2011 was a rough year for Mauao beginning with a severe storm event on 29 January. The following information was released in June and May respectively to help explain some of the issues that the Council was facing with regard to repairs.

Mauao track repair and rehabilitation timeline

What did we do, when did we do it, and why?
June 2011

Regular users of the base track may find it useful to read about some of the decisions that have been made since the mountain was closed after the 29 January storm.

Mauao is an archaeological site. Any proposed earthworks on site requires clearance from the Mauao Steering Group and in many cases from the Historic Places Trust. Before any debris is to be moved a representative from the Mauao Steering Group needs to advise staff whether or not an Iwi monitor will be required for each given section.

The main principle driving all decisions about track repair on Mauao are:

• no unnecessary earthworks
• keep machinery movements along the tracks to a minimum. 

Timeline

29 January, 2011
It rained – a lot! Major slips, closure of the entire mountain.

Map showing all of the slips that were identified after the January storm event.

3 February
Staff and geotech consultants presented a report detailing the status of the mountain to the Mauao Steering Group. At this meeting approval was given by the Steering Group for debris clearing works to proceed on the maunga. Minor works began that day, major debris shifting work began 8 February, partial base track access and one summit track opening (Waikorire) was achieved by 5pm 9 February.

February focus
Ongoing repairs to summit tracks with a focus on clearing as much debris as possible from the summit tracks and along the opened base track section on the ocean side.
Designs prepared for repair to the Oruahine summit track.

3 March
The Mauao Steering Group was presented with a range of options that could be used to repair the major base track slips. (The major slips are numbered as Slips 3, 4, 5 and 6.) Of the options available to repair the largest slip, Slip 5, a bridge was recommended as the best long term way to ensure pedestrian safety and to manage ongoing debris. The Steering Group approved the bridge as an option to pursue, and that it should be a timber bridge. They directed that a ‘soft’ approach be taken on the other slips; that debris was to be cleared without any unnecessary aggressive construction.

During February/March methodology was being assessed for repair of the base track stone wall section. The stone wall section needed to be repaired first to allow machinery access to the major slips.

Main considerations were how to fix the stone wall section in the most economic way while not adversely damaging the tracks.

A ‘fast fix’ option was considered which involved bringing in rocks to prop up the wall. This was ruled out for a number of reasons. It was not the best long term option for the integrity of the track; over 200 movements would have been required to transport the rocks along the base track between Pilot Quay and the stone wall. This would have required extensive track resurfacing. Alternatively, a barge could have been used to transport the rocks in at $5K per day with a larger set up cost. Even without the barge, the price estimate for the job was around $65K.

The method eventually used to repair the stone wall section cost $28K and has provided a better long term solution.

14 March
4WD track opened.

By 3 March designs had been completed for the first Oruahine slip repair, which required a “crib” retaining wall. Building consent was issued on 14 March. (It was issued as an “emergency works” consent which saved 4 – 6 weeks.)
Work on the first Oruahine retaining wall started 24 March and was completed on 16 May.

 Oruahine track construction pictured, April 

15 April – designs received for the stone wall and the process to draw up formal contract documents began.


March – May
Continued investigations into bridge option for Slip 5. Considerations included:
• Should it be a suspension bridge or conventional. (Settled on conventional)
• Should it have deep piles or shallow foundations?
• Surveys were undertaken to better understand the profile of the ground that was being worked with.
• What would the drilling costs be?
• How best to manage ongoing debris that will continue to be delivered down the slope.

3 May  
Stone wall contract documents sent as an invited tender to three contractors.

9 May
Stone wall tenders all received and contract awarded.

18 May
Stone wall work started on site

June
Stone wall work was completed 7 June.

By the end of May the stone wall work had reached a point sufficient to permit machinery access to the major base track slips. Slips 3 and 4 were tidied up pretty quickly.

About the same time (end of May) City Planners advised that the proposed bridge, which had not yet reached final design, would need to go through a robust resource consent process. In addition to the two months required for construction, the resource consent process would add at least three months to the timeline.

