High Precision in the Dark
The commercial and cultural hub of southern Asia, Hong Kong is home to more than seven million people--and is one of the most densely populated regions in the world. With a surface area of just 1,104 square kilometers (426 square miles), Hong Kong’s dense population could lead to congested roads and highways. Over several decades, Hong Kong has invested in infrastructure for both public and private transportation. Public transportation systems include surface and underground railways, buses and ferries. Private transportation centers around taxis, trucks and cars as well as bicycles and motorcycles. But for all its efforts, Hong Kong still struggles with traffic.
To help alleviate the traffic load, the Hong Kong government is constructing a bypass route along the north shore of Hong Kong Island. The new route, named the Central-Wan Chai Bypass and Island Eastern Corridor Link (CWB), consists of 4.5 kilometers (2.8 miles) of limited access, divided roadway that will divert traffic from the central corridor. In addition to improved traffic flow, the project will create new waterfront areas along the city’s Victoria Harbor. Roughly 3.7 kilometers (2.3 miles) of the CWB will be in a new structure built along the existing seawall. Most through traffic will travel in separate tunnels below the surface, with the top of the structure providing space for local traffic, green space and recreation areas. Construction on CWB began in 2009; final completion is slated for 2017.
Near the center of the project’s route along Hong Kong Island, CWB passes over a tunnel operated by the Hong Kong Mass Transit Railway (MTR). The tunnel carries rapid transit trains traveling between Hong Kong Island and Kowloon to the north. Passenger trains pass through the tunnel every five minutes during the day and every two minutes in peak periods. It’s an important link in the Hong Kong transit system, and project requirements called for the tunnel to maintain normal operation during CWB construction.
While the CWB structures were planned to avoid physical interference with the MTR tunnel, engineering teams needed to guard against any deformation or disturbance that might result from the construction. To do so, project managers engaged Mannars Chan & Associates (MCA) to provide monitoring services.
Prior to installing the ADMS, MCA used a Trimble GX 3D Scanner to create a detailed 3D model of the MTR tunnel. The model provided the basis for designing the monitoring system. The MCA team decided to install cross-section arrays of five 25-millimeter (1-inch) prisms at specific locations in the tunnel. The cross sections are at 5-meter (16-foot) intervals in areas directly beneath the CWB structure, and 10-meter (32-foot) intervals on each side of the CWB. At the center of the monitoring zone, two additional cross sections are 1.3 meters (4.3 feet) apart. The prisms are bolted onto small brackets attached to the tunnel walls.
Controlled by Trimble 4D Control Software, the Trimble S8 can precisely point to and measure a prism. Following pre-set instructions, each instrument measured to approximately 100 prisms in 30-minute cycles. A cycle consisted of seven sets of measurements to each prism. Each set included measurements in the Face 1 and Face 2 positions. The system measured automatically and in total darkness, stopping only for scheduled maintenance on the railway.
When looking down the tunnel, prisms can appear to be very close together in the instrument’s field of view. This can introduce challenges when attempting to measure to a desired target. To prevent problems, MCA used Trimble FineLock technology with the Trimble S8. FineLock allows the instrument to detect targets without interference from other prisms, thereby ensuring that the instruments always measured to their correct targets. “When viewed from the total stations, the distances between some prisms are very small,” says MCA Managing Director Mannars Chan. “FineLock Technology can perform very accurate centering, and Trimble MagDrive maximizes turning rate and measurement efficiency.”
Monitoring in an active railway tunnel presented some interesting challenges. There was tight clearance between the rail cars and tunnel walls, so the instrument brackets needed to be carefully designed and installed. The fast-moving trains caused heavy vibration in the tunnel, and each passing train buffeted the instruments with strong gusts of wind. The team developed a suspension system in the brackets to isolate the instruments from the vibration while maintaining a stable platform for measurement. In addition to the vibration, the instruments needed to perform in dusty conditions. Nightly track maintenance in the tunnel produced fine metal dust that soon covered the instruments and prisms.
Despite the difficult conditions, the instruments performed without difficulty. “The Trimble monitoring system gave us and our client very high confidence,” said Chan. “The system was our eyes and ears for monitoring work where no traditional method could be used.”