MAKATOTE VIADUCT – UNDERPINNING PIER 7 CHALLENGING NATURE WITH A COLLABORATIVE APPROACH

Peter Wissel1, Alastair Blackler2, Walter Rushbrook3, Matthew Callaghan4

INTRODUCTION

Located between and National Park, the Makatote Viaduct is the third highest railway viaduct in . Land instability and erosion resulted in Pier 7 of the Makatote Viaduct being vulnerable to foundation failure and was identified as the greatest risk to any of the structures on the New Zealand Railway Network by its owner ONTRACK. A project was put together by ONTRACK to underpin the vulnerable foundations. The project team faced demanding constraints and difficulties that had to be overcome to ensure a successful outcome of the project. The construction was effectively undertaken inside a National Park with numerous environmental challenges including undertaking construction in a rare blue duck habitat and in a pristine trout spawning stream. Most critically, there was a possibility that the construction activities may in themselves destabilise the viaduct. ONTRACK, Fulton Hogan Civil Ltd and specialist engineering consultants worked collaboratively based on a no-blame and best for project approach to successfully complete the $4.2M project between May 2006 and February 2007.

This paper provides an overview of the challenging work to underpin the eroded pier foundation in difficult topography and during extreme weather conditions.

HISTORY It is the combination of this exposure, continued lowering of the bed level due to scour, unknown lateral soil pressure on the upstream footing, risk of further embankment erosion at the downstream footing and the potential for the volcanic ash layer to slip against the footings that raised the stability of Pier 7 to the highest priority requiring remedial action by ONTRACK Structures Engineering.

DESIGN

Several options were considered during the scope and conceptual phase of the project such as:

ƒ The placing of rock rip rap protection at the toe of the upstream footing to prevent further scour [1] Makatote Viaduct being tested in 1908 and installation of soil nails / rock bolts along the face of the embankment. Completed on 10 July 1908 by company J. and A. ƒ The construction of a concrete retaining wall Anderson and Co. for the sum of ₤53,369, the utilising soldier piles and ground anchors which Makatote Viaduct is the third highest railway would protect the toe of the footings and viaduct in New Zealand with a total height of prevent further attack to the embankment from 78.6m. As part of the Main Trunk Line the river (NIMT) the viaduct serves as a vital link between ƒ The underpinning of the two front footings by and . installing a large pile at each footing and spanning a large cross beam between piles to Regular rainfalls and seasonal melting water from support the lattice tower legs and relieve load Mt. Ruhapehu eroded the river banks and exposed on the existing footings the toe of the upstream mass concrete footing of Pier 7. The last option was considered to be the most acceptable form of construction to meet Resource Consent conditions, provided an acceptable

1 Peter Wissel, Regional Manager, Fulton Hogan Civil Ltd 2 Alastair Blackler, Project Manager, Fulton Hogan Civil Ltd 3 Walter Rushbrook, Manager Structures Engineering, ONTRACK 4 Matthew Callaghan, Director, Novare Design Ltd reduction of risk to ensure ongoing operation of the and recognised as adjustments (plus/minus) to the Viaduct and was cost effective in comparison to TOC. the other options. In order to drive ONTRACK’s key objectives for the The proposed underpinning structure consists of contract a performance incentive was established, two 2m diameter bored concrete piles constructed which solely focused on safety, program, to a depth of 38m below beam soffit level. The environmental compliance and quality. Only piles were spanned by a post-tensioned cast-in- exceptional performance of the contractor was situ bone-shaped concrete beam 37m long, 1.5m meant to be rewarded with no incentive for wide and 3m deep. The horizontal loads are business as usual performance. transferred through three steel struts from the concrete cross beam to six ground anchors, which were installed in pairs of two and connected with a ENVIRONMENT pre-cast concrete tie beam. A major focus of the contract was the minimisation of environmental impacts and therefore all work was planned and executed with close co- ordination between ONTRACK, Fulton Hogan Civil Ltd, Horizon Regional Council (HRC), the Department of Conservation (DOC) and Fish and Game.

