Providing resilience in a Wellington waterfront development

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Published 24 November 2017
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Providing resilience in a Wellington waterfront development

Abstract

Wellington’s waterfront is in demand for new developments due to its proximity to the central business district, infrastructure, and harbour. This area can be impacted on by several hazards like earthquake shaking, liquefaction and lateral spread, see level rise and tsunami (tsunami not considered in this paper). The waterfront was reclaimed in a number of stages beginning in the 1850s, mainly by end tipping of weathered gravels. Liquefaction and lateral spread of these gravels as a consequence of a strong earthquake shaking event is likely to occur as was observed in the recent M7.8 Kaikōura earthquake in November 2016. This paper presents a case study of a five-storey building currently being constructed on the Wellington waterfront. To mitigate the impact from some of the natural hazards and to provide a resilient structure various foundation and ground improvement systems were considered. An in-ground cellular foundation solution was selected. This solution offers: liquefaction mitigation, shear resistance to resist lateral spread beneath the building, foundations for the new structure, temporary basement walls and effective cut-off of ground-water flow during construction. The foundation selection process, features and associated risks are discussed in this paper.

1 INTRODUCTION

Since the 2011 Canterbury earthquakes and 2016 Kaikōura earthquake, ‘resilience’ is one of the main focuses while designing foundations and substructures on sites prone to liquefaction and lateral spread such as the Wellington waterfront.

A five storey building with a basement is currently being constructed on the Wellington waterfront. While the waterfront provides unique location and amenity advantages for developments, the ground conditions are very challenging and complex.

The building design includes base isolation to prevent the building’s superstructure from absorbing earthquake energy, with ground improvement to support the base isolation system and provide a high level of resilience to earthquake damage.

This paper discusses the foundation options considered for this complex site, the basis of selecting the in-ground cellular foundation solution and its features and associated risks and hazards.

2 PROJECT INFORMATION

2.1 Proposed development

A five storey building with a basement beneath approximately 90% of the building footprint is currently being constructed on the Wellington waterfront (refer Figure 1 for site location). The building owner wanted to provide a high level of resilience under an extreme earthquake event, including a lower potential for damage in a severe event than a normal office building.

The building design includes base isolation to provide high seismic performance by reducing the extent to which the building’s superstructure absorbs earthquake energy. To support the base isolation system and ensure this high seismic performance, ground improvement was needed to reduce liquefaction and lateral spread.

Figure 1: Site plan

2.2 Specific site conditions

The site is on reclaimed land on the Wellington Harbour foreshore. The reclamation fill is underlain by thin layer of recent marine deposits and Pleistocene alluvial deposits, which infilled the steeply graded valley (Begg & Johnston, 2000). Changes in sea level built up the alluvial deposits in layers of gravel, sand and silt. Greywacke bedrock underlies the alluvial deposits at more than 100m below existing ground level.

The original shoreline, which ran along Lambton Quay, is approximately 300m west of the site (refer Figure 1). Prior to 1876, the reclamation fills extended up to Waterloo Quay (Semmens et al. 2010). The land beneath the site was reclaimed in the early 1900s. As illustrated in Figure 2, a mass concrete seawall was constructed immediately to the south-east of the site which formed the edge of that reclamation. It is likely that the reclamation was formed by tipping materials excavated during roading and other construction work, predominately silty sandy gravels. In the early 1970s, a reclamation southeast of the site was constructed, supported by a sheet pile wall to the east and rock revetment to the south.

2.3 Seismic shaking hazard and liquefaction risk

The seismic subsoil class for the site is considered to be ‘Class D – Deep or Soft Soil Sites’ in accordance with NZS 1170.5:2004 (Standards New Zealand, 2004). An Ultimate Limit State (ULS) of 2500 year return period has been considered. Peak Ground Acceleration (PGA) of 0.62g and a corresponding earthquake magnitude of Mw 7.1 was derived from NZTA Bridge Manual (NZTA, 2016) following the recommendation in recent geotechnical guideline (NZGS, 2016).

Liquefaction occurs when excess pore pressures are generated in loose, saturated, generally cohesionless soil (sands and non-plastic silts) during earthquake shaking. This causes the soil to undergo a partial to complete loss of shear strength. Such a loss of shear strength can result in settlement, bearing capacity failure and / or horizontal movement of the soil mass. Liquefaction of gravels within reclamation fills has also been observed following the earthquake in Kobe, Japan in 1995 (Cubrinovski & Ishihara, 2003; Hara et al. 2004; Hara et al. 2012), and the recent earthquake in Kaikōura in 2016 which greatly affected Wellington port land (Cubrinovski et al. 2017).