Underground Space Use: Analysis of the Past and Lessons for the Future – Erdem & Solak (eds)© 2005 Taylor & Francis Group, London, ISBN 04 1537 452 9

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1 INTRODUCTION

1.1 Hukou tunnel north

The Hukou tunnel is part of the 345 km Taiwan HighSpeed Railway, a US$ 15 billion project, under BOTmodel, running from Taiwan’s capital, Taipei, to the second major city, Kaoshiung located in the southof Taiwan. Operational services are scheduled forOctober 2005.

Civil works have been constructed by 12 majorJoint Ventures, for a total of 39 km of mined tunnels,8 km of Cut & Cover tunnels, 251 km of viaduct andbridges, and 31 km of Cut & Fill embankments.

For tunnels, the necessity of double track requiredthe excavation section of approximately 110�120 m2,in order to guaranty a finished tunnel cross section ofminima of 90 m2 in accordance to aerodynamic require-ments associated with the high-speed train velocity of300 km/h.

NATM has been the main excavation methodology.The Hukou tunnel, with a total length of 4.3 km, is

the third longest tunnel, after the Paghuashan tunnel(7.4 km) and the Linkou tunnel (6.4 km), and is con-sidered as one of the most difficult tunnels on the

project. It includes 2 Emergency exits (Adit A and B).This paper will review the collapse that occurred inthe Northern section of Hukou tunnel.

1.2 Geology of Hukou tunnel north

The Hukou Tunnel alignment cuts through the Hukouterrace in unconsolidated, un-lithified sedimentarymaterials, dated as Pleistocene.

The Tientzuhu Formation, mostly encountered atNorth portal section, consists of a poorly graded gravelbed and with sandy and silty lenses, ranging from afew meters to 30 m in thickness.

The Yangmei formation is mainly composed ofalternating silty clay and silty sand, with an estimatedthickness of 900 m and underlies the whole area tra-versed by Hukou tunnel.

Three major fault structures were encountered dur-ing the excavation of the northern Tunnel section andthree collapses have occurred within the vicinities of these three geological structures. The three faultsarranged according to the sequence encountered dur-ing the excavation are (1) Adit-A, (2) Yanghsiwo, and(3) S-1032 faults.

High pressure jet grouting in tunnels – a case study

A. Da Vià1, M. Marotta1 & G. Peach2

1ITAI Engineering & Construction Co.2 Independent Tunnelling Consultant

ABSTRACT: Using the authors experience gained on design and construction management on one particu-larly difficult NATM tunnel located in Taiwan this paper reviews by way of two collapses how high pressure jetgrouting (400 bars) was successful and efficient implemented, with benefits for both client and contractor. Thepaper will discuss two collapses case studies. These collapses occurred in the Top Heading of a 120 m2 tunnelbeing excavated by NATM. The first collapse was 90 m and the second collapse was 290 m in length. In bothcases the tunnel were completely blocked, and the collapse extended up to the surface. This paper will detailhow high pressure jet grouting was decided upon as the preferred recovery methodology and the managementprocess that was adopted in order to fast tract the jet grouting design. The details of the surface trials will becovered and the configuration of the vertical and horizontal jet grouted columns will be explained in detail. Theplant requirements will be covered including the additional plant required and the modifications that were car-ried out to the existing site plant. This was all carried out within an extremely tight construction programme.This paper will explain the programme arrangements planned and then in detail discuss the as built performancegiving the details of times for design, mobilization, cycle time per canopy, along with all the time performancedata for the individual jet grouting elements employed with the jet grouting option. The instrumentationrequirements will be explained and the field data recovered with be discussed and lesson learnt covered.

1.3 Hydro geological conditions

Hydro geological conditions along the Hukou TunnelNorth alignment generally were determined by geo-logical structures. The Yanghsiwo Fault in the northand S-0132 fault in the south play the role of imper-meable barriers for inclined water-bearing sandy lay-ers and divide the area in to three main zones:

First and third zone initially had low water level pro-viding dry conditions to the excavation while in thecentral portion the original groundwater level was closeto the ground surface and several aquifers with differ-ent hydraulic heads were crossed by tunnel excavation.

A total of 224 deep wells from the surface, withdiameters of 300–350 mm, drilled to a depth 20–25 mbelow the tunnel invert with a 3–5 hp pump, were con-structed from both sides of tunnel alignment at an off-set of 10–15 m from centerline.

Total pumped water quantity reached the volumeof 4,718,743 m3 by June 2004.

Distance between wells varies from 25 m to 15 mfor these areas, with high critically permeability andwater accumulation due to fault barrier, located at theborders of the Zone II.

