Modern grouting now demands skilled, probably specialist, practitioners that know the best way of using the many techniques now available. The expanding range of grouting techniques has made tunnelling more practical in situations where it has been nearly impossible; whether through mixed and broken geology in mountainous areas for base tunnels or in increasingly crowded urban underground space to prevent damage to adjacent structures.
Grouting can be used as a preventative measure, perhaps as part of the designed geotechnical structure, or as a rescue measure. The latter is legitimate if truly unexpected poor grounds conditions are encountered that have to be rectified, such as an undiscovered underground fault. However adequate site investigation should reduce the chances of such grouting being necessary. Rescue grouting often halts other tunnelling activities, increasing the project time. Grouting operations that can be carried out before or elsewhere, simultaneous with tunnelling, should not delay the project as long. In the latter case they should be carried out with sensitivity to their possible effects on tunnelling.
It should be pointed out that this article does not concern itself with grouting solely to prevent groundwater movement. However, particularly in soft ground, this can well have a structural component.
Traditional injection ‘permeation’ grouting fills naturally occurring voids with suitable grout to consolidate ground. There are now several extensions of this basic technique by which ground can be compacted or slightly moved, as examples. The advent of additives to modify the properties of cement-based grouts, colloidal grouts as silica gels, and the polymer, or ‘chemical’ grouts has broadened the range of ground types that can be treated as well as the reach of grouting. The more advanced artificial grouts however are relatively expensive and usually reserved for where there are substantial water problems or emergency requirements.
Despite the exclusion and support expectations of the performance of TBMs, grouting seems to be increasingly practiced concurrently with TBM operation to the extent that, on some critical projects such as the Gotthard base tunnel, drilling and injection grouting equipment is designed into the TBM complex. This may be an obvious necessity or a requirement of the client or its design engineer requiring greater assurance. For example, in many circumstances in the construction of the Madrid southern metro network extensions injection grouting was used, especially in crossing sensitive surface structures, despite efficient use of the EPB TBMs.
On the other hand, insufficient grouting capacity, for whatever reason, to meet the ground conditions actually encountered can lead to greater problems, whether technical, as reduced project progress or financial matters. For example, in preparation for the boring of the Port of Miami tunnel, Florida, the client has rejected a request for extra funding for grouting limestone that is more permeable than anticipated by the contractor.
An extension of injection/permeation grouting in this context is ‘compaction’ grouting in which grout is injected into loose soils to form layers or ‘bulbs’, compacting the surrounding ground into denser material with ‘engineered’ properties. The technique has been widely used by Hayward Baker, part of the Keller Group in North America, with its Denver System chiefly to correct building settlement, but also for treating rubble or loose fills, and liquefiable soils.
TAMs
Although tube-a-manchette devices, the sleeved grouting pipes also known as TAMs, have been available for many years, they have been important in allowing grouting operations to be more selective and better controlled. The effects are still being felt in such operations as compensation grouting (see below), aided by the development of new grouts. The advantages of TAMs include:
• Grout injection can be carried out in various stages. Different mixtures can be injected for each grouting stage, in order to treat the ground in the most suitable way.
• The treatment of the ground is carried out in a very accurate way using the sleeves mounted on the pipes to meet client requirements on location.
• Injections may be repeated at later stages through the pipe installation.
The Durvinil and Durvitech devices available from Sireg include sleeved grouting pipes in plastics and fitted with Dur-O-Ring rubber valve sleeves on the extrados or within the wall of the sleeve, operating at a specific ‘bursting’ pressure. The tube is installed in a bored hole filled with a cementitious grout to form a ‘sleeve’. This is broken through during grouting, allowing the grout to pass through and into the ground to be treated.
Compensation
Since its first major application on London’s Jubilee Line Extension (JLE) metro project under Waterloo mainline terminus, and then elsewhere on the JLE project, compensation (also ‘fracture’ in North America) grouting has gained growing acceptance for the stabilisation of surface structures possibly affected by settlement due to tunnelling, or to ‘compensate’ for any actual movement detected by an array of instrumentation on the surface.
