The Sumner Road Reopening Project – rockfall mitigation and road repair of an important route between Lyttelton and Christchurch

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The Sumner Road Reopening Project – rockfall mitigation and road repair of an important route between Lyttelton and Christchurch

ABSTRACT

Sumner Road between Lyttelton to Evans Pass was severely affected by rockfall as a result of the 2010-2011 Canterbury Earthquake Sequence. Approximately 2 km of Sumner Road was closed to all traffic following the 22 February 2011 earthquake. Sumner Road was used as a corridor for dangerous goods vehicles between Lyttelton and Christchurch and the absence of this route periodically required temporary closing of the Lyttelton Road Tunnel to the public to allow dangerous goods vehicles to pass. In 2015 Christchurch City Council (CCC) and New Zealand Transport Agency (NZTA) initiated a programme of works to mitigate the geohazards affecting this road corridor. A team comprising Golder and Jacobs staff provided concept designs, tender evaluation and supervisory technical expertise to the client during construction. The physical mitigation of rockfall hazard, repair of earthquake damaged retaining walls and the road pavement took 2.5 years and the road was officially opened to the public on 29 March 2019, more than 8 years following the earthquake that closed it. Significant cost savings were possible due in part to our re-evaluation of the primary rockfall source slope as not having experienced significant rock mass damage, therefore avoiding costly large-scale benching and associated earthworks.

Introduction

The project area was affected by significant rockfall on multiple occasions during the 2010-2011 Canterbury Earthquake Sequence (CES), both as a direct result of specific earthquakes and subsequently. Sumner Road was also affected by rockfall prior to 2010, with anecdotal records of rockfall occurring during most winters. Prior to 2010, rockfalls were typically of small volume, with block sizes less than about 0.5 m in maximum dimension. Debris flows also affect the road corridor, including several that occurred during a heavy rainfall event in 2014. Concept level risk assessments completed by several consultants including Golder concluded that post-earthquake rockfall presented an unacceptably high risk to road users and that mitigation measures were required to reduce the risk to an acceptable level.

The road formation and retaining walls were also damaged as a result of earthquake shaking and debris flow, the latter causing disruption of the surface stormwater system. While design of these elements was not part of the original rockfall mitigation work scope, it is relevant to note that rockfall caused relatively minor damage to the road formation (i.e., impact craters and damaged concrete parapet walls) compared to the damage from earthquake shaking.

Concept design for rockfall mitigation at the site (Aurecon, 2013) included reinforced earth bunds at two gullies at the western end of the project area, extensive slope modification and scaling (approximately 1 Mm³) to remove the potential source rock.

Slope Instability Hazard

Sumner Road traverses steep, south facing slopes adjacent to Lyttelton Harbour (see Figure 1 and 2). The slopes include prominent cliffs up to 100 m high that slope at about 70° or more. The area is underlain by Miocene age basaltic to trachytic volcanic rocks of the Lyttelton Volcanic Group (Forsyth et al., 2008). These lava, tuff and breccia beds dip at about 10° to the north, into the slope. The weaker breccia and tuff beds typically slope at about 45°.

The southern side of Lyttelton Harbour exposes basaltic lava flows of the Late Miocene Diamond Harbour Volcanic Group. This formation suggests that the general configuration of slopes around Lyttelton Harbour is at least Late Miocene in age.

Figure 1: Location plan of Sumner Road Remediation Project site

Figure 2: Aerial photograph of the eastern area of Sumner Road Remediation Project site prior to construction where Sumner Road passes below 100 m high steep rock cliffs. Yellow and blue zones denote recent, likely earthquake-related rockfall. Red zones are debris flows associated with the 2014 heavy rainfall event.

A range of slope instability hazards affect Sumner Road including rockfall (at various scales) and debris flows. Minor rockfall has reportedly affected the road on an almost annual basis prior to the CES with failures mainly coming from the road cuttings. Rockfall was widespread along the project site as a result of the earthquakes. Based on aerial photographs and field mapping it is estimated that about 9000 m³ of rockfall occurred as a direct result of the earthquakes. The dominant source areas for rockfall from the earthquakes were the natural cliffs and to a lesser extent the road cuttings, with about 90% of rockfall being sourced from these areas.

