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New Zealand Engineering 1998 June

Metropolis : Foundations wit hin a Volcano

Andrew Langbein is senior geotechnical engineer with Tonkin and Taylor, Auckland

In April 1997, Tonkin and Taylor was engaged to carry out a geotechnical investigation for Metropolis, a major high rise development sited on a relic volcanic cone. The geotechnical and construction knowledge from nearby sites led to expectations of extremely variable and uncertain geological conditions. The large column loads for the relatively slender tower configuration of the 40-storey apartment building and tight differential settlement tolerances necessitated an innovative design approach with flexibility to adapt to the possible variations.

Drilling of f900 mm and f1200 mm shafts.
Inspection casing (with windows) installed in the middle of the starter casing in the centre of the photograph. Augering of basalt rock being undertaken. Belling tool in the right foreground.

The piling contractor was closely involved in pre-award discussions to enable a full appreciation of the design approach and effects of ground conditions. Extensive proof drilling was carried out permitting a construction approach largely based on "observational methods". The pile foundations were recently completed under budget and with appreciable savings on the project. This article presents a summary of the investigation, design and construction approach for the challenging site conditions.

Metropolis is situated in the Auckland CBD, on the corner of High Street and Courthouse Lane. The high rise development is a relatively slender tower structure with a foundation plan area of 52 m H 45 m, and will be Auckland's tallest apartment building.

The project is a Krukziener Properties development. The project manager is Promanco Kenman Ltd, with Holmes Consulting Group providing structural engineering services, and architectural design by Peddle Thorpe Architects. The pile construction was carried out by Brian Perry Ltd, and Multiplex Constructors (NZ) Ltd has begun construction of the structure.

Pile cap of central tower and lift well constructed. Construction of retention piles to the north and additional "podium" piles to the south (foreground). The derelict Auckland Magistrates Court to the right of the photograph.

Geology

Approximately 60,000 years ago, a volcanic explosion occurred in the area of the present Shortland and Victoria Streets. Tuff, basalt and scoria were ejected from the volcanic crater, centred near the presently derelict Auckland Magistrates Court. Several vents are believed to exist, from which pyroclastic material was ejected.

The Magistrates Court was constructed on the present site in 1955. This six storey structure with a single basement level was founded on a raft. The founding level for this building coincided with scoriaceous gravels, which were significantly weathered. Shortly after completion of the building, some settlement was recorded, which exceeded estimates and was attributed in part to crushing of weathered scoria gravels under high contact pressures. These historic observations typified the uncertain behaviour of the formations within the relic volcano and its surrounds.

Subsurface conditions

Construction difficulties experienced on adjacent sites, together with expectations of highly variable and complex subsurface conditions, necessitated an intensive geotechnical investigation programme. This involved two stages of investigation drilling followed by proof drilling of every column location.

Results from this work showed surficial soils to a depth of up to eight metres, consisting of a melange of silts and weak scoriaceous gravels. Basaltic boulders were encountered within this deposit. This unit grades into a more competent stronger soil/rock that is described as scoriaceous lava. These materials are weak to very weak red/black vesicular rock. Occasional moderately strong basalt boulders/flows were encountered within this layer and the drilled core contained numerous fractures. It was inferred that this unit is a weak rock with possibly scoriaceous gravel layers, coinciding with poor core recovery.

Moderately strong basalt rock was encountered on the southern part of the site. This rock has weathered slightly and is weaker than other basalts within the Auckland Volcanic Field. This may in part be explained by this volcano being at least three times as old as most other Auckland volcanos. The flow is of variable thickness, becoming thinner to the north and west.

Beneath the north and east parts of the site, the scoriaceous lava is underlain by layers of volcanic ash, tuff and displaced ECBF (East Coast Bays Formation) rock which has fused with basalt in some locations. The dip of these geological units indicates that the centre of the volcanic vent is to the east of the site.

Laboratory testing consisting of unconfined compressive tests and Hoek cell triaxial compression tests were undertaken on selected rock samples of scoriaceous lava and basalt.

Pile design

The design approach, given the subsurface variability and potential for uncertain founding conditions, was to develop a range of pile configurations comprising:

• "End bearing" piles: end bearing piles were designed to be founded within moderately strong basalt underlying the southern and western part of the site

• "Load sharing" piles: using a combination of skin friction (80 percent) and end bearing (20 percent). These piles are able to provide a greater degree of confidence in the variable and uncertain geological conditions, by mobilising a larger area of soil compared with end bearing piles

• "Multiple belled" piles: heavily loaded columns, founded above shallow thin basalt flows (three to five metres thick), were constructed with an upper bell. Upper bells are able to mobilise the strength of the basalt layer. Displacements of the piles and axial shortening within the piles can also be reduced.

Stipulated design criteria required a differential tolerance of less than 15 mm (ignoring axial shortening) with column loads of up to

18 MN. Pile settlement modelling was undertaken using both finite element and finite difference software and settlements were checked for a range of pile loads and dimensions to confirm compliance with performance criteria. Design dimensions ranged up to 1.2 m shaft diameter with two metre diameter bells and pile lengths typically of 11 to 31 metres.
Pile construction
Construction of the piles began in November 1997, using two hydraulic auger rigs operated by Brian Perry Ltd. The dominant scoriaceous lava proved to be significantly more massive than anticipated, grading from a cemented scoria gravel at the surface to a weak weathered basalt at depth. The contractor maintained good progress throughout the works completing 55 piles in two months.

An "observational approach" was adopted for the checking and confirmation of the selected pile configurations. This used:

• Pile shaft inspections of partially completed piles, typically prior to belling, using a modified casing with inspection windows

• A review of pile shaft drilling records, comparing drill rates over the pile length

• A comparison of cuttings and shaft observations with on-site stored core from the proof drilling.

The above approach permitted modifications to the pile lengths and the omission of selected pile bells, providing savings in both time and cost. This approach was also useful in the eastern part of the site where severe groundwater conditions prevented full shaft inspections on a number of piles. In this area the "observational approach" was supplemented with inspection results from nearby dewatered piles.
Pile monitoring
As part of the validation of the design/construction approach, settlement monitoring devices have been installed in three selected piles. These will be monitored during the tower construction to provide settlement data and for determining the mobilised load distribution.

The settlement monitoring devices consist of a series of potentiometers connected to sleeved fibreglass rods of differing lengths. In combination with precise surveying of the pile cap these will enable the amount of axial shortening over the length of the pile to be assessed. The amount of skin friction mobilised by the soil/rock can also be calculated using this arrangement.

The construction of the foundations for Metropolis involved an innovative approach to address the geologically variable conditions. The staged investigation, development of design options, and extensive proof drilling provided for flexibility for the pile construction. A close working relationship was maintained with the structural designers and piling contractor during all stages of the design and construction, and the use of an "observational approach" allowed for revisions to the pile configurations and associated cost and time savings for the project.