WELLINGTON INTERNATIONAL AIRPORT MULTI-LEVEL CARPARK – CONCRETE RAFT SLAB CONSTRUCTION

A DAVIDSON1; C ASHBY1; C SUTHERLAND2; R TAGGART2

1 Opus International Consultants 2 Fletcher Construction Company

SUMMARY Opus International Consultants (Opus) designed the new 10 Storey car parking building at Wellington International Airport that is currently under construction. The Fletcher Construction Company (Fletcher) is responsible for the construction of this project. Included is a 10 storey concrete moment frame structure with buckling restrained braces (BRB’s) founded on a raft slab and an adjoining elevated roading structure and concourse. The scope of this paper is to summarise key design considerations behind the use of a shallow raft slab foundation. To be covered in more detail is the construction of the raft slab including phasing, methodology, risk management, placing, curing and management of site constraints. Further information on the design of the car parking building may be found in session 3 paper 1 from the NZCS conference 2016 “Wellington International Car Parking Building” INTRODUCTION Wellington International Airport Limited (WIAL) is currently implementing a major re-development plan for the airport which includes the new car parking building with an additional 1090 carparks along with a transport and bus hub within the building. The site for the new car parking building is located to the North East of the existing terminal and has a network of ramps and elevated road structures (known as the concourse) which will connect the carpark to the new hotel, existing carparking building, concourse, and terminal. Opus were engaged by WIAL to provide the structural engineering services for the car parking building, concourse and new Rydges hotel. Fletcher have been contracted by WIAL for the construction of the car parking building and the adjoining concourse structures. BUILDING DESCRIPTION The car parking building is 10 storeys including a mezzanine level between ground and the first floor. Overall dimensions of this building are 78 m x 51 m x 28 m high with a gross floor area of the building of approximately 35,000 m2, which will provide parking for up to 1090 cars. The concrete frames are largely precast cruciforms with precast double tee floor units.

The concourse structure is a single level structure with a gross area of approximately 4400m2

including ramps. The raft slab for the carpark on which this paper is based covers an area of approximately 4050m2 and varies in depth between 1.0 m and 1.5 m. The total quantity of concrete that was used in the construction of the foundation was 4316 m3 along with 704 tonnes of reinforcement. The slab also had a number of services running through it. A typical cross section of the building is given in Figure 1 below.

Figure 1: Typical cross section of building showing the location of BRBs.

RAFT SLAB DESIGN BACKGROUND Foundation Structure The foundation system for the building is a 1.0m deep raft slab with 1.5m deep perimeter thickening. The bases of the columns where the BRBs are located were also thickened to 1.5m locally to increase shear capacity in the raft slab. The raft design was carried out by finite element modelling of the soil structure interaction in both SAP2000 and LS-DYNA, which resulted in a more efficient shallow raft slab foundation. Structural Options The building was originally proposed to be a steel braced frame structure with a 30m deep bored piled foundation. It was the client’s request that the concrete option be considered due to the close proximity to the coastline, with steelwork creating a durability issue resulting in high maintenance costs for WIAL associated with exposed steelwork. The fire protection for the steelwork was also a substantial cost. With the open sight lines within the building being a key aspect of the design, a conventional shear wall system was not considered suitable so both a concrete frame with supplemental braces and the steel braced frame systems were presented as options during the preliminary design stage and cost estimates were prepared for both options. Without taking into account the on-going future maintenance of the building, the concrete option was still evaluated as

cheaper to construct. Together with concrete being more durable under exposure to adverse environmental conditions compared to steel, and architectural consideration regarding sightlines, the concrete frame with supplemental BRB braces option was selected as the preferred building form. CONSTRUCTION Statistics The area of the slab was approximately 4050m2 and varied in thickness from 1-1.5m. Table 1 shows a summary of the quantity of concrete used in the construction of the raft slab.

Table 1: Quantity of concrete used in construction of the raft slab.

Pour # Concrete Quantity (m3) Trucks (No.)

