ENHANCING THE RESILIENCE OF WATER AND ROAD TUNNELS WITH A FOCUS ON THE WELLINGTON REGION
Eleni Gkeli, Senior Engineering Geologist, Team Leader, Opus International Consultants, Wellington David Stewart, Senior Engineering Geologist, Opus International Consultants, Wellington Pathmanathan Brabhaharan, National Technical Director (Geotechnical), Opus International Consultants, Wellington
The Wellington region has a number of water and road tunnels, to convey critical lifeline utilities. The often aging tunnels are designed to lower standards and are subject to deterioration, hence require monitoring and maintenance to ensure operational integrity and resilience to earthquake and other hazard events. Authorities in the Wellington region and elsewhere, have been supported by Opus International Consultants (Opus) in managing the risk, through periodic inspections to identify key vulnerabilities, prioritisation and intervention. Best practice investigation and strengthening techniques were applied to ensure resilience and cost effectiveness. The paper presents case histories that highlight the value of geotechnical inspections in understanding risks, and how these influenced mitigation solutions. Case histories include the retrofit works of the 3.5 km long Orongorongo water tunnel, the strengthening of the concrete lining of Carey’s Gully stormwater tunnel at the Southern Landfill, the risk assessment of the Westport Water Supply Tunnels and the seismic strengthening of the Hataitai Bus and Mount Victoria road tunnel portals. Management of the safety risks during inspection, investigation and construction is also discussed.
Keywords: tunnel, risk, lifeline services, lifeline routes, resilience, maintenance, strengthening
Introduction seismicity. The risks in the tunnels relate to both challenging rock conditions and The Wellington Region is characterised by seismicity as a result, maintenance and rugged terrain. Urban infrastructure upgrading strategies should address both development on this terrain often requires issues. A number of case studies are important lifeline services, such as water presented in this paper, for tunnels managed supply pipelines and major transportation by a variety of authorities. For the locations of routes to be conveyed through tunnels. the tunnels in the Wellington Region, see Considering that most of these tunnels were Figure 1. constructed in the beginning of the last century, regular maintenance and upgrade are key elements to extend their operational lives and enhance resilience in earthquake and other hazard events.
Wellington Region is primarily underlain by a Hataitai Bus rock complex commonly known as Wellington Tunnel
Greywacke, consisting of sandstone, Carey’s Gully mudstone (argillite) and localised volcanic Tunnel rocks. The rocks are metamorphosed, highly Wainuiomat deformed and variably weathered, often with high degree of fracturing and shearing (Begg Wellington et al., 2000). Further, the Wellington Region is among the most seismically active areas in Mount Victoria Orongorongo New Zealand. The active faults located within Tunnel Tunnel the Wellington Region, such as the Ohariu, the Wellington and the Wairarapa faults, as well as the proximity to the subduction zone Figure 1: Location of tunnels in Wellington are the main contributors to the high Region Some of the tunnels have been previously uncontrolled water seepages at various discussed by Stewart (2008). locations. Regular inspections allowed recording of the most adverse locations and Orongorongo Water Supply Tunnel – monitoring their evolution over time. Wellington Water Ltd. Recommendations for remedial works were presented for consideration by GWRC. The Orongorongo tunnel was constructed in the 1920’s. It carries a 700 mm diameter water supply pipeline from the Orongorongo River to the catchment upstream of the Wainuiomata Water Treatment Plant. The tunnel was previously operated and managed by Greater Wellington Regional Council (GWRC) and currently by Wellington Water Ltd. The tunnel is 3.2 km long, 2.0 m high and 2.0 m wide and is excavated in Wellington Greywacke. About 50% of its length is concrete lined, while the rest is unlined (see Figure 2).
