Underground Space – the 4th Dimension of Metropolises – Barták, Hrdina, Romancov & Zlámal (eds)© 2007 Taylor & Francis Group, London, ISBN 978-0-415-40807-3

Cost-effective pre-injection with rapid hardening microcement andcolloidal silica for water ingress reduction and stabilisation ofadverse conditions in a headrace tunnel

A.K. BahadurUttaranchal Water and Power Department, Dehradoun, India

K.G. HolterBASF UGC Europe, Zürich, Switzerland

A. PengellyBASF Construction Chemicals, Delhi, India

ABSTRACT: During the drill-and-blast excavation of the headrace tunnel, difficult rock conditions wereencountered in a zone caused by a regional fault. Tunnelling through the zone was initially done without pre-treatment of the ground, resulting in severe difficulties and a lengthy delay. Therefore it was decided to excavatea bypass tunnel utilizing pre-excavation grouting from the tunnel face. A two-stage pre-injection scheme waslaid out in order to achieve the desired penetration of grout into the ground. First stage grouting was done withrapid hardening microcement, in which a limited grout take was achieved. The second stage of grouting wasdone with liquid colloidal silica, thus achieving penetration into the finest discontinuities. The result was anacceptably fast advance, excellent ground stability and virtually dry ground conditions. This enabled the safeinstallation of rock support with steel sets combined with sprayed concrete.

1 BACKGROUND

1.1 The project

During the drill-and-blast excavation of the 16 kmlong headrace tunnel of 4 × 76 MW Maneri BhaliHydro Electric Project, difficult ground conditionswere encountered in the stretch passing below astream. The ground cover mostly consists of river bornmaterial (RBM).

Tunneling through this section was initially donewithout any pre-treatment of the ground, resulting inthe puncturing of the small rock cover over the tun-nel crown, causing severe leakages from the watersaturated RBM zone and chimney formations.The dif-ficulties caused lengthy delays, so much so that a 56 mlong problem zone in the tunnel took nine months toexcavate.

Moreover, due to problems faced in negotiating thissitaution, the tunnel could not be excavated to fullrequired cross section. As a result the designers ofthe tunnel lining advised the reduction of the tunnelsection and the installation of steel lining.

The work on the project was stopped in earlynineties. When it was resumed in 2002, the projectmanagement did not feel comfortable with the idea

of having a reduced tunnel section with steel liningin this portion due to the of extra time required forthe steel lining, as well as the increased friction lossesassociated with this arrangement.

1.2 Geological conditions

The challenge in this situation was to ‘again’ crossthis fault zone in the bypass tunnel, without groundcollapse and in dry conditions, thereby allowing lin-ing construction in dry and stable conditions. Therock mass consisted of heavily jointed quartzites andmetabasics on each side of a regional fault zone.

A river valley at surface followed the fault zonealignment with subsequent thick deposits of riverborn materials over lying the quartzite and metaba-sics. The fault zone exhibited highly crushed materialwith associated high water seepages that resulted insignificantly reduced stability of the excavated rock-mass. Joint fillings consisted of fine grained quartzitematerial of clay and silt fractions.

1.3 The problem

During the initial phase of the project, the tunnel wasexcavated through this zone without any pre-treatment

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Figure 1. Longitudinal geological section with the tun-nel alignment passing under the weak zone. Note that therock overburden is indicated as uncertain. The vertical linesindicate exploratory holes.

of rock mass. Large water ingress and cave-ins wereexperienced. A very irregular tunnel contour withless than required excavation diameter was the result.The tunnel was supported with steel sets and bak-fill concrete. The tunnel has suffered from majorseepages in this section since the excavation in theeighties.

After the work was restarted in 2002, it was decidedto reconsider the design of the tunnel lining in thissection, with a intent of reducing the lining construc-tion time and friction losses. The first option was tore-excavate this part of tunnel to the fully requiredcross section. This was not implemented since thiswould have caused stoppage of all excavation/liningactivities further downstream.

Moreover, with the rate of water seepage beingobserved it was considered uncertain if a structurallysufficient support of the tunnel could be achievedin the already excavated tunnel. For these reasons, itwas decided to excavate a by-pass tunnel around theproblem area. Pre-treatment ahead of the excavationwas decided in order to improve the properties of theground as well reduce water ingress to a minimum.

