North Canterbury Transport Infrastructure Recovery (NCTIR) State Highway 1, Peketā
Key words: Rockfall, Canopy, Construction, NCTIR, Kaikōura
Abstract
The Rockfall Canopy (SCC-500) installed above State Highway 1 (SH1), south of Kaikōura extends for 104m in length and is the first of its kind in the Southern Hemisphere. The site is located 2 km south of Peketā on a corner of SH1 with a near vertical cliff that extends up 90 m above the road. Multiple rockfalls occurred at this location and it became clear to Waka Kotahi (NZTA) that something needed to be done to keep the travelling public safe. As part of the North Canterbury Transport Infrastructure Recovery (NCTIR) project, the SCC-500 canopy system was developed and selected due to its ability to self-clean and withstand multiple rockfall impacts up to 500kJ before requiring maintenance. The specific system has 13 posts ranging from 10 to 15m in length, anchored 6m above the road. The system was installed using a carefully choreographed combination of roped access personnel, helicopters, cranes, hiab’s and scissor lifts. The canopy project was the final and most complex of the NCTIR rockfall protection solutions.
1. Background
During the November 2016 Kaikōura Earthquake, the North Canterbury region experienced significant ground shaking and numerous slope failures. The transport corridor north and south of Kaikōura suffered extensive damage and was inundated by numerous rockfalls and more than 750,000m2 of debris from 85 landslides. The NCTIR Alliance was tasked with reinstating this damaged coastal corridor and re-connecting the communities of the South Island. The NCTIR Alliance was made up of two owners, New Zealand Transport Agency (NZTA) and KiwiRail, and four non-owners, namely Downer, Fulton Hogan, Heb and Higgins. The NCTIR Alliance was also supported by a design sub-alliance of Aurecon, Tonkin & Taylor and WSP.
The subject site is approximately 2 km south of Peketā on SH1, at the first prominent sea-cliff corner (Figure 1). The site comprises a 90 m high near vertical Greywacke cliff. The highway is located at the base of the cliff roughly 10 m above sea level supported on a seawall. The road is narrowly confined between the cliff and the sea.
Figure 1: Sea-cliff site, prior to construction
The site suffered minor rockfall as a result of the main earthquake event but remained relatively active during and following rainfall events throughout the NCTIR project. It was anticipated that the amount of rockfall would reduce with time, however the opposite occurred at this site, with incidents of rockfall increasing. The continued rockfalls posed an unacceptable risk to the travelling public and a solution was required to mitigate this risk.
A number of options to mitigate the risk were explored in conjunction with NZTA. The option of a self-cleaning rockfall canopy was ultimately considered the best option for the scenario and subsequently adopted for design and construction.
2. Designing a Rockfall Canopy
The rockfall mitigation system required for this site needed to protect the road without encroaching onto its narrow footprint, equally it was required to be quickly and efficiently installed with minimal future maintenance.
Unlike a traditional rockfall barrier, rockfall canopies are manufactured specifically for each unique site and the design is developed based on a full-scale 1:1 tested system. The first step of the project was to understand the hazard and then to assess the site to determine the canopy layout.
2.1 Understanding the Hazard
Using over three years of rockfall data collected during the NCTIR project (Figure 2), the design team were able to develop a working geological model and hazard profile for this section of State Highway 1.
Figure 2: Photo of rock trajectory from scaling
Detailed rockfall modelling was undertaken to quantify the rockfall hazard and determine design inputs. To start, the rockfall modelling software RAMMS, was used to conduct 3D rockfall trajectories analysis of the whole site to determine any preferential rockfall paths (Figure 3). This was then followed by a series of detailed 2D rockfall sections using Rocscience RocFall. The rockfall modelling allowed the team to determine suitable mitigation solutions and confirm that a rockfall canopy (such as the SCC-500) was a suitable option. The design philosophy of a self-cleaning canopy (SCC) is to use the falling rock’s kinetic energy and the structure’s elasticity to redirect rocks into a safe zone. The system is thereby low maintenance. The canopy system is only suited to steep rock faces and is an alternate to concrete galleries or tunnels.
Figure 3: RAMMS and RocFall Simulations.
Currently, the only full-scale tested rockfall canopy available is the Geobrugg SCC-500. This canopy is tested to a self-cleaning energy level of 500KJ and a maximum energy level of 1000kJ. The SCC-500 design was developed with 1:1 field testing in conjunction with the WSL (Swiss Federal Institute for Forest, Snow and Landscape Research). Although a canopy cannot be EAD 340059-00-0106 certified (formerly ETAG 027) the SCC-500 testing was still carried out in accordance with the guideline. The testing outputs were then used to calibrate Geobrugg’s finite element simulation software, FARO, which is then used to simulate future structures (Figure 4).
Figure 4: Geobrugg Canopy Testing (EOTA 500kJ block) and FARO simulation.
2.2 Designing the Concept
A number of alternative mitigation measures were considered during design development, including a concrete rockfall shelter and various extents of active mesh. However, with the design focus on minimal disruption to the community, a cost-effective solution, and minimal impact on the coastal environment it became clear a rockfall canopy was the best option.
The canopy size is broadly determined by three key factors:
- The slope geometry, the steeper the slope, the shorter the canopy net will be.
- The width that the canopy needs to span (i.e. the width of the road or rail)
- The allowable deflection of the canopy net, this is determined by the clearance height required above the road (or other infrastructure).
An initial 3D design model of the canopy was created (Figure 5) to determine the above metrics and confirm the suitability of the structure. This was followed by a calibrated FARO simulation from the SCC-500 testing to confirm anchor forces, post buckling resistance requirements and confirm that the seismic, snow and wind loads could be structurally satisfied.
Figure 5: (Left) concept design drawing, (Right) Concept 3D Design Model.