Figure 3: Jigger used for staff access in Orongorongo tunnel
In 2013, GWRC secured the required funding, and carried out considerable stabilisation and maintenance works in the tunnel. Opus designed the stabilisation measures and supported GWRC in the tendering and construction phases. The stabilisation works were carried out at the areas of highest risk identified in the previous Opus inspections (2005 to 2012) and at additional locations identified during Figure 2: General view of the Orongorongo construction. The areas of highest risk were Tunnel selected for stabilisation, based on experience of the tunnel behaviour through Opus has provided regular geotechnical the years, practical observation during inspections and subsequent assessments of construction and available funds. risks in the tunnel since 1997 when the water pipe was constructed. The inspections occur The risks previously identified in the tunnel every two to three years and are also part of were addressed through the stabilisation / the safety requirements for the small gauge maintenance works shown in Table 1. With railway line that goes through the tunnel to the completion of the stabilisation works, the allow jigger access for staff (Figure 3). risk of rock fall and roof / lining collapses in the tunnel is lower. Some residual risks The main geotechnical risks identified in the remain in the tunnel, and on-going tunnel were rock falls and lining failures of geotechnical inspections are being carried- various volumes (up to 1-2 m3). Factors out, to observe the overall tunnel condition indicating higher degrees of deterioration and and the behaviour of stabilisation measures risk of collapse are localised roof collapses and identify possible new risks. The most and loosening of rock mass in the unlined recent inspection in November 2015, sections; cracks and other damage in the concluded that the tunnel is generally in good concrete lining sections and significant condition. between the lining and the surrounding Table 1: Mitigation measures applied in bedrock, with the potential to impose loading Orongorongo Tunnel on the lining due to either ground convergence or roof collapse. The risk of Identified Risk Applied mitigation collapse of sections with substandard Small rock falls removal of loose concrete lining was considered high and the blocks (scaling) possible consequences are unacceptable for Potential rock falls of scaling where the operation of the landfill and subsequently moderate to appropriate and for the environment. WCC undertook significant size application of spot rock immediate measures to mitigate the risk of bolts to stabilise the collapse, such as re-construction of the roof blocks lining along the highest risk 160 m long Roof collapses localised pattern of section at the downstream end and rock bolts and steel supporting the rest of the tunnel with timber mesh installed on the tunnel roof (Figure 4) braces. WCC also engaged Opus to assess Areas of significant drainage holes the risks along the length of the tunnel and water flows develop options for managing them. Damage to the Sealing of cracks, tunnel lining localised stabilisation with rock bolts and installation of drainage holes
Figure 5: A general view of Carey’s Gully Stream Tunnel in 2005
Opus, carried out inspections in 2002, 2004 and 2005 aiming to understand the behaviour Figure 4: Stabilisation of tunnel roof of the tunnel and identify areas of distress, cracking and deformation of the lining that Carey’s Gully Stream Tunnel might indicate a high risk of collapse (Figure 6). Following the assessments of 2002 and Carey’s Gully Tunnel was constructed in the 2004, it was concluded that although there 1990’s to carry the Carey’s Gully stream was no immediate risk of collapse, due to its under the Wellington City Council (WCC) inadequate thickness, the lining was not able Southern Landfill in Happy Valley. The tunnel to carry the anticipated long term static and is 530 m long, fully lined, with a rectangular seismic loadings. Options considered to section 1.8 m wide and 1.6 m high, and was mitigate the risks included: excavated through highly fractured • Installation of a new circular pipe Wellington greywacke (Figure 5). Soon after throughout the substandard section the construction of the tunnel, the risk of • Formation of a new concrete arch along substandard concrete lining was identified. the roof of the tunnel • Grouting of the void between the concrete The lining was of inadequate thickness in lining and the surrounding rock many sections, indicated to be as little as 50 to 75 mm, while voids were suspected • Construction of a new tunnel over the substandard section • Installing beams to support the roof lining
Figure 7: Investigation works in Carey’s Gully Tunnel
The findings of the investigation indicated Figure 6: An example of a problematic that the wall lining was largely adequate section identified during inspections (>150 mm thick) however the roof lining was thin and unreinforced. The concrete quality As part of the proposed remediation strategy was generally satisfactory and the roof Opus recommended and carried out cavities were small (<0.8 m). The final investigations to better understand the risks solution developed in conjunction with WCC, and refine the mitigation option (Figure 7). involved installation of replaceable The investigations comprised cored galvanised steel roof beams supported on boreholes through the concrete lining, to brackets attached with stainless steel bolts to measure the thickness of the lining and the concrete wall lining (Figure 8). This option retrieve samples for testing the strength of is significantly cheaper than the other options concrete. 40% of the boreholes were drilled considered and sufficiently robust to maintain into the surrounding bedrock, to examine its the tunnel integrity under possible loadings quality. CCTV viewing was carried out in the from the surrounding ground, including boreholes to investigate the condition of the seismic loads. A 5 yearly inspection in 2014 primary tunnel support and the existence of confirmed that the solution is working well. voids between the lining and the surrounding rock. Finally, chemical tests were carried out on seepage flows to investigate its aggressivity.
Significant health and safety issues were addressed during both inspections and investigations. High risks from explosive methane gas and potential leachate seepages, cramped confined space conditions and flooding in heavy rain events were considered, assessed and safety measures were put in place. Due to the hazardous nature of the site and the difficulties in communication, measures included forced ventilation, continuous Figure 8: View of Carey’s Gully Tunnel monitoring with gas detectors, intrinsically during the construction of remediation works safe equipment and a large safety team to support the team working in the tunnel.
