Dredging the Hong Kong Airport

Dredging the Hong Kong International Airport

Brantz von Mayer and The Writers for Hire

 

The 7,153,519 residents of Hong Kong inhabit a total area of 686 square miles. 1   Despite its size, this small island nation boasts one of the busiest airports in the world – an airport that would never have been realized without the modern technique of capital dredging.

Capital dredging significantly alters the underwater or shoreline terrain for a commercial purpose. When the goal of capital dredging is land reclamation, the dredged aggregate (or “spoil”) is not simply discarded but used for some beneficial purpose. In contrast to other dredging endeavors, the spoil is considered the desired product here, and the water deepening is merely a byproduct. The “leftover dirt” from capital dredging projects has been used to create land for recreation, farming, industrial use, civil projects, and even forestry.

Hong Kong International Airport at Chek Lap Kok is a shining example of a major capital dredging project that involved creating land for its own sake. Because land is a commodity in short supply on this densely populated island nation, land reclamation through dredging was a cornerstone of the construction process.

Planning

The need to replace Hong Kong’s Kai Tak Airport was evident since the 1960s, as the existing facilities from 1925 could no longer support the burgeoning international travel. 2   Because Hong Kong is composed of more than 200 small islands, however, there was no acceptable tract of land large enough to hold the footprint for a substantial international airport. 3   The solution: plan the formation of 1,248 hectares of land off the coast of Lantau, Hong Kong’s largest island. Three-quarters of the new airport’s platform would be reclaimed land dredged from the sea, and the remaining quarter would be the result of excavating two other islands. 4  

To minimize noise over the city of Hong Kong, the desired new airport would enable aircraft flight paths over the sea. 5   The island of Chek Lap Kok was identified as early as 1974 as a possible site due to its clear airspace and proximity to major urban areas, although it took 15 years and many studies to officially confirm the site as the official location for the new airport. 6 ;

Site Studies

The design of the reclamation required extensive knowledge of the depth of the material to be dredged and the compressibility of the underlying marine sediment from the proposed fill, as well as soil strength and base stability for seawall foundation. 7   Because conditions on the ocean floor vary drastically with regard to water depth, marine aggregate to be dredged, and underlying compressible strata, the questions of settlement and stability of the reclaimed land that would form the airport platform were of utmost importance. 8

Large-scale site investigation (SI) involved a plethora of field and laboratory tests that were included in a site-wide geotechnical database. Land-based SI aided reclamation efforts and provided data for designing the airport building, tunnels, and runways. Results of marine-based SI supplied information for temporary ferry piers, an inter-island sea channel, and temporary sewage outfalls. 9

Various organizations conducted SI over a number of years to establish a plan with a comprehensive geotechnical basis, including thorough studies of the seabed at the desired location. From 1989-1991, offshore studies used seismic reflection surveys, cone penetration tests (CPTs), vibrocores, and marine boring techniques to determine dredging levels for the removal of the soft marine mud at the proposed reclamation site and to establish the nature of the underlying sediment. 10

According to Civil Engineering Computer Aided Learning (CIVCAL), a joint venture of the University of Hong Kong, Hong Kong University of Science and Technology, City University of Hong Kong, and the Hong Kong Polytechnic University, “Dredging levels were established from 3,000 CPTs on a square grid. The criteria for determining the base of dredging was a cone resistance (qc) of 500kPa, over 1m depth. Below proposed seawalls, the criteria were increased to 680kPa.” 11  

(The cone penetration test [CPT] determines the geotechnical engineering properties of soils and delineating soil stratigraphy. The CPT is one of the most used and accepted tests in soil methods for soil investigation worldwide. 12   The kilopascal [kPa] is a unit of pressure; 1 kPa is approximately the pressure exerted by a 10-g mass resting on a 1-cm2 area). 13

