Tailor Made Concrete Structures – Walraven & Stoelhorst (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-47535-8

Design and construction of an immersed concrete tunnel using anintegrated dock facility

C. BauduinBESIX, Brussels, BelgiumUniversity of Brussels, Brussels, Belgium

P. DepuydtBESIX, Brussels, Belgium

ABSTRACT: The construction of the A73 highway (the Netherlands) involved the construction of a 2.4 kmlong, 2*2 lane tunnel to mitigate the impact of traffic on the city of Roermond and to cross the 1 km widevalley of the river Roer. Given the ground conditions (mainly dense sand and gravel), required cross sectionand tunnel depth, an immersed tunnel appeared to be the most economic solution to cross the valley. However,a facility to permit precasting of the RC tunnel elements needed to be established. The small depth and widthof the Roer prevented transportation of tunnel elements by use of the river and dewatering was not permitteddue to the associated environmental impact. The analysis of the geotechnical data indicated the presence of a5 m thick local loam layer over a length of 350 m along the tunnel alignment, located at the eastern part ofthe Roer valley. This impervious layer offered the opportunity to excavate a 350 m long dock along the axis ofthe tunnel between temporary anchored sheetpiles that were installed into the loam layer. Two precast tunnelelements of approximately 158 m long could be constructed in this dock. A trench was excavated between andtemporary sheetpiles were installed 5 m outside the future tunnel location, thus permitting the transportationand immersion of a total of four such tunnel elements in two installments from the dock to their final location.The remaining part of the tunnel was constructed in what was previously the dry dock after the immersion ofthe elements. This paper describes how the concrete structure was designed and specified (concrete weight andtolerances) with allowances for the specific geotechnical and hydraulic conditions (water depth, concrete weightand freeboard, water level management in the trench, excavation depth and uplift of the impervious layer etc.).The paper describes the behavior and provisions of the concrete structure on the gravel bed foundation, whichwas preferred to sand flow to minimize the risks of liquefaction as the area is mode-rately seismic.

1 INTRODUCTION

As part of the new 40 km A73 Highway in the southof the Netherlands, the Ministry of Transport Pub-lic Works and Water Management (Rijkswaterstaat)planned to build a 2*2 lanes, 2.4 km long tunnel, partlyadjacent to the city of Roermond and partly under the1 km wide valley of the river Roer. The tunnel solutionwas selected in order to minimize the impact of thehighway in the urban area and to preserve the naturalenvironment of the Roer valley.

The Ministry prepared the tender for the tun-nel works as a Design and Construct contract andthe client’s specifications were stated on a ratherabstract level, leaving a large degree of design free-dom to the competing contractors. In 2004 the contractwas awarded to the lowest bidding contractor thatconformed to the client’s specifications. The design

and the construction were the contractor’s responsi-bility, under process supervision by the Ministry. Thecontract was awarded to Besix, the works were exe-cuted by Besix-Strukton Betonbouw JV, the designwas undertaken by Besix Design Department and thepreparation, design and execution of immersion oper-ations was undertaken by Mergor (part of StruktonBetonbouw).

Severe environmental constraints applied to thedesign and construction of the tunnel:

– To preserve nature and to avoid settlement of neigh-boring structures, with no permitted changes toground water levels

– The construction should not influence the courseof the River Roer and not reduce the dischargecapacity of the Roer Valley in case of flooding

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Figure 1. Location of the tunnel.

Figure 2. Typical CPT in construction pit.

– Vibrations and noise should not adversely impactthe natural environment or cause hindrance ordamage to neighboring structures and persons.

The ground conditions are characterized by mediumdense sand and sandy-loam top layers, overlying denseto very dense, highly pervious sand and gravel layers.Stiff loam layers with low permeability were foundlocally between these layers.

