CAISSON INNOVATION

DAVID PATTINSON

Project Manager – McConnell Dowell

ABSTRACT Auckland’s natural environment and infrastructure are key shapers of our city’s liveability that have faced significant pressure from recent population growth. In this growth environment, Watercare awarded McConnell Dowell a $15M contract to construct a new two million litre concrete wastewater attenuation tank in Glen Eden, West Auckland. The tank and pipelines were designed to service the growing population (from 24,700 to approximately 45,600 by 2051) predicted for the Upper Glen Eden catchment in West Auckland and reduce the number and frequency of wet weather overflows to the environment. In many ways, we enjoy a luxury of space in New Zealand compared to many other countries. Nevertheless, infrastructure upgrades in urban environments are inevitably challenged by space constraints, and methodologies with a smaller footprint and lower impact on the surrounding area are likely to become increasingly popular. In our largest and most populous city, McConnell Dowell used a method rarely seen in this country to construct the circular underground attenuation tank in the constrained West Auckland site. The project attracted attention from infrastructure planners, designers, and delivery organisations all over the country, fascinated to see the technique in action. The original tender design for the storage tank was for an open excavation, constructing the tank from the bottom upwards and backfilling around the completed structure. However, space at the site was severely restricted on all sides, so the McConnell Dowell engineering team proposed an alternative caisson design, which significantly reduced the work space required, maintained high safety standards for the work crew, and minimised risk and cost for the client. The tank was constructed at ground level, with 400 mm thick concrete walls, cast within a bespoke formwork system in three separate lifts. The key was a steel cutting shoe which was designed and fabricated in house by McConnell Dowell. Following the casting of each wall lift, an excavator was used to dig out the central area of the shaft and by carefully undermining the steel cutting shoe around the shaft perimeter, allowed the caisson to sink under its self-weight. A total of 2,500 m3 of spoil was removed, while 50,000 litres of bentonite mix were pumped into the 50 mm gap (annulus) between the outer wall face and the cut face to reduce friction and prevent ground collapse. Three and a half months after commencement, the completed caisson arrived in its final resting place - 18.5 metres in diameter, 12 metres high and weighing 750 tonnes. Floor slab anchor piles, a 1.2 m thick floor slab, self-cleaning vacuum chamber, pre-cast tee-beam roof, mechanical and electrical installation, and network pipe connections completed the job. Once the tank had been commissioned and the existing carpark and nearby sports field reinstated, the tank was brought into service and all that remains visible are seven manhole covers and an electrical control box.

The existing wastewater network in the area did not have sufficient capacity to manage current flows during wet weather events, so during heavy rainfall the network was overflowing diluted wastewater into the surrounding environment. The new attenuation tank is designed to reduce these overflows from an average of 10 per year to two or less and the community is already seeing benefits of cleaner waterways during storm events. Key words Caisson, cutting shoe, wastewater, tank, risk, safety, Watercare, engineering, construction, commissioning INTRODUCTION This paper describes how we achieved “progress through collaboration” on Watercare’s Glen Eden Storage Tank and Sewer Upgrade to develop an innovative design and construct solution that addressed common risks associated with construction in an urban environment and provide a quality, reliable solution that reduced overflow frequency to the environment. Watercare issued a Request for Proposal with NEC3 as the form of contract and hosted a tender stage workshop with all shortlisted contractors to promote early collaboration. This process was designed to ensure that project drivers and objectives were adequately communicated to all parties for inclusion at Tender Stage. The original tender design for the storage tank was for an open excavation, with a ‘bottom up’ construction. However, in recognition of Watercare’s objective to minimise disruption, McConnell Dowell identified an alternative proposal to construct the tank using an innovative caisson (sunk shaft) construction method, a first for Watercare and New Zealand. The alternative tender was accepted as a partial Design and Construct contract in April 2016. Developed in conjunction with designers, Stantec New Zealand, our alternative proposal delivered on Watercare’s business needs, whilst providing the following benefits:

• Reduced construction footprint due to a reduction in the working space required

• Reduced overall community impact from less time required on-site

• Less noise and vibration impact as no significant backfilling required

• Minimised risk and cost for the client by avoiding open excavation to install the tank

• Cost and environmental savings in excavation, disposal, importing, backfill activities and associated fuel and transport usage

The caisson construction method is not commonly used for permanent structures and infrastructure in New Zealand. McConnell Dowell developed and advanced the detailed design through a number of design workshops and made subsequent adjustments to the construction method, which resulted in a successful installation and delivery of a first-class asset to Watercare.

