur systems were used to form a protection ‘dome’ against seawater flow into a water transmission tunnel at Mumbai, India. Ingress of up to 1.5m3/h was halted. Keller carried out extensive permeation grouting from the surface for tunnelling works on London’s Docklands Light Railway Extension2.
Safety & environment
Concerns over the possible toxic effects of incorrectly operated chemical grouts have led to the introduction of new types based on acrylates. A recent entrant to the international market is Rhm, a member of the Degussa Hls Group, offering Bautaxa Plexilith 322 and 324 acrylic grouts. Both have been approved by the UK’s WRc water research body and have passed a London Underground Limited engineering department fire test. In water-stop applications, Plexilith has high chemical resistance for work in polluted areas and sewerage applications as well as sea and fresh groundwater. Its low viscosity enables it to be easily carried by moving groundwater or pumped along water paths.
De Neef acrylate grouts have been employed to consolidate ground and keep out water in the stacked tunnel construction under the Boston South Station as part of the Central Artery/Tunnel Project. The low viscosity chemical grout limited heave from grouting while achieving deeper penetration. Grout holes were placed only 1.5m apart to ensure there were no gaps in the grout wall, and the operation was successful in keeping groundwater in loose soil out of the main excavation up to 30m below the water table. De Neef’s AC 400 acrylate grout is a monomer system for water control applications, containing no acrylamide monomer. It is claimed to have toxic exposure levels of only 1/100 that of acrylamide grout and 1/50 that of NMA grout.
CarboTech says that only a few simple precautions are required in the use of its chemical grouts to ensure safety.
Jet grouting has developed in the last two decades for soft ground and overcomes limitations imposed by grain size by replacing the existing soil structure. Cementitious grout is ‘blasted’ into place in a rotary motion, mixing in-situ material. Originally developed to form vertical cut-offs and underpinning, the technique has been applied more or less horizontally to protect tunnels and to install other structural support before excavation, especially in cut+cover work.
Sub-vertical ground treatment by jet grouting has been used by Nicholson Construction on two projects in San Francisco4. For the Islais Creek Transport/Storage Project, around 1000 overlapping jet grouting columns totalling 7500 linear metres were used to treat squeezing ground, including soft clay. Angled columns were employed where the 4.5m diameter sewer tunnel passed under a double-track commuter railway embankment.
Keller has been installing jet grouted columns (the Soilcrete process) on the Hull sewer tunnelling work in the UK with main contractor Miller. These are used to form entrance and exit blocks adjacent to shafts for TBM passage in weak or water-bearing ground.
The growth in sprayed concrete tunnelling methods for large section tunnels has led to the need for structural security in poor ground. Various forms of umbrella arch have been developed for creating a structural canopy in front of the face. There are also many methods for supporting the actual face with spiles or grout, which can be removed by excavation.
Rodio has employed jet grouting for the Turin Railway Interchange Project5 both horizontally to arch a pilot tunnel crown and in a pattern of fans to buttress the final invert excavation. Both were used in conjunction with permeation grouting around the crown and side of the tunnel, and sometimes with thin steel driven reinforcements in an arch over the main tunnel excavation and at a shallow depth below the main Turin-Milan surface railway. Rodio has also employed its grouted arch technique (Rodinjet) for pre-excavation of the Girsberg Tunnel, Switzerland. The company is using Dorodur H50 cementitious grout from Rohrbach Zement to form an arch by horizontal jet grouting over the planned excavation (7.7m x 4.5m).
Control of the effects of ground movements by high-pressure grout injection is now an important tool when tunnelling is planned beneath important structures. James Wheeler of Hayward Baker3 (June ’98, p44) says compaction grouting was used below surface structures on the Baltimore Metro 20 years ago.
Several suppliers of monitoring and recording equipment are meeting the sophisticated require-ments of modern grouting operations. Atlas Copco Craelius has announced a new recorder, the Logac 4111, that is designed for simple and reliable surveillance of grouting work. The new recorder is part of the Logac 4000 computer based logging system, which includes the Craelius Flow Pressure (CFP)) meter, which measures both flow electro-magnetically and flow in separate.
Ground freezing
Ground freezing and subsequent thawing can now be controlled with great accuracy. The method is often set aside as being too expensive, but its proponents claim that, if ground freezing is included in the project plan from the start, it often proves cheaper than other options7. Its planned use also avoids emergency situations in which even more expensive unplanned ground freezing or extensive chemical grouting has to be used in any case.
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