Pre-injections or injections ahead of the excavation face in underground construction can, in many situations, offer significant advantages. This is particularly the case in difficult ground conditions like severe water ingress or mechanically poor ground or soil, in which pre-injections can contribute to avoid mishaps and serious delays.
Pre-injections in underground construction have two main goals; either the reduction of the permeability in the ground and hence the reduction of the water ingress into the underground structure, or the mechanical improvement of a jointed or weathered rock mass or a soil by the penetration of a grout into the ground. Modern pre-injection technology involves the design of methods adapted to the encountered situation. This includes proper drilling techniques and drilling geometry for the best delivery of the grout into the ground, determination of relevant working criteria during the actual injection, as well as the employment of state-of-the-art injection products like special microfine cements and colloidal silica.
Several challenging underground projects have recently been successfully conducted due to proper use of pre-injections which requires a proactive mindset, since it represents an upfront investment of time and costs for the drilling and injection works. This investment, however, is usually of a small order compared to the possible consequences which can result from the lack of pre-injections, like e.g. groundwater draw down, heavy water ingresses or collapse of the tunnel due to bad ground.
Tunnel excavation involves a certain risk of unexpected ground conditions. One of the risks is the chance of hitting large quantities of high-pressure ground water. Smaller volumes of ground water ingress can also cause problems in a tunnel or its surroundings. Water is the most frequent reason for grouting the rock that surrounds tunnels. Ground water ingress can be controlled or handled by drainage, pre-excavation grouting and post-excavation grouting.
Rock or soil conditions causing stability problems for tunnel excavation is another possible reason for grouting. Poor and unstable ground can be improved by filling discontinuities with grout material which has sufficient strength and adhesion.
Collapses at the tunnel face or unexpected high-water inrushes is not an uncommon experience when tunneling in geologically difficult ground like fault zones in alpine terrain or tunnels with shallow location influenced by weathering or low rock stresses.
Tunneling in urban areas often involve shallow location of tunnels, proximity to existing underground structures, as well as establishing connections between underground 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.
The state-of-the art technology within rapid hardening microfine cements and liquid colloidal silica can improve the cost-effectiveness and technical feasibility of tunneling in difficult ground significantly.
The pre-injection concept
The basic idea of pre-injection is to treat the ground prior to the excavation by injecting a grout into the ground. Injection in this context means the introduction of a grout into the ground through drill holes or pipes by pumping with pressure.
Pre-injection basically consists of the following main steps:
- Drilling of holes or pipes for the injection (placement of the grout by pressure)
- 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
- exploratory 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, mix designs
What can be achieved by pre-injections?
Pre-injections can have two main goals
- Reduce the permeability of the ground and hence, reduce the flow of water into the tunnel after excavation
- Improve the mechanical properties of the ground and hence, provide improved stability of the ground during the excavation and support of the tunnel
In the case of permeability reduction, the main goal will be to fill the water bearing discontinuities with a stable grout that seals off the water flow.
In the case of mechanical improvement of the ground properties, the grout eventually will need to have a certain final mechanical strength.
A combination of these two effects is also desirable in many cases.
Before the technical details of a pre-injection 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 detailed 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 like drilling to desired length and eventual drilling equipment installed on TBMs (Figure 1 below).
The main issues to consider in this process:
- Ground properties and their capacity to be drilled
- Method for the drilling of holes or pipe installation
- 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
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 pressure build-up in the ground, and hence lead to a risk of hydrofracturing or undesired penetration of grout far away from the location of the injection. There however may be good reason for low pressure thresholds in low cover urban tunnel situations
Barton et al (2004) demonstrates that injection pressures measured at the injection lance (at the collar of the drill hole) do not correspond to the pressure of the grout in the actual ground. There is a significant drop of pressure in the immediate vicinity of the drill hole into the ground, as long as there is a flow of grout.
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 (pre-defined or subject to adjustment). These decisions are made at each step of the injection cycle. Therefore, pre-injection is a type of work which requires experienced hands-on management on a continuous basis during the works.
State-of-the-art Grouts for Penetration in soils and Fine Joints in Rock
Injection in difficult ground requires the complete method to be especially adapted in order to achieve penetration in the ground as desired. A very 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 a difficult parameter to measure or verify directly. Penetrability describes the ability of a grout to penetrate 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 the following three measurable material properties:
- Grain size distribution (if the grout is a granular medium)
- Viscosity of the grout
- Stability of the grout (resistance to separation over time or when exposed to pressure)
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 injection in soils the following indications have been given by Karol [2.1]:
k = 10-6 or less
k = 10-5 to 10-6
groutable with difficulty in grouts under 5 cP viscosity
k = 10-3 to 10-5
groutable by low-viscosity grouts but with difficulty when viscosity is more than 10 cP
k = 10-1 to 10-3
groutable with all commonly used chemical grouts
k = 10-1 or more
groutable by suspended solids grouts
Rapid hardening microfine cements
These microfine 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
- Excellent penetrability due to the two above mentioned issues
- Lower setting, hence eliminating waiting time for the next step in the injection cycle
This 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 cP, which is slightly higher than water. This offers particularly good penetration properties, which otherwise only chemical agents like silicates (water glass) or acrylates can offer.
