A case study in earthquake recovery and resilient design, Slip NRP5, Kaikōura

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A case study in earthquake recovery and resilient design, Slip NRP5, Kaikōura

A case study in earthquake recovery and resilient design, Slip NRP5, Kaikōura

J.R. Grindley1, M. Engel2, G. J. Saul1

1 WSP, Christchurch; 2 RDA Consulting, Christchurch

ABSTRACT

The NRP5 Mass Movement is one of the 10 Primary slope failures that affected the Transport corridor North of Kaikōura, following the 14 November 2016 Kaikōura Earthquake. During the earthquake, an initial slope volume of 10,000 m3 failed onto the State Highway and Main North Rail Line. On-going instability became a significant issue that affected long-term performance of the slope, particularly for the Railway, post debris clean-up. The main geotechnical issues at the site included translational slope failure, face erosion, debris inundation, seepage, and surface water erosion.

Immediate response included removal of debris off the rail and road corridors, shaping of the head scarp and installation of a debris fence at the slope toe. Post-earthquake storm events, including ex-tropical cyclones Gita, Debbie and Cook, resulted in debris flows and advancement of the debris fan by 50 m which further affected the transport corridor. Continuing erosion and regression also occurred.

Longer term recovery works included earthworks and horizontally drilled drain holes to improve stability and reduce seepage erosion. This was further supported by anchored netting and ballasting of the face with gabions. Earthworks were also used to catch debris and direct water flow into adjacent natural drainage channels.

This paper presents a case-study of the practical geotechnical considerations encompassed in the immediate response, as well as the design and construction phases to ensure reliability of the transport corridor in rainfall events as well as resilience to future extreme events and earthquakes.

Introduction

The magnitude 7.8 Kaikōura earthquake on 14 November 2016 caused significant damage along State Highway 1 (SH1) and the Main North rail line (MNL) over a 150 km zone between Culverden to Seddon.

The transport corridor was particularly affected along a 15 km section between Mangamaunu Bay and Clarence River, North of Kaikōura, where the road and railway line are narrowly constrained between the sea and the Seaward Kaikōura Ranges (Figure 1). This section of transport corridor was significantly affected by 10 large “Primary” mass movements, and numerous areas of “Secondary” slope damage. Areas of secondary slope damage did not reach the transport corridor but were identified as features where further movement may occur.

Waka Kotahi Transport Agency (Waka Kotahi) and KiwiRail established the North Canterbury Transport Infrastructure Recovery (NCTIR) Alliance with the goal of re-opening the infrastructure links along the Kaikōura coast as quickly as practically possible. The MNL was reopened after 10 months with monitoring and temporary closures during significant rainfall, while the SH1 corridor remained closed for 13 months.

This paper outlines the engineering response and recovery works to Primary Slip NRP5, a shallow 10,000 m3 earthquake triggered debris avalanche that reached the SH1 and MNL, and a Secondary slide (NS14) at a higher elevation on the slope. Early post-event response included debris removal from SH1 to provide construction access, and later debris removal from the MNL. Small instabilities occurred during relatively moderate rainfall, affecting the MNL and hence a temporary fence was installed for railway reopening followed by further recovery works for increased reliability and resilience.

Transport Corridor

Figure 1: Site location (after the November 2016 Kaikōura earthquake)

Ground and Groundwater Conditions

Geomorphological and Ground Conditions

The NRP5 site is located 25 km north of Kaikōura immediately adjacent to SH1 and the MNL and affects 155 m of the Transport Corridor. SH1 and the MNL run parallel to the slope along this section, close to sea level, with the road on the seaward side.

The NRP5 and NS14 slips occurred with the seaward slope of a fan of accumulated colluvium deposited in prehistoric debris avalanches or debris flow events. Instability at the NRP5 occurred as a shallow debris slide within the seaward slope steepened by sea erosion and/or the transport corridor. The landslide scarp is approximately at RL46 m and is inclined at 50° to 60°.

NS14 occurred at an approximate RL of 400m, with the debris running out onto the shallow slopes above NRP5. It formed a debris accumulation zone 150 m long and 100 m wide. (Figure 1). This debris stopped 200 m above the head of the Slip NRP5 head scarp but advanced 100 m in subsequent rainstorm events, burying surrounding vegetation (Figure 2).

Figure 2: – NRP5 during response phase with NS14 debris advance and flow paths.