The discharge of the River Roer is governed byrainfall and the melting of snow in upstream regions.The water level is mainly governed by the discharge,the water level in the Meuse and by the opening orclosing of the upstream dams. In normal dischargeconditions, the Roer is approximately 15 m wide and4 m deep; in extreme conditions, however, the valley isflooded over its whole width between the winter dikes(approximately 1 km). The water level varies from18.75 m NAP (minimum level) to 20.74 m NAP (return

Figure 3. Tunnel cross section TE1/TE2, concrete pouringstages.

period: 1/10 years), 21.55 (return period: 1/100 years)which corresponds to a river width of 1 km.The designwater level was 22.44 m NAP (return period 1/10000years). The maximum top ground level is 20.5 m NAP.The ground water piezometric head reacts moderatelyto variations in the river level.

The geometric requirements of the tunnel were:

– The cross section was to consist of two tubes(two carriageways each) separated by a centralemergency evacuation tunnel (fig. 3).

– Internal free height requirements: 5.0 m (includingan allowance for electro-mechanical installation)and width between the walls: 10.20 m minimumto be increased to allow for construction toler-ances. The height between bottom and top slab is5.6 m to allow for ballast concrete, road pavementconstruction and settlement tolerances (fig. 3).

– Specific to the tunnel in the Roer Valley: the toplevel of the roof slab was to be located at 14.85 mNAP maximum over approximately 200 m width ofthe valley in order to avoid adverse effects of thetunnel on the groundwater flow. The overburdencombined with the design water level governed themaximum values of the actions for designing thetunnel cross sections (fig. 4).

– The design specifications required that a minimumcontact stress of 5 kPa due to the absence of over-burden over approximately half of the tunnel lengthto accommodate possible future natural changes ofthe river bedding be included in the design. Thisgoverned the volume of structural and ballast con-crete that was required for the internal volume of thetunnel, given the unit weight of the concrete and theamount of reinforcement.

– The concrete volume and internal net open vo-lume determined the free board. The smaller thefree board, the easier the immersion and ballastexchange operations.

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2 GENERAL DESCRIPTION OF THE TUNNEL

Four main areas can be identified (fig. 7):

– An open access ramp at the western side wasdesigned as a polder construction formed by a PEsheet placed underwater on the bottom and theslopes of a deep dredged open trench. Backfillon the sheet compensated for uplift ground waterpressure.

– Piled open ramp and tunnel section, west of theRoer valley. The deepest part of the access rampand a 60 m long part of the tunnel west of the dike,including the main technical building and sumppit, were designed as reinforced concrete struc-tures supported by GEWI piles and constructedbetween permanent anchored sheet piles or combi-walls.A temporary unreinforced concrete floor wasdesigned below the RC structure between the sheetpiles and anchored by the GEWI piles to avoiddewatering. This design was chosen because therequired width in the trench for the PE sheet wasnot available and also due to the complexity of thestructures to be built.

– Crossing of the valley, as far as the eastern dikes(see chapter 3).

– An urban section of the tunnel, which is locatedpartly under the urban ringroad to the south of Roer-mond; the alignment being crossed at ground levelby local access roads at several locations.The urbanpart was designed as a cut and cover cast in-situreinforced concrete structure.

3 TUNNEL CROSSING THE VALLEY

The design for the crossing of the RoerValley was a keyelement to the project due its very important economicimpact on the total construction cost.

Several solutions complying with the fundamentalrequirements were predesigned and evaluated in termsof cost and risk. Immersed tunnel solutions appearedto be the most appropriate because they would requireno piling works and construction materials could beused efficiently provided that:

– the trench could be constructed using temporaryfacilities such as recoverable sheet piles

– an efficient and economic solution for the area forprecasting the immersed elements could be foundwhile preventing the need for dewatering

The trench along the tunnel alignment was sup-ported by temporary anchored sheet pile walls located5 m beyond the planned tunnel, thus permitting recov-ery of the sheet piles after completion of the works.

The use of the Roer to transport elements was notfeasible as it has insufficient depth and width. Care-ful analysis of the ground conditions indicated the

presence of a 5 m thick loam layer along the tunnelalignment at the eastern 350 m of the valley, the bottomof the layer being at approximately −2 m NAP.

This geotechnical feature was the key to the designof the construction area for the precast RC elements ofthe immersed tunnel, as it permitted the construction ofa construction pit along the axis of the planned tunnel,i.e. in the same alignment as the trench, without theneed for dewatering provided that the retaining wallcould penetrate sufficiently deeply into the imperviouslayer.

