Hong Kong International Airport (HKIA) is one of the world’s busiest – carrying about 74 million passengers and around five million tonnes of cargo in 2019. However, with the existing airport reaching capacity, an expansion was needed to cater for long-term air traffic demand and to strengthen Hong Kong’s status as an international aviation hub.
Once its US$18 billion ‘Three Runway System’ reaches completion by 2024, the Hong Kong International Airport will be able to serve 30 million additional passengers annually and handle increased cargo volume.
To enable the third runway and new passenger and cargo facilities, reclamation of 650 hectares is required to the north of the existing airport island to create a safe and stable platform for the airport extension.
WSP was engaged by Airport Authority Hong Kong (AA) to provide specialist independent geotechnical advice, initially for the reclamation design and construction and subsequently for all the geotechnical aspects of the expanded airport facilities.
We advised on the HKIA reclamation expansion from the feasibility studies and through the preliminary and detailed design between 2011 and 2015. Once the detailed design was complete, we continued our advisory role in design and construction, which is expected to continue until the completion of the project.
Reclamation presents risks and opportunities
Throughout this large-scale project, WSP has helped AA to identify and manage geotechnical risks and opportunities. One major risk to the airport extension project was the presence (in the reclamation area) of highly disturbed, contaminated subsea deposits of mud from previous reclamation projects around the Hong Kong area. This contaminated mud needed to be stiffened and strengthened in place without affecting the marine ecology, rendering conventional dredging solutions unsuitable.
To overcome regulatory challenges and achieve the project goals, a deep cement mixing (DCM) technique was applied across the reclamation area. Cement was injected and mixed through up to 40 m of soft, contaminated mud to create cement clusters to support the seawall and the newly reclaimed land. Each cluster is comprised of four overlapping columns, approximately 1.2 m in diameter and between 20 and 40 m long. With 250 000 cement clusters, this DCM project is the world’s largest and it has more than doubled the number of DCM columns ever created. It was also the first marine project in Hong Kong to apply the technology at such a large scale.
WSP’s specialists identified a significant opportunity for safe optimisation of the DCM works by halving the depth to which the columns extended into firmer deposits beneath the soft clay. This optimisation saved vast volumes of cement, greatly improving the project’s sustainability. It also shortened the amount of drilling time required, reducing project costs.
The WSP team was also instrumental in developing the geotechnical solution that was adopted for this complex reclamation and advised on geotechnical issues as they arose during construction. With WSP’s specialist geotechnical advice, the reclamation works which began in 2016 have been successful and were substantially completed by the end of 2020.
To identify any potential problems and to predict future performance of the reclamation for our client, the WSP team is interpreting all available instrumentation and monitoring data, including from innovative subsurface instruments such as movement sensors at the seabed level and load cells that were buried in the ground before the reclamation. We have completed a pilot trial in collaboration with HK Poly U university to monitor ground settlement using interferometric synthetic aperture radar (InSAR). The use of InSAR in such a recent reclamation within a very active construction environment is novel and challenging; however, trial results are encouraging, and AA will continue to employ this method during construction and in the longer term.
From reclamation to take-off
WSP is acting as AA’s independent geotechnical advisor on all aspects of the design and construction of the expanded airport facility, including the geotechnical aspects of the runway, taxiways, parking areas and airport facilities such as the new passenger terminal and tunnels for the automated people mover and baggage handling system. We are contributing to greater safety and robustness of the temporary works excavations and are identifying appropriate opportunities for optimisation.
For the Western Vehicular Tunnel, we are exploring optimisation options to reduce, where appropriate and safe, the additional ground improvement quantities proposed by the contractor. Our input has increased the robustness and safety of the temporary works for the various cut and cover tunnels on the project, after we identified potential stability risks caused by the complex ground conditions. For the Third Runway Concourse basement, we have analysed a complex excavation interface between two different adjacent contracts and identified an optimised scheme, saving six months on the construction program.
Having been involved in this major infrastructure project since its inception, from feasibility through to construction, WSP has become a trusted advisor to our client. Throughout the project, we have helped AA to foresee challenges, make effective decisions to manage their risk, and achieve significant savings through safe and effective optimisation. WSP will continue to advise AA through to completion of the airport extension project by 2024.
* This work was performed by Golder professionals who joined WSP in an acquisition completed in 2021.