Site investigations included three boreholes drilled up to 50 m deep from the landslide surface with piezometer monitoring to better understand the groundwater regime (Figure 3). Drilling indicated variation of materials over short distances, consistent with a debris flow, debris avalanche or landslide deposition but typically comprised very dense (SPT N >60) bedded cobbly sand, gravel and silts (Figure 3). The cobbles and boulders are sub-rounded to sub-angular and are typically 0.1 m to 1 m diameter. This colluvium was also exposed over the full slip height of NRP5.

Bedrock is observed in the shoreline and was encountered at more than 50 m depth in boreholes drilled from the fan above and is interpreted as a gently sloping, possibly wave cut platform. It comprises greywacke bedrock from the Pahau Terrane (Rattenbury et al, 2006).

Figure 3: Geological section (showing final benched profile and drainage drill holes)

Groundwater Conditions

Significant water seepage was evident on the steeply sloping face where the NRP5 slip occurred. This typically occurred at two levels, high on the face between approximately RL 30 to 35 m and just above rail level at RL 10 to 15 m. In both locations, it appeared to extend approximately 30 m across the face.

On the debris surface above NRP5, test pits were terminated at 3 to 4m due to groundwater inflows while piezometers installed below the surface observed groundwater 10 to 20 m below the slope surface which was inferred as the likely source of the face seepages.

Although groundwater was observed in screened piezometer zones, observations during drilling and supporting permeability tests indicated that the groundwater system is likely to be a complex system of localised thin, aquifers perched on low permeability silty horizons which can be difficult to drain effectivity. The observations did, however, highlighted the presence of a deeper zone of water and possible recharge zone that could become a target for drilling 40 to 60m horizontal distance from the slope face.

Mass Movement types Considered in Design

Surficial Slope Failure

Following initial debris clearance after the 2016 Kaikōura Earthquake, surficial failure of colluvium on the slip face occurred during relatively frequent rainfall events. In the period before recovery earthworks were undertaken (April 2017 to June 2018), 14 incidents of instability occurred. Two of these occurred during significant rainfall events where up to 220 mm rain fell over 24 hours. This is consistent with average recurrence intervals (ARI) of 10 to 20 years. The remaining 12 incidents occurred during smaller rainfall events with 30 to 100mm of rainfall in 24 hours (ARI of 0.5 to 2.5 years).

Failure debris for each of these incidents was typically < 15 m3. Generally, this resulted in blocking of the water table and on some occasions inundation of the railway track disrupting rail operations until a catch fence was installed (Section 5).

Deep Seated Failure

No evidence of deep-seated failure was observed post-earthquake. Possible tension cracking was observed 10 m upslope of the NRP5 head scarp during tree clearance however further assessment discounted this as a likely deep-seated failure mechanism.

Debris Flows

Debris flows from the de-vegetated scarp of NS14 (above the NRP5 slip) resulted in debris advancing to within 100 m of the NRP5 head scarp and down shallow gullies at the lateral margins to reach the transport corridor and foreshore.

Hummocky and channelled terrain was also observed in vegetated areas between NS14 and NRP5, indicating debris flows had historically advanced past the NRP5 headscarp.

Recovery

Recovery was carried out in two stages. While resilient long term solutions were desirable, this had to be balanced against the urgent need to re-establish and open the transport corridor. While the transport corridor remained closed there was a significant impact on South Island freight haulage, with KiwiRail freight, and other Picton to Christchurch road freight, being diverted to an alternate 120 km longer route. This increased road traffic on alternate routes not design for that level of traffic.

The immediate recovery (Stage 1) focused on re-opening the transport corridors and re-establishing freight rail services with acceptance of weather related delays. This was achieved on 15th September 2017 for railway operations and 15th December 2017 for road traffic.

Stage 2 focused on increasing the reliability and resilience of the transport corridor. This approach also allowed for monitoring of all sites to gain better insight into the type and frequency of ongoing instability and identified those sites where the biggest gains could be achieved in terms of reliability.

Stage 1 – Immediate Recovery

At NRP5, the primary works in Stage 1 involved clearance of failed material. In this area, transport corridor realignment was not possible due to nearby rail tunnel and coastal constraints.

Trees within 5 m of the head scarp were trimmed with the root balls left in place. A 3 m wide bench was formed to unload the southern half of the head scarp and intercept overland water flow to reduce slope erosion and riling observed on the slip face below.

To minimise railway disruption from smaller, relatively frequent instabilities, a 3 m high catch fence was constructed at the toe of the slope. Remote monitoring devices were also installed on the fence to communicate to Train Control when debris had impacted the fence. The catch fence was considered temporary and comprised readily available materials to avoid delays in importing a proprietary off the shelf system.

Drain holes (12 m long) were drilled at 10 m centres into the base of the slope where strong seepage was observed. Initial flows were up to 15 litres/min, but subsequently reduced to a few litres/min within a few weeks.

