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

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1 Tailor Made Concrete Structures Walraven & Stoelhorst (eds) 2008 Taylor & Francis Group, London, ISBN Design and construction of an immersed concrete tunnel using an integrated dock facility C. Bauduin BESIX, Brussels, Belgium University of Brussels, Brussels, Belgium P. Depuydt BESIX, Brussels, Belgium ABSTRACT: The construction of the A73 highway (the Netherlands) involved the construction of a 2.4 km long, 2*2 lane tunnel to mitigate the impact of traffic on the city of Roermond and to cross the 1 km wide valley of the river Roer. Given the ground conditions (mainly dense sand and gravel), required cross section and 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 width of the Roer prevented transportation of tunnel elements by use of the river and dewatering was not permitted due to the associated environmental impact. The analysis of the geotechnical data indicated the presence of a 5 m thick local loam layer over a length of 350 m along the tunnel alignment, located at the eastern part of the Roer valley. This impervious layer offered the opportunity to excavate a 350 m long dock along the axis of the tunnel between temporary anchored sheetpiles that were installed into the loam layer. Two precast tunnel elements of approximately 158 m long could be constructed in this dock. A trench was excavated between and temporary sheetpiles were installed 5 m outside the future tunnel location, thus permitting the transportation and 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 of the elements. This paper describes how the concrete structure was designed and specified (concrete weight and tolerances) with allowances for the specific geotechnical and hydraulic conditions (water depth, concrete weight and 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, which was 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 south of the Netherlands, the Ministry of Transport Public Works and Water Management (Rijkswaterstaat) planned to build a 2*2 lanes, 2.4 km long tunnel, partly adjacent to the city of Roermond and partly under the 1 km wide valley of the river Roer. The tunnel solution was selected in order to minimize the impact of the highway in the urban area and to preserve the natural environment of the Roer valley. The Ministry prepared the tender for the tunnel works as a Design and Construct contract and the client s specifications were stated on a rather abstract level, leaving a large degree of design freedom to the competing contractors. In 2004 the contract was awarded to the lowest bidding contractor that conformed to the client s specifications. The design and the construction were the contractor s responsibility, under process supervision by the Ministry. The contract was awarded to Besix, the works were executed by Besix-Strukton Betonbouw JV, the design was undertaken by Besix Design Department and the preparation, design and execution of immersion operations was undertaken by Mergor (part of Strukton Betonbouw). Severe environmental constraints applied to the design and construction of the tunnel: To preserve nature and to avoid settlement of neighboring structures, with no permitted changes to ground water levels The construction should not influence the course of the River Roer and not reduce the discharge capacity of the Roer Valley in case of flooding 781

Figure 3. Tunnel cross section TE1/TE2, concrete pouring stages. period: 1/10 years), 21.55 (return period: 1/100 years) which corresponds to a river width of 1 km. The design water level was 22.

The geometric requirements of the tunnel were: Figure 1. Location of the tunnel. Figure 2. Typical CPT in construction pit.

2 Figure 3. Tunnel cross section TE1/TE2, concrete pouring stages. period: 1/10 years), (return period: 1/100 years) which corresponds to a river width of 1 km. The design water level was m NAP (return period 1/10000 years). The maximum top ground level is 20.5 m NAP. The ground water piezometric head reacts moderately to variations in the river level. The geometric requirements of the tunnel were: Figure 1. Location of the tunnel. Figure 2. Typical CPT in construction pit. Vibrations and noise should not adversely impact the natural environment or cause hindrance or damage to neighboring structures and persons. The ground conditions are characterized by medium dense sand and sandy-loam top layers, overlying dense to very dense, highly pervious sand and gravel layers. Stiff loam layers with low permeability were found locally between these layers. The discharge of the River Roer is governed by rainfall 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 or closing of the upstream dams. In normal discharge conditions, the Roer is approximately 15 m wide and 4 m deep; in extreme conditions, however, the valley is flooded over its whole width between the winter dikes (approximately 1 km). The water level varies from m NAP (minimum level) to m NAP (return 782 The cross section was to consist of two tubes (two carriageways each) separated by a central emergency evacuation tunnel (fig. 3). Internal free height requirements: 5.0 m (including an allowance for electro-mechanical installation) and width between the walls: m minimum to be increased to allow for construction tolerances. The height between bottom and top slab is 5.6 m to allow for ballast concrete, road pavement construction and settlement tolerances (fig. 3). Specific to the tunnel in the Roer Valley: the top level of the roof slab was to be located at m NAP maximum over approximately 200 m width of the valley in order to avoid adverse effects of the tunnel on the groundwater flow. The overburden combined with the design water level governed the maximum values of the actions for designing the tunnel cross sections (fig. 4). The design specifications required that a minimum contact stress of 5 kpa due to the absence of overburden over approximately half of the tunnel length to accommodate possible future natural changes of the river bedding be included in the design. This governed the volume of structural and ballast concrete that was required for the internal volume of the tunnel, given the unit weight of the concrete and the amount of reinforcement. The concrete volume and internal net open volume determined the free board. The smaller the free board, the easier the immersion and ballast exchange operations.