The Makatote River is protected by a National Water Conservation Order. The Order protects the river’s outstanding wild and scenic characteristics, the outstanding qualities provided by the gorge and riparian margins, the unique wildlife habitat for the blue duck (whio), and the outstanding recreational trout fishery. The construction had to be programmed around the breeding season of the endangered whio and the trout spawning season. Final design of structure Besides continuous monitoring of water quality the construction team developed together with DOC a detailed monitoring program of the whio pairs in CONTRACT particular during the breeding and hatching season. The basis of the contract document was NZS 3915 with no appointed Engineer to the Contract. Due to the significant construction risks including ground CONSTRUCTION conditions, weather, topography, environmental considerations and the requirement to keep the Temporary Work viaduct fully operational at all stages of the construction a simple Alliance Agreement acted as Access to the construction site was gained in May an umbrella over the standard NZS 3915 contract. 2006 from an existing track constructed in the This proved to be enormously successful as it 1980s on the southern side of the gorge. The meant that ONTRACK, Fulton Hogan Civil Ltd and track was covered in thick scrub and had washed the expert advisors could literally act as one team out in several places and required widening along to address and successfully resolve engineering the length of the track to allow access for and construction issues. The simplicity of the construction equipment, in particular the crawler Alliance Agreement meant that administration cranes, to the bottom of the gorge. The installation costs from this process were low and a lot of value of sediment control measures like cut-off drains, was gained from the close working relationship sediment ponds and hay bale barriers was carried between the principal, consultants and contractor. out in parallel to the earthworks associated with the access track. Following a registration of interest, Fulton Hogan Civil Ltd was invited by ONTRACK to develop, together with ONTRACK, the best suited construction technique, program, risk register and finally the Target Outturn Costs (TOC). Any deviations from the original contract scope had to be agreed between the parties as scope changes

proposed ground investigation included two 40m deep boreholes holes at each pile location and three 12m deep boreholes at each ground anchor set, with cores recovered and SPTs carried out every 1.5m. The downstream pile borehole was the first borehole to be drilled. Andesite boulders and gravels cemented in a sand matrix were generally encountered for the length of the bore. At 38m below ground level (7m below pile founding) the investigation drilling struck an aquifer. The borehole was drilled a further 13.5m to 51.5m total depth, to determine the consistency of the aquifer and to investigate artesian water pressures at lower level. From the time artesian water was encountered at 38m depth the head was measured Installation of downstream bridge consistently at 6m above ground level, equating to a pressure head of 44m. Levelled storage at the bottom of the gorge was minimal and confined within the boundaries of Immediate consultation between the designer, ONTRACK owned land. Truck and trailer access to geotechnical engineer and contractor commenced the bottom of the gorge was difficult and trailers and it was concluded to plug the borehole by had to be turned around by a crane with trucks means of grouting, ensuring that no artesian water often being towed back up the hill. Often it was would penetrate into the pile during the excavation. easier to offload plant and materials at the top of Another borehole about 5 metres to the side of the the hill and transport it in a 6 wheel drive dump pile was drilled to determine the consistency of the truck to the bottom. artesian water pressure and to install a piezometer and inclinometer to measure any changes in water Once access to the bottom of the gorge was pressure and ground stability during piling. Tilt gained, a bridge across the river and an access meters were also attached to the four existing track to Pier 7 itself had to be constructed. Thick viaduct footings of Pier 7, to monitor any rotational vegetation on the 15m high bank to Pier 7 was movement during piling operation. cleared by hand until an excavator could start with the forming of the access track to Pier 7. The The 12m ground anchor boreholes were the next ground consisted typically of a 2-3m deep layer of boreholes to be drilled with fewer surprises, though sensitive wet ash overlying the lahar boulder on the upstream anchor, all of the drill water was deposits. Springs and ground water were lost at about 9.3m depth, identifying potentially a frequently discovered during the excavation of the very porous layer or possible fissure in the road. The 120m of access track from the lower cemented lahar deposit. bridge to Pier 7 required 9 tons of cement, which was hand spread, to sufficiently stabilise the Access to the upstream pile borehole was not subgrade before the road could be used. available at the time of the initial investigation, as clearing the vegetation around the upstream footing of Pier 7 unveiled that the footprint of the Site Investigation and Geology upstream pile was part way up the 17m high river bank. The upstream pile borehole had to be The surrounding geology is described as a laharic excavated from the river bed once access to the conglomerate of coarsely bedded poorly sorted upstream pile was in place. andesitic boulders, cobbles and pebbles set in a matrix of compact partly cemented medium to fine Pile Design and Plant Selection andesitic sands [2]. The conglomerate depth is unknown however 100m vertical faces are The presence of artesian water in the pile bore exposed throughout the Makatote Gorge. The raised a number of concerns in relation to the conglomerate is generally stable and competent existing pile design and envisaged piling however it is covered with a 1-4m deep layer of air methodology. Uplift pressures on the pile had to be fall volcanic ash which is weak and susceptible to considered, as did potentially balancing the head slipping in wet conditions. Due to the inaccessibility of water in the pile with the artesian water of the right-hand bank around Pier 7, no pressure, limit the flow and migration of fines from investigation drilling was carried out prior to the the ground into the pile. Another major concern commencement of work. As a result, no detailed was potentially destabilising the downstream knowledge of the ground conditions prior to footing, in case artesian water forced itself up the commencing construction was available. The