Aerial view of the constrained site at Harold Moody Reserve

BACKGROUND Auckland’s natural environment and infrastructure are key shapers of our city’s liveability that have faced significant pressure from recent population growth. In this growth environment, Watercare awarded McConnell Dowell the $15M contract to construct a new two million litre capacity attenuation tank in Glen Eden, West Auckland. The tank and pipelines were designed to service the growing population and reduce the number and frequency of wet weather overflows to the environment. During the tender phase, McConnell Dowell identified that the area available for construction of the storage tank in the Harold Moody Reserve was excessively limited and there were a number of constraints presenting considerable safety and project risks. The main constraints were:

• The tank was to be constructed less than 6 m away from a 910 mm dia water main – the major supply to the North Shore – to the west of the tank

• A children’s playground to the north of the tank

• A sports field to the east of the tank (which needed to remain in use)

• Car park entrance to the south of the tank, and

• Restrictions on working hours because of the nearby residential area The original tender design for an open excavation would have required construction of the tank from the bottom up, necessitating 15 m of working space around the circumference of the tank to allow for a safe batter, resulting in a 50 m diameter open hole in the ground – there was simply not enough room. McConnell Dowell recognised there was an opportunity for an alternative construction method to overcome these constraints and reduce the project risk profile and submitted an alternative caisson proposal to Watercare. The caisson methodology eliminated open excavation and associated potential hazards to the public and our team by decreasing the frequency and range of working at height and significantly reducing working in confined spaces. Environmentally, reductions in excavation, disposal, and material import were vastly reduced.

In addition, this minimised the risk to the nearby 910 mm dia watermain and to structures from vibration and soil movement, made best use of the limited construction area and allowed for access to be maintained to the playground and sports fields. Due to the high recreational value of the tank site, Watercare undertook extensive consultation during the design and consenting phase. Following the acceptance of our alternative tender, we allocated an additional three months to re-design the tank construction (caisson) at the project outset and to allow for further consultation and the preparation of consent documentation, planning, and procurement- this resulted in a well-organised, systematic project mobilisation with no consenting issues. COLLABORATION AND DEVELOPMENT OF OUR RISK MANAGEMENT APPROACH NEC3 requires a high level of Client and Contractor collaboration to manage document transfer and response. With our alternative proposal, it also required transparency to demonstrate our performance against the original programme and outturn cost. This was achieved by taking a shared risk approach and taking steps to break down organisational barriers by critically assessing and agreeing to each party’s objectives in terms of risk management for the project. The early identification, communication and management of project risks as part of the ‘early warning’ process is at the very heart of the NEC3 contract. As a result, risk management was a whole-of-project process, used initially to support the scheme optioneering process to provide the optimal solution for the client, from a design, operational, reputational and cost viewpoint. It also ensured we were actively managing and controlling our own operational risks around the construction of the project from an early stage. A single Project Risk Manager facilitated the process of establishing one over-riding risk management approach and a sole living risk register. Transparency was key. Monthly risk review meetings facilitated by the Project Risk Manager provided a forum for all parties to sit down once a month and table issues in an environment where everyone was looking for a solution. Risks were identified and ranked according to consequence, monitored and regularly reassessed with the objective being to negate risks over time. The caisson sinking illustrates this process. At the outset we had minimal experience of the method, in particular, the rate at which the caisson would sink in the ground conditions. Contingencies were put in place if the caisson proved difficult to move – these risks were ranked as “high”. As the caisson began to sink, risks such as the efficiency of the cutting shoe were reduced. Conversely, when the tank then hit harder ground, there was increased risk of cost to resolve the issue. The risk then reduced again as the tank continued to sink under the additional weight of the concrete walls and all risks associated to the caisson sinking were closed out when the tank reached its final position.