The by-pass tunnel was located at approximately25 meters distance running parallel to old Head RaceTunnel alignment. Limited relief in the form of alreadyhappening seepage in the original Head Race Tunnelalignment was anticipated, though it could not be con-sidered adequate to create desirable dry excavationconditions.

When excavating through the zones, the exploratoryholes drilled ahead of the tunnel phase always showeda significant amount of water seepage. Hence, apre-injection scheme was considered necessary, toconsolidate the ground and reduce the seepage to aminimum.

2 PRE-INJECTION SCHEME

2.1 Required results of the pre-injections

The main goal of the pre-injection works was toachieve penetration of the ground with a grout witha significant mechanical strength. The extent of thepenetration had to be sufficient in order to reduce thewater seepage to a virtually dry rock mass.

The densely fractured competent quartzitic schistwhich was the rock mass to be encountered, wouldrequire an improvement of the joint characteristics inorder to achieve a general improvement of the rockmass. General weathering of the rock mass or voidsin the rock were not encountered. Hence, the pre-injection scheme was a pure issue of achieving goodpenetration into fine joints with fillings of in-situcrushed rock material.

2.2 Pre-injection strategy

The first attempts with pre-injections with ordinaryPortland cement (OPC) were not convincing. Penetra-tion of the ground was not achieved. In most cases, oneonly achieved filling of the drill holes with the grout. Itwas therefore obvious that one had to employ a moresophisticated injection system, including a well laidout method in order to achieve the desired result.The strategy for the injection works consisted of thefollowing main elements:

a) Injection with pressure up to 60 bars to achievesufficient penetration

b) Injection rounds up to 15 m ahead of the tunnel face,covering the full tunnel circumference

c) A method for the establishment of drill holes todesired length in the fractured rock

d) Injection materials which would penetrate into therock mass to the necessary extent to provide thedesired improvement result

e) A second stage of injections (if necessary) with lowviscous grout

f) A simple method to verify the achieved result of theinjections, and hence, decide further advance of thetunnel face

2.3 Method considerations

After the initial attempts with injection with OPC aswell as microcements, it was obvious that a two-stageinjection scheme would be required. Basically thisconsisted of a first stage injection with microcementswith a limit to the injection pressure.

Following the first stage of microcement injections,one drilled a few holes ahead of the tunnel face to verifythe achieved water seepage reduction and improvedproperties of the rock mass (by drillability). In the casethe result were unsatisfactory, one would decide thesecond stage injection.

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Figure 2. Principal sketch showing coverage of the twoinjection stages ahead of the tunnel face. The dark areaindicates the coverage of the first stage with microcementinjections. The lighter area shows the second stage withcolloidal silica injections.

The second stage of injections was designed as aninner injection fan which was entirely covered by therock mass volume treated under the first stage of injec-tions. Figure 2 shows the layout of the two stages ofinjections.

With this method it was possible to advance the tun-nel face approximately 7–8 m before a new injectioncycle had to be undertaken. This provided a ‘safe’ vol-ume of treated ground of minimum 3–4 m ahead ofthe tunnel face which had been injected during bothinjection stages.

The jointing of the rock mass imposed a limit tothe feasible drilling length with normal percussivedrilling. In untreated rock the holes would partially col-lapse and create difficulties with the retracting of thedrilling rods. It was also difficult to achieve good andtight placements of the injection packers in open drillholes under such high extent of jointing and unstablerock.

For this reason it was decided to establish the drill-holes through grouted steel pipes. Furthermore, thedrilling and injection through the steel pipes was donein advancing steps with lengths of approximately 3 m.Hence, one would re-drill and inject through the samesteel pipe several times, advancing longer for eachtime. In this way one would be re-drilling through astabilized length and hence, one could drill further intoun-treated rock.This method allows making long holesbut limiting the drilling length in untreated ground tosections of 2–5 m.