3 2 GENERAL DESCRIPTION OF THE TUNNEL Four main areas can be identified (fig. 7): An open access ramp at the western side was designed as a polder construction formed by a PE sheet placed underwater on the bottom and the slopes of a deep dredged open trench. Backfill on the sheet compensated for uplift ground water pressure. Piled open ramp and tunnel section, west of the Roer valley. The deepest part of the access ramp and a 60 m long part of the tunnel west of the dike, including the main technical building and sump pit, were designed as reinforced concrete structures supported by GEWI piles and constructed between permanent anchored sheet piles or combiwalls. A temporary unreinforced concrete floor was designed below the RC structure between the sheet piles and anchored by the GEWI piles to avoid dewatering. This design was chosen because the required width in the trench for the PE sheet was not available and also due to the complexity of the structures to be built. Crossing of the valley, as far as the eastern dikes (see chapter 3). An urban section of the tunnel, which is located partly under the urban ringroad to the south of Roermond; the alignment being crossed at ground level by local access roads at several locations. The urban part was designed as a cut and cover cast in-situ reinforced concrete structure. 3 TUNNEL CROSSING THE VALLEY The design for the crossing of the RoerValley was a key element to the project due its very important economic impact on the total construction cost. Several solutions complying with the fundamental requirements were predesigned and evaluated in terms of cost and risk. Immersed tunnel solutions appeared to be the most appropriate because they would require no piling works and construction materials could be used efficiently provided that: the trench could be constructed using temporary facilities such as recoverable sheet piles an efficient and economic solution for the area for precasting the immersed elements could be found while preventing the need for dewatering The trench along the tunnel alignment was supported by temporary anchored sheet pile walls located 5 m beyond the planned tunnel, thus permitting recovery of the sheet piles after completion of the works. The use of the Roer to transport elements was not feasible as it has insufficient depth and width. Careful analysis of the ground conditions indicated the presence of a 5 m thick loam layer along the tunnel alignment at the eastern 350 m of the valley, the bottom of the layer being at approximately 2mNAP. This geotechnical feature was the key to the design of the construction area for the precast RC elements of the immersed tunnel, as it permitted the construction of a construction pit along the axis of the planned tunnel, i.e. in the same alignment as the trench, without the need for dewatering provided that the retaining wall could penetrate sufficiently deeply into the impervious layer. This construction pit was used to construct two tunnel elements (each of which was approximately 135 and 158 m long), after which the pit was inundated and the tunnel elements were floated out of the pit and subsequently immersed (fig. 4). The pit was then dewatered and used again to construct the next two tunnel elements which were similarly transported and immersed at their final location. Finally the pit was dewatered again to construct a remaining part of tunnel approximately 350 m long as a cast in-situ cut and cover tunnel using the framework of the immersed parts. After backfilling the trench and the construction pit, the sheetpiles located adjacent to the trench and the construction pit were removed. In order to achieve the conceptual design described above, it was necessary to successfully balance a number of oppositely interacting parameters, as shown in table 1. Analysis of the interaction between these parameters has indicated that, for a given level of the underside of the loam layer and thus the allowable excavation depth, the feasibility of the concept was governed by: The required tunnel height for resistance with full overburden 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 reinforced concrete adequately and the control of this value during construction, in addition to the control of dimensions during construction Several scenarios were analyzed, which finally led to the following design (fig. 4): For the deepest and thus most heavily loaded elements 1 and 2, the required slab thickness was too large to be compatible with the probable low water levels in the Roer and maximum allowable excavation depth in the construction pit. It was decided to install a lock wall in the trench which allowed the water level in the construction pit and the adjacent part of the trench to be controlled independently of the water level of the Roer. The elements were then towed to the deepest parts of the trench and finally the lock was opened after equating the two water levels.the elements 1 and 2 were designed for a free 783