INNOVATION FOR LARGE CONCRETE STRUCTURE The concept of caisson sinking relies upon a steel cutting shoe to cut through the underlying ground. This was designed in-house by McConnell Dowell, fabricated in their mechanical yard and delivered to site for setting up. The steel cutting shoe was constructed on a narrow strip of blinding concrete, and concrete fill place under its sloping face to prevent any settlement during concreting operations. Starter bars were then fixed within the ‘shoe’, which was subsequently filled with concrete – this formed the base of the walls which were to be cast above. An external concrete hard standing was also cast to allow safe access. In conjunction with formwork supplier PERI, we designed a bespoke formwork system to sit on top of the cutting shoe for the cast in-situ walls. The system was formed with curved primary walers (preformed to the curvature of the tank), PERI lattice girders, curved plywood, and three levels of tie-rods for each pour. Each wall was cast in 4 m high sections, so pressures of up to 100kN/m2 were catered for. For each lift, eight sets of internal shutters were fixed in place to form the inside face, followed by fixing of pre-fabricated panels of wall reinforcing steel. Finally, eight sets of external shutters were fixed in place, an access platform erected around the top of the shutters, and the wall cast. An early start was arranged to ensure Auckland’s traffic did not compromise the pour. The pour was carried out over a 5-hour period, with 5 lifts of 800 mm completed at a time - this ensured that the foundations were loaded equally (thus preventing differential settlement) and a cold joint between layers was avoided. Settlements were monitored as the wall was cast – only minor, insignificant, settlement was recorded. Upon completion of the pour, the shutters were left in place for 48 hours prior to stripping. Once stripped, hessian sacking was draped over the exposed concrete faces, and a watering feature set up on top the wall to allow a steady spray of water to soak the sacking, keep the concrete surface moist and free from any thermal cracking. With the wall shutters stripped from the first pour, excavation began to sink the caisson under its own weight. A 12 tonne excavator cleared the bulk load from the central area of the tank, followed by a more sophisticated under-mining of the steel cutting shoe with a 5 tonne machine. Regular monitoring of the levels of the

Starter bars fixed in the cutting shoe ready for concrete

Excavation inside the tank

Design of the steel cutting shoe

structure was undertaken to ensure a smooth, even descent. The steel cutting shoe was designed with a 50 mm step at the outer face, thus leaving a 50 mm gap on the outside of the sinkling walls - a bentonite mix was pumped into this gap (annulus) to help reduce friction and prevent ground collapse onto the wall. Excavated spoil from within the tank was removed by a combination of crane and clamshell grab, and conventional skips. The wall continued to descend until, after a three week period, the top of the new wall reached the level of the external concrete hardstanding. At this stage, the internal shutters were re-erected using Peri’s jump-form system, wall reinforcement added, and external shutters re-erected. For the second lift, the team were unsure as to what settlement the walls would experience as the additional pour was completed – too much settlement would sink the walls and potentially pull the external shutters off as they clashed with the external hardstanding. Additional spoil was place against the cutting shoe, within the excavation, to apply a ‘brake’ and prevent settlement. This proved a huge success as only 20 mm (uniform) settlement was realised. The process of sinking the second wall lift, re-setting of formwork / rerinforcing, casting the third lift, and shaft sinking continued until the tank reached its final resting place – a total of 300m3 of concrete was used in the tank’s wall construction, with vertical tolerances met. With the walls in place, works to the foundation (tension piles) and floor slab were undertaken – these activities are traditionally carried out prior to wall constru ction. Part of Stantec’s design was to provide a structural tie between the floor slab and walls – couplers had been installed in the first wall pour and starter bars were now screwed into the couplers to form the mechanical tie. A further feature of the tank was the provision of a central column, used for a vacuum flushing system (to self-clean the tank following operation), and providing support for the roof. Further innovation was applied to the construction of the tank roof using precast, pre-tensioned units with an in-situ concrete topping. This precluded the use of large amounts of formwork, falsework and scaffolding within the completed tank, providing a safer environment in which to work. ENGINEERING AND DESIGN We applied a staged approach to engineering constructability by facilitating a series of Construction Risk Assessment Workshops (CRAWs). These workshops enabled constructability to be considered at each stage and helped us to identify areas where special control measures would be required to ensure construction would be right first time - despite the complexities of manufacture, associated methods of construction, and the fact that this method had not been used in New Zealand previously. Risk was quantified in terms of effect, likelihood and cost and provided strategies to mitigate potential construction, environmental and H&S and commercial risks. This process provided the framework for the development of subsidiary documentation such as CEPs, JSEAs, and the Risk Register. As an example, we scheduled major lifts into the tank CEP to ensure all lifts were coordinated appropriately and executed safely.

