DESIGN AND CONSTRUCTION OF JACANAUNDERPASS

J. DEARAuGOCONNElL WAGNER

G. NasONVlcROADS

ABSTRACT: The paper describes aspects of design and construction of what is believedto be the largest example of installation of precast concrete units by hydnmlic jacking inAustralia. The two separate carriageways of the Jacana Underpass, consisting of 18 precastinverted U shaped units each, were successfully jacked up to 110 m through materialsconsisting of residual clays and boulders through to medium to slightly weathered basaltbeneath a busy arterial road, railway tracks and a suburban street without disruption tothese overhead services.

PAPER NO. 44

1.0 INTRODUCTION

Jacana Underpass is a 110m long underpass on Melbourne's Western Ring Road where itpasses beneath a transport corridor consisting of Pascoe Vale Road, a major urban arterialroad carrying approximately 40,000 vehicles per day, four train lines including thestandard gauge line to Sydney, the broad gauge line to Wodonga, and twin urbancommuter train lines, and Electric Street, a local residential street.

The Underpass consists of twin concrete units, each sufficiently wide for three trafficlanes. The units were cast in work areas to the east and west of their final locatio.n andjacked hydraulically into position along guide rails cast into footings constructed in theworks areas and also in pilot tunnels under the corridor. Typical dimensions are Shownin Figure 1. The units were cast in approximately 13m lengths and had a mass of around1200t each.

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Typical Cross Section F1pre 1

Whilst pipe jacking is not a new concept it is believed that these units are certainly thelargest placed in Australia by this method. Adoption of this installation method enabledthe grade separation of the Western Ring Road and the existing transport corridor to beconstructed without major disruption to the existing road, rail and pedestrian traffic.

A feature of the method adopted was the very low cover of 690 mm minimum betweenthe underpass roof and commuter and broad gauge rail tracks. The low cover wasnecessitated by gradeline considerations which required the Western Ring Road profile tobe as high as possible at Pascoe Vale Road.(Fig. 2). It was not economically feaSible tolift the levels of Pascoe Vale Road and the adjacent rail corridor. The resultant lowcover meant that a traditional tunnel solution was not feasible. Bridging options werealso investigated but the adopted solution had the advantages of eliminating majordisruption and providing flexibility for possible future track relocations.

The Western Ring Road approaches the underpass site in cut excavated into medium tohighly decomposed basalt of Quaternary origin. The ground is typified by large residualbasalt boulders and clays over a rock profile of variable weathering. The basalt boulderswere up to 600mm diameter while the underlying basalt varied from completely toslightly weathered.Joint spacings varied from extremely close « 20mm) to medium spaced (200mm to6OOmm). A significant sub-horizontal red clay layer existed at about foundation level inplaces and this had a major effect on the structural design of foundation systems.Figure 3 shows the general geological profile.

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Site Plan and Loqitudlaal Section

OFF RAMP TOPASCOE VALE ROAD

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(a) Construction of pilot tunnels shown in Figure 1.(b) Construction of permanent underpass foundations within the pilot tunnels

incorporating guide rails for later skidding of units into position.(c) Construction of casting bays to east and west of the pilot tunnel portals to support

newly cast units and provide a jacking and guidance system for later installation of

units.(d) Casting underpass units in approximately 13m sections.

The eoncept developed to overcome the geological and gradeline constraints involved the

following:

2.0 : JACKED UNDERPASS CONCEPT

(e) Attaching a part tunnelling shield to the leading units to permit safe nurnngoperations when removing ground ahead of the units and allow advancement ofunits by jacking.

(f) Complete installation of all units and joint closure. Addition of portal details afterfinal placement of units.

FigA shows construction details adopted by the Contractor (Candac).

TRAILING PLATCS MOVE wITHSTATIC PLATE FOREPOlES OVER UNIT 1

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PERMANENT FOUNOA nON ANDGUIDEWAY IN PilOT TUNNEL

Contractors Front Shield, Forepole & Jacking Arra.ngement F1gure 4

The casting bay details were indicated schematically in the tender documentation and leftto the Contractor to detail requirements. The Contractor elected to prestress thetemporary casting bay footings to the permanent footings to provide jacking resistance forunit installation.