1 958 167

2 951 166

3 1165 203

4 612 110

5 630 111

Total 4316 757

The total quantity of reinforcing used for the raft slab was 704 tonnes. Maximum Cruciform Weight 24 Tonnes. Crane: Liebherr 450 T Crawler with Fly Gib. Planning and Staging The car parking building was split into three zones for the superstructure construction to fit with the construction and crane lifting methodology. This formed part of the basis for the raft slab pour sequence. The raft slab was split into 5 zones that were consistent with the building superstructure zones, these zones are shown in Figure 2 below. Pours 1 and 2 were under superstructure zone A and upon completion allowed the superstructure build to commence in that zone using a mobile crane. Pour 3 was to allow access for the crawler crane on site to start lifting precast in for zone A as the capacity of the mobile crane was limited. Pours 4 and 5 were not program critical and hence were smaller in size to reduce the level of resourcing required to complete them. Concrete supply was the critical component determining maximum slab pours in the case of this project. Therefore the concrete supplier, Allied Concrete (Allied), was heavily involved with the planning of these pours. The pour sizes were limited by the capacity available from Allied who had to consider factors such as maximum driver hours, maximum concrete plant output, and potential transport delays due to traffic. Due to the size of the first three pours there were 3 concrete plants operating, two of Allied’s and one other, to provide the required supply, this also created a redundancy as a risk mitigation so that the pours could be completed if there was a malfunction with any one of those plants. At the planning stage a lot of consideration was given to the timing of the concrete pours as the timing was heavily dependent on avoiding both busy times at the airport and on the road through the Mt Victoria tunnel (state highway 1) which resulted in the pours occurring at night time. Saturday evenings were chosen with the pours starting at around 3pm in the afternoon

and continuing into the early hours of Sunday morning. The planned pour duration was up to 13 hours, the time chosen allowed the pour to occur without coinciding with a peak traffic flow between the concrete plants and the site. Starting on Saturday afternoon also avoided Saturday morning sports traffic. Additionally disruption to Allied’s normal supply activities were also minimised. Given the size of the pours, Allied had to bring in truck drivers from their other plants from around the country. By choosing a Saturday evening the pour could be completed within the maximum number of driving hours permitted for these drivers without affecting projects serviced by their home plants. Weekends with big events in Wellington were also avoided due to accommodation constraints for out of town staff, increased airport and traffic management issues. Consultation with Wellington City Council the NZ Transport Agency and Go Wellington was also carried out to make them aware of the additional traffic that we were putting on the road. This also allowed us to take account of any specific road closures that we would need to avoid. Alternative routes were also considered along with their time implications, for example we calculated that it would take concrete trucks an additional 5.5 minutes to get to site via Oriental Parade and Evans Bay Parade should there be an incident in the Mt Victoria Tunnel. During the planning phase other aspects that were considered included specific health and safety requirements, site set up and pour plan, plant and equipment including contingencies for breakdowns, requirements for placing and curing and placing required subcontractors personnel on standby. Other key considerations included the requirement for compliance inspections, machinery operation below OLS and noise restrictions. A number of contingency plans were put in place to ensure that all pours could be completed even if something was to go wrong. The pours were programmed with 2 concrete pumps with a spare concrete pump on standby, and required a total of 3 concrete batching plants supplying concrete and a number of spare concrete trucks. This ensured that if something was to break down then the pour could continue and be finished in time before driver hours ran out and before the airport started to get busy again for Sunday morning flights.

