PIPE ABANDONMENT BENEATH OPERATIONAL RUNWAY AT BRISBANE AIRPORT James O'Grady, Business Development Manager, Mainmark, Melbourne Abstract Mainmark has completed permanent abandonment works of a 280 metre 1650 internal diameter concrete enveloper culvert beneath an operational runway. The company was contracted by Brisbane Airport Corporation for the project, via a main works contractor. The project was completed in two stages and appropriately adhered to the site's safety standards. First, the pipe was sealed using Mainmark's structural resin injection technology, Teretek. This was to protect the pipe against further water ingress. The second stage of the project required Mainmark to completely fill the pipe with a cementitious grout. Given the critical nature of runway operations, the specifications for a strong, yet lightweight, fill material were rigorous. It required a lightweight and flowable solution with no shrinkage and no bleed. The cementitious fill material required high sulphate resistance to resist saline and acidic ground water. Mainmark developed a bespoke grout mix using their proprietary product, Terefil, an air- entrained lightweight cementitious mass void fill material, to meet the project specifications. The filling operation was monitored in real time by sixteen separate sacrificial cameras installed within the crown of the pipe, which ensured the pipe was completely filled. Key Words: Pipeline abandonment, Cementitious fill, Grout, Pump Introduction Brisbane Airport (BNE) is the primary international airport servicing Queensland and is Australia’s third largest capital city airport by passenger numbers. BNE’s network spans 79 destinations across the nation and the world, with nearly 23 million passengers travelling through the airport each year. The passenger numbers are forecast to grow to 50 million by 2035. To service the increase in passenger numbers, Brisbane Airport Corporation (BAC) is constructing another runway, running parallel to its existing main runway. Early works for the new runway involved sand being pumped to site to surcharge the alluvial soils. The sand was pumped beneath a shorter cross runway through an enveloper pipe. At the completion of the sand pumping, Jan de Nul engaged Mainmark to fill the enveloper pipe with a high mobility, light weight, cementitious grout fill.

BNE’s Runway System The airport currently has two runways; a main runway suitable for larger aircraft and a smaller cross-runway capable of landing aircraft from the smaller general aviation aircrafts to 737s if required. A new runway is being constructed 2km west, and parallel to, the existing primary runway. This will enable BAC to deliver the capacity needed to meet the forecast growth in passenger numbers. Construction commenced in 2012 and the runway is on track to be opened in 2020. The smaller cross runway will be decommissioned in 2020 when the New Parallel Runway (NPR) opens. The A$1.35 billion NPR at Brisbane Airport is 3,300m long, 60 m wide with up to 12 km of taxiways. Brisbane Airport is located on an old, low-lying Brisbane River Delta, resulting in extremely poor strength soils up to 35 metres deep in some parts. In order to prepare the foundations for the new runway, significant ground preparatory works were required. This involved

the placement of 11 million cubic metres of sand in a geotechnically engineered design across the runway site. This, in conjunction with the strategic placement of 330,000 wick drains (largest wick drain project in Australia) will assist in the consolidation of the site foundation, extracting pore water from the underlying alluvial soil. Enveloper Pipes The 11 million cubic metres of sand was dredged by Jan De Nul from Middle Banks in Moreton Bay using a jumbo class dredge, the Charles Darwin. The dredge then transported the sand to the mouth of the Brisbane River, from where the sand was pumped up to 9 kilometres at its furthermost point through a steel pipe to the third runway site.

Figure 1: Sand preloading of alluvial soils The sand was transported in the steel pipe under the existing cross runway through a 1650 mm diameter precast concrete enveloper pipe. The 280 metre long enveloper pipe runs at an angle under runway two, from a launch pit on the east side to a receiving pit on the west side. The concrete enveloper pipe is quite shallow, being located only one metre below ground level at the ends. The crown of the pipe is level with the ground water level. During the works, local dewatering maintained the ground water level below the base of the pipe. Once the pumping operations were completed, there was no need for the enveloper pipe to remain in service. However, the integrity of the enveloper pipe is critical, as any failure will impact the runway above.