This construction pit was used to construct twotunnel elements (each of which was approximately135 and 158 m long), after which the pit was inun-dated and the tunnel elements were floated out of thepit and subsequently immersed (fig. 4). The pit wasthen dewatered and used again to construct the nexttwo tunnel elements which were similarly transportedand immersed at their final location. Finally the pitwas dewatered again to construct a remaining part oftunnel approximately 350 m long as a cast in-situ cutand cover tunnel using the framework of the immersedparts. After backfilling the trench and the constructionpit, the sheetpiles located adjacent to the trench and theconstruction pit were removed.

In order to achieve the conceptual design describedabove, it was necessary to successfully balance a num-ber of oppositely interacting parameters, as shown intable 1.

Analysis of the interaction between these parame-ters has indicated that, for a given level of the undersideof the loam layer and thus the allowable excavationdepth, the feasibility of the concept was governed by:

– The required tunnel height for resistance with fulloverburden and uplift safety without overburden,including tolerances on settlement

– The required water level in the Roer– The ability to predict the unit weight of the rein-

forced concrete adequately and the control of thisvalue during construction, in addition to the controlof dimensions during construction

Several scenarios were analyzed, which finally ledto the following design (fig. 4):

– For the deepest and thus most heavily loaded ele-ments 1 and 2, the required slab thickness was toolarge to be compatible with the probable low waterlevels in the Roer and maximum allowable excava-tion depth in the construction pit. It was decided toinstall a lock wall in the trench which allowed thewater level in the construction pit and the adjacentpart of the trench to be controlled independently ofthe water level of the Roer. The elements were thentowed to the deepest parts of the trench and finallythe lock was opened after equating the two waterlevels.The elements 1 and 2 were designed for a free

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Figure 4. Overview of floating – immersion operations.

board of 20 cm to 50 cm depending on the assumedtolerances on concrete weight and dimensions.

– For the two highest and thus less loaded elements 3and 4, a design with a high free board (greater than1 m) was adopted, as the possibility of using the lockwall no longer existed following the immersion ofelements 1 and 2

– After immersion, the fourth element was struttedand preloaded under water against the longitudinalwalls of the construction pit in order to avoid loss ofcompression in the Gina profile, which could occurduring the subsequent dewatering of the construc-tion pit which was necessary for the construction

of the final in-situ part of the tunnel under the RoerValley

The structural design of the tunnel and temporarystructures was undertaken so that the aforementionedrequirements were accommodated.

4 CONCRETE

Concrete grade C28/35 was selected as:

– Its spalling behavior is more favorable when com-pared with concrete of a higher strength

– Because it develops manageable internal tempera-tures in its initial hardening stage (hydration pro-cess). In order to avoid cracking, it was specifiedthat the maximum tensile stress in the hardeningconcrete during the hydration process was not toexceed half of the average instantaneous tensilestrength. FE heat and stress calculations were per-formed for the selected concrete mix to check thiscriterion and to determine the amount of watercooling that was necessary to concrete the pouringstages (stage 1: floor slab, stage 2: internal walls;stage 3: external walls and roof).

The specified design life for the tunnel is 100 years.Durability of the structure was achieved by adoptinga concrete cover of 50 mm, specifying environmentalclass XD3/XF4, using CEM III cement with a min-imum slag content of 50 % and a maximum waterpenetration of 20 mm.

The value and range of variation of the unitweight of the concrete are of large importance forthe immersed tunnel. Values were specified on theinitial concrete mix type tests. The unit weight ofthe concrete (γ = 22.85 kN/m3 − 23.46 kN/m3) andthe as built dimensions were measured continuouslyduring the construction in order that they remainedwithin the acceptable limits for successful floating andimmersion operations.

5 FOUNDATION DESIGN

Due to the seismic risk in the Roermond area, a gravelbed foundation was preferred to classical sand flowin order to minimize the risk of liquefaction duringan earthquake event. This foundation consists of nineridges, 1.65 m wide at their top level and 0.5 m highper 22.6 m long tunnel section (fig. 5).

