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Rock Anchor & Drilled Pile Retaining Wall, Lewis Pass.


Construction of Rock Anchor and Drilled Pile Retaining Wall

Client: NZTA

Duration: 10 weeks, April 2011—June 2011

Consultant: Opus ConsultantsRock anchor drilling

Project Overview

This project involved the rehabilitation of approx 40m of SH65 near Shenandoah (between Springs Junction & Murchison) where significant subsidence had occurred due to inadequate road pavement foundation. The original foundation of weathered granite was overlaid by soft fill material, the subsidence was caused due to movement that occurred at the interface between the two layers.

Opus Consultants provided the upgraded reconstruction design which included; rock anchors, and drilled piles secured to timber retaining / steel support whalers. This anchored retaining wall design provides improved durability to the road surface.

Rock Anchors
Construction of Rock Anchor and Drilled Pile Retaining Wall

  • RB32mm Reid Bar
  • Installed depth 10m
  • Number of anchors; 17
  • Double Corrosion protected via 2 casings of grout 
  • Drilled into moderately weathered granite
  • Installed 20 degrees above horizontal
  • Permeability tests  on anchor holes
  • Proof Testing of trial sample anchors
  • Suitability and Acceptance testing of anchors

Construction Methodology

Rock AnchorsRock anchors

An excavator mounted, Bulroc "Down the Hole" percussion hammer drill was used to form the anchor holes.

Depth: 12 metres total depth with 6m embedment into granite bedrock, and 20 degrees above horizontal.

This drill rig is a 20 tonne directional drill and uses a J Drive casing system that is suitable for greater diameter casing installations—as required in the soft overlying soils encountered on this project.

Our excavator mounted drill rig concept proved ideal for the constraints of this project, i.e; inadequate drilling access outboard of the drill holes necessitated the drill rig be located directly over the drilling area.

Proof Testing

Three test anchors were installed using 40 mm MacAlloy stressbar with minimum UTS 1295 KN. They were drilled 3m into competent rock then grouted. After a 28 day curing period these were proof-tested to BS8081 using approved hydraulic testing equipment. The proof test to 890 KN determined the “grout to ground” bond strength for the drilled granite material encountered.

Suitability and Acceptance Testing

We undertook onsite Suitability and Acceptance testing of the production anchors. Production rock anchors were subjected to a testing equivalent 85% of the design load. This both ensures the build quality of the bonded section and the free length function correctly.

Testing involved use of a hollow steam hydraulic jack capable of producing sufficient force to yield the bar. During testing the cyclic loading of the anchor was progressively increased while the elasticity over the unbounded section of anchor is measured. Measurements are taken using fine tolerance instruments, and the results measured against design criteria produced from the preceding Proof Testing exercises.

Rock Anchor case study PDF print version download.

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

Christchurch Address 

Greywacke Road extension,
New Zealand 


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Rock Anchor & Drilled Pile Retaining Wall, Lewis Pass.

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