A Leica Geosystems robotic total station is mounted on one of the tunnel’s sidewalls; clearance between the train and sidewall is less than 18 inches.

Transporting six million passengers each year through the South Ferry subway station in New York City could be a lot easier. The station, situated at the tip of Manhattan, adjacent to the Staten Island Ferry terminal and Battery Park, was originally built in 1905 when subway trains were much shorter than today’s models. Its platform can only accommodate the first five cars of each train. Disembarking passengers must walk forward from the rear cars to exit, which causes delays. Unlike most modern stations, which have two or three tracks, the South Ferry terminal has only a single track, and the sharp curvature of the platform and track slows train operation and generates excessive noise. In addition, the single entrance and narrow stairs can cause congestion, and do not provide easy access for disabled passengers.

To remedy these limitations, the New York Metropolitan Transit Authority launched a $400 million improvement program for the South Ferry Terminal. The effort will result in the relocation of the station platform and mezzanine to a new location beneath Peter Minuit Plaza and State Street at the foot of Manhattan. The rebuilt station will incorporate a number of improvements, including a full-length platform that can accommodate two 10-car trains, additional station entrances and station accessibility compliant with the Americans with Disabilities Act. The new terminal will have the ability to handle up to 24 trains per hour.

The construction work is being done under a design-build contract by Schiavone Construction Company of Secaucus, N.J., and includes strengthening and stabilizing the foundation of the existing tunnel structure using the mini-pile foundation technique and cut-and-cover excavation (a method of construction for shallow tunnels where a trench is excavated and roofed over) of a new tunnel. Underpinning stabilization and excavation work commenced in 2005; the project is expected to be completed by the end of 2007. It was clear from the outset of the project that a rigorous deformation monitoring regimen would be required to protect nearby historic structures. To support this phase, Schiavone looked to high-precision total station technology.

A Rigorous Monitoring Regimen

Schiavone contracted with Mueser Rutledge Consulting Engineers (MRCE) and Geocomp Corporation, both of New York, to provide specialized geotechnical design and monitoring services for the underpinning and tunnel construction of the new South Ferry subway station.

“The monitoring regimen developed for this project focuses on the existing tunnels and nearby structures, including some very important historic buildings and facilities within Battery Park,” says W. Allen Marr, CEO of Geocomp. “Movement tolerances for this job are fractions of an inch. This meant we needed the most reliable, rugged and repeatable measuring systems.”

The monitoring solution delivered by Geocomp and MRCE uses 10 Leica Geosystems (Norcross, Ga.) TCRP1201 automatic robotic total stations, which monitor more than 450 prisms across the site. Inside the tunnel, the total stations are mounted on brackets on the tunnel walls, hanging from the tunnel ceilings near crossovers, between structural columns in stations and on “crash walls” between opposing direction tracks. The prisms are affixed to the walls near the concrete anchors every 10 ft along the length of the tunnel near the construction work to detect absolute and differential movements of the walls during excavation and underpinning. Above ground, the total stations are mounted on buildings with monitoring prisms affixed to building structures, as well as the walls that support the cut-and-cover excavation.

The robotic total stations are controlled and synchronized using Leica GeoMos monitoring software. Two local computers in the tunnels communicate via wireless radio links to the total stations. The computers are remotely accessible via IP-enabled phones for data retrieval from GeoMos.

In addition to the Leica automatic total station network, MRCE and Geocomp also deployed an array of other instrumentation. Seismographs installed both inside the tunnels and affixed to the foundations of nearby structures record vibrations caused by construction activities. Tiltmeters on the external walls of the exposed tunnels monitor any twist of the tunnel structure while they are undermined. And in-place inclinometers installed in the support-of-excavation walls and in the ground in front of nearby historic structures monitor lateral deflections during excavation.

