Domain Road Interchange, Tauranga Eastern Link –Design in Liquefiable Ground

DOMAIN ROAD INTERCHANGE, TAURANGA EASTERN LINK –DESIGN IN LIQUEFIABLE GROUND DAVE IDLE1, ANDREW WALKER1, RALF KONRAD2 1 Opus International Consultants 2 Peters and Cheung Ltd SUMMARY Soft ground conditions, combined with high seismicity and the risk of large liquefaction settlements have resulted in the development of an innovative design solution for Domain Road Interchange, where the new motorway diverges from the existing State Highway Two, on the Tauranga Eastern Link. The design was optimised from a single bridge over the main alignment to two bridges and three independent expanded polystyrene (EPS) embankments carrying the alignment over Domain Interchange. INTRODUCTION Tauranga is the major town in the Western Bay of Plenty with the largest port by volume in New Zealand. Together with the Auckland and Hamilton regions the area is expected to be home to more than 50% of the New Zealand population by 2031 (Statistics New Zealand, 2012). To meet the future demand the Government identified seven Roads of National Significance (RoNS) to support economic growth, improve operational efficiency and increase safety. A new eastern link was identified as one of these RoNS, providing a more direct route to the port, reducing journey times between Paengaroa and Tauranga and supporting managed growth of the area. The Tauranga Eastern Link (TEL) (Fig 1) is a $450million highway to meet this and at the time was the largest roading contract awarded in New Zealand (September 2010). The project comprises 12 bridge structures, four interchanges, 6km of upgraded highway and 17km of new road all designed within the Principal’s Requirements. The design and construct (D&C) contract was awarded to a Fulton Hogan HEB construction Alliance with URS, Opus, Peters & Cheung and Bartley Consultants providing full design services. One of the advantages of this form of contract is the ability to collaborate which enabled the integration of the construction methodology during the design phase. The construction manager and design team were co-located in a project office to aide this cooperation. Additionally, risk and constructability workshops were held throughout the design process to provide opportunity for formal feedback. This paper covers the design of Domain Road Interchange. DOMAIN INTERCHANGE TEL diverges from State Highway Two at Domain Interchange approximately 11km east of Tauranga, near the existing roundabout between the State Highway and Domain Road. The proposed structure contains two bridges; the 92m long three span Interchange Bridge crossing the realigned roundabout; and the single 20m span Off Ramp Bridge crossing the westbound off ramp. Additionally, three separate EPS embankments are provided to make up the rest of the 450m long structure. Figure 1 Location Plan Figure 2 3D View Domain Road Roundabout (Courtesy of NZTA) GROUND CONDITIONS The key geological feature of the whole site is an old river channel which contains approximately 20m of estuarine silts. Above the silts is a varying thickness layer of peat (up to 6m) and below the silts are sands of varying density. See Figure 3. Figure 3 Elevation and Geological Profile At the Interchange Bridge the design is significantly constrained by the soft silts which have very low strength and stiffness providing little resistance under lateral seismic loads. This low stiffness ensures the ground is classified, class E, the lowest soil classification in accordance with NZS1170.5 (Standards New Zealand 2004). The key geotechnical issue at the Off Ramp Bridge was the loose sands with high liquefaction potential. Seismic and liquefaction vertical settlements are expected to be greater than 300mm in an ultimate limit state (ULS) event. This causes potential down drag on the piled foundations and affects the interface between bridge and embankment. The piles at both bridges are founded at a depth of 40-45m in very dense sands which are not expected to liquefy. The ground conditions over each EPS embankment vary, but a common feature is the highly compressible peat. Under vertical loads the peats lose up to 50% of their volume, therefore any solution for the embankment has to manage the short and long term settlement risks. ALTERNATIVE TENDER DESIGN The specimen design for the interchange comprised a single span bridge over the main TEL alignment. During the tender design, significant opportunities for cost savings at the interchange were identified. Our solution was to take TEL over Domain Road, allowing a shift in the geometric alignment to retain the existing substation to the South, which provided significant cost savings. This option led to a requirement for spanning the 450m length of the interchange. Initially the bridge options considered a multi-span