Master Plan

In April 1990, the Provisional Airport Authority (PAA) was established to plan, design, and build a replacement airport for Hong Kong. That July, the New Airport Master Plan (NAMP) study was initiated to carry out the PAA’s goal of “a comprehensive and environmentally acceptable scheme for the planning and implementation of an operationally safe and efficient new airport.” Completed that December, the NAMP was presented in early 1992. 14

Because of the enormous environmental impact of this endeavor, the NAMP included an Environmental Impact Assessment (EIA). (Following design changes to the airport platform, a subsequent EIA Supplement was released in October 1992.) 15

The NAMP forecast passengers, cargo, and aircraft movements, based on a comparison to the 1990 statistics (18.7 million passengers, 802,000 tons of freight, and 106,000 aircraft movements). It also investigated runway configuration and terminal layout, including the facilities required to meet growing passenger demand levels. Its ultimate recommendation was for two parallel runways, each 3,800 meters (2.36 miles) long, to be separated by the passenger terminal. The NAMP called for a phased development, with the airport to open with a single runway at an annual capacity for 35 million passengers and 3 million tons of cargo. 16

The NAMP was completed in December 1991, followed by the design of the airport platform. By the next December, construction had begun and the airport began to take shape. 17

Implementation

The entire platform for the new Hong Kong International Airport at Chek Lap Kok was created in just 2 1/2 years. At the peak of activity, land was being formed at the rate of 2 hectares per day. During this land reclamation process, material moved at the average rate of 10 tons per second. 18   Upon completion, 90 million pounds of marine fill had been moved. 19  

The airport platform, constructed from 250 million cubic yards of dredged sand and clay, is “new land” that did not exist before the early 1990s. 20   The 1,248-hectare platform comprises 938 hectares of reclaimed land and 310 hectares from the two original islands of Chek Lap Kok and Lam Chau. 21

Construction of the platform required a complete synthesis of several complex operations that, independent of each other, were major undertakings: dredging mud from the sea floor, constructing seawalls to maintain structural integrity, excavating and flattening the existing headlands of Chek Lap Kok and Lam Chau islands, and introducing offsite marine sand for land reclamation. 22,   23

Dredging the Footprint

Before the platform could be built on reclaimed land, 68.8 million cubic meters of aggregate from the ocean floor had to be removed within the airport’s planned footprint. Barge-mounted cranes dug up the soft and granular seabed soil and loaded it into dump barges, while dredged marine sand discharged through floating pipelines provided fill material for land reclamation. As is common after dredging, the seabed was irregular and pockmarked. The crew used trailing suction hopper dredgers and cutter suction dredgers to level and smooth the sea floor surface. 24

As the project progressed, the scope of the dredging work increased. The original dredging expectation was for the removal and disposal of some 4.5 million cubic meters of soft marine clay from the coastal areas west and north of Chek Lap Kok; however, this was increased to 13 million cubic meters of dredge to include an access channel into the center of the airport platform. 25

Constructing Seawalls

To protect the reclamation, a 11.5km-long sloping seawall was constructed of 2 million cubic meters of rock, underlay, fill materials, rock fill, and marine sand. Rock-armored seawalls line most of the perimeter of the airport platform to maintain its structural integrity, except for sections of natural coastline or pre-existing concrete quay walls. The bulk fill used to construct the seawalls was bottom-dumped by split barges, bottom dump barges, and trailers, in addition to end-tipping from dump trucks and bulldozers. 26

Prior to constructing the seawall, the areas were dredged to remove soft marine mud unsuitable for supporting the walls. Insufficiently cleared areas could impact the safety and stability of the entire seawall. Dredgers were used to dig these foundation trenches all the way to “competent material,” at dredge levels that varied from location to location, to a maximum depth of –29mPD (meters above Principle Datum, the height above average sea level).