Further modifications made by the Contractor included the use of a static sheet as afriction cutter along the tunnel roof. This replaced the schematic construction proposal ofcontinuous modified steel sheet piles jacked from the rear unit as •forepoles" to alsoprovide protection for mining. The static steel sheet was unrolled from spools attached tothe front of the first trailing unit and anchored at the underpass portal, to the Contractor'sdetail.

The tunnelling shield provided access for jacking of individual forepoles. Trailingfriction cutters of sheet steel extended over the leading unit and were fixed to the ends ofeach forepole.

The units of each carriageway were jacked entirely from one direction, with the northernunits being jacked from the east to the west and the southern units vice versa. The unitlength varied, with a typical length of 13m chosen to limit jacking force. Recesses weredetailed in the ends of units to form intermediate jacking stations for advancing the units.The Contractor elected to jack units in pain.

Side ground support was achieved by shoterete with rock hoIting and insertion of "peagravel" as jacking proceeded. This medium provided complete support to overburden andwalls, including live loading, and also provided a medium along which the unit wallscould slide.

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3.0 STRUCTURAL DESIGN DETAll.S FOR UNITS

During jacking the unit legs were designed to be restrained only by the pea gravel backfillin the overexcavation slot adjacent to each leg. Skid plate keepers slid against the skidrail and limited the lateral movement to ±25mm.Bending moments and shears whichcould be developed in the unit walls and roof were limited by this tolerance betweenkeeper plates and skid rails. Lateral loads were developed on the skid rails andfoundations when leg deflections were such that the clearance was taken up.

The loads considered to act on the units consisted of:• Dead load.• Live load (T44 and M250 and suspended excavation plant) acting both directly on

units and on forepoles.• Temperature - differential and uniform.• Shrinkage.• Soil and rock lateral pressures.• Differential movement due to vertical and lateral construction tolerances on guide

rail installation.• The effect of crabbing (Le. advancement of units not being parallel to the guide

rails due to uneven jacking or differential restraint to movement.• Sway of individual units (due to differential earth pressures on each side caused by

the variation in leg heights or when pea gravel support to one side was lost thenunits approaching from east and west passed.)

• Jacking forces.

Lateral loads on the unit legs during installation resulted from the effect of units'crabbing'. This effect would result from the unit not maintaining a parallel COlirse. Adiagonal or crabbing force would be set up. These effects were analysed by consideringthe overalJ statics of the unit taking all forces into account Le., roof and wall friction,skid rail friction, pressure due to rock wedges, passive and active pea gravel pressure,jacking forces and lateral contact pressures. The distribution of the contact pressureswhich develop between the leg skid plate and the foundation rail were assessed using afinite element structural analysis to account for flexure of the tunnel unit leg andinteraction with the foundation.

Finite element analyses were also carried out to determine the distribution of live load fora range of unit positions relative to the rail track and road traffic lane locations.Thesefinite element analyses were carried out for a portal frame with "pinned" bases. Theresults were then modified to take account of the range of possible leg positions relativeto the guide rails and of lateral pressures due to varying pea gravel height.

Finite element modelling was also used to determine the twisting effects due to adversetolerances-on guide rail level with the worst scenario of opposite out of tolerance on eachleg, thus producing maximum twist, being investigated. Twisting moments derived wereadded to flexural movements, as per the recommendations of R H Wood.(ll

Large bending moments develop at the wall-roof junction due to the actions on the units.In order to increase the efficiency of the junction a large radius bend was added to theprincipal outer leg reinforcement with supplementary reinforcement to resist inducedsplitting effects within the junction. (2) Extensive use of welded reinforcing mats was madein order to limit congestion in the junction zones and a system of alternating bar spacingswas used to allow access for concrete tremie tubes down the full height of the walls, withthe units being cast monolithically.

The units were designed to resist jacking forces at each joint and at the end jacking pointof SOOOkN per leg concentrated on the smallest practical area. This force was required toovercome frictional resistance made up of friction along the leg skid plate/rail interface,between the wall concrete and pea gravel, at the roof concrete/steel friction cutterinterface and due to drag on the inside of the legs caused by spoil from the main tunnelexcavation spilling into the pilot tunnels. The specification limited the difference betweenjacking forces applied to opposite legs in order to control crabbing of the underpass units.The jacks on opposite legs were linked to control differential jacking effects.