This particular method was applied for injectionsin adverse ground with rapid hardening microcementsduring the construction of the Bjoröy subsea road tun-nel in Norway in 1994. A controlled improvementof the ground was achieved and safe drill-and-blast

Figure 3. Sketch showing the main steps in the method ofestablishing drillholes through steel pipes for injection in dif-ficult rock. Repeated drilling and stepwise advance throughgrouted steel pipes.

excavation through the zone could be realized. (Holteret al. 1995, Holter et al. 1996)

The method of stepwise advancing repeated drillingand injection through grouted steel pipes was oneof the key issues to achieve any success with pre-injections under these ground conditions. This methodis therefore explained in detail below. Figure 3 belowillustrates the method graphically.

– Step 1: Drilling of 65–70 mm diameter hole to 3 mlength

– Step 2: Installation of a steel pipe with internaldiameter 50 mm (or diameter to fit the expandablepackers)

– Step 3: Placement of packer (diameter 48 mm) atthe very end of the steel pipe and injection of a stiff(very viscous) cement based grout which fills theannular space between the rock and the steel pipe.Hardening for approximately 12 hours

– Step 4: When the grout is hardened, drilling throughthe steel pipe to feasible length (approximately 3 mbeyond the steel pipe)

– Step 5: Placement of packer and pressure injec-tion with microcement grout for penetration intothe rock mass in the drilled length of the hole.

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Termination criteria for the injection set to 60 barsinjection pressure, or 300 kg injected cement permeter drillhole

– Step 6: After hardening of the injected grout,re-drilling through the pipe and injected area toapproximately 3 m beyond last drilled length

– Step 7: Placement of the packer in the pipe and inject(repetition of step 5)

The length of the drilling steps was adjusted to theencountered rock conditions.

Normally it was possible to advance the drilling to adepth 4–5 m beyond the preceding injection depth.Thesame method was used when injecting the liquid col-loidal silica. However, drilling lengths were normally5–6 m in untreated rock due to the improvement effectin the rock of the preceding microcement injections.

2.4 Injection materials

Bearing in mind the relatively low rock cover and thepoor ground, it was necessary to inject with mod-erate pressures. In order to achieve penetration ofthe different joint features with moderate injectionpressures, one recognized the need for two differentgrouts. One grout was designed to penetrate into thelargest fissures and the other grout type to penetrateinto the fine joint features which were partially filledwith the in-situ crushed rock material (silt and clayfractions).

2.4.1 Microcement groutIn order to achieve the best possible penetration inthe first stage of injection, a microcement with par-ticularly good penetrability properties was chosen.Furthermore the chosen microcement had rapid hard-ening properties. Initial set of the grout mix took placeafter approximately one hour with water/cement ratio1:1 at 20◦C (typical temperature in the tunnel). Thegrain size characteristics of the microcement was ad95 of 16 µm.

The standard utilized mix design of the microce-ment grout had a water/cement ratio 1:1 with thedosage of a special dispersing agent. This gave anexceptionally good stability combined with low vis-cosity of the grout (Marsh cone time 32 sec) which wasconsidered essential for the penetrability properties.The rapid hardening properties of the microcementgrout enabled the continuous work without any delaybetween the different steps in the injection operation.For the microcement a maximum injection pressure of60 bars was experimentally established, based on whenhydrofracturing with a grout would with water/cementratio 1:1 would occur. Furthermore, at 60 bars injectionpressure with this mix a relatively limited grout takewas experienced. Hence, a risk of a significant pres-sure build-up in the ground fed by an injected volumeof grout could be neglected.

2.4.2 Colloidal silica groutFor the second stage injection an injection gel basedon liquid colloid silica was chosen. The reason for thiswas the penetrability properties that this gel offered.The chosen gel had a viscosity of 5 mPas in liquid state(during injection) and a particle size of 0.016 µm. Thisallowed for extremely good penetration under difficultground conditions.

The workability properties of this grout were excep-tionally favourable under these conditions. One couldeasily utilize the same injection equipment for themicrocement and the colloidal silica grout, a cementinjection plant with a high pressure single componentplunger pump.

The open time of the colloidal silica grout could beeasily and precisely controlled from approximately 10minutes up to 2 ½ hours.

Colloidal silica is a pure mineral grout. The gellingprocess takes place by a physical reaction between par-ticles of silica (SiO2). The accelerator for this grout isa solution of sodium chloride. Hence, this grout is veryuser and environmentally friendly, imposing no healthrisks due to chemical reactivity or toxicity during theinjection works.