4 Parameter If parameter is It favorably affects It unfavorably affects Parameter is initial condition or design parameter Deep excavation level construction pit Thickness of loam layer Deep Water depth required in Roer to enable floating of TE Thick Excavation depth (higher uplift resistance) Total tunnel height High Strength of roof and bottom slab Allowable thickness of ballast concrete Vertical tolerance for immersion and accuracy of predicted settlements Total tunnel height Low Required water depth for floating conditions (relative levels between the Roer and the depth of excavations) Water level Roer High Floating conditions (low free board) Clearance under TE Uplift of loam layer Dimensions retaining structure of pit Design parameter - To be determined by critical ground investigations Required water depth for floating conditions (relative levels between the Roer and the depth of excavations) Strength of roof and bottom slab Allowable thickness of ballast concrete Vertical tolerance for immersion and accuracy of predicted settlements Determined by free height, ballast thickness and roof/ bottom slab thickness Determined by free height (design requirement), ballast thickness and roof/bottom slab thickness Variable, slightly predictable, unmanageable Water level Roer Low Requires high free board Variable, slightly predictable, unmanageable Free board TE Low Floating conditions Ballast quantity Equilibrium after immersion Free board TE High Required depth of construction pit. Required depth trench Required minimum level of Roer to enable floating of TE Depth of tunnelr element in final position (thus maximum value of overburden on roof) Clearance for ballast concrete in tunnel Weight of reinforced concrete Maximum level of tunnel roof after immersion No overburden on tunnel roof Follows from vertical alignment The smaller the depth, the smaller the floor and roof thickness thus total element height High Uplift in absence of overburden Allowable uncertainty on settlement Low High free board, thus required depth etc Design requirement Design requirement Required water depth for floating conditions (relative levels between the Roer and the depth of excavations) to enable floating Immersion (high volume of ballast water) Exchange of water ballast to permanent ballast Design parameter, minimum free board (0.2 m) to enable floating of TE Design parameter, limited by strength requirement for applied load Allows optimization of roof thickness Total tunnel height Design parameter limiting deviations to gravel bed Effective contact stress under bottom slab Height of the tunnel (limiting level of tunnel roof) Minimum thickness of ballast concrete (moderately) Design parameter Design requirement to minimize effects of tunnel on ground water flow Design requirement 784

5 of the final in-situ part of the tunnel under the Roer Valley The structural design of the tunnel and temporary structures was undertaken so that the aforementioned requirements were accommodated. 4 CONCRETE Concrete grade C28/35 was selected as: Its spalling behavior is more favorable when compared with concrete of a higher strength Because it develops manageable internal temperatures in its initial hardening stage (hydration process). In order to avoid cracking, it was specified that the maximum tensile stress in the hardening concrete during the hydration process was not to exceed half of the average instantaneous tensile strength. FE heat and stress calculations were performed for the selected concrete mix to check this criterion and to determine the amount of water cooling that was necessary to concrete the pouring stages (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 adopting a concrete cover of 50 mm, specifying environmental class XD3/XF4, using CEM III cement with a minimum slag content of 50 % and a maximum water penetration of 20 mm. The value and range of variation of the unit weight of the concrete are of large importance for the immersed tunnel. Values were specified on the initial concrete mix type tests. The unit weight of the concrete (γ = kn/m kn/m3 ) and the as built dimensions were measured continuously during the construction in order that they remained within the acceptable limits for successful floating and immersion operations. Figure 4. Overview of floating immersion operations. 5 board of 20 cm to 50 cm depending on the assumed tolerances on concrete weight and dimensions. For the two highest and thus less loaded elements 3 and 4, a design with a high free board (greater than 1 m) was adopted, as the possibility of using the lock wall no longer existed following the immersion of elements 1 and 2 After immersion, the fourth element was strutted and preloaded under water against the longitudinal walls of the construction pit in order to avoid loss of compression in the Gina profile, which could occur during the subsequent dewatering of the construction pit which was necessary for the construction Due to the seismic risk in the Roermond area, a gravel bed foundation was preferred to classical sand flow in order to minimize the risk of liquefaction during an earthquake event. This foundation consists of nine ridges, 1.65 m wide at their top level and 0.5 m high per 22.6 m long tunnel section (fig. 5). The interaction between the foundation and the tunnel was determined by the stiffness of the ridges and by the uniformity of their top level. The load-settlement relationship of a single ridge was established using FE geotechnical calculations assuming a hardening soil model for the natural ground and Mohr-Coulomb behavior for the gravel ridges. The behavior that was predicted for the ridge was confirmed by plate load 785 FOUNDATION DESIGN