Figure 2: Pour Zones

Health and Safety Management of health and safety is a vital component of construction in which a number of significant hazards exist. These pours were very large activities with numerous subcontractors all working together at once and a large number of heavy machinery movements at night all of which required specific management. It was also important to recognise that working people at night time when they are otherwise used to working during the day adds the potential for fatigue. Lighting also becomes an important factor to ensure that everyone working on the site can see. Inductions and toolbox talks were carried out with all subcontractors’ personnel prior to each pour commencing which identified protocols for moving around site, how trucks would move through the site and what would be used to identify these paths through the site. This was treated as a new activity for each pour as they had different site constraints and traffic movements as the construction progressed. The toolbox talks and inductions were carried out prior to each pour stage with every staff member participating. To help manage fatigue personnel were encouraged to take frequent breaks and food and drink was provided throughout the evening. Also once the pour was complete a fresh dayshift crew was brought in to implement the curing requirements. In the case of unexpected excess wind posing an unacceptable health and safety risk there was a contingency plan in place to stop the pour, this plan could also have been implemented for any other reason that may have required the pour to be stopped before it was completed. The plan was to create a construction joint across the slab parallel to the direction of the pour. It was estimated that should this occur there would be approximately 80m3 of concrete in trucks either at site or on their way to site at the time such a decision was made. This would be sufficient concrete to ensure a construction joint was formed in an appropriate location. The top of this construction joint would be formed using a timber edge to get a straight line with the remainder of the joint going down at an angle through the slab. The concrete at this joint would be properly vibrated and finished and then the sloping face would be sprayed with Rugasol prior to later being water blasted to achieve a type B construction joint. This contingency plan was not required. Set Up and Pour Plan – Pour 3 The site was set up to best maintain the constant flow of trucks delivering concrete to the pumps. The set up was configured in the most suitable way for each individual pour. Figure 3 below shows the dual entrance and single exit setup that was employed for pour #3. Drivers were advised prior to their arrival at the WIAL site the designated entrance for their incoming load. There were two entry points for trucks during pour #3. Gate one located on the northern end behind the sound barrier containers with the second entrance created off Stuart Duff Drive as shown in Figure 3. Both entrances together allowed for up to 6 trucks to wait before unloading allowing a buffer of trucks to be maintained throughout the pour. The storage area was limited to six trucks as it allowed the entrance beside the airport gates to be maintained for WIAL emergency services accessing the airport. All concrete trucks entering the site entrance unloaded their concrete into the respective pump. Pump #1 was located at the northern face of the pour #3 and pump #2 was located on the slab (pour #2). Once the concrete was delivered the trucks exited the site through the single site exit.

Throughout the pour WIAL operations were aware of what was happening and that Fletcher were managing the entrance for emergency services vehicles to the airport.

Figure 3: Pour 3 Site Plan

Reinforcement Fixing The reinforcement fixing was a standard operation, nothing specific of particular note. Stop Ends For the intermediate stop ends Stremaform was used as this is a convenient stop end that does not need to be removed. This does away with the need to apply retardant to regular formwork and to have that formwork removed and the surface water blasted to achieve an acceptable construction joint. Mix Design Given the depth of the raft slab there were concerns that the heat of hydration while the concrete was curing could lead to excessive shrinkage cracking should a standard mix design be used. Calculations were completed by Opus to show that a standard mix design would work for this pour but needed to be controlled well. However, Fletchers opted to mitigate risks further and provide a reasonable margin by using a fly ash mix. There are a number of benefits of using a fly ash mix, the most significant in this case was that the hydration reaction is slowed down and therefore the heat of hydration is reduced. The lower temperature in turn results in a lower risk of drying shrinkage. A further benefit is that the pouring face remains open for longer while placing which allows there to be a very large open face as was the case with these pours. Durability is also enhanced.

There are also some downsides with using a fly ash mix, the most significant was that it is difficult to place and move due to its sticky consistency. It is also a much more expensive alternative. In order to allow the concrete to flow adequately between the more heavily reinforced parts of the foundation an initial slump of 160mm was used and refined throughout the pour. The slump was achieved through the use of admixtures. Environmental Controls Given the timing of the pours the key environmental consideration, additional to day to day environmental requirements, was noise emissions. Noise travels well at night time and given the lower background noise any specific noisy operations can be quite disruptive to residents particularly when they are trying to sleep. Noise restrictions meant that a specialist consultant was required to be engaged to provide a “Construction Noise Management Plan” to assess noise levels at surrounding property boundaries and advise measures to be implemented to keep the noise below the night works mitigation threshold. Acoustic screening as shown in Figure 4 below, was provided to ensure these limits were met. Throughout the pours monitoring was carried out with noise levels recorded at various property boundaries to ensure compliance with the “Construction Noise Management Plan”. Prior to the pour a letter drop to approximately 400 residents within the nearby area was also carried out to inform them of the works.