The specification required the Contractor to lubricate the skid plate and rail to ensure theassumed friction factor was achieved in the field. SimiIarly, lubrication was requi!ed atthe roof/steel friction cutter interface. Lubrication of the skid rail was achieved bycasting in grease nipples and tubing and pumping grease into the interface.

4.0 DESIGN OF FOOTINGS

The permanent foundations were constructed within the pilot tunnels. Where suitablebasalt founding material existed, the foundations were designed as spread footings. Tolimit overbreak in the mining and jacking forward of the main underpass units, the pilottunnels were set out such that the extreme northern and southern walls of the north andsouth pilot tunnel were approximately 150mm beyond the outside walls of the underpasslegs.

In zones of high lateral loads on the underpass legs shear keys were incorporated in thespread footings. These were excavated in short lengths from the soffit of the pilot tunnelsto ensure pilot tunnel stability. Hold downs consisting of reinforcing bars in groutedholes up to 5.5m deep below the soffit of the pilot tunnels were typically required toresist footing overturning due to lateral loads on the guide rails caused by earth pressuresor r.T';\hhi ng ~fff'Cts

For the northern footing it was necessary to cast in a 850mm diameter sleeve for laterinsertion of a HOPE drainage pipe. This necessitated the design of a "Vierendeel" typeconcrete box which surrounded the circular void as shown in Figure 5. Weldedreinforcing mats were adopted to reduce congestion and allow better concrete placementin this vicinity.

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In areas where highly weathered rock and clay was present at the base of the pilot tunnelsit was necessary to adopt micropiles to support the foundation. These consisted of 36mmdiameter reinforcing bars placed central1y in a col'l'Upted polyethylene tube within a200mm grouted hole. Micropiles were drilled with air operated percussion machines.The grout consisted of API Clasa G cement mixed in a colloidal mixer at a water cementratio of 0.36. This cement was selected by the Contractor to satisfy bleeding andshrinkage requireme!lts and achieve the high strength req~.

The micropiles cxtended from 6.Sm to I7.Sm below footing level. They were analysedas pin ended and designed as a composite soillgroutlbar system (based on modular ratios)with limitations on soil, grout and steel strain and bond stress into the rock. This methodtreated the micropiles asentially as a ground improvement system.

The applied loads produced a series of axial compressive and tensile loads in the bars,depending on the direction of the lateral loads. Anchor plates were welded to themicropiles to resist these forces and the footing designed on a strut and tie basis to resistthese micropile actions. In order to incorporate the SSOmm diameter pipe sleeve thenorthern footing was deepened in the micropile zone as the 'Vierendeel' system did nothave sufficient capacity in this instance.

5.0 DESIGN OF PILOT TUNNELS

Construction of the pilot tunnels was carried out under a separate contract thatcommenced prior to the main underpass contract. The dimensions of pilot tunnels weredetermined by the requirement for the northern footing to include the drail1ll8e pipediscussed above and for the southern footing to provide ICCesS for movement by truck ofexcavated spoil from east to west of the site.

Tunnel support was based on the New Austrian Tunnelling Method (NATM) usingrockbolts and shotcrete. Conventional end anchored rockbolts 204m long and 2Smmdiameter stressed to S6kN were specified, supplemented by dowels or 'SweUcx' andforward spiling.

Early in the construction of the south pilot tunnel it became apparent that the end anchorbolts were crr.eping under 10ld and would DOt maintain the specified tension.Load cells were used to monitor the loss of 10Id on Idected anchors. 'Swellcx' rockboltshad been performing well and were test 10Ided with a view to using them in lieu of endanchored stressed rock bolts. Connell Waaner then developed an alternative groundsupport system using the 'Swellcx' rockbolts. Extensive finite element analysis of thepilot tunnela wu carried out to investigate the effect of both the end anchored and'Swe1lex" rockbolts on ground movements.

Ground monitoring included triangulation measurement within the pilot tunnels using aninvar tape cxtensometer and precise levelling of the crown as well as surface monitoring.Crown settlement was generally around 3mm for the Swe1lcx reinforcement with iso1aledmaximums up to Smm

The shotcrele was steel fibre reinforced and of thickness varying from SOmm to lOOmmin the crown and 2Smm to SOmm in the walls. Extensive trials were carried out todetermine the optimum shotcr= mix with various additives such as air entrainer andaccelerators. Core tests and cylinders were taken to control quality.

