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Rocksaw Road Widening, Matiri Narrows


Principal Stakeholder: Tasman District Council (TDC)

Client: Fulton Hogan

Duration: 14 February 2014–28 February 2014

Project Overview

Matiri Narrows road rock saw road wideningThe widening of a 275m stretch of the two-way road beside a rock wall, known locally as the "Matiri Narrows". This is a short section of the Matiri Valley Road cut into the steep cliffs just above the Hope River's western bank beside "submarine rock". The location is just north of Murchison off SH6. Prior to widening the carriageway was too constricted for large vehicles to pass by each other with an adequate lateral margin of safety. 

There were two recent non-injury accidents reported involving trucks where the northbound truck closest to the cliff face had drifted off the seal, and the narrow shoulder and then collided with the wall—dragging along the wall for several metres—both trucks that collided with the rock wall were extensively damaged and written off. A local school bus operator also claimed that one of his drivers had a close shave with a stock truck. 

School bus passes work in progressPrior to these incidents Barry Whitnall (Fulton Hogan's Murchison area supervisor) had been urging TDC to widen The Narrows, and so asked one of the affected operators to write to TDC and express his concerns. In turn TDC then approached Fulton Hogan for an estimate to widen the 275 metre stretch of road. 

Barry knew that TruLine Civil (TLC) had an excavator-mounted single-blade rock saw based in Greymouth, so he contacted us, explained the project and invited us to visit the site. TDC accepted Fulton Hogan's estimate for the work and as Truline Civil had just completed rock-saw trenching through 32 km of solid rock along the West Coast for the Ultra-Fast Broadband fibre optic cable lay, we were able to widen the Matiri Narrows commencing  Friday 14 February 2014.   

Rocksawing/Road Widening Methodology

The work advanced rapidly with a combination of earthmoving and hauling equipment working together in unison; 

  • TruLine Civil's 2.1m dia. hydraulically driven rocksaw "quick hitch" attached to the boom of a 24T Volvo excavator (EC240CL) 
  • While a second CAT excavator was kept busy filling dual roles (described below)  

TruLine Civil's 2.1m rocksawIntermittently, after the rock saw made a series of roughly 1m deep parallel cuts down into the rock parallel to the road, approximately 50cm apart, with the length of the cuts out to the working reach of the boom, the second excavator "nibbled" out the resulting 1m tall vertical slabs to clear more rock for cutting.

Subsequently the second excavator prepared an elevated ramp of rock spoil 2m high for the rock sawing excavator to advance onto and safely cut back the rock face up to the 6m ceiling. To leave the best aesthetic finish on the cut rock wall face the operator worked the rocksaw blade in a manner similar to an over-size grinder, cutting back the face in swathes of about 20cm width that rendered the conglomerate rock of the cliff face down to small rocks and dust and left the rock wall with an attractive, smoothly dressed finish.

TruLine Civil's Rocksaw at work on the Matiri NarrowsOtherwise the second excavator was kept busy loading the fully recoverable rock spoil onto the Fuso truck.

The final cleanup of the widened road was achieved with a Hitachi Wheel-loader. 

Over the job's two week duration the 275m stretch of rock wall was cut away vertically up to a ceiling of 6m, and laterally back from the existing road shoulder to a width of about 3m. 

Site Preparation

The Fulton Hogan crew prepared the site first with an excavator, cutting the bulk of the vegetation off the rock wall, in order to expose the rock which was to be removed as spoil and ensure it was sufficiently clean for re-use at a later date. The site prep. also made it straightforward for the rock saw operator to see his cut. 

1,400m3 of usable rock material was hauled away from the site for use later on, while some of the cut rock was used on-site, and compacted to level the shoulder and maximise the width of the carriageway.

Rock composition

The hydraulically powered rock saw attaches to the excavator boomThe rock wall / cliff face is mostly graded beds of sandstone and mudstone conglomerate rocks (equivalent to 30MPA which was the softer rock}, there was also a bed of very hard Separation point granite in one area—the saw operator successfully cut through this but it was much slower going. 

Rock saw blade operation

The Rock saw blade is 2.1m in diameter, fitted with Sandvik or equivalent tungsten carbide tips. The blade rotates at 22rpm, resulting in a 2.4m/sec cutting velocity at the blade edge which was sufficient to cut through the mudstone and sandstone rock with relative ease. 

Rocksawing continues as spoil is loaded Road is finally reinstated with a wheel loaderCutting generates some airborne dust but because of the relatively slow rotation of the blade, there is very little fly-rock cast off either laterally or forward of the blade. This meant the operator only needed to cease cutting during the very brief period that motorists were passing in close proximity to the excavator on the single lane adjacent to the work.

Traffic management

North and south-bound traffic ingress/egress was managed with radio communication to rocksaw operator ensuring the objective risk of the work to road users was negligible and very well managed.

Road reinstatement 

The reinstated road 30m behind the cutThe workflow of removing the spoil immediately behind the leading cut meant that all that was required to reinstate the carriageway was a final cleanup with a Hitachi wheel loader. The road reinstatement was completed in-full right behind the Rocksaw's last cut, meaning the benefit of the safer widened dual carriageway was immediately available to road users when the earthmoving equipment rolled off the site at the end of the final day. 


Representatives of the TDC visited the site at the conclusion of the work and were very happy with the results they saw. Matiri Valley locals, the School Bus operator and his driver have all given excellent reviews of the now safely widened Matiri Narrows.

Download the Matiri Narrows Rocksaw road widening PDF print document.

Contact us

TruLine Civil is determined to incrementally improve the service we deliver our clients, and the quality and range of civil engineering services that we offer to owners and stakeholders who expect excellence.

Excellence in Infrastructure.

TruLine Civil.

Interested in our services? You can contact us +64 (21) 228-5090 and ask for Geoff Powell or e-mail me at 

Christchurch Address 

Greywacke Road extension,
New Zealand 


Click to view the Matiri Narrows road widening slideshow >>

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Credits: Photos and case study documentation by Shaun G. Waugh

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Rocksaw Road Widening, Matiri Narrows

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