A Web of Wireless Data

All monitoring data from the site--from the robotic total stations and the additional instrumentation--was transmitted to Geocomp’s Web-based iSiteCentral monitoring and reporting system. Authorized project users could access the monitoring data from iSiteCentral via password-controlled Internet portal in real-time 24/7 using a landline, RS232 serial port, mobile telephone or satellite connection. The iSiteCentral system can provide immediate notification of potential issues by mobile phone, E-mail or pager whenever out-of-tolerance conditions are detected. This way, problems can be addressed before they become critical. At any given time, a user can log onto the iSiteCentral website and see a status report on the condition of every sensor on the site and examine graphs showing the complete history of data for the sensor or group of sensors.

“The project presents unique challenges for monitoring implementation and logistics,” Marr says. “For example, the existing tunnels need to be monitored before excavation begins to collect baseline data. This means that nearly all of the instrumentation has to be installed inside the active subway tunnels. These tunnels are normally in operation 24/7--even during the construction--which limits access to the tunnels to once every few weeks. For that reason, it is necessary to create a system that will operate reliably and automatically around the clock under the most difficult environmental conditions.”

“The wireless component was a very desirable factor,” adds Rob Nyren, the Geocomp instrumentation engineer. “However, in active tunnels, wireless communication is a challenge even in the best conditions. New technologies like spread-spectrum radios with powerful transmission capabilities allowed us to overcome many of the obstacles. By going wireless, we avoid installing several thousands of feet of cabling for sensors, communication lines, etc., and save both time and extra union labor costs.”

Benefits of Automated Monitoring Systems

“An effective performance monitoring system can play a major role in controlling costs on a large infrastructure construction job,” Marr says. “The monitoring system can reveal potential problems and deficiencies in time for corrective action to be taken. It can reduce the possibility that construction will adversely affect neighboring people and facilities, and also reduce possibilities for claims arising from construction-related damage.”

“In this instance,” Nyren adds, “the vibration monitoring led Schiavone to change their blasting practices in the underlying rock. The vibrations were exceeding the project thresholds, and the robotic total stations showed that very little deflection occurred in nearly all of the tunnel sections affected by construction.”

Precise deformation monitoring is increasingly becoming a standard requirement for major engineering projects, especially those involving underground excavations, to protect nearby structures from damage during the construction process. More importantly, it provides a critical margin of safety for workers on the job as well as people going about their daily business in the vicinity of the excavation.

Manufacturer Information:
Leica Geosystems,www.leica-geosystems.com


Geodetic Monitoring
The monitoring of construction objects and dangerous areas is becoming more and more important. Monitoring involves periodically and automatically measuring reference points in or around the active area to determine deformation. When movement tolerances are exceeded, it is often necessary to immediately analyze the measured data to activate response events. Monitoring tasks and deformation analysis present some of the most sophisticated challenges in the surveying profession today because they require the highest accuracy, maximum reliability of the sensors, automatic measurements, and highly flexible computation and analysis tools.

Leica GeoMoS is an open, scalable and customizable software suitable for a wide range of monitoring applications. All measurement and result data are stored in an SQL database so it can be accessed and configured from anywhere in the world. Data can be exported to other systems in ASCII, SQL, DXF and BMP formats.

A wide range of sensor types are supported, and additional sensors can easily be added. Combining data from a range of geodetic and geotechnical sensors helps to ensure the lowest possible risk. For combined GPS/total station monitoring systems, GeoMoS can be used with Leica GPS Spider reference station software for advanced GPS monitoring using GNSS technology. Sensors and measurements are automatically controlled over cable, radio LAN, WLAN and GSM communications. Connection of the different sensors, such as GPS, total stations and levels, is handled by a Leica Sensor Manager component. It also provides an interface for sensor configuration. GeoMoS features real-time data synchronization and security between client and server using standard data transaction technologies.

The software consists of two main applications: Monitor and Analyzer. The online function, Monitor, is responsible for sensor control, collection of data and event management; Analyzer is in charge of analyzing and reporting the measured data, plus editing and post-processing where required.