outside of the pile between casing and the To suit the oscillator methodology the initial pile excavated ground. casing had to be re-designed. A casing with 20mm wall thickness was selected with an additional 1200mm high, 32mm thick plate added to the base to create a 52mm wide shoe. The shoe was castellated and hard faced to reduce wear during oscillating.

Ground anchor with tie beam and strut

After analysis of the available data, the downstream pile design was reviewed and a new design and methodology was determined whereby a telescopic pile was chosen, with the pile reduced Oscillator working at downstream pile to 1.8m diameter over the lower 10m. This would allow the 2m diameter pile section to be The 20mm casing was further utilised as structural constructed to a safe depth of 17m above the casing to assist with the flexural strength reducing aquifer without the danger of pressured water reinforcing quantities and therefore costs. As a penetrating into the pile bore. Prior to commencing pre-requisite 100% ultra sound testing of the lower with the lower 10m section of the pile the annulus 10m section of the 1.8m diameter casing welds of the upper 2m diameter section was sealed with had to be carried out. cement grout preventing artesian water rising uncontrolled between the pile casing and the Piling ground. Once pile excavation commenced it soon became The borehole results confirmed in principle the apparent that this would be a very arduous consistent deposit of lahar andesite boulders operation with daily productivities varying between cemented in a sand matrix. The size of boulders 40mm and 3000mm resulting in an average nevertheless was significantly larger than expected production of 400 to 500mm per day. While the and being visible along the exposed river banks. It oscillator ensured that the casing was always at was agreed that driving and/or advancing the pile the level of pile foundation, grabs and chisels were excavation in front of the casing would likely lead used to split and remove the pile spoil. Ground to a significant over excavation of the pile bore with water was encountered 1.5m below the working boulders collapsing into the pile beneath the platform, which was very surprising as piling casing, potentially destabilising the existing footing. commenced 17m above river level and 4m away The combination of very difficult ground conditions from the edge of the near vertical bank. paired with the presence of artesian water led to the directive to minimise any vibrations in the close “Freezing-up” of the casing in the lahar material vicinity of the downstream foundation. was a significant risk and piling operation was carried out in extended shifts. Furthermore the The use of an oscillator drill rig was identified to be casing was lifted at the end of the shift by several the best method of excavating the pile. The 100mm. oscillator drill rig reduced the amount of over break along the pile walls by keeping the pile casing at all times at the base of the pile excavation, thus reducing the risk of collapse of boulders into the pile. The oscillator also enabled excavation of the pile with sufficient casing above ground level to balance the head of the artesian water expected to be encountered.

the reinforcing cage the pile was poured as a wet pile using tremie pipes.

While the piling operation continued on the downstream pile, staging was constructed over the Makatote River to access the upstream pile. The staging platform was 8m above the river bed and had to be strengthened to withstand the imposed reaction loads from the oscillator.

Driving piles into the river proved very difficult and a down-the-hole hammer rig was used to break up the larger boulders at the staging pile locations, to ease the driving of the staging piles.

