The New Hong Kong International Airport at Chek Lap Kok Island (HKIA) is said to be one of the largest public works projects in history. Constructing HKIA was such a massive project because the island/airport had to be built literally from the ground up. This process required that a vast amount of fill material be dredged-up from the surrounding waters. Another source of material was the island of Chek Lap Kok itself along with other smaller surrounding islands. These islands were steep and mountainous requiring engineers to level off the peaks and use them as fill material to provide more area on which to build the airport.

Even though every effort was made to minimize settlement, HKIA engineers knew that through the course of time and use, settlement would occur. The objective was to minimize differential settlement and to track the pavement’s profile as the pavement settled. Great consideration was given as to how they would track differential settlement. The principal concern being that differential settlement would create areas of roughness causing pilot and passenger complaints along with a high dynamic loading environment. Engineers decided to implement a profile measurement program of their runways and to determine the rideability of these pavements by using aircraft simulation. Once the profile data was measured and analyzed, the engineers would incorporate this information into their overall pavement management practices.

As mentioned, there are several areas on the airport site where the reclaimed land borders bedrock from the original islands. There was concern of differential settlement at these areas of transition. A decision was made to establish a baseline survey of the pavements prior to the airport’s opening. In January of 1998, a topographical profile was made establishing a baseline profile of the Southern runway (07R-25L). The Auto Rod and Level profiler (AR&L) used to establish this baseline measured the pavement’s true profile; capturing all the grade changes and long wavelength bumps and dips.

This true profile would allow the engineers to see any changes caused by settlement, and allow them to compare the most recent profile to the baseline survey. The pavement’s profile would then be evaluated for aircraft ride quality by APR Consultants.

Since 1998, HKIA engineers have measured the elevation profile of their two runways annually. That data is then sent to APR and undergoes a complete ride quality analysis. This analysis consists of a visual profile analysis comparing the current year’s profile to that of the baseline profile. Being that the profile captures all events embedded in the profile, changes are easy to identify.

Once the profile has been visually assessed, the profile is then analyzed for ride quality by performing aircraft simulations. For this, two types of aircraft are typically used; a narrow body aircraft such as the Boeing 737-800 and a wide body aircraft such as a 747-400. Using these two aircraft, the range in gear spacing (distance from nose gear to main gear) allows for the detection of both short wavelength roughness and that of long wavelength roughness. The gear spacing on the 737 aircraft is 51 feet (15.5 meters). It will respond to wavelengths from 15 feet (5 meters) up to 150 feet (50 meters) in length. The 747-400 has a gear spacing of 84 feet (26 meters). It will respond to wavelengths up to 300 feet (100 meters) in length if encountered at a sensitive speed. Using these two aircraft provides the analysis with the ability to monitor events at both the long and short end of the wavelength spectrum.

Once the aircraft analysis is complete, a straight edge analysis is then conducted. HKIA requires that an ICAO’s 3-meter straight edge be used to compare their runways with that of the ICAO specification. However, this straight edge is relatively short and will not necessarily identify roughness events that will greatly affect aircraft response. To identify roughness events that cause deep loading, the straight edge needs to be longer, more representative of the aircraft’s gear spacing. For this, HKIA also requires a 100 foot (30-meter) straight edge analysis using a 1-inch (25mm) allowable deviation. This analysis generally verifies higher accelerations registered with the aircraft simulation portion of the analysis.

Finally, to identify and track settlement, HKIA uses a 500-foot (152-meter) straightedge. The deviations identified with this analysis are then integrated with the same analysis performed on the previous year’s profile data and that of the baseline profile data. The resulting tri-colored area plot clearly identifies the settlement that has occurred since the baseline was measured and as well as since the last profile measurement occurred.

Since the initiation of this process in 1998, the runways at HKIA have been found to produce settlement. However, every analysis technique used to assess the ride quality of these runways concludes that, to date, there are no ride quality problems resulting from settlement. This fact is reinforced by the lack of pilot or passenger complaints.

As a world-class airport, engineers at HKIA have tirelessly pursued the best capability and technology available to ensure their tremendous engineering and financial investment is protected, safe and will be serviceable for years to come.