Based on observations of the scars from rockfalls on the slope, the failure mechanisms for individual source areas have mainly comprised toppling, which is consistent with the layered volcanic formation dipping gently into the slope. However, evidence has been recognised for discontinuity-controlled wedge failures, planar failures and isolated blocks being thrown directly into the air. The largest observed slope failures resulting from the earthquakes have extended about 5 m into the slope, but generally the failures are limited to 2 m into the slope behind the pre-earthquake face.

Debris flows followed the 2014 heavy rainfall event affecting several locations within the project area (see Figure 2). However, the debris comprised mainly fine-grained silty material, with relatively minor boulder fraction. Consequently, vehicles impacting debris flow material is relatively minor compared to boulder dominated rockfall debris. As such, the risk to road users associated with debris flows is relatively minor compared to other geohazards.

Concern was previously raised about the potential risk of collapse of the cliffs above Sumner Road during a future earthquake. Previous work on the site suggested that the rock mass strength had been significantly degraded as a result of the CES (e.g. Massey et al., 2012). Our observations during field mapping did not support this theory; we observed no significant tension cracks along the crest of the bluffs and relatively minor dilation of the rock mass in the slope during abseil inspections. We concluded that, despite the bluffs being steep rock slopes, rock discontinuities in the slope are generally favourably oriented. More importantly, the bluffs have not been actively eroded by wave action during the late Quaternary. This contrasts with the well documented location of cliff collapse around Sumner, where coastal erosion had occurred during the Holocene.

Design Approach

Risk Assessment

CCC’s approach to the mitigation of rockfall risk along Sumner-Lyttelton Road Corridor was to meet a risk based criteria of better than ARL2 (Assessed Risk Level – using the semiquantitative risk assessment approach of RMS, 2011) with respect to road user safety and to ensure that the road could be reopened (at ARL2 or better) within three days of a future earthquake similar to the CES events.

Golder and Jacobs undertook a semi-quantitative risk assessment for rockfall risk affecting Sumner Road users following the RMS (2011) method. We divided the corridor into 100 m long segments and calculated an ARL for each. This assessment indicated that about 90% of the road corridor was estimated to be ARL1, the highest risk level assigned by the RMS method.

Preliminary design

Golder and Jacobs developed a preliminary design for rockfall mitigation incorporating:

  • Scaling of the main source areas above Sumner Road using abseil crews,
  • Construction of an approximately 400 m long rockfall catch bench below the bluffs which was recognised as the main source of rockfall material (see Figure 3).
  • Construction of a rockfall protection structure in ‘Gully F’ above the Lyttelton Port Coal storage facility where rockfall boulders from steep cliffs concentrated in the natural gully (see Figure 4).
  • Scaling and trimming of the road batters adjacent to Sumner Road.
  • Significantly, previous advice to the CCC was that the slopes providing the main rockfall source area above Sumner Road required quarry-style benching due to earthquake-related damage to the rock mass. The Golder/Jacobs preliminary design described above did not adopt this approach as the rock was assessed to be reasonably intact.

Figure 3: Conceptual design of the 15 m wide, 400 m long cut rockfall catch bench

Figure 4: Location of rockfall protection structure above Sumner Road at ‘Gully F’

CCC let the rockfall mitigation works tender on a design build NZS3916 contract basis using the Golder/Jacobs preliminary design as a specimen design for tenderers. McConnell Dowell (MacDow) was selected as the successful tenderer for the rockfall mitigation works, with Beca providing design services. Beca and MacDow developed detailed designs for the works that largely followed the preliminary design.

Rockfall mitigation works

Scaling was completed by multiple (up to six three-person crews at a time) rope access teams working from the crest of the escarpment. The scaling work took approximately six months to complete and required only minor high-velocity blasting effort to remove selected large rock pinnacles that were too large to remove by hand tools, airbags or low-velocity blasting. Physical support was available to support unstable areas of slope, but this was not required as rock removal by scaling was considered as being adequate to achieve the necessary safety threshold.

The 400 m long catch bench was excavated using conventional excavators and 6-wheeled ADTs with in-ground blasting needed to improve excavatability. The bench is approximately 15 m wide and has a minimum 2 m high rock wall lip left in place along the outside edge to improve retention of rockfall (see Figure 5). The floor of the catch bench comprises ripped in place rock debris to provide a substrate to mitigate bouncing and rolling of rockfall. The bench falls to the east towards Sumner Road at a gradient of 1V:8H. Stormwater is diverted off the bench at seven intervals to prevent the full catchment of the bench from discharging directly onto Sumner Road. In line with the Principal’s requirements, rockfall modelling estimated that the bench would arrest 95% of rockfall from the escarpment and also has ample capacity to accommodate a large-scale failure, should that occur.