Maintenance work to grab The river bank near the upstream pile was excavated to staging level to provide a levelled The 2m diameter section of the pile was advanced surface and sufficient working room for the to founding level and the annulus of the pile was oscillator and welder to splice the casing. The sealed using a combination of 12m3 cement grout working space was secured with a 6m high rock and 40m3 block fill, which was gravity fed between fall protection around the pile. Steel mesh was also the steel casing and the ground. fixed to the bank to contain any lose boulders.

In order to continue with the excavation of the lower portion of the pile the 1.8m diameter casing had to be lower into the already excavated pile and extended above the 2m pile casing to allow for the oscillator drill rig to be connected. Excavation continued at very slow rates but with no artesian water entering into the pile bore.

Staging to upstream pile

It was with some trepidation and a lot of careful planning that the drilling of the upstream borehole was started. The river bed from where the upstream borehole was drilled was only 23m above the level where on the down stream pile the artesian water with 44m pressured head was

Concrete pour at downstream pile encountered. Sealing the bore hole would be very difficult if during the investigation drilling artesian Once the entire pile was excavated, the 1.8 water of this pressure was struck. diameter casing was cut off at the bottom of the 2m diameter pile and grout pipes were installed At 11m depth artesian water was encountered but inside the 2m diameter pile, passing into the the pressure head was only 12m. Drilling annulus of the 1.8m diameter pile. This task was progressed to the maximum required depth of 25m carried out with utmost care as the joint between (10m below pile founding) and the artesian water the 1.8m and 2m diameter pile was 20m below the pressure fortunately did not increase and the ground water table and the pile was not yet sealed borehole was easily grouted and sealed. at the bottom. 14.9m3 of grout was used to grout the annulus of the 10m section of 1.8m diameter Without the presence of high water pressures near pile. The pile base was then cleaned and founding level, the upstream pile was constructed inspected using CCTV. Following the placement of as per the original 2m diameter pile design. The boulders encountered were however larger than

experienced on the first pile and as a result the substantial weight of the false work on the annulus around the pile was significantly wider surrounding ground and therefore the stability of than on the downstream pile. In order to seal the the bank had to be assessed. Also whether the gap, 60m3 of block fill were required. This volume existing footings could carry the additional load of grout equated to an average annulus around the from the concrete crossbeam was unknown. pile of 457mm and resulting in more concrete placed outside of the pile casing than inside the casing.

Following the completion of the upstream pile to staging level, the remainder of the pile was constructed above ground level by extending the casing to the beam soffit elevation, installation of the reinforcing cage and pouring the concrete in conventional matter using a mobile concrete boom pump.

Monitoring of the ground conditions was continuously carried out during piling operations. Inclinometer and tilt metre readings on the existing footings were carried out daily and the piezometer data was recovered and reviewed weekly. No Installation of formwork for last beam section movement of the existing footings was detected during the construction of the piles. It was decided to use a combined support system transferring part of the loads onto the existing viaduct footings and the remainder onto the surrounding ground. 10 x 610UBs were used, spanning between the existing footings and supported from the ground with shore loading. Both systems were designed to take 100% of the full weight of the wet concrete. The shore load would initially take the weight with the load being transferred back to the existing viaduct footings through the 610UBs in case the ground settled under the weight of the wet concrete. For the upstream section of the beam, part of the loads were transferred into the completed pile by supporting the 610 UBs on brackets attached to the permanent pile casing.

© Piling operation at upstream pile Doka Framax formwork system was used for the straight runs of the beam walls with conventional timber formwork around the tower leg bracings and Beam Construction the tapered section.