Figure 2: 1 metre diameter sand pipeline

Figure 3: Enveloper pipe location Problem The steel pipe used within this project was supported off the concrete enveloper pipe on caster wheels. During pumping, the casters locally abraded the pipe wall to a depth of approximately 100 mm. In addition, there was a continuous scour the length of the pipe caused by temporary dredge pipeline decommissioning activities.

Figure 4: Scour of the pipe

When dewatering ceases, the pipe will become inundated with the high sulphur and chloride groundwater. This will cause degradation of the pipe as the exposed reinforcing steel corrodes with the resulting spalling of the concrete. Proposed Solution In assessing options for the decommissioning of the enveloper pipe, the following goals were identified:

- Undertake any remedial works in a safe manner;

- Ensure that the runway is not affected by structural failure of the pipe;

- Ensure that the runway is not affected by settlement of the pipe;

- Does not require a closure of the runway to implement;

- Cost effective Completely filling the enveloper pipe was identified as the option that removed the risk of any future pipe collapse and had minimal impact on runway operations. Localised patching options for repairing the pipe were rejected as they required access chambers, dewatering and ventilation for ongoing pipe inspections. The parameters for the filling material for the enveloper pipe were:

- Pumpable cementitious material; - Low density, not exceeding that of the

surrounding embankment material, to avoid future subsidence of the pipe;

- Fill procedure to ensure no air void at crown.

- Suitable for existing ground water conditions.

With the soft underlying soils, the fill material density had to be not greater than the density of the runway subgrade. There was a risk of standard density cementitious fill material causing settlement of the soft soil with resulting localised subsidence of the runway. Mainmark proposed the use of Terefil, a highly flowable minimum 1 MPa cementitious grout with a density of 1000 kg/m3. The low density, equivalent to the weight of the ground water that filled the pipe when not dewatering,

removed the risk of future localised subsidence. The Terefil does not require vibration and will self-level. In addition, with no bleed water released during cure the Terefil cementitious grout reduced the risk of any void at the crown of the pipe. Terefil has less than 300 micron shrinkage, lower shrinkage than typical cementitious fills. The Terefil neat grout is also compressible. By compressing the in-place grout to approximately 100 kPa, the shrinkage can be effectively negated by release of compressive force during cure. The low shrinkage and lack of bleed water reduce the risk of voiding between the fill material and the crown of the pipe. In addition, contact grout tubes were placed along the crown of the pipe to address this risk.

Figure 5: Contact grout tubes to pipe crown To ensure long term performance in the groundwater, a sulphate resisting cement was used. Solution Delivery The works to abandon the enveloper pipe under the runway occurred over three weekends. The first weekend access was to seal the pipe, install the grouting infrastructure. Teretek geopolymer was injected into the ground from inside the pipe. The Teretek

provided structural void filling behind the pipe and stopped inflows to the pipe. Six grouting lines and two contact grout tubes were installed in the enveloper pipe. Each grouting line was pressure tested after installation to confirm integrity.

Figure 6: End forms on, works ready The second two day access period was to install a camera monitoring system, form the pipe ends and place the Terefil1000. A series of video cameras were installed along the crown of the pipe. The cameras allowed the filling of the pipe to be monitored in real time to confirm complete filling of the pipe. The ends of the pipe were formed and 564 cubic meters of cementitious fill was placed in a continuous pour over 30 hours, placing from the low end of the pipe to the high end. Placing

from the low end ensures air is expelled from the pipe by the leading face of the grout, removing the risk of air pockets being trapped against the crown of the pipe.

Figure 7: Terefil placement underway The 28 day compressive strength of the fill material was approximately 7.5 MPa with typical density 1026 to 1089 kg/m3. Conclusion The project appropriately adhered to the sites’ high safety and quality standards and was accomplished in three separate two day shutdowns in a very busy operational airfield environment with limited access to the aircraft pavements along the alignment of the culvert. Mainmark completed the filling of the enveloper pipe, meeting the requirements of the head contractor and the airport.

Author Biography James O'Grady is a civil engineer with over 25 years' experience in design and construction. He is a Chartered Professional Engineer and a Fellow of Engineers Australia. He commenced with Mainmark in 2010 bringing over 20 years of experience in civil engineering and construction, from design of structures to supply of material to construction projects. James is responsible for business development of new products and new markets.