Furthermore, colloidal silica offers long term chem-ical stability and durability (resistance to washout orleaching). For this reason colloidal silica was preferredto silicate (waterglass) based gels.

3 ACHIEVED RESULTS

During injection it was necessary to limit the max-imum injection pressure to approximately 60 bars. Thereasons for this were occurrence of backflow of groutinto the tunnel and sliding of the packers in the steelpipes.

The first stage injections with rapid hardeningmicrocement showed a relatively limited grout takeof only 100–150 kg per m drillhole when the termina-tion pressure of 60 bars was reached. Bearing in mindthe seepage which was encountered in the drillholesone would expect a higher grout take.

The reason for the relatively low grout take wasjoint fillings which consisted of the silt and clay parti-cles, which in turn limited the penetration of the groutcreated by filtration.

The first stage injection fan with microcement wasalways completed in the full circumference of the tun-nel before the secondary fan was attempted.The reasonfor this was that the first stage injection providedpenetration of grout into the joints with the largestapertures. Hence, the second injection stage with anextremely low viscous grout could be targeted for thefiner joints and the joints which were partially filledwith clay and silt.

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Figure 4. A typical situation during ongoing injec-tion works. The steel pipes for injection are seen along theperimeter of the tunnel face.

Figure 5. Tunnel under excavation through successfullytreated ground in the weakness zone. Litterally dry and stableconditions at the tunnel face, as well as a favorable shape ofthe tunnel contour have been achieved.

The secondary fan was drilled and injected withliquid colloidal silica with a termination pressureof 25 bars, or approximately 100 kg per m drillholelength.

Injection beyond a pressure of 25 bars usuallyshowed signs of hydrofracturing. The control of theachieved result was done in two ways. Firstly, the waterseepage situation after the injection of the two stageswas controlled in drillholes. Secondly, the result wasobserved in the tunnel contour after the excavation ofthe first round starting from the injection location. Inthis way the detailed criteria for termination of theinjection were fine tuned and continuously adjusted.

The result was a literally dry and stable tunnelcontour. No excessive breakouts of rock or cave-ins occurred during excavation through the weaknesszone. (Figure 5).

4 CONCLUDING REMARKS

This proactive approach to pre-grouting fault zonesahead of the excavation face resulted in the tunnelbeing excavated without a single incidence of collapseand with virtually dry conditions. The effects could bedirectly compared to the poor conditions and heavywater flows observed in the adjacent tunnel where pre-injections were not used. A direct comparison of theeffectiveness of microfine cements and colloidal silicagels versus OPC was also clearly evident.

This exercise also showed that a proactive approachto drilling and grouting with a clearly defined methodstatement and utilizing materials with the proper pen-etration, viscosity and rapid hardening characteristicscan significantly reduce the construction time throughsuch highly faulted and unstable zones (6 months inby-pass tunnel versus 18 months in original tunnel).The low cost of pre-injection in comparison with post-injection techniques can also be clearly seen. The costfor materials on this project came to a material costof approximately Euro 1,200/m versus a similar con-dition in another tunnel in India where polyurethanepost-injections were utilized at a cost of approximatelyEuro 12,000/m and currently almost 2 years behindschedule.

The post-injection then cost about 10 times morejust in materials. Considering that materials are mostlyless than 10% of the total cost, post-injection as amethod may easily cost 20 to 30 times more thanpre-excavation injection. (Stenstad, 1998)

REFERENCES

Stenstad, O. 1998. Execution of injection works. (inNorwegian), Proceedings of Post Graduate TrainingCourse sponsored by the Norwegian Chartered EngineerAssociation and the Norwegian Rock Mechanics Group1998. Fagernes. Norway

Holter, K.G., Johansen, E.D. & Hægrenæs, A. 1996. Tun-nelling through a Sandzone. Experiences with GroundImprovement from the Bjoröy Subsea. North AmericanTunnelling,WorldTunnelling Conference,Washington D.C1996. Rotterdam. Balkema

Holter, K.G., Aagaard, B. & Johansen, E.D. 1995.Geomechanical Considerations and Methods for GroundImprovement during Tunnelling through a Sandzone. TheBjoröy Subsea Road Tunnel. (in Norwegian) Norwegianrock Mechanics Conference 1995 (Norwegian) Oslo.Tapir

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