The in-situ roof segments in place

Undertaking the storage tank design under a Design and Construct contract had a number of benefits including fostering a collaborative design culture. McConnell Dowell was able to design the steel cutting shoe, allowing Stantec to incorporate this temporary works design into their permanent works, including temporary loading conditions, climbing formwork, access requirements etc. The Watercare’s designer remained responsible for the process elements of the tank, and all other civil / structural elements. An in-house engineering resource carried out the design of the steel cutting shoe and bentonite supply tubing which allowed the best optimisation of permanent designs, best safety in design and ensured constructability and cost checks were regularly monitored. COLLABORATION ON THE GROUND Integration of Watercare’s induction procedure with our own resulted in McConnell Dowell leading most of the project inductions collaboratively on-site instead of Watercare carrying out their own inductions. One of our project objectives was to foster highly collaborative and effective relationships with suppliers to optimise efficiencies and provide excellent quality materials and services to the project. All subcontractors were managed by McConnell Dowell under our management systems and regular progress review meetings were conducted to review and optimise safety, environment, and quality performance, plus scheduling and project coordination. Commissioning was a critical activity for Watercare. Consequently, discussions commenced with their operations staff at an early stage where an understanding of the constraints and requirements were established. A Commissioning Plan was drawn up and submitted to the Watercare for approval – once approved, the document became the methodology all parties adopted.

Water tightness inspection inside the tank

COMMUNITY IMPACTS

The location of this project was in a built up residential area containing many community amenities meaning there was a high level of public interface and local attention. Extensive consultation was undertaken by Watercare through the design phase with identified stakeholders including the Waitakere Ranges Local Board, Mana whenua, Auckland Council, sports clubs, landowners and local residents to ensure the project was planned to minimise environmental effects and disruption during construction. Due to the importance and sensitivity of customer satisfaction McConnell Dowell employed a dedicated stakeholder manager to work with Watercare’s Stakeholder liaison advisor. Working together, we regularly communicated with the local residents and invested stakeholders, produced informative newsletters, arranged notification signage, and dealt directly with the infrequent customer complaints. We also invested in community initiatives to foster public interest and involvement. Some examples of community initiatives we instigated were:

• Provided viewing windows cut into the hoardings surrounding the site where interested parties could safely watch the tank sink over the duration of construction

• Visited the local school and arranged for the local children to paint ‘environmental’ pictures which were displayed on the site compound hoardings

• Maintained safe access to the public playground alongside the tank construction site

• Liaised with the Glenora Bears League Club in winter and the Glenora Softball Club in summer to ensure access was maintained for sporting events

• Organised an “Open Day” for the general public to see inside the tank before it was covered over with a roof

• Liaised with the residents most affected by the works to minimise the noise by installing haybales and noise walls and maintaining access with temporary paths and ramps.

CONCLUSION

The caisson method of construction attracted significant attention from infrastructure delivery organisations all over the country interested in learning about how new techniques can be employed to overcome the challenges of building infrastructure in big cities where space is at a premium and stakeholder engagement is a priority. Methods like the caisson that have a smaller footprint and impact on communities will likely become more popular and Glen Eden has provided a positive legacy for the industry. Three and a half months after commencement, the completed caisson arrived in its final resting place 14m under the ground. With reinstatement completed to a very high standard, all that can be seen are four manhole covers and an electrical unit in the carpark. Since these new infrastructure upgrades have been completed, overflow frequency has reduced through increased capacity of the network. View a time-lapse of construction of the tank here: https://www.youtube.com/watch?v=a1IniH7FThM

Harold Moody Reserve carpark reinstatement over the tank