The interaction between the foundation and the tun-nel was determined by the stiffness of the ridges and bythe uniformity of their top level. The load-settlementrelationship of a single ridge was established usingFE geotechnical calculations assuming a hardeningsoil model for the natural ground and Mohr-Coulombbehavior for the gravel ridges. The behavior that waspredicted for the ridge was confirmed by plate load

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Figure 5. Plan view ridges for one TE and detail of ridges.

tests that were undertaken on full scale ridges whichwere constructed using the intended gravel. Deviationsof the actual top levels of the ridges from the theo-retical levels provoked non-uniform reactions whichhad to be accounted for. Deviations of the top levelsmay be stochastic or systematic, but are not knownduring the design. They interact with the structure dif-ferently depending on the scale (area over which localdeviations are averaged) of the deviation:

– Stochastic deviations act at a local level; particu-larly at “stiff points” as protruding ridges in the spanof the bottom slab are unfavorable when comparedwith a uniformly distributed soil reaction. Theseeffects have been mitigated by reducing the thick-ness of the lower face of the bottom slab in thecentre of the span below the tunnel cells and by thedesign methodology

– Systematic deviations over areas of a quarter to halfof the area of a section mainly act at a global levelin a single tunnel section provoking torsion or crossspanning of the tunnel from one outer wall to theother

– Systematic deviations over areas that are approx-imately the area of a whole tunnel section(22.6 m*27.64 m) result in the shear forces beingtransferred from the poorly supported section tothe well supported section

Figure 6. Flow scheme of design and specification of gravelbed

Separate non-linear analysis models wereconstructed to simulate each of the types of aforemen-tioned deviations. In all of these models, the non-linearbehavior of the spring and possible gaps between thetop of ridges were modeled. A large number of cal-culations have been undertaken with these non-linearmodels in order to develop a full understanding of theeffects of the assumed deviations. These calculationsled to envelopes of internal forces that were used toconstruct equivalent deterministic simple 2D-modelswhich were then used for routine design, of which theresults had been proven to be conservative when com-pared with the envelopes that were obtained from thenon-linear models.

The different simplified models that were used forthe design have been incorporated into the permanentworks by introducing:

– Specifications for the placement of the ridges– Acceptance criteria for the gravel bed as placed

in-situ, by introducing limits on the measured devi-ations of the as built levels when compared with thetheoretical.

The flow diagram shown in fig 6 indicates thedesign steps that were followed to establish the possi-ble effects of any deviation, to convert these models toeasy-to-handle routine design models, and to specifyallowable mean deviations of the ridges over areas ofdifferent sizes.

The non-linear models were used to predict themost probable value of settlement due to backfillafter immersion as well as the upper and lower lim-its of these settlements (as approximated 5% and 95%

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Figure 7. Overall view of the tunnel and design solutions adopted.

ranges).The theoretical level of the gravel bed has beencorrected on the basis of the calculated most probablevalues of settlement (35 mm in the deepest sectionreducing to 5 mm in highest section). After backfill,the deviation between the as built level and theoreti-cal level was for 90% of the measurements less than5 mm, extreme deviation was 15 mm.

6 ACCEPTANCE PROCEDURE FOR GRAVELBED

The ridges were placed by an underwater scrader thatwas specially developed for the project.The top level ofthe ridges was measured in-situ by use of a multi-beam.Transforming the measured data into a calculated aver-age level per meter length, the level of the ridges andthe levels of the assumed areas in the design couldbe easily checked against the acceptance criteria. Ifthe three criteria (fig. 6) were met, the foundation bedwas accepted. If one or maybe several criteria were not

satisfied, the actual measured in-situ ridge levels wereintroduced in the relevant calculation models to checkthe design (slab reinforcement, shear force in shearkey, etc.) for the actual as built situation. If necessary,profile of the gravel bed had to be adapted.

7 CONCLUSION

Strict client’s specifications on environmental condi-tions and geometric boundaries were applied in thedesign and construction of the tunnel.

Considering the local ground conditions, animmersed tunnel was the optimum design solution interms of costs and risks. Integration of design, con-struction methods, specifications and monitoring wasthe key issue for a successful project.

The construction pit was started in November 2004and tunnel elements were immersed in May andOctober-November 2006. The project was deliveredin November 2007.

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