The canopy’s main components consist of:
- A plinth that connects the post to the rock face.
- A top and bottom rope that extend the entire length of the system.
- A series of upslope ropes that support the hinged post and the canopy net.
- A downslope rope from the end of each post to keep the system tensioned and help self-cleaning.
- Brake elements that absorb energy by deforming plastically and reduce maintenance.
- Upslope, downslope and lateral anchors that secure the structure in place.
Using a digital design approach the canopy was optimised for the site resulting in a length of 104 m made up of 13 spans of 8 m widths. The post length ranged from 10 to 15 m with a canopy net length between 12 to 26 m at 35 degrees upslope. The posts were anchored at 6 m high above the road and inclined at 15 degrees. The primary net used is the high tensile (1770MPa) ROCCO ring mesh with a secondary TECCO 65/3 for aperture control. The structure also has a central support rope separation to enable a split construction for more efficient installation and maintenance.
2.3 Completing the Design
After completion of the concept design during lockdown in April 2020, a site visit was scheduled during Level 2 Covid-19 restrictions to mark out the proposed anchor locations and assess the local topographic irregularities to determine if posts needed to be modified or moved. One of the highest risks identified in the early design stages was ensuring the post locations were accurately transferred to site within the 3-dimensional space on a highly irregular rockface. If a post ended up being too long, short or misaligned, it would have a significant effect on the geometry and performance of the structure. To mitigate this risk an extensive digital design model was developed and additional measures such as an innovative plinth with the ability to vary thickness was designed to allow minor tweaks to occur during construction to account for challenges on the rockface.
Another critical challenge in the design was securing the downslope anchorages. This needed to be independent of the existing seawall and remain outside the narrow road corridor. The innovative solution developed was to install isolated plinths beneath the road that would transfer the downslope forces from outside the seawall down to the underlying rock.
The final design of the rockfall canopy was completed in a 3D digital design model (Figure 6) and translated to a series of construction drawings. Geobrugg also provided a site-specific installation manual to assist in the construction of this unique structure. The digital model was continuously used during the project to assess the impacts of any new site geometry information and enabled the 3D structure to be accurately surveyed throughout construction which instilled confidence in the team. The digital design approach was extremely useful for assessing interactions with adjacent structures, in manufacturing the components and educating all stakeholders of the design intent.
Figure 6: Digital Design Model
In addition to the design benefits, the digital model was also developed into a detailed visualisation by the NCTIR visualization team (Figure 7), this allowed for communications and videos to be developed for stakeholder and community engagement. The advantage of these visualisations is that they are built from the design model, and therefore illustrate an accurate representation of the structure.
Figure 7: Digital visualisation
2.4 Below and Adjacent to the Canopy
As part of the finished design, the areas of the cliff below and adjacent to the canopy structure were carefully considered and systems were designed to mitigate rockfall hazards from these areas. Beneath the canopy several areas were actively supported with either TECCO 65/3 mesh or anchored and sprayed with shotcrete to reduce the minor rockfall from beneath the canopy mesh. Active mesh was extended to the south of the canopy in an area where the geometry was not suitable for the canopy. To the north of the canopy where the slope becomes much shallower a rockfall attenuator (ATT-60) was constructed, as the slope angle was unsuitable for a canopy.
3. Constructing a Canopy
The construction of the rockfall canopy was completed by the NCTIR team. Rock Control were brought in as the specialist contractor to install all elements of the structure including ground anchors, concrete structures, shotcrete, rock mesh and canopy construction. Selection was based on their extensive experience in building rockfall protection structures and past experience along the Kaikōura Coast. Early engagement with contractors in the canopy project was crucial to establish a construction methodology, identify challenges and establish solutions, this process continued for the duration of the project. The majority of the construction was completed during nightshift, which allowed the road to remain largely open during the day, minimising disruption to the public.
All works were completed by an experienced crew with a range of specialist skills to undertake all aspects of the project. The system was installed using a combination of rope access personnel, helicopters, cranes, hiab’s, elevated working platforms (Figure 8) and various drill rigs.
Figure 8: Canopy construction during nightshift road closures
Due to the high variability of anchor heights along the alignment, a range of drilling equipment was utilised and included an excavator mounted rig, rope access and hiab assisted drilling techniques. Where possible the larger drill rigs were used to increase productivity with light weight drill rigs limited to the higher extents of the canopy. Rig movements were minimised where possible to optimise productivity.
NCTIR’s strong culture of safety was carried through to this project, and from the start the team’s focus was on protecting those tasked with building this challenging structure. A top down approach was adopted, where initial scaling of identified hazard areas was completed, followed by the installation of draped temporary mesh and erosion control matting to minimise rockfall impacting the canopy area during the construction (Figure 9).
Figure 9: Temporary rockfall protection above the canopy
Construction supervision during the canopy installation was critical in communicating information between the construction and design team. NCTIR had a dedicated onsite team who monitored all aspects of construction night and day. Geobrugg also provided support during critical installation steps and completed full climb over inspections and a final sign off (Figure 10).
Figure 10: Final site walkover
4. Completing the Canopy
The Kaikōura Rockfall Canopy project was completed in June 2021 and formed the last project of the NCTIR programme. The complexity of this structure and dedication of NCTIR’s One Team through to the end epitomises the overall success of the NCTIR project (Figure 11).
Figure 11: Completed Canopy
Figure 12: The on-site Canopy Team
Design and constructing the Southern Hemisphere’s first rockfall canopy is a testament to the hard work and dedication of all those involved.
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The authors are grateful to the North Canterbury Transport Infrastructure Rebuild (NCTIR) and New Zealand Transport Agency (NZTA) for permission to publish this paper. The dedication and contribution of all members of the design and construction teams is respectfully acknowledged.
