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New Zealand Engineering 1999 July Construction Underneath the Royal SunAlliance Centre The Royal SunAlliance Centre is a 34-storey tower on a six-storey podium that is currently under construction in Auckland City. It is founded on shallow footings and includes major retained cuts of up to 20 metres. The building is located on a site between Fort and Shortland Streets in downtown Auckland where the tower will be a major feature on the Auckland skyline. Left: The north elevation of the building at 3 May 1999. Note the unglazed lower podium structure which approximately delineates the retained height of soil. The development is being undertaken by the Kiwi Development Trust, with Royal & SunAlliance as one of the key tenants. Murray Jacobs Ltd has carried out structural design for the main contractor Fletcher Construction Ltd, with Brian Perry Ltd as a specialist foundation contractor. Tonkin & Taylor has provided specialist geotechnical design services over the course of the project. Underlying the site are relatively stable and predictable geological materials, however, the local topography has raised a number of geotechnical issues that challenged both designers and contractors. Fort Street follows the line of the historic coastline, now 300 metres from the sea due to reclamation operations. Shortland Street is at the top of the old sea cliffs and hence is over 15 metres higher than Fort Street. The site was already occupied by the old L.D. Nathan building and several other structures including the existing SunAlliance Building. These buildings stepped up the slope from Fort to Shortland Streets, with a major gravity retaining structure between the roads, following the line of the historic cliffs. Apart from the existing SunAlliance Building, which was to be retained until completion of the main tower in the new development, all of these structures were to be demolished prior to construction of the tower block. Tonkin & Taylor carried out a site investigation programme including 10 machine boreholes, backed by a laboratory testing programme. Also available to support the investigation were seven historical boreholes from the Tonkin & Taylor database and testing associated with these investigations. Soils underlying Fort Street consist of general uncontrolled fill to a depth of four metres over the old Waitemata Group bedrock wave platform at RL 0 m. Beneath Shortland Street the soils generally consist of 10 metres of ash and tuff volcanic soils overlying Waitemata Group bedrock. The Waitemata Group bedrock consists of weak interbedded siltstone and sandstone deposits extending several thousand metres below existing ground levels. This rock is generally a stable and predictable founding material for highly loaded foundations. The volcanic soils on site consisted of clayey silt ash materials interbedded with slightly welded, gravelly sand tuff. Excavation in these materials around Auckland shows they are very competent in an undisturbed state and may stand up to 10 metres in near vertical temporary cuts. Founding level for the proposed structure was below RL 0, which placed foundations directly in the Waitemata bedrock over most of the site. With the close proximity of bedrock most of the foundations were detailed as "shallow footings" bearing on unweathered rock. Due to the high loads placed on the foundations these footings were over two metres wide and deep. Foundations in one corner of the building were piled due to the rock dipping lower in that area. The difference in levels across the site resulted in up to 20 metres of retained soil in some locations. Despite the stable nature of the retained soils there are many problems associated with retention of soils to these heights. A major concern in the initial design process was the possibility of a weathered contact layer between the Waitemata Group rock and the overlying ash. Any design concept had to be considered for the ability to retain a block failure triggered by sliding on a low strength layer. A team design approach between Murray Jacobs, Fletcher Construction, Brian Perry and Tonkin & Taylor resulted in a cost effective and very secure design for the main retention system. This consisted of bored 750 mm � concrete piles placed at 2.0 m centres with three levels of tie-back anchors. A series of meetings were held with other consultants representing neighbouring building owners to agree on appropriate design parameters for site retention. Most importantly these examined deflection limits and continued support for the adjacent structures. Tonkin & Taylor performed a staged analysis of the proposed wall construction using finite element computer software, and Murray Jacobs performed detailed structural design. Some major issues covered in design were seismic deformations, effects on adjacent buildings and long-term wall loadings. The assessment of loads at each stage of construction also identified the optimum size and spacing of the anchors to achieve capacity for the critical maximum loading, which was frequently an intermediate step of construction. Long-term soil loads were of particular concern to Murray Jacobs because of the change in ground levels across the site. The variation in level results in long-term soil loads much greater than the design base shear for seismic events. All of these loadings have to be transferred to the foundation through the structure. Because of the magnitude of these loads Tonkin & Taylor performed a detailed assessment to ensure the design was not overly conservative.
The southwest corner of the site after bulk excavation. Note the main wall to the left and the sloping temporary wall to the right. Also visible are strip footing excavations at the wall base and the core jump form in the foreground. Constraints of the construction programme required the construction of two temporary retaining walls within the site. A temporary wall was required around the foundations for the tower core to allow construction to start while the bulk excavation and anchor installation was still under way. To achieve this Tonkin & Taylor designed an anchored, bored concrete pile wall retaining up to 11 metres with a back slope of 0.7 to 1. This wall was installed by Brian Perry very early in the construction process and allowed Fletcher Construction early excavation of the core footings and installation of the jump form. One of the piles can be seen in the photo above, in foreground adjacent to the jump form. Further retention was required around the existing SunAlliance Building as this is to remain in place until the new tower is complete. After the existing building is vacated, it is to be demolished and the remainder of the basement and podium structure completed. To achieve this Tonkin & Taylor and Brian Perry designed an anchored, bored pile retaining wall to retain the soils beneath at the north and east faces of the existing building. A significant feature of this system was the severely restricted space available between the new footing and the existing building. This resulted in a wall that overhangs the work area at an angle of 17 degrees, a strange concept for a geotechnical engineer. The existing building piles have also been tied back to assist in retaining the soils. The above photo shows this wall and the footing excavation at the toe. Monitoring is essential for retention of this scale to measure wall performance during the life of the structure. With comprehensive monitoring in place it is realistic to assume remedial measures may be implemented at any time during construction and before significant stability problems occur. Based on this it is possible to use lower factors of safety than may otherwise be considered. Tonkin & Taylor monitored wall deformations using inclinometer tubes cast within four bored piles around the perimeter wall. These provided full deflection profiles for the wall at instrument locations. Confirmation of the results was provided by control surveys undertaken by Fletcher Construction. Results from the monitoring indicated actual deformations were very close to design predictions both in magnitude and profile. This provides a high level of confidence in the assumed soil parameters for further construction work on the site. 
The southeast corner of the site after 2/3 of bulk excavation. To the left of the view is an existing portion of wall anchored back as part of the retention system. The piling rig is installing underpinning for the wall. The close working relationship between the design and construction team on this project has encouraged the production of efficient designs that are appropriate to the builder’s construction methods. This has provided efficient solutions to the considerable construction and design challenges presented by the site geology. Ongoing monitoring has also provided excellent information on the performance of the retention systems. This information has confirmed the design parameters and will allow further refinement in design as the project continues. Chris Bauld is a geotechnical engineer with Tonkin & Taylor Ltd |
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