Collapses at the tunnel face or unexpected high water inrushes are not uncommon experiences when tunnelling in geologically difficult ground like fault zones in Alpine terrain or tunnels with shallow location influenced by weathering or low rock stresses. Tunnelling in urban areas often involves shallow location of tunnels, proximity to existing underground structures, as well as establishing connections between these structures. The consequences of a groundwater drawdown or deformations in the ground caused by instabilities are particularly unacceptable due to the possible impact on buildings with sensitive foundations.

Pre-injections in shallow located tunnels can strongly reduce the risk of mishaps. The state-of-the art technology within rapid hardening microcements and liquid colloidal silica is particularly emphasised. This technology can improve the cost effectiveness and technical feasibility of tunnelling in a sensitive environment in difficult ground significantly.

The recent improvement of the preinjection technology employing controlled accelerated setting of microcements opens new possibilities for the use of cementitious injection technology.

The pre-injection method
The basic idea of pre-injection is to treat the ground prior to the excavation by injecting a grout into the ground through drillholes and pumping at pressure.

Pre-injection basically consists of:
• Drilling of holes for the injection
• Injection until the termination criteria are reached
• Evaluation or control of the injection result: decision regarding repeated injections or to commence excavation through the treated ground

In addition to these three main steps in the actual injection cycle, there is an up front process of determining the main scope and location of the injection works. This includes the following important issues:
• Eploratory drillings to determine the initial state of the ground to be treated
• Exact location to establish the drillings for the injection
• Injection method, main features
• Grout types, grout mix designs

It is important to understand a pre-injection scheme in tunnelling as a sequence of decisions made at several stages in the injection cycle. Management of the injection works is therefore a crucial factor in order to achieve cost-effectiveness.

Pre-injections can have two main goals:
• To reduce the permeability of the ground and hence, reduce the flow of water into the tunnel after excavation
• To improve the mechanical properties of the ground and hence, provide improved stability of the ground during the excavation and tunnel support

Before the technical details of a preinjection scheme are designed, one should make overall considerations regarding the method. The layout of the method comprises decisions at a strategic level for the pre-injection works, including the fundamental approach as to how to achieve the desired result. The basic framework of the operations in the injection cycle is laid out in this process.

This planning process needs to give the necessary input for the specification of equipment, such as drilling to desired length and cycle of probe drilling and injection fan drilling (figures 1 and 2). Themain issues to consider in this planning process are the following:
• Ground properties and their capacity to be drilled
• Drilling method
• Packer system to suit the chosen hole diameter or pipe
• Grout mix designs to suit the required penetration, early strength and long term material properties
• Considerations regarding injection pressures
• Termination criteria for the injection process
• Simple control measures to verify that the injection result meets the requirements

It is important to understand an injection operation as a complete method. All the above mentioned issues should always be considered during the planning process.

A frequently misunderstood feature is the grouting pressure. Very often, maximum permitted grouting pressures are specified at far too low a level. The main reason for this is frequently said to be that a higher grouting pressure might feed a global pressure buildup in the ground, leading to a risk of hydrofracturing or undesired penetration of grout far away from the location of the injection.

Barton et al (2003) demonstrates that injection pressures measured at the injection lance (the collar of the drillhole) in most cases do not correspond to the pressure of the grout in the actual ground. As long as there is a flow of grout, there is a significant drop of pressure in the immediate vicinity of the drillhole into the ground. Another important issue is to always have in mind that an injection operation is a cycle in which decisions are made with regards to specific criteria. These decisions are made at each step of the injection cycle. Therefore pre-injection is a type of work that requires continuous and experienced hands-on management during the works.

Characteristics of grout
Injection in difficult ground requires the complete method to be adapted in order to achieve desired penetration. An essential detail in this context is the choice and design of the proper grout characteristics with special emphasis on penetrability.