Figure 4: Acoustic Screening in Front of Concrete Pump

Placing / Vibration For each of the pours the placing plan was varied to ensure that the concrete was placed in a manner that the smallest practicable open face was able to be maintained throughout the duration of the pour. The pumping rate was targeted at 100m3/hr to be able to achieve this. To allow this to happen effectively the planned location of the pumps took into consideration the ease of getting concrete trucks to them along with the ability of the pumps to be able to reach the required areas of the pour without clashing with each other.

The concrete was placed in layers of up to 500mm deep and then vibrated with immersion vibrators at approximately 300mm to 500mm centres to ensure adequate vibration. As well as ensuring adequate vibration, the pour face was continuously monitored to ensure that it remained open throughout the pour. In some cases this meant it was necessary to apply anti vap throughout the pour prior to reaching the float surface. Figure 5 and Figure 6 below show the commencement of placing for pour 1 and the completion of placing for pour 3 respectively.

Figure 5: Commencement of Pour 1 Concrete Placing

Figure 6: Pour 3 Concrete Placing

Finishing Particular attention was required during the finishing process to prevent early age plastic cracking. Pouring at night when the temperature was within acceptable requirements helped mitigate the risk to an extent. However, wind in particular the stiff Wellington Northerly winds on this site was the biggest issue which was causing the surface to dry out after the initial float. Therefore anti vap was applied as required, using a trailered mobile industrial pressurised spray unit with a 70m extension hose, to ensure that the surface did not dry out prior to final finishing. The importance of this in preventing early age plastic cracking cannot be emphasised enough. Where some cracking does occur it is important to get to it as soon as possible to revibrate it prior to the concrete setting to the point where this is not possible. Final finishing is a crucial step and is required even where there are architectural slabs going over the top of the main slab. During the finishing process the application of anti vap was required to continue to ensure that the surface remained damp. Curing Careful consideration was required to be given to the curing process to ensure that undesirable shrinkage cracking was prevented. In order for the curing process to be most effective the concrete needed to be finished well, in a manner that early age shrinkage cracking was managed. At no point between placing and completion of the specified curing should the surface of the concrete be allowed to dry out. This meant the use of anti vap up until the concrete had set adequately to allow the specified curing to occur. For this project the slab was required to be water cured and insulated for 7 days. By insulating the slab the heat from the hydration reaction was retained to improve the effectiveness of the hydration and build early age strength more quickly. The insulation also ensured that the temperature throughout the depth of the slab was relatively consistent thereby reducing the likelihood of built in stresses from the slab curing at differing rates throughout the depth. Figure 7 below shows the slab for pour 2 being cured. To achieve the insulation and water curing a network of trickle hoses through hessian cloth was used to hold the moisture before being covered by 20mm of polystyrene that was fixed to 12mm plywood to weight it down. There were minimal gaps in the polystyrene provided around the locations of the trickle hoses. The plywood was then fixed together to prevent individual sheets from lifting if the wind came up, in some cases additional weight was added around the edges, if it was particularly windy, to prevent the edges lifting as it was critically important to prevent the risk of the insulation being picked up by the wind and becoming Foreign Object Debris (FOD) for the adjacent live airport.

Figure 7: Curing of slab for pour 2.

For interest a single series of temperature sensors were placed within the slab for pour 2 to record the temperature within the slab over the first two days of curing. Figure 8 shows the readings from these sensors including one for site air temperature. Three sensors were placed within the 1.0m thick slab; one near the bottom, one in the centre and one near the top. It can be seen that the maximum temperature in the slab seems to be reached after about one day. What was good to see was that the temperature near the top of the slab experienced insignificant effects from the changes in ambient temperature assisted by the insulation provided to the slab.

Figure 8: Temperature readings from pour 2.

CONSULTANT / CONTRACTOR RELATIONSHIP The early engagement and teamwork between the Contractor and Consultant design team to tailor the solutions for the raft slab was important and contributed to the overall success of these challenging concrete pours. Fletcher’s strong commitment to teamwork and providing a quality product in a zero harm workplace made this a safe and enjoyable project to work on.