The southern and eastern seawall trenches employed grab dredgers and bottom dump barges with higher draft to access shallower areas. The northern and western seawall trenches utilized the larger and more powerful trailer suction hopper dredgers and cutter suction dredgers. 27

Leveling Chek Lap Kok and Lam Chau

Excavating Chek Lap Kok and Lam Chau islands fulfilled two purposes: to level the site in preparation for construction and to generate fill for use in land reclamation. Of the total 197 million cubic meters of fill, 108 million cubic meters resulted from excavating the small, rocky islands of Chek Lap Kok and Lam Chau. 28

Because the airport construction site was to be situated on the mountainous terrain of Chek Lap Kok, excavation was required in order to flatten the area for building. Giant earth movers removed the mountains, resulting in 200 million tons of rock. The excavated rubble was used in land reclamation to join the two small islands into one big island. This proved to be one of the world’s biggest land-moving exercises ever completed to date: 600 million tons of material was removed, enough to fill Rome’s Colosseum 200 times. 29

Introducing New Sand

Reclaimed land currently accounts for some 6% of the onshore area of Hong Kong. Many past reclamation involved vast amounts of weathered rock obtained by cutting into hillsides. While this method had the advantage of speed in reclaiming land, using weathered rock as placed fill requires a long settlement period because the rock fragments are not uniform. In addition, the material particular to Hong Kong’s terrain has a high clay content, which causes slow drainage. For these reasons, offshore sand became the preferred fill option in the 1980s. 30

More than 75 million cubic meters of marine sand was dredged from various borrow areas around Hong Kong’s waters to provide the supplemental fill needed to round out reclamation for the airport platform. This quantity allowed for settlement and compaction. Sand placed above +3mPD was compacted to 95% maximum dry density, followed by a further vibrocompaction for even denser fill for greater stability. In addition to the platform itself, the seawall construction utilized another 7 million cubic meters of marine sand dredged from these borrow areas. 31

The designated marine borrow areas were located from between 4km and 45km away from the platform build site. Trailing suction hopper dredgers pulled up sand from these borrow areas, at maximum depths of -40mPD, and transported it to either the reclamation area or the rehandling pits. They dumped it directly into the reclamation site until they reached an average level of -6mPD (-9mPD for the northern runway section and the northwestern seawall). Cutter suction dredgers took over the work with sand deposited into the rehandling pits, pumping this fill onto higher levels of the reclamation. 32

Hong Kong International’s Dredging Fleet

The enormity of the Hong Kong International Airport project, coupled with the swift desired completion deadlines, required a large and varied fleet dredgers. This fleet – in fact the largest international fleet of dredgers ever assembled to date – included 15 trailing suction hopper dredgers, 5 cutter suction dredgers, and 7 grab-and-bucket dredgers that worked around the clock. 33,   34

Trailing suction hopper dredgers use powerful dredge pumps to suck material into their hoppers via a draghead fitted onto a suction pipe that is dragged over the sea floor. The hopper holds the dredged material and has bottom doors to dump the material at the designated dump site once full. 35   Alternately, a pressure pipe over the bow hydraulically pumps aggregate onto shore. These highly sophisticated machines use global positioning systems (GPS) to monitor the dredging area such as the seabed depth, tidal information, dredge location, and target dredge levels. Trailing suction hopper dredgers carried out the bulk of the dredging of marine mud from the airport footprint, due to their dredging capacity (200,000-500,000 cubic meters per week), their flexibility to adjust to variations in the sea floor, and their ability to be relocated. To remove the soft mud of the seabed and bring in sand from marine borrow areas, all told, the trailer dredgers sailed the equivalent of 40 times around the world. 36

Cutter suction dredgers are anchored to the seabed. At the suction inlet is the cutter head, which cuts through marine aggregate and transports it to the suction mouth. Submerged or onboard dredge pumps move the material from the sea floor to either a barge for transport to the dump site or directly to the shore via floating pipelines. Because they are stationary, they can achieve high accuracy with greater control over varying the dredge levels as required by different seabed conditions. Cutter suctions dredgers at Hong Kong International Airport dredged an average of 300,000 cubic meters of aggregate per week. 37