viaduct, varying span lengths and superstructure types (e.g. concrete Super Tee beams or steel plate girders). The most economical solution for foundations was for piled foundations to the deeper very dense sands, giving pile depths of approximately 45m. Due to the number and length of the piles, foundation costs were a high proportion of the total structure cost and a balance between increasing span lengths, beam depths and approach embankment heights was required. To mitigate future post construction settlement risks, the underlying peat and silt required extensive ground improvements. Due to the soft ground conditions, potential 8m height embankments and consequential seismic stability issues and settlements, a 450m viaduct structure would have been required along the interchange. However, an alternative option which used a lightweight EPS fill embankment was considered which provided a cost reduction of approximately half compared with an equivalent viaduct structure. The final tender design comprised two bridges; a three span Super Tee form crossing the interchange roundabout (Interchange Bridge) and a single span single hollow core bridge crossing the westbound off ramp (Off Ramp Bridge). Following contract award, the design team refined the solutions and some examples of the design are discussed in the following sections. GEOTECHNICAL DESIGN As noted above, the soft ground of peats and silts undergoes significant settlement under a vertical load. To manage the settlement, in both the short and long term, a dense sand raft and extensive preload regime was implemented. Where conventional embankment heights exceed 3m, the required ground improvements and potential settlements meant that an EPS embankment was more economic. This was founded on the sand raft with geogrids and high-tensile geotextiles provided in the raft to ensure seismic slope stability was maintained. The raft is expected to settle up to 4m below existing ground level, thus providing significant lateral support to the piles and pile caps of the Interchange Bridge (see figure 4). Figure 4 Sand raft sequence To mitigate post construction settlements of the low permeability silts, vertical (wick) drains were installed up to 25m and more than 3m of preload fills placed. Use of wick drains allowed the consolidation settlements to be achieved within 7 months of construction. A high ratio of preload fills to final loads reduces the creep settlements in the peat to acceptable limits. Ongoing settlements are expected to be less than 30mm over the 25 year post construction period. During construction, settlements in the area have required a sophisticated instrumentation and monitoring scheme, the design of which was based on an observational approach requiring verification during construction. The expected settlements of more than 4m have matched monitored settlements. This result required thorough planning and observation to mitigate potential drag settlement effects on the existing infrastructure. PILE DESIGN Early engagement with the Contractor allowed the pile installation methods and sizes to be confirmed. A job wide pile was selected to allow competitive production and procurement rates to be achieved. A 710mm diameter driven concrete filled steel shell pile was the preferred option with a minimum casing size of 12mm. Given the low soil stiffness, relatively high seismicity and structural form, the piles were working at high stresses. The design team took advantage of the required form to reduce the reinforcing steel content lower in the piles. Relatively benign soil conditions were not expected to have significant corrosion risk, but an allowance of 2mm to external surfaces was used in accordance with AS2159 (Standards Australia 2009), HERA (2011) report R4- 133 and NZS3109 (Standards New Zealand 1997). The steel and concrete section was assumed to be composite after a calculated depth. This was calculated on the basis of bond strength, allowing for shrinkage away from the casing and the horizontal pressures due to Poisson’s ratio and the load from the structure above. Additionally, due to the long pile length and low lateral stiffness, the piles were checked for buckling using the method in Annex C of BD74/00 (Department for Transport 2000). This was back checked using a linear buckling analysis in LUSAS with Winkler springs representing the lateral soil stiffness. The geotechnical pile design was carried out using a limit state design method in accordance with AS2159 (Standards Australia 2009) including allowance for drag loads imposed on the piles from potential liquefaction settlements. Based on the geotechnical ground conditions and analyses, the depth of the neutral axis to which negative skin friction loads are considered is between 27m and 30m below ground

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