Westport Water Supply Tunnels – 2016 Health and Safety Act. The variety of Buller District Council remedial options available to council have been clearly assessed with BDC, along with Opus took advantage of the experience likely maintenance and capital cost gained in the Wellington Region to assist obligations.to enable wise choices to be Buller District Council (BDC) in investigating made for the future of the water supply. This maintenance and repair options for their work is currently in progress. water supply tunnels. The Westport water supply is sourced from a pristine mountain stream via a unique system of four hand dug tunnels excavated in 1903 through to a series of water races to the reservoirs and the water treatment plant on the hill overlooking Westport. The tunnels total 1.9 km in length with water running as open channel flow through the tunnels, which are only partially lined.
While the tunnels have performed extremely well for Westport, they periodically have had collapses which have disrupted the water supply and present safety hazards for staff. An alternative pumped source installed in 2007 has been used to cover when the tunnels have been out of service. Opus were involved in a comprehensive review of the Westport Water supply in 2009, which included a risk assessment of the tunnels and supply line from source to town. The tunnels were noted to provide a high risk to the water supply with a significant cost of remediation. Figure 9: Partial collapse of water supply
Individual sections were assessed as low to tunnel in 2014 high risk depending on the condition of the tunnel walls and tunnel supports (where Mount Victoria Tunnel, New Zealand present). The highest risk tunnel (Tunnel No Transport Agency 4) which had a history of collapses was remediated by BDC in 2009 by enlarging and Mount Victoria Tunnel is an integral part of pulling an 800mm dia. PE pipe through. State Highway 1 (SH1) in Wellington. The Tunnel No 2 is a shallow cover tunnel which tunnel connects the eastern suburbs of is encompassed by a large previously active Hataitai, Miramar, Lyall Bay and the regional landslide and as such presents significant airport to the Basin Reserve, city centre and risks to Council if reactivated. Tunnel No 1, northern motorway. The tunnel is 623 m long previously Council’s best performing and and 5 m high with a two-way traffic lane longest (1.2 km long) tunnel had a partial configuration. The construction of the tunnel collapse event in mid-2014 (Figure 9). was completed in October 1931.
The logistics of accessing and repairing the The New Zealand Transport commissioned tunnel in the wake of the Pike River coal mine Opus to assess the earthquake performance disaster have caused significant challenges of the tunnel. The first preliminary stage of for BDC. Opus has engaged with experts in assessment identified the parapet walls mines safety and underground safe work above both portals as key vulnerabilities of practices to ensure that procedures for the tunnel, in earthquake events. As a result, inspection and repair of the tunnel are the parapet walls were considered to be compliant with the requirements of the new potential hazard to the safety of road users. Any failure and/or substantial movement of the walls could cause tunnel closure between Poor quality, highly fractured and loose rock a few days to a week to allow site clearance was encountered in several anchor locations and to secure the site temporarily. which presented challenges and difficulties in Strengthening options were proposed to the installation of rock anchors. The problem improve the earthquake performance of the was addressed by applying innovative portal walls. ‘External rock anchors and grouting techniques, use of PVC pipes, beams’ was the preferred option to installation of inflatable packers to plug the strengthen the portal walls. This option hole, extending the length of anchor holes, involved anchoring the portal to the rock reducing the hole inclination and relocating behind the wall by constructing anchors and two of the anchors new locations with better a concrete beam in front of the parapet wall. ground conditions.
Opus carried out the detailed design for this option. Special consideration was given to the historic nature of the tunnel and the concrete beam on the portal walls was architecturally designed to match the original cornice in order to minimize interference to the portals façade. Further, exposed anchor heads installed on either end of the concrete beam were covered with an architecturally designed capping. Wellington City Council was happy with the architectural solution and issued an Exemption from Resource Consents.
Opus carried out monitoring of construction. Figure 11: View of construction site above The construction works required intensive Mount Victoria Tunnel portal coordination and collaboration of all parties to address the challenging environmental and All works were delivered on time and budget health and safety conditions of the project, despite the demanding ground conditions and involving working at heights above live traffic, construction issues associated with the wall. disruption of traffic, noise mitigation for night A view of the architecturally designed portal time work, site visibility, containment of debris walls following the completion of construction and equipment and working in a high profile is shown in Figure 12. area (see Figure 10 and Figure 11).