6 Figure 6. Flow scheme of design and specification of gravel bed Figure 5. Plan view ridges for one TE and detail of ridges. tests that were undertaken on full scale ridges which were constructed using the intended gravel. Deviations of the actual top levels of the ridges from the theoretical levels provoked non-uniform reactions which had to be accounted for. Deviations of the top levels may be stochastic or systematic, but are not known during the design. They interact with the structure differently depending on the scale (area over which local deviations are averaged) of the deviation: Stochastic deviations act at a local level; particularly at stiff points as protruding ridges in the span of the bottom slab are unfavorable when compared with a uniformly distributed soil reaction. These effects have been mitigated by reducing the thickness of the lower face of the bottom slab in the centre of the span below the tunnel cells and by the design methodology Systematic deviations over areas of a quarter to half of the area of a section mainly act at a global level in a single tunnel section provoking torsion or cross spanning of the tunnel from one outer wall to the other Systematic deviations over areas that are approximately the area of a whole tunnel section (22.6 m*27.64 m) result in the shear forces being transferred from the poorly supported section to the well supported section Separate non-linear analysis models were constructed to simulate each of the types of aforementioned deviations. In all of these models, the non-linear behavior of the spring and possible gaps between the top of ridges were modeled. A large number of calculations have been undertaken with these non-linear models in order to develop a full understanding of the effects of the assumed deviations. These calculations led to envelopes of internal forces that were used to construct equivalent deterministic simple 2D-models which were then used for routine design, of which the results had been proven to be conservative when compared with the envelopes that were obtained from the non-linear models. The different simplified models that were used for the design have been incorporated into the permanent works 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 deviations of the as built levels when compared with the theoretical. The flow diagram shown in fig 6 indicates the design steps that were followed to establish the possible effects of any deviation, to convert these models to easy-to-handle routine design models, and to specify allowable mean deviations of the ridges over areas of different sizes. The non-linear models were used to predict the most probable value of settlement due to backfill after immersion as well as the upper and lower limits of these settlements (as approximated 5% and 95% 786

7 Figure 7. Overall view of the tunnel and design solutions adopted. ranges).the theoretical level of the gravel bed has been corrected on the basis of the calculated most probable values of settlement (35 mm in the deepest section reducing to 5 mm in highest section). After backfill, the deviation between the as built level and theoretical level was for 90% of the measurements less than 5 mm, extreme deviation was 15 mm. satisfied, the actual measured in-situ ridge levels were introduced in the relevant calculation models to check the design (slab reinforcement, shear force in shear key, etc.) for the actual as built situation. If necessary, profile of the gravel bed had to be adapted. 7 6 ACCEPTANCE PROCEDURE FOR GRAVEL BED The ridges were placed by an underwater scrader that was specially developed for the project.the top level of the ridges was measured in-situ by use of a multi-beam. Transforming the measured data into a calculated average level per meter length, the level of the ridges and the levels of the assumed areas in the design could be easily checked against the acceptance criteria. If the three criteria (fig. 6) were met, the foundation bed was accepted. If one or maybe several criteria were not CONCLUSION Strict client s specifications on environmental conditions and geometric boundaries were applied in the design and construction of the tunnel. Considering the local ground conditions, an immersed tunnel was the optimum design solution in terms of costs and risks. Integration of design, construction methods, specifications and monitoring was the key issue for a successful project. The construction pit was started in November 2004 and tunnel elements were immersed in May and October-November The project was delivered in November

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