Grab-and-bucket dredgers are stationary, essentially barge-mounted cranes that dredge seabed material with a clamshell bucket suspended from the crane. 38   The grab tool is mounted on either a dragline or a hydraulic backhoe excavator. Aggregate is dropped into a bottom dump barge, which is typically towed to the disposal site. The extraction capacity, determined by the size of the grab bucket and the power of the crane, varies between 1.0 and 20 cubic meters. 39   The grab-and-bucket dredgers used in the Hong Kong International Airport project had grab capacities of 6-20 cubic meters and were primarily used for dredging mud in the shallow areas of the coastline, seawall foundation trenches, and sea channel. 40

Successful Opening and Ongoing Expansion

Hong Kong International Airport represents one of the world’s largest civil engineering endeavors of this type. 41,   Upon completion, the new airport had increased Hong Kong’s landmass by nearly 1% and earned the distinction of the world’s most expensive airport by Guinness World Records. 42

On July 6, 1998, one of the "Top 10 Construction Achievements of the 20th Century" opened its runways. The new Hong Kong International Airport had ability to handle 35 million passengers and 3 million tons of cargo a year. A second runway (the North Runway) was added in May 1999, and the Northwest Concourse extension opened in January 2000. 43,   44

Air cargo through Hong Kong International has been growing at 6% a year and express cargo at 13% a year. To keep up with this enormous growth, 2004 saw the opening of a new air express cargo facility and cargo terminal. Just four years later, work began on a new cargo terminal, to be completed by 2013. At 246,000 square meters, the facility will have an annual capacity of 2.6 million tons, increasing the airport’s total annual cargo handling capacity to 7.4 million tons. 45

A second passenger terminal (called T2) officially opened in February 2007 46,   and a second passenger concourse, the North Satellite Concourse (NSC), opened in December 2009. 47,   By 2010, after only 12 years in existence, the Hong Kong International Airport served 50.9 million passengers and ranked as the world’s busiest airport by cargo traffic. 48,   49

In 2011, the Airport Authority began construction on the last piece of land on the airport island available for large-scale development. The new midfield concourse, located between the two existing runways, is expected to be fully operational in 2015. 50

Despite the near-constant expansion since its initial opening, Hong Kong International Airport expects to reach or exceed capacity in the next two decades. The International Air Transport Association (IATA) has estimated that annual air traffic will reach 97 million passengers, 8.9 million tons of cargo, and 602,000 flight movements by 2030, growing at a compound annual growth rate of 3.2%. 51,   52

In a proactive measure to keep up with the growing use of its facilities, the Airport Authority released its Master Plan 2030 for major expansion to add a third runway through additional land reclamation of an estimated 1,606 more acres from the waters around Lantau and construction of a new platform. 53,   54,   55,   No final decisions have been made regarding the 2030 expansion – in fact, the current expansion is still 2+ years from completion – but the Airport Authority is continually working to maintain’s Hong Kong International’s status as one of the world’s top airports.

1. CIA World Factbook Entry, Hong Kong: https://www.cia.gov/library/publications/the-world-factbook/geos/hk.html

2. Wikipedia Entry, Hong Kong International Airport: http://en.wikipedia.org/wiki/Hong_Kong_Airport

3. CIA World Factbook Entry, Hong Kong: https://www.cia.gov/library/publications/the-world-factbook/geos/hk.html

4. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

5. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

6. CIVCAL: Hong Kong International Airport: http://civcal.media.hku.hk/airport/introduction/default.htm

7. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

8. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

9. CIVCAL: Hong Kong International Airport: http://civcal.media.hku.hk/airport/investigation/default.htm

10. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

11. CIVCAL: Hong Kong International Airport: http://civcal.media.hku.hk/airport/siteformation/reclamation/marine/_hidden/content.htm

