An automated monitoring system plays a key underground role in a massive construction project in Hong Kong.

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.

A view down the MTR tunnel. The monitoring prisms are visible as small points of light on the tunnel walls.

MCA devised a plan to install an Automated Deformation Monitoring System (ADMS) based on Trimble technology. The system--which combines total stations, automated measurement and sophisticated data management and analysis--can provide continuous, 24-hour monitoring of the tunnels without disrupting MTR operations and maintenance activities. For the initial phase of the project, the ADMS would monitor a section of tunnel 120 meters (390 feet) long.

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.

A Trimble S8 Total Station is ready for measurement in the MTR tunnel. The customized mount isolatesthe instrument from the vibration of passing trains. A single cable supplies power and communications to the instrument.

To measure the prisms, MCA installed seven Trimble S8 Total Stations on steel brackets near the center of the monitoring zone. Next to each instrument, a metal box housed a computer and communications equipment. Power came from 12V lead-acid batteries that were changed once per week. A wireless network connected the instruments to an MTR server located outside of the tunnel.

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.”

A Trimble S8 Total Station installed for monitoring in the Hong Kong MTR tunnel. The metal boxes contain power supplies and communications equipment for the instrument.

As raw measurements arrived at the MTR server, Trimble 4D Control stored the data in an SQL database. Project teams configured Trimble 4D Control to compute results for each prism in real time, and to compare results against pre-set alert levels. Based on the amount of measured motion, the software could issue email or SMS text messages to the management and project teams. During the first six months of monitoring, the Trimble system did not detect any motion that would trigger an alert.

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.”