Stage 2 –Long Term Resilience and Reliability

Optioneering

Opening of the MNL highlighted some concerns around the performance of the Stage 1 works. To address these concerns debris flow mitigation was implemented initially. This included a series of bunds formed in a herringbone pattern to divert flows from NS14 into side gullies and under new bridges along the transport corridor. Debris flow bridges were designed to allow access underneath for clearance machinery.

Surficial failures continued to occur even in dry weather due to seepage. Although the catch fence was in place, delays for clearing and damage repair were frequent. The face also continued to deteriorate with significant erosion and riling of the surface.

The mobilisation of debris from Slip NS14 in relatively small rainfall events also raised the possibility that debris may advance over the slope crest and down onto the transport corridor below. This would remove established vegetation exposing the corridor to ongoing and more frequent instability.

Transport corridor realignment was not possible due to rail tunnel and coastal constraints and there was insufficient room between the MNL and slope to catch potential failures. Other mitigation options were investigated that had minimal disruption on railway services that were now operating. The works also needed to provide a low whole of life cost and to minimise ongoing maintenance.

Initially anchored netting stabilisation of the 6000 m2 slip face was considered for concept planning, however, a value engineering exercise was undertaken by the project team. This resulted in a more cost effective option being adopted as described in the following section.

Earthworks and Drainage Solution

The final solution adopted to manage the ongoing slope instabilities issues involved earthworks to reprofile and form benches in the slope combined with drainage drilling as stability modelling confirmed water seepage as having a significant impact on the stability of the unvegetated slope. This would be supported with minimal anchored face support where required (Figure 4). The focus on earthworks and drainage, enabled a relatively cost effective mitigation solution with an acceptable design life.

This solution provided support for the slope at areas of strong seepage and slope saturation while providing catch benches to prevent failure material from reaching the transport corridor. The fence remained in place at the toe to protect the railway.

The earthworks methodology was designed to allow access across the MNL and formation of access ramps for top down construction whilst leaving two sloped benches in place long term (figure 4). The lower bench was formed 7 m wide to enable horizontal drainage drilling without impacting the MNL. Drainage included surface run off measures and 29 sub-horizontal drain holes up to 60 m long, drilled at 5m centres into the slope.

Drain hole and surface run-off water was collected on the benches and a fill bund was formed to prevent overspill onto the slopes below. Cross fall on the benches lead overland flow into gravel lined side drains leading to drainage gullies. A Geosynthetic Clay Liner (GCL) was laid on the lower bench to prevent water infiltration, minimising seepage on the slope below.

3m Catch Fence

SH1

Railway

Horizontal Drains

Anchored Mesh

Lower Bench & Drainage Channel

Gabion Ballast

Top Bench & Side drain

Access to NS14

Figure 2 : Final earthworks profile with anchored mesh, gabion support and drainage installed

Construction Challenges

There were numerous challenges with the earthworks solution at this location, however, the Alliance approach enabled early contractor consultation to address some of these challenges during design. Some of these challenges included continued safe rail and road access below the site, design of steep gravel cut slopes between benches and potential destabilisation of slope material by seepage.

Maintaining Railway and Road Access

When the works were being undertaken at NRP5, both the MNL and SH 1 had been re-opened to public and freight movement. The NCTIR delivery team was required to maintain, as a minimum, single lane vehicle access past the site and four train movements per day.

Installation of the lower 40 m long horizontal drains, 1.5 m above the rail level, was achieved with minimal disruption to the MNL by trenching temporary steel conduits below the rail ballast enabling drilling to be undertaken from the road level and under the railway line without blocking rail operations.

The 40,000 m3 of excavated material had to be removed from the site across the MNL and SH 1. Small heavy-duty off-road Hydrema dump trucks proved ideal in traversing the steep benches and passing under the highway and the railway at the southern side gully (debris flow channel A, Figure 3) to a stockpile on the foreshore above the high tide line. These processes saved significant time in construction and eliminated interaction with, and disruption to, transport operations.

Notes

1. Hydrema off-road dump trucks moved excavated material under the road and rail bridge for the Debris Flow Channel A.

2. The material was loaded onto road trucks on the foreshore.

1

2

Figure 3: Site during earthworks

Site constraints

There was very limited space for the earthworks with the MNL and SH1 immediately at the toe of the slope and with NS14 located above NRP5. Part of the protection to minimise the potential of debris from NS14 overtopping NRP5 were the trees at the top of NRP5. During Cyclone Gita, the trees helped to direct debris flow towards the debris channels. From slope stability modelling, a suitable batter angle was determined that would achieve the required level of stability while maintaining as many trees as practicable.

