Design and Construction Issues of the Soft-Eye Headwall for Brisbane AirportlinkM7 s 12.48m Diameter Tunnel Boring Machines

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1 Design and Construction Issues of the Soft-Eye Headwall for Brisbane AirportlinkM7 s 12.48m Diameter Tunnel Boring Machines Joseph Donohue 1, R. Greg Eberhardt 2 and Ildiko Juhasz 3 1 Associate, Arup 2 Senior Engineer, Arup 3 Principal Engineer, Parsons Brinckerhoff Abstract: The Tunnel Boring Machine (TBM) launch box for the Brisbane AirportlinkM7 consists of diaphragm walls with the use of both steel and reinforced concrete for the various levels of temporary and permanent props and whalers. The launch box was constructed in a top-down sequence to a depth of approximately 23m, which resulted in the 1.2m thick concrete sections being heavily reinforced. Glassfibre reinforcement was required in the soft-eyes to allow for the launching of the two TBMs through the headwall. Under lateral loading predominately arising from soil and water pressure, the glass-fibre reinforcement needed to resist the short-term loading until the TBM s were launched. The permanent regions of the diaphragm wall above and below the soft-eyes required steel reinforcement to satisfy the long-term loadings and to meet the 100 year design-life requirements. The use of the glass-fibre reinforcement entailed several design issues that had to be dealt with, including: a low modulus of elasticity and shear strength of the material, creep effects and limited ligature shapes. Three-dimensional analysis was undertaken to optimise the headwall design in both the short and long-term conditions. The quantity of reinforcement, which was required to withstand the temporary loading, has led to significant construction constraints, especially with congestion of the GFRP and steel reinforcing at the interfaces between the soft-eyes and the permanent headwall. Careful detailing, a close working relationship with the contractor and a strong, engineering site-presence during construction, allowed for the successful integration of the design and constructability requirements. Keywords: AirportlinkM7, Soft-Eye, Headwall, GFRP 1. Introduction This paper discusses the soft-eye headwall for Brisbane AirportLinkM7 Tunnel the largest infrastructure project in Australia. The detailed design of the soft-eye headwall had to incorporate many considerations including the construction sequence, which had numerous load-combination cases under both short and long-term conditions. The construction sequencing incorporated ground improvement behind the headwall, in the upper layers of soil strata, which reduced the lateral loading and assisted with launching the TBMs. Another important consideration was accounting for the design issues which were specific to the glassfibre reinforcement. The quantity of reinforcement, which was required to withstand the temporary loading, resulted in the need for careful detailing of the GFRP and steel reinforcing at the interfaces between the soft-eyes and the permanent headwall. 2. Project description AirportlinkM7 Tunnel, Brisbane, QLD The AirportlinkM7 Tunnel is a 6.7km, multi-lane road connecting Brisbane s inner-northern suburbs and the north-south CBD bypass tunnel with the airport and the north-south Gateway Motorway. The tunnel has three connections to the existing surface-road network. Thiess-John Holland JV were appointed as principle contractors in May 2008 following the successful tender-bid; Parsons Brinckerhoff-Arup JV were the design team. The project s completion date was set at June These tight timelines meant that the design was required to progress in an accelerated schedule to keep pace with construction. Major design inputs and issues such as: design criteria, soil testing, construction loads and sequencing etc had to be developed and updated without any delay to the construction schedule. The authors were structural engineers and design lot leaders for cut and cover structures on the Toombul or Eastern Connection of this project. These structures were on the critical path due to their temporary use