This prompted a directive to the contractor to construct a temporary track in the hope that a reasonable alternative could be achieved as a short term option for public access. The aim was to create a path at least 1m wide. The main obstacle was a 4 tonne boulder. The heaviest excavator able to be used for work on the base track was a 3 tonne digger. (Blasting or breaking of rocks on Mauao is not permitted by Tangata Whenua.)

As it turned out, the contractor had a superb piece of luck with the way the boulder shifted during the track construction. The result exceeded all hopes, enabling a 2m wide track.

In light of this result the bridge option has been put on hold for reassessment. It remains the best long term option. Preparation and design to date for the bridge option is $26K. If the contractor had not had such a good result with the Slip 5 boulder we would continue with the bridge.

Ongoing issues

It should be noted that there have been ongoing weather events throughout the year. The 4WD track had several reactivated slips and has needed to be closed more than once. Slip 6 has failed five times – the most recent being Saturday 4 June.

The Oruahine track will remain closed for at least three months. Design, building consent and resource consent preparation is still being finalised for a second crib retaining wall that is needed for additional washouts that occurred in the intervening months since January’s storm event.

We have installed two warning signs, one at the approach to Slip 6 on the ocean side and at the approach to Slip 3 on the Pilot Bay side.

Future communication about Mauao on the website, brochure and general information will state that the mountain is an unsafe area in wet weather.

We should also expect that track closures will be a part of life in future.

Tauranga City Council June 2011

Previously published information, May 2011:  Some design and construction challenges facing the Mauao fixup job since the Jan 2011 storm event

While repairs have been ongoing higher up the mountain on the 4x4 and Oruahine summit tracks, extensive planning and design work has been underway in preparation for repairing the base track washouts. The biggest challenge is that it is almost impossible to get the necessary machinery and materials around the base track. This has put huge constraints on what might otherwise be a more straightforward construction job.

The largest washout 

The largest washout on the Mauao base track is a combination of three slips that all exit at the same point. This newly created washout gully will continue to deliver large amounts of debris down the slope in future storm events. The challenge is how to reinstate the track while limiting damage from future debris.

A 15 m long bridge will be built across this washout. The aim is to allow future debris to wash under the bridge with minimum damage to the bridge and track. The challenge is to find a solution that is the most economic for the long term; to design for most storm events while accepting that ‘minimum damage’ in this environment is no guarantee. (There are a number of large trees further up the slope which could potentially wipe out a bridge if they came down the slope.)

The bridge must be designed for a maximum load, about 4 tonnes including its own weight. This is not just something you plonk onto the track. To support this bridge, piles need to be drilled deep into the track through rocks and boulders. This requires the sort of drilling rig that can not currently access the site. Access is a huge problem: access for vehicles to deliver the bridge itself and access for the machinery that is required to install the bridge. So the solutions to two problems are still being assessed: 1) how to get the bridge to the site and 2) how to install the bridge when none of the necessary equipment can get there. The bridge will probably need to be delivered in sections. This still requires some logistical problem solving to ensure that the rest of the track can handle whatever vehicle movements are needed.

Track stability 

The Mauao base track is basically just a pathway that has been cut into the mountainside; it was never designed for heavy machinery. The repair programme must be planned in a way that aims to keep machinery movements to a minimum over the base track. Therefore a lot of planning has gone in to limiting the distance that the landfill from each slip will transported around the base track. The repair programme as a whole has had to be planned in its entirety before any work can start.

The most significant weak spot is a stone wall that runs along the base track, about 50m from the sandy beach. This wall is still intact but storm damage has eroded and undermined it at the base. The conventional method of reinforcing the erosion with boulders and landfill has had to be ruled out because of the access issues. (Ocean access is not as simple as it might seem, nor cheap. The sort of barge that is suitably sized to transport the material and machinery required to repair the stone wall, for example, is not available locally, would come at a cost of $30K per day, and would still not guarantee easy access to the site.)

Once the stone wall is fixed the smaller slips can be addressed fairly quickly – about a week of work per slip (except for the bridge). Debris from each slip will be cleared and placed in a location where it can be reused. A method for fixing the stone wall has now been designed and tendered. Work will start in the next week, take up to three weeks and cost around $30K, after which time the remaining slips can be addressed. Hopefully the base track can reopen after August.