1.4 Possible effects of dewatering on collapses

The Zone II mainly consists of alternate sandy layers(aquifer) and less permeable beds, but permeableenough to transmit water, similar to silty-clay layers(aquitard). Prior to any dewatering, an internal equi-librium between the “wet” and “dry” side of each faultwas in place. Dewatering causes a compression of the aquifer system, where the support previously pro-vided by the pore fluid is transferred to the granularstructure.

When the load of the granular structure remains lessthen any previous maximum load (pre-consolidationstress), dewatering creates only a small elastic defor-mation of the aquifer system with a small displace-ment at the surface.

Long term pumping raises the stress on the aquitard’sbeyond the maximum pre-consolidation stress, theaquitard compact in an irreversible mode.

The dewatering was carried out only on the “wet”side of each fault for a period long enough to cause a

migration of the internal stress on the aquitard’s. Asmore fluid is squeezed from the interior of the aquitard,larger internal stress propagates into the aquitard, untila point where the internal stress exceeds the pre-consolidation stress and the compressibility dramati-cally increases.

This could have caused an interruption of the inter-nal stress equilibrium between the wet and dry side ofeach faults.

For the South Collapse area, deformation monitor-ing carried out during construction did not show anysignificant alarm values until a few hours before thecollapse occurred. This could be because of the natureof the clayey soil surrounding the tunnel being able tosuddenly release the stress.

2 COLLAPSES

2.1 Introduction

During the construction of the Hukou tunnel, three col-lapses occurred.

A first minor collapse happened during the exca-vation of the Adit A (emergency exit tunnel), whileapproaching the main tunnel alignment.

The major two collapses (named North and Southin relation to their position from the Adit) both occurrednearby the two previously described faults.

2.1.1 North collapseNorth collapses occurred at chainage 64 � 520 on2nd Aug. 2002 (nearby the southern “wet” side of theYanghsiwo fault) when top heading had already holedthrough, and during bench excavation with perma-nent invert concrete just 10 m behind and with tempo-rary ring closure of the heading in front assured by astructural temporary invert shotcrete. During collapsethe ground surfaces (overburden 60 m) sank and acrater, with diameter of 16 m by 9 m depth, was clearlyvisible.

The collapsed area covered a total length of approx-imately 90 m with a total volume of collapsed soilestimated as 7000 m3. Visible cracks occurred in innerlining, showing a total collapse-affected area of approx-imately 180 m.

2.1.2 South collapseSouth collapse started on 31st October 2002 whilst topheading excavation was ongoing, with cracking andstripping of shotcrete, nearly 50 m behind the face.Collapse itself started at chainage 66 � 012, approx-imately where the S-1032 fault intersect the tunnel atthe East side around 80 m behind the face, and devel-oped by a domino effect until the 3rd November 2002,with a total of 14.000 m3 of material entering the

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Table 1. Homogeneous hydro geological zones.

Zone limits

Zones From To Remarks

Zone I 64 � 200 64 � 402 Low water tableZone II 64 � 402 66 � 000 High water tableZone III 66 � 000 66 � 200 Low water table

tunnel. Emergency measures including reinforcingrings, timber props and an air mortar backfilled bulk-head stopped the advance of the collapse some 271 mfrom the original tunnel face.

Affected area was however judged as far as 350 mbeyond the collapsed area, as settlements of 40 mmwere measured in the area between the already exe-cuted invert concrete and the top heading. Installationof additional rings and timber supports was performedimmediately, stopping the ongoing deformations.

2.2 Loss mitigation work

For both collapses a similar approach was taken inorder to stop further failures, consisting in the back-filling of surface craters, in the installation of addi-tional support rings and in the strengthening by meansof cement grout through self-drilling bolts, in theaffected areas.

2.3 Preliminary void filling

Initially for the north collapse, pressure grouting fromsurface was planned in order to fill voids and forimprove soil characteristics in the surrounding of thearea to be re-excavated.

However, the result of the site investigation showeda limited efficiency of this methodology for the actualsite conditions, caused by leaking from inside thetunnel and by the impossibility to recover consolidatesand samples. Therefore initial plan was reduced andgrouting from surface considered for void filling only;same principle was applied to second south collapse.

2.4 Redesign of supports

2.4.1 North collapseRedesign finalized two support classes, for re-profiling(RD-2) and for re-excavation in full collapsed area(RD-1 ref. Figure 1)

New designed support classes, differs from theoriginals by the use of H beam instead of lattice gird-ers, the increased thickness of inner lining, the use ofhigh strength shotcrete, the removal of rock boltingand the introduction of jet grouting technology asmain consolidation/support element.