2 as the launch box for the tunnel boring-machines (TBMs), and also served as the receiving pit for two large, reinforced-concrete culverts that were jacked under the North Coast Rail Line and airport rail link. 3. TBM Launch Box The focus of this discussion is on the headwall of the TBM launch box, which consisted of reinforcedconcrete diaphragm walls with in-situ roof, base and intermediate slabs. It was constructed by a top-down construction technique utilising two layers of temporary steel props and whalers between the intermediate and base slabs. The roof and intermediate slabs featured large temporary voids to allow segments of the TBM to be lowered into position. In cross-section, the TBM launch box incorporated a haunched base-slab to accommodate the TBM outline. The road level is at a higher elevation than the TBM invert, and the haunched profile of the base slab reduced the vertical span-length of the typical, diaphragm wall-panels. The diaphragm wall at the headwall needed, however, to span to below the invert of the TBM. Figure 1 shows a section through the portal wall with two layers of temporary props and whalers between the intermediate level slab and base slab. Figure 1. Section through headwall with two layers of temporary support. After the TBM launch, a non-structural infill was provided in the base slab up to the underside of pavement level. The upper layers of soil strata, to a depth of 12m at the headwall, were soft silty-clays with clayey-gravels and very stiff-to-hard silty-clays below. Low to medium strength siltstone was present towards the bottom of the soft-eye region at close to the final road level. The typical, diaphragm wall-panels for the TBM launch box were 1200mm thick. The objective to maintain the same thickness of diaphragm wall-panels at the headwall to span the extra length, even for the shortterm design-life, was a challenge during the design phase.

3 3.1 Soft-eye Headwall In the soft-eye regions of the headwall, glass-fibre reinforced polymer (GFRP) bars were used to provide zones without steel reinforcement to allow the TBMs to break-through. The soft-eyes were centred about the TBMs and had a clear diameter of 13.10m. This provided construction tolerance and clearance between the steel reinforcement and the 12.48m diameter TBM cutter-head. To reduce the lateral loading and the corresponding bending effects on the portal wall and, to assist with launching the TBMs, ground improvement was carried out behind the portal wall. See Figure 1. The extent of this ground improvement had, however, to be limited due to the proximity of property boundaries and a newly-diverted sewer line. 3.2 Factors Affecting Construction Sequence The timing of the works meant that the ground improvement behind the portal wall could not be completed until after the sewer line had been installed; this sewer diversion was required to enable the TBM launch box diaphragm-walls to be constructed. A further input into the construction sequence was the need for a permanent overland water-flow path diversion, which was incorporated into the roof slab of the TBM launch box at the opposite end to the headwall. This was required to be operational to enable the commencement of early works for the jacked box under the North Coast Rail Line and airport rail link. Another significant factor on the construction sequence was the lead times for the GFRP bars and, especially, for large diameter bars and bent bars. These factors contributed to the headwall diaphragmwall panels being constructed last for the TBM launch box, although the design was undertaken in parallel with the TBM launch box diaphragm-wall and roof slab. 4. Design Criteria Over the extent of the soft-eyes, the glass-fibre reinforcement had to satisfy the short-term loading until the TBM s were launched. Steel reinforcement was utilised in the permanent regions above and below the soft-eyes to satisfy the long-term loadings and to meet the 100 year design-life requirements. The design life of the GFRP is an important design consideration for it is subject to creep effects. Based on the projected schedule for construction and excavation of the launch box and for the assembly and launching of the two TBM s, the design life of the GFRP reinforcing was established at six months. The design of the GFRP was based on the American Concrete Institute (ACI) Guide for the Design and Construction of Structural Concrete Reinforced with Fibre Reinforced Polymer Bars, ACI 440.1R-06. Though there are other international codes, the ACI method had a separate check against creep rupture using unfactored loads. This was critical, since creep rupture can be the governing failure mechanism under sustained loadings such as under earth and water pressure (1). The design parameters for the GFRP bars were verified by tests; these were undertaken to ACI test method B5 as described in ACI Analysis The analysis showed that the construction sequence and ground improvement were critical. The governing locations for bending were positive moment at mid-span of the soft-eye and negative bending at the intermediate slab-level, which is largely locked-in during the excavation sequence. For the chosen construction sequence, the extent and strength of the ground improvement influenced the profile of the bending in the headwall. The extent of the ground improvement was largely determined by the sewer location and by the TBM launching requirements. The strength of the adopted ground improvement was chosen to balance the peak positive and negative bending, since both these regions needed to be resisted with concrete reinforced with GFRP bars. It was thus important that the analysis considered both upper and lower values for strength of ground improvement. A number of soil-structure interaction-models were created for the headwall with the intent to model the 3D-effects of the headwall and adjoining diaphragm wall-corner panels, with the intent being to reduce the vertical bending by accounting for load spanning in both directions. This was used to accurately determine

