The Hong Kong Airport Authority’s survey database, with its geographic land-information system and dynamic cadastral plan, has another important function. Work is in progress on the island around the clock at hundreds of places. To ensure that these activities do not get in one another’s way and that results fit perfectly together, it proved necessary to monitor the results carefully at every stage.
For this purpose, the area of Chek Lap Kok is divided into so-called works areas. The Airport Authority allots these to a given contract consortium, so that it can get on with its job. After a given period, the contractor "returns" its works area to the Airport Authority, accompanied by detailed as-built information of the work done and the changes made. Ayson’s staff also monitors these data by a process of continual as-built surveys and land management. For that task they use GPS, Leica TC2002, TC1700, and TC1010 total stations, and Leica NA2002 and NA3003 digital precision levels.
Electronic field to finish
To quote Ian Ayson: "The survey section has established a fully electronic system to gather, record, progress, and report settlement monitoring. The readings of the levelling staff are digitally recorded by the level by means of a bar-code system. A RAM card records the readings and makes it possible to calculate a "close" before the survey staff returns to the office. The data on the RAM card are downloaded to a PC, then a network adjustment is carried out.
The adjusted results are transferred directly to a database and validation checks are made. These data are then immediately available to other users for ongoing analysis." The tabulated data are downloaded once a month to a CADD system, and plans are plotted of the locations of markers and of the total settlement. Other plans are then produced that show the current rate of settlement over an eight-week period and settlement contours. These plans and reports are then distributed for further evaluation.
Documentation of forecast subsidence
One of the many great challenges for the surveyors on Chek Lap Kok is to keep track of subsidence in newly reclaimed areas. In close cooperation with the Head of Engineering, Dr Graham Plant, and his specialist team of geotechnical engineers, Chief Surveyor Ian Ayson and his survey team, in conjunction with Geotechnical Instrumentation Engineer Colin Lang, have covered the length and breadth of the island’s 12.4 square kilometres with 1878 survey points. Some of the sensors reach down to a depth of 90m. These important markers in the upper strata help determine subsidence in all three planes by double levelling and surveying.
Other instruments used for this purpose are piezometers to determine pressure, inclinometers that measure the horizontal displacement of the soil strata, and extensometers that record vertical changes. In all, 77 inductive Soundex inclinometers and extensometers, 24 magnetic extensometers, and eleven MPBX multi-borehole point extensometers have been installed.
To date the subsidence that has occurred has been largely as computed and forecast. For the first six months, each area of newly reclaimed land subsided by about half a metre, mostly at the beginning, and progressively less later; during the next eighteen months it settled further by a monthly average of 5mm; and finally it continued to compact at a monthly rate of only 1mm or had come to rest.
These data are gathered monthly from one hundred to two hundred of the remaining total of five hundred survey points. The surveyors use Leica NA3003 digital levels to determine height.
Leica total stations and GPS to determine position
Stable benchmarks are used as fixed reference points, solidly anchored and secured in Chek Lap Lok’s granite. The coordinates of the benchmarks have been known accurately to within 2ppm ever since construction work began, and they are still being continually monitored.
The control network has been extended by so-called datum piles on newly reclaimed land, at a density of about one control point every two kilometres on reclaimed land and of less than 1km on original land.
Even so, because of the vast size of the site, the constant changes that occur, and the presence of visual obstacles, triangulation and level traverses of several kilometres are the rule.
Besides Leica TC1010, TC2002, and TC1700 total stations, three mobile stations of GPS equipment of American make are used to determine XY coordinates. These make it possible to obtain a kinematic positional fix of the antennas, in real time to centimetre accuracy, or by post-processing to normal GPS sub-centimetre accuracy.
Ian Ayson comments: "GPS heighting is well-known to be less accurate than XY positioning, and all manufacturers specify accordingly. We have experimented with GPS for settlement monitoring, but have not as yet found a procedure reliable enough to replace standard levelling methods. When we have used GPS in real-time kinematic mode, we have noted some ionosphere anomalies over the island that resulted in height readings which varied within the specified accuracy range. It proved possible to eliminate this problem by surveys in rapid-static mode or by post-processing analysis. However, in my view, GPS should not at present be used for very precise heighting without further extensive research and testing."