2.4.2 South collapseNew designed supports for south collapse applied thesame principles already adopted for the north col-lapse redesign, but dimensioned bigger due to, morecritical collapse distance and higher overburden. Sixto nine meters rock bolts were included on the tunnelshoulders.

For both collapses the reinstatement of dewateringsystem by wells was mandatory prior to re-excavationcommencement.

2.5 Introduction to jet grouting methodology

Considering the nature of a collapsed area, and theresult of the site investigation, it was necessary tochoose and design a consolidation system able to:

– Guaranty a certain geometrical accuracy of thetreatment area.

– Fit different ground conditions– Avoid impact on the dewatering system– Be easily estimated in term of execution time,

material consumption and costs.– Use existing plant wherever possible.– Be verified in field trials before implementation.– Be robust and flexible.

High pressure jet grouting was judged as the mostfeasible technology able to fit the listed requirements.

In the jet grouting methodology, first a borehole,drilled up to the desired depth, is carried out with arotary drilling machine with the end element of thedrilling rod provided of both cutting tools and nozzlefor injection.

At the completion of the drilling the treatment isthen executed by gradual withdrawal of the rod withcontrolled step time, and at same time, by the rotationof the rod and the attached head with nozzle that radi-aly jets the cement slurry in to the soil with highenergy and impact (400 bar), mixing it with injected

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CONTOUR & FACECONSOLIDATION BY JET GROUTING

PRE EXCAVATIONFOOT CONSOLIDATIONFROM SURFACE

Figure 1. RD1 jet grouting consolidation scheme.

material and creating a solid cement column. The vol-ume of treated soil can vary in accordance to the speedof rotation, the number and diameter of the nozzles,and the pressure of injection. Those parameters aredefined at first by site trials.

Jet grouting effectiveness changes in accordancewith the soil characteristics. Its efficiency increasesfrom cohesive to un-cohesive soils (typically the cir-cumstances occurring in a collapsed area).

Therefore the jet-grouting technique is more effi-cient exactly where is more necessary to execute soilimprovement.

It should be noted that execution of jet grouting didnot affect the dewatering system during the two col-lapses reconstruction work.

2.5.1 Details of arch canopy and faceconsolidation

The redesign proposed a ground treatment by meansa reinforced jet-grouting canopy, able to create an arti-ficial arch to contain the area to be excavated.

The columns reinforcement by grouted steel pipesgives continuity to the structure in materials where jetgrouting is less efficient. The flexibility of this method-ology is mostly in creating a double structure: an archstructure guaranteed by the longitudinal co-penetrationof the columns, and a longitudinal beam structuregiven by the steel pipes.

Face consolidation by mean of horizontal jet grouting columns reinforced with fiberglass rods wasdesigned in order to limit a possible face extrusion andwith the purpose to reduce the pre-convergences aheadof the face and avoid further failures.

Fiberglass rod, as column reinforcement, providesthe same continuity as the steel pipe in the arch canopy;however the nature of fiberglass material (high tensilestrength and low shear strength) allows an easy cut-ting by mining equipment while excavation proceeds.

Design parameters are showed in Table 2 & 3.

2.5.2 Details of foot consolidationJet grouting arch canopy itself could act as a load ifpoor bearing capacity of the soil occurs at the top

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Table 2. Jet Grouting parameters for canopy and face consolidation.

Parameters Value Unit measure

Expected column diameter 600 mmPressure of injection 400 baFlow of injection 3–10 m3/hSpeed of raising rod 24–48 cm/minSpeed of rotation 8–15 RPMNozzle numbers 1 setNozzle diameter 2.2–3.8 mm

Table 3. Crown/face consolidation – North & SouthCollapses.

Class North South

Crown protection Jet grouting Jet groutingcanopyType Sub-horiz. Sub-horiz.Execution from Tunnel TunnelModule length (m) 5–7* 7Number of columns 48 48per moduleExpected column 600 600dia. (mm)Columns length (m) 15 15Overlap (m) 8–10* 8Overlap (numbers of) 2–3* 2

Face consolidation Jet grouting Jet groutingExecution from Tunnel TunnelExcavation module 10 7length (m)Number of columns 16–20 20–35per moduleExpected column 600 600dia. (mm)Columns length (m) 15 15Overlap (m) 5 8

* First and second values are related to Re-excavation (RD-1) and Re-profiling (RD-2) classes.

Table 4. Jet grouting parameters for vertical columns fromsurface.