It should be noted that the injection of dense grout under high pressure in order to support or lift the structures above, then there could well be a reaction on the structure(s) below, especially the new tunnel support lining. This type of situation has been the subject of research at Cambridge University. This affirms that such grouting operations have to be widely instrumented to judge reactions.
When necessary it is usual to optimize stability of the ground prior to tunnelling by injecting a ‘slab’ of treated, perhaps creating a slight ‘heave’. The grout holes can also be used to ‘compensate’ for settlement to introduce more grout to eliminate potential voids and so maintain support. Most importantly the process deters differential settlement from tilting structures, likely to cause severe damage.
Compensation would be impossible without the sleeved grouting pipes, as described before, together with a wide variety of geotechnical instrumentation.
Preconfinement
Grouting plays and important part in the preconfinement method within Rocksoil’s ADECO-RS system. In addition to the wider use of face and advance ground stabilisation with injection grouting, preconfinement also employs glass-fibre elements in holes also used for grouting.
The Durglass range of such elements from Sireg including various profiles and bars, sometimes with grouting tubes, composed of continuous glass fibres embedded in a polymer matrix with a high bonding surface to key in with the grout. The glass-fibre elements promote high tensile strength in advance of the tunnel face, as well as allowing easy excavation due to low shear strength.
Jet grouting
Having little in common with other forms of grouting, jet grouting relies on mixing with the existing ground to form consolidated, cemented columns in the ground with known structural properties. These can be linked to form a wall around an excavation in a similar way to piling or diaphragm walling. There may also be multiple rows of columns to treat greater areas.
More recently, inclined or horizontal jet grouting has been employed from either the surface or underground to form a canopy over the tunnel excavation.
Jet grouting can only be used in softer deposits or soils since the grout jet has to break up the existing structure and mix it with the grout to form a new structure on curing. The boring and jetting drill string can carry one to three jetting heads that are rotated on withdrawal from the bottom of the borehole to the surface in a carefully controlled and monitored procedure.
Use of jet grouting associated with tunnelling has increased rapidly and is now being used in London for the first time on London Underground’s Victoria metro station upgrading by Keller UK for the Vinci–BAM Nuttall contracting joint venture.
Control and monitoring
In the more specified and reported environment of modern tunnelling it is important that there is a good system of feedback on grouting and its effects. In traditional grouting, if the necessarily permeable ground could ‘take’ the injected grout, then excavation and support would be greatly improved. However, not all ground can accept grout. Indeed some ground could be made worse if the ground is friable as well as impervious, or the grout pressures and hence delivered volumes are too high, causing ground heave.
Therefore the nature of the ground being treated should be understood, with sampling and testing, before grouting. And then the correct volumes and applied pressures of grout can be decided to achieve the required results. So instrumentation is essential, with automatic reporting usually required. A wide variety of instrumentation is now available for accurate and reportable grouting. Soletanche Bachy’s in-house control systems is called Spice handles data including borehole geometry and volumes of grout required at the set-up stage, injection settings, monitoring of the grout plant including pumps, flowrate and injection pressure, and tracking of grouting quality and production. It works in conjunction with programmes such as Caster, which groups together all the exploration data and establishes the location of grout holes, and Sphinx, to assemble actual grouting data and presents them graphically.
Excavating a tunnel previously strengthened with Sireg Durglass continuous glass-fibre and polymer elements including grouting tubes Operation of a sleeved grouting pipe – tube-a-manchette – in situ using a Sireg grouting tube At the control panel of Atlas Copco’s Unigrout truck-mounted grouting system The principle of compensation grouting whilst tunnelling to reduce surface settlement [After Dept of Civil Engineering, University of Cambridge] Use of Sireg glass-fibre and polymer elements, including a grouting tube, in anchors installed in a tunnel portal face. Using jet grouting semi-horizontally, as developed by Rocksoil, to form a canopy around the excavated tunnel using a special rig