Figure 5: Photo of completed rockfall catch bench above Sumner Road. Stormwater drains denoted by grey narrow gravel-filled trenches on catch bench.

At Gully F MacDow and Beca opted for a 55 m long and 7 m high reinforced-earth ‘Green Terramesh’ rockfall bund as the rockfall protection structure where significant rockfall was concentrated (see Figure 6). The maximum design boulder size was in the order of 2 m in the maximum dimension.

Figure 6: Green Terramesh rockfall bund above Sumner Road at Gully F

Finally, the road cuttings were trimmed and scaled to remove obvious rockfall source material and to improve the line of site at some locations. Many of the road cuttings are almost vertical, in excess of 10 m high and were cut during the 1930s when Sumner Road was originally constructed. These road cuttings have typically performed well and only minor trimming and scaling was undertaken as part of the project. Significant reprofiling of the cut slope at Windy Point was completed due to slope failure below the outside (supporting) road edge and to optimise the road alignment (see Figure 7).

Following completion of the rockfall mitigation works, we completed a ‘post-works’ risk assessment. The assessment concluded that all of the road corridor now meets ARL2 or better and that most of the corridor is better than ARL3.

Post rockfall remediation road level repairs (road formation, retaining walls, stormwater and pavement)

Following completion of rockfall mitigation works, safe access was available to Sumner Road for repair of retaining walls and road pavement. Golder and Jacobs prepared an inventory of retaining walls and developed concept designs and cost estimations for repair. Many crib retaining walls were found to be damaged, while concrete retaining walls performed well. Informal guard rails and parapet walls were extensively damaged from rockfall. In all, work was undertaken on 30 retaining walls along Sumner Road some of which were more than 50 m long and 8 m high. An example of a retaining wall under repair is shown in Figure 8.

Concrete crib retaining walls were typically repaired by anchoring back the structure into rock and placing multiple coatings of structural shotcrete to reinforce the crib elements (see Figure 9). Some gabion basket retaining walls were removed and entirely replaced and some new retaining walls were required where no retaining wall was previously present. New ground beams were constructed to support crash barriers.

Stormwater control was typically designed to replicate the pre-earthquake condition and this required replacement of many culverts, sumps, flumes and gutters.

Figure 7: Windy Point, which is located above Lyttelton Port in the southwestern end of the project. Excavation of the slope was undertaken to enable realignment of the road away from the failed supporting edge (red dashed line) to the present location (yellow lines).

Figure 8: A retaining wall in the process of being repaired. Sumner Road is above the retaining wall.

Figure 9: Rock anchoring in progress as part of the repair of an existing, damaged wall.

Where appropriate, the pre-earthquake pavement was retained, however, most of the pavement was damaged by rockfall and retaining wall deformation requiring widespread replacement with stone mastic asphalt (SMA) overlying bitumen treated basecourse (BTB). In areas of over excavation cement treated basecourse (CTB) was also utilised. During pavement works historically placed coal tar was excavated and disposed of at an appropriate offsite facility.

Conclusions

Implementation of rockfall mitigation works including scaling of rockfall source areas and a rockfall catch bench have been assessed to meet CCC’s risk-based criteria for road users and resilience against road closure following a large earthquake. The mitigation includes an innovative catch bench beneath the main source of rockfall during the earthquakes. The construction cost for the project was about $20M below the pre-construction cost estimate of $60M, due in part to the re-evaluation by Golder/Jacobs of previous assumptions that the largest rockfall source required large-scale benching.

References

Aurecon. 2013. Sumner Road Stage 3 A&B Concept Design Report. Report for Christchurch City Council.

Forsyth, P.J.; Barrell, D.J.A.; Jongens, R. (compilers) 2008. Geology of the Christchurch area. Institute of Geological and Nuclear Sciences 1:250 000 geological map 16. 1 sheet + 67 p. Lower Hutt, New Zealand. GNS Science.

Massey, C.I.; McSaveney, M.J.; Yetton, M.D.; Heron, D.; Lukovic, B and Bruce, Z.R.V. 2012. Canterbury Earthquakes 2010/11 Port Hills Slope Stability: Pilot study for assessing life-safety risk from cliff collapse, GNS Science Consultancy Report 2012/57.

Roads and Maritime Services (2011) Guide to Slope Risk Analysis Version 4 (Draft)

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