Parameters Value Unit Measure

Expected column 2000 mmdiameterPressure of injection (air) 20–25 barPressure of injection 450 bar(cement)Flow of injection (air) 10–15 l/min(�1000)Flow of injection (cement) 150–190 l/minSpeed of raising rod 6–12 cm/minSpeed of rotation 10–15 RPMNozzle numbers 1 setNozzle diameter 2.2–3.8 mm

Table 5. Foot consolidation for North & South Collapses.

Class North South

Foot consolidation Jet grouting Jet groutingType Vertical VerticalExecution from Surface SurfaceExecution prior to Heading HeadingExpected column dia (mm) 2000 2000Column length (m) 8.00 10.00Columns spacing (m) 1.50 1.50

heading foundation. For this reason it was necessaryfor pre-excavation foot consolidation methodology tobe adopted this enables transfer of the load into depth.The full collapse area disturbed soil is below the topheading foot, sub-horizontal columns, executed fromtunnel below the heading, were not judged as suffi-cient and excluded because of the difficulty of drillingat the optimum angle. The time taken to drill thesesub-horizontal columns would greatly impact uponthe cycle/programme time.

Bi fluid technology was then applied in order toinstall large diameter vertical columns from the sur-face with sufficient bearing capacity. In each of thecollapses world record depths were achieved.

In the bi-fluid system, the shearing action is accom-plished by the high-pressure injection of cement slurrycontrolled by a ring of compressed air at approximately20 to 25 bars. It reduces the dispersion of the cementslurry, and thus increases the penetration action, permitting the execution of larger diameter columns(up to 2 m in diameter)

For the previously described jet grouting process itwas decided to execute the pre-drilling, the jetting rodinstallation, and the jet grouting operation with sepa-rate plant in order to speed up the cycle time.

The temporary filling of the pre-drilled holes, bybentonite-cement slurry, solved the problem of keep-ing their medium term integrity.

In additional the execution of a pre-drilling pro-vided a chance to check, by inclinometer instrumen-tation, the actual deflection from theoretical verticalaxis of each drilling. Actual average deviation on Northcollapse has been 0.29% corresponding to 17 cm at58 m depths, with a maximum deviation of 0.74%,within the required specifications (less then 1%).

The section applied for south collapse re-excavationconsisted in a double canopy jet grouting, 15 m in

length for 7 m excavation. The layout, including verti-cal column from surface, differs from the one appliedfor north collapse mainly for the elevation of the ver-tical columns, extended at an elevation of 2 m higherthen top heading foot and executed with a 90 m aver-age overburden, this protected the sides of headingduring excavation. A higher number of face jet grout-ing was necessary in order to stabilize the face. Anaverage number of 36 (with a maximum of 43) fiberglass reinforced columns, 15 m in length, were per-formed each 7 m-excavation module, with columnspattern studied time by time in accordance to the actualface observation.

3 DEFORMATION MONITORING

Monitoring was executed with extreme attention dur-ing the entire project.

Settlement and convergence were measured by highaccuracy optical survey, with at least twice daily nearthe areas affected by construction activities (excava-tion, grouting, etc.), including surface settlement.

In additional, geotechnical instrumentation frominside the tunnel (pressure cells, Steel ribs and shotcretestrain gauges, rock bolt axial force meter), and fromsurface (extensometers, inclinometers), was locallyutilized in accordance to special conditions, as duringcollapses re-excavation.

A comparison between maximum-recorded defor-mations is shown in Table 6.

The tunnel deformations behavior, and as a conse-quence allowable limits before failures, was variablein similar overburdens in accordance with the geolog-ical conditions. Settlement increased for top heading,bench, or invert in accordance to the bearing capacityof their foundation, causing rotation in case of non-homogeneous material in the tunnel sides. The allow-able deformation before failure is not only dependingon those factors, since the south collapse occurred inan area with low value deformations and with goodgeological conditions able to support an excavationrate that was for the previous 4 months over 5 m/day,including arch protection by pipe roofing.

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Figure 2. 3D model – RD1 Jet grouting 5 m modules.

Table 6. Maximum settlement and convergence.

Max Settl. Max Conv.Area Stage (mm) (mm)

North collapse before collapse 350 330North collapse re-excavation 60 40South collapse before collapse* �50 40South collapse re-excavation 80 80

* Related to top heading only where collapse started.

4 PLANT AND EQUIPMENT FOR JETGROUTING EXECUTION

In order to optimize the arrangement, for both collapsesa single plant was installed on the surface, for the exe-cution of jet grouting from either surface and insidetunnel. This was possible by the drilling of servicesholes from surface to the tunnel roof, from where high-pressure pipes, service pipes and cables were installed.

The surface arrangement frees the required spacein tunnel, therefore it could be applied also to smalltunnels, or where top heading is shortly followed byother excavation and/or concreting phases.