The penetrability of a grout is difficult to measure or verify directly. Penetrability describes the ability of a grout to enter into a medium like a granular soil or fine joints in a rock mass under a certain injection pressure.

The penetrability of a grout for injection purposes in underground construction is mainly influenced by three measurable material properties: grain size, viscosity and stability. The grain size is decisive for the penetrability into fine joints, as well as permeating between the grains in a soil. Figure 3 illustrates graphically the importance of grain size for the penetrability in joints in rock.

The viscosity is important since it will directly influence the shear stresses in a grout when it is flowing through joints or between the grains in a soil. The lower the viscosity, the lower the shear stresses in the grout and hence, the lower the injection pressure which is required to sustain the flow of the grout into the ground.

The stability of the grout is important since it will directly influence the capability of the cement in the grout to penetrate into the fine discontinuities.

The process of bleeding, in which the cement grains separate from the grout mix and clog the entrances to the fine joints, does not occur with a stable grout mix. For soil injection purposes one must consider closely which intended improvement effect and penetration mechanism that is planned when designing the grout mixes and the method. Four examples of soil improvement mechanisms are shown in figure 4.

These cements were originally designed as tunnel injection cements with the main emphasis being on cost-effectiveness in tunnelling. They are available in the grain size range typical for microcements down to the ultrafine microcement range. These cements offer particular advantages in a tunneling situation. The main advantages are:
• Small grain size
• Excellent stability and low viscosity even at relatively low water/cement ratios (e.g.1.0)
• Excellent penetrability due to the grain size distribution, stability and viscosity characteristics
• Setting within 1.5 to 2 hours (w/c ratio 1:1, 20 degrees C), eliminating practically all waiting time for setting
• Rapid setting characteristics also in cold groundwater conditions

Liquid colloidal silica grout type consists of silica grains (SiO2) in the nanometric scale in a colloidal solution in water. The typical grain size is 0.016 µm. Its viscosity is 5-6 m.Pa.s, which is slightly higher than water. These technical properties offer a unique technical performance in a number of injection situations. Colloidal silica, contrary to silicates and acrylates, is a completely non-toxic product, which makes it unique in terms of environmental friendliness and health and safety. Colloidal silica is a mineral grout designed for permanent longterm purposes, whereas silicates only can have a temporary function.

The penetrability of colloidal silica in jointed rock and soils is illustrated in figure 4. The simplicity during application, using the same equipment as for cementitious grouting makes this system very suitable as a supplement to cementitious grouting.

Maneri Bhali hydropower project
The headrace tunnel for the Maneri Bhali hydroelectric power project phase 2 (Uttaranchal Province, India) underpasses a valley with low rock cover. The tunnel was excavated by drill and blast and supported by steel sets and lagging with concrete backfill.

The situation with valley underpass and main geological formations is shown in figure 5.

The valley corresponds to a regional weakness zone which intersected approximately a 300mlength of the tunnel. The weakness zone exhibited densely jointed and partially crushedmica-quartzite schist. In-situ fine grained crushedmaterial in the silt fraction occurred as joint fillings. The entire weakness zone was highly permeable; hence high water inflows were encountered.

The valley had been underpassed with serious difficulty during a construction attempt several years earlier. No pre-injections were carried out at this stage. The highly jointed and crushed rock combined with the low overburden of only 20-25m(of which only ca 5m rock) at the shallowest imposed a critical point in the headrace tunnel.

The maximumwater pressure in the tunnel during the operation of the power plant would be 10 bars.This would create a potential risk of hydrofracturing and leakages out of the headrace tunnel. There would also be a risk of charging of water in the surrounding ground resulting in a danger of landslides.