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 and designed for permanent long-term purposes, whereas silicates only can have a temporary function.
The penetrability of colloidal silica in jointed rock and soils is illustrated graphically in figure 2.
Note the size of a grain of colloidal silica to the far left. In a soil injection situation, colloidal silica can offer permeation (D) in soils down to the coarse silt fraction (0.01 mm) with the proper injection method.
Case Example A: TBM Breakthrough into a Ventilation Shaft, CTRL Contract 220 Corsica Street, UK, 2002
The construction of the Channel Tunnel Rail Link (CTRL) in the UK, contract 220, involved several difficult situations with complex geometries of tunnels, cross-passages and ventilation shafts which had to be excavated in sandy silts. Mostly these situations were dealt with by dewatering the sand through vacuum drainage.
At the ventilation shaft at Corsica St a shielded TBM was to break through into the already constructed shaft. The ground conditions were loose sands and silts below ground water table. Grain size tests of the sands showed that the 80% of the grains were in the range of 0.1 to 1 mm, with approximately 10% uniformly distributed on each side of this range.
In this case it was evident prior to the construction, that the dewatering of the sands was not sufficient for achieving the required stability of the sands for a safe and controlled breakthrough of the advancing TBM into the shaft. An injection campaign for stabilization of the loose sands was therefore necessary.
The technical solution
The main contractor undertook a full scale in-situ trial with injections of liquid colloidal silica into the sands in order to determine the possible achievable effect of such an injection campaign.
The trial injections were done in the main shaft floor during construction using sleeve port pipes (tube a manchettes). The trial showed thorough permeation of colloidal silica to radial distances of 0.5 to 0.7 m around the sleeved pipes. A significant improvement of the mechanical strength was observed, however no measurements of the UCS were done at this stage. The results were convincing and an injection campaign with colloidal silica was decided.
During construction, a fan of sleeved pipes was drilled from the shaft into the sands at the location of the breakthrough of the TBM. The fan of pipes was laid out in order to cover the ground around the whole perimeter of the TBM, as well as completely covering the intersection between the shaft wall and the TBM excavated tunnel. The configuration of this situation is shown in figure 3 above. Holes with tubes-a-manchette pipes with lengths 4-6 m and 1.2 – 1.5 m spacing were drilled covering the entire perimeter of the advancing TBM. A total of approximately 60 tons colloidal silica was injected.
Case Example B: Maneri Bhali Hydropower Project, Headrace Tunnel, India 2005-2006
The headrace tunnel for the Maneri Bhali hydroelectric power project phase 2 (Uttaranchal Province) 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. During the last phase of the construction wet-mix steel fiber reinforced sprayed concrete was also used for rock support in the most adverse rock conditions.
The valley corresponds to a weakness zone which intersected approximately 300 m length of the tunnel. The weakness zone exhibited densely jointed and partially crushed mica-quartzite schist.
In-situ fine grained crushed material 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 under passed with serious difficulty several years earlier. No pre-injections were carried out at this stage. Hence, large water inflows in combination with a very irregular tunnel contour resulting from cave-ins were the result. In this phase, the earlier excavated tunnel portion through the weakness zone was bypassed, in order to create feasible conditions for the support and waterproofing of the headrace tunnel.
The highly jointed and crushed rock combined with the low rock cover of only 20-25 m at the shallowest imposed a critical point in the headrace tunnel. The maximum water 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.
A technical solution with the following main goals had to be laid out:
- Stabilization and permeability reduction 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
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 feature of the injection method was to inject through grouted steel pipes with a length of 2.5 m. The high degree of jointing and crushing of the rock mass severely limited the drilling operation. Each of the steel pipes was therefore used for repeated drilling and injection. The first drilling and injection step through the steel pipes reached 6 m in front of the tunnel face. The second step reached 8-11 m, and the third and final step reached 13 m in front of the tunnel face. The first stage consisted of injection of rapid hardening microfine cement. The rapid hardening of this cement allowed for a continuous operation with drilling, injections and the subsequent re-drilling to larger 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. In this way the injection of the low viscous grout would entirely take place where the microfine cement already had been injected, thus only utilizing the colloidal silica for the fine joints.
Pre-injection in underground construction is an efficient and cost-effective tool to avoid serious delays and costly mishaps. In urban situations, with shallow locations with difficult rock conditions and soils the state-of-the-art technology within rapid hardening microfine cements and colloidal silica offers certain advantages in resolving potentially difficult water ingress situations as well as providing effective ground improvement.
Barton, N and Quadros, E., 2004. “Improved understanding of high-pressure grouting effects for tunnels in hard rock” ISRM 2003 – Technology road map for rock mechanics, South African Institute for Mining and Metallurgy.
Garshol, K., 2002. “Pre-injections in Tunneling – a Useful Measure.” Christian Veder Kolloquium, Injektionen in Boden und Fels, Technical University Graz, (German).
Holter, K.G., Johansen, E.D. and Hægrenæs, A, 1996. “Tunneling through a Sand zone: Ground Treatment Experiences from the Bjoröy Subsea Road Tunnel”. ITA World Tunnel Congress/North American Tunneling, Washington D.C.