5 OVERALL PROGRAM

South collapse was the most critical in term of overallschedule.

When the collapse happened the completion of exca-vation was scheduled within 3 months.

At start of re-excavation the gained delay wasalready of over 5 months.

At completion of lining work the delay was reducedto 1fi month, and it was fully recovered by the trackwork execution.

Break trough of headings occurred after 358 days (north collapse) and 461 days (south collapse)respectively.

It’s therefore clear how the South collapse recon-struction, extending for a length 50% higher then thenorth one, was positively beneficed by the first col-lapse experience, in term of management, design (22months for North Collapse VS 11 months for Southcollapse), workmanship and plant.

Major technical improvements consisted in the useof a two booms drilling machine for the execution ofthe jet grouting from tunnel, in the use of an addi-tional guide lattice girder with pre-installed steel pipe through which jet grouting drilling could be exe-cuted without penetrating the shotcrete layer, and in asystematic void filling from surface through the jet

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Table 7. Equipment and plant.

Category Characteristics Set Location

Pre-drilling 1 SurfaceRod installation 1 SurfaceCrane 1 SurfaceDrilling/ jet grouting 2 booms 1 TunnelDrilling/ jet grouting 1 boom 1 TunnelHigh-pressure pump max 400 bars 3 SurfaceHigh-pressure pump max 700 bars 4 SurfaceSilos 2 SurfaceMixer plant 2 SurfaceAgitator 2 Surface

Figure 3. Collapse #2 surface plant arrangements.

Table 8. Chronology of North Collapse area (180 m).

ProgressDate Status days

2nd Aug 2002 Collapse 031st Aug 2003 End of emergency measures 291st Oct. 2002 Start of site investigation 60October 2002 Start re-design14th Jan 2003 Start excavation from S to N 1651st Mar. 2003 Start pre-drilling from surface 21125th Mar. 2003 Start re-profiling from N to S 23526th July 2003 Break trough of heading 358July 2003 End of re-design27th Sep. 2003 Break trough of bench 42110th Nov. 2003 Invert concrete completion 46511th Jan. 2004 Arch concrete completion 527

grouting pre-drilling holes, in order to create betterconditions for jet grouting execution.

6 CONCLUSIONS

6.1 General consideration

The great engineering and management experiencegained during the Hukou tunnel construction is agood example of fully recovery of very unfavorableand unexpected conditions.

With the occurrence of two major collapses and atight construction schedule in front, finally no delaywas caused to the further line development.

The use of the Jet grouting methodology was suc-cessfully implemented from management side to con-struction side using the experience of all the qualifiedpersonnel involved in the project.

Effectiveness of this consolidation methodologyhas been confirmed, proving it as applicable even toworst soil conditions.

The possibilities of utilize existing plant, withsome minor improvement, provides a fast mobiliza-tion time with benefit on the construction schedule.

A remarkable issue is that jet grouting didn’t affectthe dewatering pumping system, considering thesmall difference (�7 m) between the alignment of thepumping wells and the one of the vertical jet groutedcolumns.

For future projects, a special consideration shouldbe taken on effect of dewatering in particular geolog-ical conditions.

Effect of speed of excavation in relation to thedesigned type of supports should also be further analyzed.

With an optimum arrangement, the consolidationand excavation time for a 7 m module can be around 8 days, permitting an excavation progress rate greaterthe 26 m/month.

Use of jet grouting technology permits the devel-opment of standardized procedures, resulting in aquite constant excavation ratio, providing benefits onscheduling and cost estimations.

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Table 9. Chronology of South Collapse area (271 m).

ProgressDate Status days

31st Oct 2002 Collapse 06th Nov 2002 End of emergency measures 6December 2002 Start of re-design22nd Jan 2003 O.S.C.* heading completion 8331st Jan 2003 End of loss mitigation works 9215th Apr 2003 Start pre-drilling from surface 16631st May 2003 Start Jet Grouting from surface 21225th June 2003 Start heading excavation N to S 23719th July 2003 End of surface jet grouting 2619th Oct 2004 Start heading excavation S to N 343October 2004 End of re-design26th Dec 2003 Start bench excavation N to S 4212nd Feb 2004 O.S.C.* for invert concrete 4594th Feb 2004 Break trough of heading 4618th Mar 2004 O.S.C.* for arch lining 4949th Mar 2004 Break trough of bench 49510th Mar 2004 Break trough of invert 49631st Mar 2004 End of Invert Concreting 51727th Apr 2004 End of Arch lining concrete 544

* O.S.C. as Original Schedule date of Completion.