12. http://en.wikipedia.org/wiki/Cone_penetration_test

13. http://chemistry.about.com/od/chemistryglossary/a/kilopascaldef.htm

14. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

15. CIVCAL: Hong Kong International Airport: http://civcal.media.hku.hk/airport/environment/default.htm

16. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

17. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

18. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

19. CIVCAL: Hong Kong International Airport: http://civcal.media.hku.hk/airport/siteformation/_hidden/form1.htm

20. Modern Marvels, Season 11, Episode 38, Aired 8/3/05

21. Building Journal: The World's Single Largest Building Project www.building.com.hk/downpdf.asp?id=172

22. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

23. CIVCAL: Hong Kong International Airport: http://civcal.media.hku.hk/airport/siteformation/default.htm

24. CIVCAL: Hong Kong International Airport: http://civcal.media.hku.hk/airport/siteformation/reclamation/marine/_hidden/content.htm; http://civcal.media.hku.hk/airport/siteformation/reclamation/marine/_hidden/marine5.htm; http://civcal.media.hku.hk/airport/siteformation/reclamation/marine/_hidden/marine3.htm

25. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

26. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

27. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

28. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

29. EnggPedia – The Engineering Encylopedia: http://www.enggpedia.com/civil-engineering-encyclopedia/megastructures/hong-kong-airport

30. Hong Kong Geology: http://hkss.cedd.gov.hk/hkss/eng/education/GS/eng/hkg/chapter9.htm

31. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

32. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

33. CIVCAL: Hong Kong International Airport: http://civcal.media.hku.hk/airport/siteformation/reclamation/marine/_hidden/content.htm

34. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

35. Merwede: About Trailing Suction Hopper Dredgers: http://www.ihcps.com/tshd-equipment/about-trailing-suction-hopper-dredgers

36. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

37. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

38. Wikipedia Entry, Dredging: http://en.wikipedia.org/wiki/Dredging

39. European Dredging Association: Types of Dredger: http://www.european-dredging.eu/Mechanical_dredger

40. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

41. Airport Authority of Hong Kong: Site Preparation for the new Hong Kong International Airport (1998)

42. Wikipedia Entry, Hong Kong International Airport: http://en.wikipedia.org/wiki/Hong_Kong_Airport

42. CIVCAL: Hong Kong International Airport: http://civcal.media.hku.hk/_projects/default.htm

44. Airport Authority News, Issue 21, March/April 1999

45. Airport Technology: Hong Kong International Airport: http://www.airport-technology.com/projects/cheklapkok_new

46. MTRC Corporation: Airport Station New Platform Opens for Operation

47. Hong Kong International Airport Media Centre: http://www.hongkongairport.com/eng/media/press-releases/pr_981.html

48. Airports Council International and Centre for Asia Pacific Aviation: http://www.centreforaviation.com/news/2011/03/16/world-airport-rankings-2010-hong-kong-eclipses-memphis-as-the-worlds-busiest-cargo-hub/page1

49. Hong Kong International Airport: http://www.hkairport2030.com/en/masterplan/demand_hkia.html

50. Hong Kong International Airport Media Centre: http://www.hongkongairport.com/eng/media/press-releases/pr_1020.html

51. Airport Technology: Hong Kong Approves $17.5bn Third Runway at City Airport: http://www.airport-technology.com/news/newshong-kong-approves-175bn-third-runway-at-city-airport

52. Hong Kong International Airport: http://www.hkairport2030.com/en/masterplan/demand_hkia.html

53. Airport Authority: New Reclamation Method: Airport Expansion to Minimise Impact on Marine Ecology

54. Hong Kong International Airport: http://www.hkairport2030.com/en/masterplan/demand_hkia.html

55. Airport Technology: Hong Kong Approves $17.5bn Third Runway at City Airport: http://www.airport-technology.com/news/newshong-kong-approves-175bn-third-runway-at-city-airport

Copyright © 2018