4 the both vertical and transverse bending in the soft-eyes. Both 2D and 3D Plaxis models were created along with corresponding structural stiffness matrix models. The 2D model assisted with the calibration of the 3D model, especially, with the seepage modelling which was more readily modelled in 2D Plaxis. Since the water pressure during construction was based on the seepage modelling, a sensitivity analysis was carried out on the horizontal permeability of the soil and rock layers. The structural stiffness matrix models considered the locked-in effects from the Plaxis model in conjunction with the numerous load combination cases for both the short and long-term loading conditions (2). The transverse bending was limited due to the vertical joints in the diaphragm wall-panels. The design accounted for transverse bending with co-existing axial compression, which was induced from the lateral earth and water pressure on the corner diaphragm wall-panels. It was, however, sufficient to provide a working solution for the 1200mm thick diaphragm wall-panels. Analysis showed, after the TBM break-through, that there was an increase in negative moment above the soft-eye when combined with the long-term effects and other loading such as fill on the roof slab and longterm lateral loading, which was applied after the TBM launch. 6. Design The design was required to consider the induced tension in the GFRP bars during the time taken for each excavation stage; this time included allowing for casting the reinforced concrete slabs and for installing the temporary props and whalers. Following the completion of the excavation to the underside of the base slab, the GFRP bars had a six-month design-life to allow for the casting of the base slab, de-stressing and removal of the temporary supports and for the assembly and the launch of the TBMs. There was, approximately, a two-month lag between the launch of the first and the second TBM, although the excavations on each side of the launch box were done concurrently. An internal ring beam was cast against the excavated face of the headwall, which extended from the underside of the intermediate slab to the top of the base slab in height, and between the internal faces of the corner diaphragm wall-panels in width. This was cast after the temporary steel props and whalers were removed. The ring beam was used to connect the seal ring of the TBM, which seals the TBM and launch box from water during the TBM break-through into soil and rock. See Figure 2. Figure 2. Photograph of headwall showing ring beam prior to TBM assembly.

5 The L-shaped corner-panels were stiffer than the adjoining diaphragm wall-panels with GFRP. As a result, the analytical model indicated a deflection differential between these panels. The ring beam was designed to act in shear to reduce the deflection of the panel with GFRP and, thus, to prevent local failure of the diaphragm wall panel joints. Since the ring beam did not extend over the full extent of the soft-eye, the 1200mm thick panels with GFRP were still spanning vertically. The design of shear reinforcement was also affected by the anisotropic characteristics of glass fibre. When used for ligatures, the GFRP bars were required to be manufactured with bends. A strength reduction of 14 to 23% compared with the tensile strength of a straight bar was used for the ligatures to account for fibre bending and for stress concentrations in the bend. The values used for the reduction in tensile strength due to bending were based on test results, which had been provided by the manufacturer, that were performed in accordance with test methodologies cited in ACI 440.3R. The durability of GFRP reinforcement is influenced by its exposure to the environment; the creep rupture endurance time can irreversibly decrease under adverse environmental conditions. Following the ACI recommendations, we used an environmental reduction factor of C E = 0.7 for concrete exposed to earth and weather. 7. Detailing Special consideration had to be given to the detailing for a number of reasons. The GFRP bars have a significantly-lower modulus of elasticity of 40.8 GPa in comparison with steel reinforcement, which is approximately five times larger. A much greater cross-sectional area of these are thus required to give the same bending capacity as that of steel reinforcement. This resulted in the need for five GFRP bars to be arranged in two bundles of two and three reinforcement bars respectively. See Figure 3. This arrangement was comparable in bending capacity to a single bundle of two N40 bars, which were adopted outside the soft-eye region. Figure 3. Section showing GFRP bar arrangement. The splice region between the GFRP and the steel bars required significant attention. The design of the launch-box diaphragm-walls was optimised by positioning the intermediate slab and the base slab with a minimum clearance to the TBM. This, however, involved having haunches in the underside of the intermediate slab and on the top of the base slab to provide assembly access around the perimeter of the TBMs.