Previously, pre-injections with locally manufactured ordinary Portland cement had been attempted, but with very limited penetration into the ground. In order to address the difficult ground conditions, a two-stage pre-injection scheme with two different grout types was undertaken. The main goals of this pre-injection scheme were:
• Water ingress reduction and stabilization of the ground in the weak zone to facilitate the establishment of a sufficient rock support as well as the structural and waterproofing lining of the tunnel
• Facilitate safe conditions for excavation and immediate support of the tunnel as well as constructability for the final lining

The main feature of the injectionmethod was to inject through grouted steel pipes with a length of 2.5m. The high degree of jointing and crushing of the rockmass severely limited the drilling operation. Each of the steel pipes were therefore used for repeated drilling and injection.

The first drilling and injection step through the steel pipes reached 6m in front of the tunnel face.

The second step reached 8-11m, and the third and final step reached 13min front of the tunnel face.

The first stage consisted of injection of rapid setting microfine cement. This allowed for a continuous operation with drilling, injections and the subsequent redrilling to greater depth through the same steel pipes without damaging the result of the previously injected volume.

The second main stage was the injection of liquid colloidal silica. Featuring extremely low viscosity and grain size in the nanometric scale, the finest joints as well as joints with fillings were grouted. A very satisfactory result in terms of water ingress reduction and ground improvement was achieved.

The second stage was geometrically laid out in a way that it would be enveloped by the grouted rock mass from the first stage, as shown in figure 6.

In this way the injection of the low viscosity grout would occur where the microfine cement was already injected.

The first stage injections with rapid hardening microcement showed a relatively limited grout take of only 100-150 kg per m drillhole when the termination pressure of 60 bars was reached.

Bearing in mind the seepage, which was encountered in the drillholes, one would expect a higher grout take. The reason for the relatively low grout take was joint fillings which consisted of the silt and clay particles, which in turn limited the penetration of the grout created by filtration.

The first stage injection fan with microcement was always completed in the full circumference of the tunnel before the This was due to the first stage injection providing penetration of grout into the joints with the largest apertures.

The second injection stage with the low viscosity grout could therefore be targeted for the finer joints and the joints which were partially filled with clay and silt. The secondary fan was injected with a termination pressure of 25 bars, or approximately 100kg grout per m drillhole. Injection beyond a pressure of 25 bars with liquid colloidal silica usually showed signs of hydrofracturing.

The result was a literally dry and stable tunnel contour. No excessive breakouts of rock or cave-ins occurred during excavation through the weakness zone.

The proactive approach demonstrated in this case, that pre-injection of the difficult ground resulted in safe, cost-effective and predictable construction conditions. The effects of the pre-injection works could be directly compared to the poor conditions and heavy water flows observed in the adjacent tunnel where pre-injections were not used.

A direct comparison of the effectiveness of microfine cements and liquid colloidal silica grout versus ordinary Portland cement was also clearly evident.

The experiences from Maneri Bhali show that a proactive approach to drilling and grouting with a clearly defined method statement and utilizing materials with the proper penetration, viscosity and rapid hardening characteristics can significantly reduce the construction time through such highly faulted, water bearing and unstable zones (6 months in by-pass tunnel versus 18 months in original tunnel).


Figure 1 – Drilling ahead of the tunnel face for pre-injections using a conventional drilling jumbo for hardrock drill & blast excavation Figure 2 – Schematic layout of pre-injection in hard rock tunnelling. A: percussive probe drilling pattern. B: Injection drillhole fan. Figures are typical and given in m. Figure 3 – Graphical representation to scale of grain sizes of cement grouts and colloidal silica with respect to a relevant joint aperture in rock for penetration (0.02mm) Figure 4 – Simplified graphical representation of four main types of penetration of a grout in a soil. Pure grout is shown as blue A:replacement, B:compaction, C:hydrofracturing, D:permeation Figure 5 – Headrace tunnel of the Maneri Bhali hydropower project. Longitudinal section of the difficult zone. (Bahadur et al 2007) Figure 6 – Layout of the injection method (longitudinal vertical section), The outer dark area in front of the tunnel face represents the volume which was treated in injection stage 1 with rapid setting microcements