Arras Tunnel Project

Spencer Holmes Limited was contracted by Memorial Park Alliance to complete high accuracy monitoring of existing buildings, ground levels and the temporary wall piles around the State Highway 1 Arras Tunnel underpass and Memorial Park on Buckle Street.

Installation of horizontal & vertical control network:

We installed a high accuracy horizontal and vertical control network over an area of approximately 5ha including:

  • 25 primary control marks,
  • 165 building and ground monitoring marks and
  • 115 monitoring marks on the temporary wall (the retaining wall offset from the tunnel wall).

The survey marks used for the monitoring comprised steel nails drilled and epoxied into concrete walls, building foundations and kerbs.

The horizontal survey control network was installed by closed loop traverse observations using a Leica total station. Survey lines were observed multiple times and adjusted by least squares adjustment.

The vertical level control network was installed using a Leica DNA3 digital level. Levels were transferred by closed loop levelling runs completed multiple times and adjusted across the site. Subsequent monitoring was completed using a Leica Sprinter 250M digital level.

Due to the active nature of the work site primary control marks needed to be installed well clear of the site, in the adjacent streets, to ensure accurate control points for measurement of movement.

Initial baseline readings of all monitoring marks were measured twice and subsequent monitoring was compared to these baseline readings. Results were uploaded to an online spreadsheet which automatically notified Memorial Park Alliance each time it was updated.

Temporary wall pile monitoring:

The temporary retaining walls were constructed to facilitate the construction of the tunnel walls, and were offset from the actual walls by approximately 1 to 2 m. These walls were constructed by driving steel piles into the ground and incrementally placing horizontal timber posts between the piles as the earth was excavated. This eventually created a retaining wall that was as deep as the tunnel.

The monitoring marks in the temporary wall were punch marks installed in the tops of 45 of these piles. Up to two rows of reflective targets were mounted in whaler beams below these marks.

The position and level of these pile monitoring marks were measured for each round of monitoring and X, Y and Z coordinates were calculated in terms of the control network. These coordinates were then uploaded to the online spreadsheet.

Building and ground level monitoring:

Monitoring marks were installed in all buildings and along streets within approximately 100m of the tunnel.

Levels were transferred to these monitoring marks from the control network for each round of monitoring. These levels were then uploaded to the online spreadsheet.

Settlement limits:
Settlement limits for each type of monitoring mark were determined by Memorial Park Alliance engineers. Warning and alarm levels were then determined as a percentage of these settlement limits. The online spreadsheet would report if warning or alarm levels were reached.

Conclusion:
Monitoring was undertaken over a period of 18 months and continues on a reduced basis until the opening of the Memorial Park. Over this period of time we encountered various obstacles including high winds, rain and plant exhaust. This required extra measurements and slight changes to methodology to maintain monitoring tolerances.

Two major earthquakes around magnitude 6 occurred in Wellington over the monitoring period. A full round of monitoring of all marks was completed after each earthquake to establish any movement caused by the earthquakes.

The equipment and methodology employed by Spencer Holmes for the duration of the monitoring proved to produce accurate, reliable and repeatable results to the required accuracy tolerances.