Advancement of a typical cycle of rockbolting, excavation, and shotcreting generallyoccurred at each face each day and varied in distance from approximately 0.8m to l.Smdepending on the ground conditions.

As mentioned previously, the soffit of the northern pilot tunnel had to be lowered by upto 700mm to accommodate the deepened footing necessary to incorporate the micropilesand drainage pipe in the red clay zone. Finite element analyses were also carried out toinvestigate the effect of the lowering on crown and surface deflections. The lowering wucarried out in sections in order to minimise deflections. As each section was excavated itwas shotcreted and additional rock bolts installed to provide further wall support.

6.0 PRELIMJNARY CONSTRUCTION WORKS

Preliminary works included:

(i) alterations to Iacana Station platform;(ii) relocation of signals for the electrified suburban tracks clear of the underpass;(iii) relocation of broad gauge track turnout and signals clear of the underpass.(iv) removal of rock material to the forepole level from under the broad gauge and

suburban tracks.(v) lowering to below the foundation of the underpass of the 350mm diameter pipeline

that supplies aviation fuel to Melbourne's Tullamarine Airport.

7.0 MAIN CONTRACT WORKS

7.1 PrecastiDg of Units

Formwork for precasting included a soffit and internal formwork system 33m long toenable two units to be cast in line, and side forms for one unit. The precasting cycle fortwo units could typically be achieved in approximately 2S working days.

Main transverse reinforcement for the portal frame units was prefabricated in modules ina jig and then stored in line. When the units were jacked clear of the forms, thereinforcement modules for the next units were placed in position and longitudinalreinforcement and fitments added prior to form closure. Each unit includedapproximately 130 tonnes of reinforcement.

The concrete was a 40MPa mix using 13mm aggregate, 380kg cement and 80kg flyashper cubic metre. Superplasticizer was added onsite to improve workability and to achievehigh early strength. The 1200 tonne portal frame units were cast monolithically in acontinuous cast over a period of approximately 8 hours. A helicopter float finish wasapplied to the top surface to reduce friction. Curing consisted of the application of acuring agent to the exposed top surface.

7.2 Excavation Shield, Forepoles, Trailing Plates and Static Plates

The excavation shield at the front of the leading unit (unit 1) consisted of 29 individualforeholes each approximately 0.6m wide and 3.5m long supported on a steel box girderwhich was fixed to the upper section of the legs of the concrete unit (Fig.4). Individualforepoles were jacked 0.75m using a 75 tonne jack against the front of the concrete unit.

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Attached to the rear of each forepole was a O.6m wide trailing plate that covered the topof the tint unit. A steel static plate system was restrained at the embankment of the entryportal and fed through the joint between units 1 and 2 to the top of all trailing units as theunits were jacked forward. (Refer to Figure 4.)

7.3 Jacking Arrangement

Units were jacked in pairs with a set of intermediate jacks at the back of each pair ofunits. A typical set of jacks consisted of 4 No. 300 tonne jacks at each leg Le. a total of1200 tonne per leg and 2400 tonne per unit. During occasions of high resistance thenumber of jacks was increased to a maximum of 6 jacks per unit leg. Jacks at the rearunits trailed the unit and struts were added behind the jacks in increments to transfer theforce to the rear buttress as units moved forward.

A typical cycle for unit installation included:• Jacking individual forepoles forward O.75m.• Excavation forward of O.75m below the forepoles.• Rock bolting ("Swellex") and shotcreting side walls.• Jacking units forward O.75m in pairs.

7.4 Excavation and Surf'ace Monitoring

The bulk of the excavation removal was by mechanical plant including rockbreakers withsignificant manual excavation initially at the front around and just below the forepolelevel. Rock splitting was used when necessary to avoid excessive overbreak of the frontor sides. No blasting was necessary.

Surface monitoring of vertical and horizontal movements was carried out during theforepoling process and unit jacking.Adjustment of railway tracks was carried out whennecessary to correct deviation in the horizontal and vertical geometry.

During construction, settlement and loss of material occurred particularly in line with thesides of the units above the gap between the unit and the excavated side wall. As aprecaution the rail tracks were strengthened and steel plates were placed over the roadwayin line with the sides of the units.Some modifications were made to the static plate systemto reduce lateral movements and also to the side shields at the joints to prevent loss of peagravel and to limit the associated settlement.