Figure 12: View of Mount Victoria Portal and Figure 10: Construction works in progress at cornice following the completion of the Mount Victoria Tunnel portal strengthening works design and construction management of works for the strengthening of the portal and Hataitai Bus Tunnel, Wellington City associated wing walls. The strengthening Council included ground beams behind the portal walls and reinforced concrete buttresses in The Wellington City Council (WCC) has a front of the portal and wing walls that were strategy to manage the risks to its road tied back with rock anchors. network from hazards, and to deal with such hazards in an integrated manner. As part of The design of the strengthening works had to this strategy, the WCC has carried out take into account the heritage value of the strengthening works to the Hataitai Bus portal structures. The buttresses were Tunnel and Karori tunnel and has been adopted as being discrete and not detracting investigating and assessing the Seatoun and from the heritage value of the portal Northland tunnels, which are part of its road structures. The architectural outlook of the network. The Hataitai Bus Tunnel is strengthening option was reviewed and Opus approximately 300 m long, with an arch proposed and agreed with WCC to raise the shaped section about 5.25 m high and 4 m eastern portal wing walls to match with the wide, with brick lining. The portals comprise existing portal face (Figure 14). unreinforced concrete face and wing walls at either end (Figure 13). The western portal is in the Mt Victoria suburb and eastern portal is in the Hataitai suburb.
Figure 14: 3D model developed for the architectural design of the bus tunnel portals
During the detailed design phase, a rock fall hazard was identified on the slopes above the Hataitai (eastern) portal, causing a maintenance issue and potential hazard for the road and tunnel users. The source of the rock fall was identified at the upper part of the south slope above the portal structure (see Figure 13). Rock falls were generated by unfavourably oriented combinations of rock defects forming rock blocks that became loose with time, and fell from the slope under Figure 13: View of Hataitai Tunnel portal storm events or even under static conditions. (Hataitai end) prior to strengthening The risk was assessed as high, and rock fall protection measures were implemented, The first stage seismic assessment identified comprising a steel mesh fixed on the slope the vulnerability of both portals to with a pattern of rock bolts 4 m long. earthquakes and proposed strengthening measures. Opus carried out the detailed
Conclusions
• A number of aging tunnel structures exist in the Wellington Region conveying important facilities, such as water supply or drainage services and transportation corridors. • Monitoring, maintenance and upgrading of these tunnel structures are important to manage safety hazards, extend their operational life and enhance their resilience in earthquake or other hazard events. Figure 15: Rocks accumulated at the toe of • An integrated strategy is recommended to the south slope at the Hataitai Portal. be followed by Authorities managing these tunnel assets in order to carry out Construction of the stabilisation works started appropriate and cost-effective safety in November 2014 and was completed in maintenance and strengthening works, April 2015. Additional works were carried out where required. to further improve the condition and resilience • Regular inspections in lined and unlined of the portals, including crack and other tunnel structures assist in monitoring and concrete condition repairs, drainage works, understanding the behaviour of the tunnel repairs of poor condition plastering, seismic structure and assessing the risks. strengthening of existing architectural • Best practice site investigation can further features and road berm reinstatement. assist in quantifying risks and designing Construction was completed without appropriately targeted mitigation significant issues, with Opus providing measures. construction monitoring and assisting in • Appropriate consultation during the managing the environmental issues (traffic, construction phase helps overcoming noise, dust, silt control) and coordinating construction difficulties, as well as carry various stakeholders (Bus Company, local out additional required minor works that community) in an effective manner. A view of will enhance operational integrity and the Hataitai (eastern) portal following the resilience in seismic and other hazard construction of the strengthening works is events. provided in Figure 16. Acknowledgements
The authors acknowledge the permission to publish this paper from Wellington Water, Wellington City Council, Buller District Council and New Zealand Transport Agency. Other participants in the projects included in this paper are also acknowledged for contributing to the successful outcomes.
References
Stewart, D. (2008). "Management of geotechnical risks for some tunnels in the Wellington Region", Proc. 18th Geotechnical Figure 16: View of the Hataitai portal Symposium on Soil-Structure Interaction. Ed. following completion of the strengthening CY Chin, Auckland. works.
Author Biography
Eleni is an Engineering Geologist, with 20 years’ experience in the geotechnical profession. Before coming to New Zealand in 2012, Eleni worked in Greece, in the Design Department of the managing authority for the design and construction of a 600 km long high standard motorway across northern Greece. Through that role, Eleni gained experience in the design and construction of large scale infrastructure and was exposed to the latest developments in geotechnical and seismic design in Europe. Eleni currently works for Opus International Consultants in Wellington heading the Engineering Geology and Rock Engineering team. She has been involved in a variety of infrastructure projects across New Zealand, including Transmission Gully motorway, Petone to Grenada link scheme assessment, Hataitai Bus and Mount Victoria tunnels, Orongorongo tunnel and various seismic assessments for water reservoirs, bridges and buildings. Eleni is an elected member of the New Zealand Geotechnical Society management committee.