Early contractor involvement was undertaken to determine the amount of space required to safely undertake the earthworks and drilling of the horizontal drains. A minimum bench width of 7 m was designed which included a 1 m high windrow on the outside edge of the benches to provide a visual safety barrier for machine operations and to allow the control of storm water on the bench.

Observational Approach

The complex, and heterogenous nature of the geology and evidence of local perched groundwater, created uncertainty in the effectiveness of drainage drilling. The boreholes identified a deep groundwater table and a series of shallow perched groundwater horizons within higher permeable layers that were expected to result in strong seepage of the cut face.

The success of the horizontal drainage was dependent on the drains intercepting discrete pockets of high-water flow. A staged drilling pattern was proposed where every second or third drain would be drilled initially. The effectiveness of the horizontal drains in reducing water seepage was uncertain and therefore face support was planned for areas of high seepage and saturation.

Where required, additional face support was provided by gabion baskets or anchored Tecco mesh solutions. These were designed such that they could be installed as work progressed or soon after completion of the excavation. Mapping of seepage and saturation zones was undertaken as earthworks progressed which allowed optimal support of these localised seepage zones.

This support was primarily required when, a band of strong seepage flow was encountered in the lower half of the main cut face. This appeared to be associated with a 3 to 10 m thick, highly permeable layer of clean gravels. It was identified that this seepage if unsupported could cause localised slumping that would undermine the slope above and saturate the cut face below. This area required both forms of treatment and where practical the Tecco mesh was continued down to the gabion level and was wrapped into the backfill. The site was also hydro-seeded following construction to help vegetation re-establish, and where Tecco mesh was used, a biodegraded erosion fabric was included as part of the active meshing system.

Design Performance

Since the Stage 2 construction was completed in 2019 the site has performed well. There has been a significant reduction in railway outages during smaller more regular rainfall events.

This design involved steep 50° degree sloping cuts in older colluvial gravels based on precedence. This would normally be considered marginally stable and would possibly require substantial stabilisation such as the fully anchored netting option initially considered. However, the more optimal solution of combining earthworks benching to catch debris with additional drainage and support measures prove more cost effective. It also allowed an observational approach to provide targeted support for the cut face in zones of strong seepage and saturation.

Horizontal drain holes proved effective. They intercepted groundwater with high flows up to 270 litres per minute. Over time the flows reduced but continue to flow. Piezometers responded to drainage indicating 5 m decrease in thin perched aquifers in piezometers install in the upper the upper bench.

Although drainage measures intercepted significant groundwater, face seepage still occurred during construction. This was allowed for with the observational approach and 760 m2 of anchored mesh was installed to support these zones plus 85 m of gabions up to 3 m high, slightly more than originally anticipated. These wet seepage patches on the face have since appeared to reduce although conditions since construction have been unusually dry.

The herringbone pattern of fill bunds at NS14 has been tested in one rainfall event and adjusted. The new bridges for the transport corridor at the two debris flow paths provide improved resilience to future debris flows by reducing both the size and frequency of debris inundation and expedite debris clearance and maintenance.

Conclusions

The NRP5 Slip comprised an earthquake triggered slope failure within a steep colluvial slope mainly comprising a dense silty gravel. Subsequent instability occurred during rainfall events causing disruption to the railway.

Transport corridor realignment was not possible due to rail tunnel and coastal constraints and there was insufficient room between the MNL and slope to catch potential failures. Therefore, an earthworks and drainage solution was developed using an observational approach. This enabled optimisation of the extent of expensive anchored netting and allowed for relatively step cut slope profile in gravel (50°) to be adopted for slope stabilisation.

Drainage drilling targeting groundwater seepage was also carried out. It was effective and will improve long term performance of the slope. To date this cost-effective mitigation solution has proven to be successful to date. It has been effective in increasing reliability during smaller rainfall events, although it should be noted that there it has been relatively dry conditions since construction.

ACKNOWLEDGEMENTS

The authors wish to thank Waka Kotahi NZ Transport Agency, KiwiRail and The North Canterbury Transport Infrastructure Alliance (NCTIR) for permission to publish this paper. Also, Dan Ashfield, and Mark Beijeman are acknowledged for their contribution to geotechnical solution development.

References

Rattenbury, M.S., Townsend, D.B., Johnston, M.R. (compilers) 2006: Geology of the Kaikōura area. Institute of Geological & Nuclear Sciences 1:250 000 geological map 13. 1 sheet + 70p. Lower Hutt, New Zealand. GNS Science.

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