7.5 Joint Closure

External roof and wall drains were installed in all joints. Following closure, the roofs ofadjacent units were connected with a shear key, installed by sealing the joint with the~nt roof seal and grouting a cast in recess, to ensure load sharing between units.

After installation of the units was completed, a concrete traffic barrier, to New Jerseyprofile, was constructed against each wall. This acts as the locking member to restrainthe unit leg which until this time has rested on a steel to steel sliding arrangement. Thelower section of pea gravel on the rear of the wall was also grouted to ensure leg fixity.

Fire mains and secondary internal drainage system to the underside of the roof and insideof the walls were installed, in addition to a collection system for seepage.

7.6 Surf'ace Coatlna

The inside of the tunnel has been finished with two coats of an epoxy resin baledmaterial, applied to the concrete substrate following preparation of the surface with highpressure water jetting. This was substantially cheaper than other finishes consisting offixed metal clad panels or tiles.

The coating has very good adhesion and chemical resistance qualities and is expected towithstand regular cleaning for a period of 10 years.

7.7 Road Pavement

A continuously reinforced concrete pavement of 35MPa concrete strength and 200mmthick was constructed on the approaches and through the tunnel over a l50mm thick layerof cement treated crushed rock (4.5 percent cement by mass).

7.8 IJ&btlna

To provide satisfactory driving conditions through the tunnel, the internal luminancelevels are maintained at an acceptable ratio to the external luminance levels. The levelsrange from a nominal 30 lux at night to a nominal 5000 lux maximum during daytime.

In Australia tunnel lighting has generally involved the use of a mixture of lamp types thatare switched in stages to provide an acceptable lighting level and uniformity as the tunnellighting is varied in response to external conditions.At Jacana a sophisticated steplesscontrol system has been utilised that continuously monitors external lighting levels andadjusts the output of each of the 400 watt high pressure sodium lamps to nominatedrequirements.

At the lower night levels of 30 lux only 25 percent of the lamps are required in theirdimmed condition. Uniform usage of the lamps has been achieved over a four nightperiod by a rotating switch control that powers a different 25 percent of the lamps eachnight.The overall system results in luminance levels being optimised, high uniformitythroughout the tunnel, reduced maintenance costs, greater control, and expected savingsin energy usage.

7.9 Rock Dowelled Walls

An innovative and cost effective approach was used to stabilise the slopes o~ the deepcutting on the approaches to the underpass.The excavation is up to 9.5m deep with batterslopes of 1 horizontal in 4 vertical in material of basaltic origin.Rock dowels andreinforced shoterete facing have been used to stabilise -the cutting surface area. Thedowels are typically on a I.3m grid and typically 3.5m long, but up to 5.5m long in someareas.

Galvanised 24mm diameter bars (rock dowels) were grouted into a 155mm diameter holeat 15 degrees down from the horizontal. Galvanised mesh was placed at the surface andencased in a HOmm thick layer of shotcrete. This "rock dowelling" method is simpleand provided substantial cost savings over more conventional reinforced concrete retaPUngwalls. To enhance the appearance of the facing, the cutting walls were finished With a300mm thick broken rock facing using local basalt. For the wingwalls the shoteretingwas screeded to a flat finish and painted.

8.0 CONCLUSION

ACKNOWLEDGMENTS

Respective contractors to VicRoads were:

Aviation fuel pipeline diversionPilot tunnelsUnderpass construction

Codelfa ConstructionsKiddMiningCandac

l. R H Wood, "The reinforcement of slabs in accordance with predetermined fieldsof moments." Concrete, February, 1968.

The successful installation of the lacana underpass units represents an innovative approachto construction of grade separations with minimal disruption to existing traffic. Theproject fonns the basis for further development of the method of jacked installations.

The authors gratefully acknowledge the permission of VicRoads and Connell Wagner topublish this paper. The opinions expressed are those of the authors and not necessarilythose of their respective employers. They acknowledge the contributions of colleagueswithin VicRoads and Connell Wagner. Acknowledgment is also made of the cooperationand assistance given to the project by Public Transport Corporation both during thefeasibility study stage and also during construction.

REFERENCES

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2. E Leonhardt, "Lectures on Structural Concrete Part 3. Principles for reinforcingof concrete structures."

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