6 This resulted in the diaphragm wall-panels, which required reinforcement couplers for these slabs, being positioned immediately adjacent to the soft-eye regions. The maximum negative bending occurs at the intermediate slab-level; this was also the splice zone from steel to GFRP, so the region was locally congested. Special detailing was carried out to minimise this congestion. This included having the diaphragm-wall steel reinforcement end with a coupler at the intermediate level slab and staggering the length of other steel bars to match the circular profile (in elevation) of the soft-eyes. A 10mm concrete mix was specified for these diaphragm wall-panels to allow for better consolidation in the congested areas. The reinforcement in the panels was detailed with 600mm clear zones to allow concrete placement using tremmie pipes. In addition, bending of GFRP bars is not possible on site; the factory bending results in the bent portion having a considerably lower tension-capacity. A further limitation of GFRP bars is that they can only have a maximum bend of 90 rather than a hook. This resulted in a reduced capacity of the shear ligatures. Detailing of these followed ACI 440-1R recommendations, that is, the use of 90-degree cogs with a minimum r b /d b ratio of 3, and a minimum tail length of 12 d b. This required lapping the bent bar with a horizontal spacer bar, in order to provide sufficient rigidity for lifting the reinforcement cage during construction. See Figure Construction Most diaphragm wall-panels for the TBM launch box had reinforcement-cage splice locations between the intermediate and the base slab levels. This location was not, however, suitable for the splice location in the soft-eye and, instead, the splice was located at the base slab level. This resulted in a longer panel length with these panels being assembled near the launch box. Prior to excavation, instrumentation that included inclinometers and prisms were installed and calibrated. These were recorded and compared against predicted values at each excavation stage. The 3D Plaxis model correlated reasonably well with the site-measured deflections, which were 24mm and 20mm respectively, for the stage after the removal of the temporary supports, and correlations at the other construction stages were similar. 9. Conclusions The break-through of the TBM required the use of GFRP bars, which greatly affected the design of the headwall in several aspects. The soil and pore water pressures, ground improvement extents and the construction sequence varied the peak bending moment positions on the headwall. Haunch beams on the underside of the intermediate slab and on top of the base slab were adopted to reduce the effective length of the headwall panels. The diaphragm wall headwall panels were subject to greater deflection than the adjacent corner-panels, which were inherently stiffer due to their L-shape. This effect was modeled by using 3D models and by calibrating them against the 2D models. The differential deflection created shear forces across the diaphragm wall panel joints; the shear forces were greater than the joint s capacity. These panels thus needed additional restraint; this was provided by utilising the internal ring beam to span over the joint and to effectively connect the panels together. GFRP bars are susceptible to creep effects, with creep rupture being one of their critical failure mechanisms. The adopted design life for the soft-eye was six months; when combined with the exposure classification, this established the design parameters for the reinforcement. The GFRP bars had approximately 20% of the modulus of elasticity of steel reinforcement, which meant that more bars were required to resist the bending moments. To substitute the typical steel reinforcing of two N40 bars at 180mm centres, in the soft-eye regions five 32mm GFRP bars at 180mm centres were arranged in two bundles of three and two bars. This caused congestion, which was amplified at the splice region between the steel and GFRP zones. The congestion of the reinforcement was mitigated by using concrete with 10mm aggregate to allow for better consolidation in the these areas.

7 Bends in the shear ligatures were limited to a 90, and reduced their strength by 14 to 24%. The use of GFRP bars was further complicated by long lead-times for fabrication, and the necessity for bends to be made by the manufacturer. The need for replacement bars was eliminated by careful detailing in close liaison with the contractor s construction team. The 1200mm diaphragm wall was designed and detailed without the need to widen the wall in the GFRP zone. The recorded on-site maximum deflection at mid-span, immediately after the removal of the temporary supports, for the diaphragm wall-panel measured 20mm. It compared closely to the predicted 24mm from the 3D Plaxis model. This provided greater confidence in the methods used for the design and detailing of the diaphragm wall soft-eyes using a combination of steel and GFRP reinforcing bars. 10. Acknowledgement The authors would like to thank Thiess-John Holland and Parsons Brinckerhoff-Arup for their support. 11. References 1. American Concrete Institute, Guide for the Design and Construction of Concrete Reinforced with FRP Bars, (ACI 440.1R-06) ACI Committee 440, 2006, Michigan. 2. Donohue, J,. Eberhardt, R. G., Kuhn, M., The adoption of soil co-efficients from FEA (Finite Element Analysis) models for use in stiffness matrix models to allow for multiple load cases generating large numbers of load combinations, Proceedings, World Tunnel Congress 2013, Geneva, Switzerland.