The early concept design of the cross beam was a The nearest certified batching plant was Byfords 3m deep x 2.5m wide rectangular beam for the full Ready Mix in with a travel distance of one 3 length of the cross beam. By shaping the beam hour from the site. With a maximum output of 25m into a dog bone shape (in plan) and tapering the per hour the batching plant size satisfies the day- beam from 2m deep at the ends to 3m deep at the to-day requirements in this rural area but was tower legs a reduction of concrete volume of considered not large enough to provide the approximately 40% was achieved. This reduction concrete quantities to pour the beam in one had a major affect on the seismic loading and continuous operation. With a calculated casting subsequent ground anchor requirement and also time of 8 hours to pour the beam as one, the risk of significantly reduced pile vertical loading. forming a cold joint during the pour was too high. The concrete crossbeam was constructed in three 3 Work on the 38m long post tensioned crossbeam sections of 60m to reduce the quantity of concrete which transfers the load from the tower legs to the needed per pour and therefore the time required to piles commenced once the down stream pile and carry out each pour. pier were completed. The effect of the weight of wet concrete (184m3) combined with the

The cross beam was post tensioned with 8 BBR CONA Type M3 Anchors with 19 x 12.7mm strands to a point slightly above load balanced conditions. This resulted in a slight upward deflection (or hog) when the beam was post tensioned which ensured that the self weight of the new underpinning structure and train loading was relieved from the existing footings. A lift off test was used to determine friction of the strands in the ducts prior to stressing the tendons. Holding down bolts between the tower leg and the original mass concrete footing were released prior to stressing. No noticeable positive deflection was observed during the stressing of the beam, or after removal of the supporting structure. Completed structure in February 2007

Ground Anchors and Struts After 10 months construction and spending just over 26,000 man hours, the project was completed The anchor holes for the six 12m long 40mm solid in February 2007 within program and budget. The Macalloy bar ground anchors were drilled with safety performance was exemplary and the temporary casing using a down-the-hole hammer environmental compliance exceeded the consent drill. Drilling through the andesite boulder material conditions. was difficult but all 6 anchors were installed within a tolerance of less than 50mm. This allowed for a CONCLUSION smooth installation of the pre-fabricated concrete anchor head walls and tie beams which were The construction of the Makatote viaduct nearly a fabricated off site. century ago was a milestone in New Zealand railway construction and only written reports can provide us today with an insight of the challenges the construction pioneers encountered in this rugged alpine environment.

The team involved in the project Makatote Viaduct Underpinning Pier 7 experienced these challenges first hand using today’s technology and can only marvel at what our pioneering engineers and construction teams achieved.

Drilling of ground anchors

The double corrosion protected ground anchors were grouted and tested after curing to 75% UTS and locked off at 500kN. The tie back struts consisting of 400mm diameter steel tubes were then lifted into place to connect the main cross beam to the ground anchors.

Winter conditions

Computerised equipment, electronic communication and sophisticated construction techniques have enabled us to build structures where man seems often to be of secondary nature. The Makatote Viaduct project has taught the team involved, that the most important ingredients for a successful project is the identification of risk and opportunity within the project, the determination of the people, open communication on all levels and a common goal.

ACKNOWLEDGEMENTS

A few words at the end of a technical paper are far from sufficient to acknowledge the team, which was involved in this project and made it a success from start to finish. The authors would like to thank all individuals and organisations who were involved in this challenging project for their co-operation and contribution towards this and in particular:

ƒ The men on the ground who were working away from their homes and families throughout the harsh winter months. ƒ Main Subcontractors Construction Techniques Group Ltd (ground anchors, and post-tensioning), ProDrill Ltd and Geovert Ltd (investigation and instrumentation drilling) ƒ Sally Marx and Emily Grace (Resource Management Consultants from Tonkin and Taylor) for their involvement in the consent application and the support during the construction period. ƒ Maurice Fraser (Geotechnical Advisor) for his ideas and being a sounding board in difficult technical situations. ƒ Tony Joyce (Consultant Engineer) for his design of the temporary structures. ƒ Ross Fisher, Craig Giaccio, Guy Grocott and Bill Graham for their involvement in the initial planning phase of the project. ƒ George O’Brian – Clerk of Works for being available if and when required and creating a vital link between the contractor, the client and all other parties involved. ƒ The staff from HRC, DOC and Fish and Game for their pragmatic approach and cooperation.

REFERENCES

[1] R.S.Fletcher (1978), Single Track – The Construction of the Line, William Collins Publishers Ltd [2] N.Z. Geological Survey, Engineering Geology Report EG231, Unpublished