Vital surveying work aided recovery operations in the wake of the World Trade Center tragedy.

Outline of slurry wall.
The recovery, demolition and clearing undertaken at the World Trade Center site (dubbed “Ground Zero”) in the wake of the September 11th tragedy was a massive job requiring dozens of consultants and hundreds of workers. The work was hazardous with shifting remnants of buildings and unknown conditions. Critical to the success of the assessment efforts was the complicated surveying work that enabled crews to locate building elements and utilities while monitoring the movement of unstable structures.

The New York City Department of Design and Construction (DDC) had surveying teams working 24 hours a day, seven days a week. The Department also called in three firms to assist in surveying operations at the site of Ground Zero: Vollmer Associates LLP of New York, N.Y., Tectonic Engineers of Mountainville, N.Y., and Lockwood Kessler & Bartlett (LKB) of Syosset, N.Y. Each of the firms provided surveying teams for 12-hour shifts on a seven-day-a-week basis. A professional land surveyor from one of the three firms acted as supervisor and was onsite for each shift.

The majority of the work was performed with total stations, including the Nikon DTM 520 (Melville, N.Y.), the Topcon GTS 700 (Pleasanton, Calif.) with on-board TDS data collection (Tripod Data Systems, a Trimble Company, Corvallis, Ore.), the Topcon GTS 313, Topcon ITS-1 and Topcon AP-L1A, the Zeiss ELTA 3 (Trimble, Sunnyvale, Calif.) and the Leica TC110 and TCRA1103 reflectorless/robotic total station (Norcross, Ga.). Instruments were manned at numerous stations around the site. As the purpose of these instruments was originally to simply detect movement, they were independent of each other and uncontrolled.

To detect any movement that might jeopardize the rescue workers or the buildings, repetitive angles were measured to corners of buildings adjacent to the site in some cases. Other instruments were sited on the corners of some large sections of debris that were still standing. The surveying teams were equipped with aerosol air horns to sound an alarm, if necessary. During the initial stages of the recovery, the horns were used on occasion as large pieces of debris were being removed, sometimes posing a threat to the recovery teams below.

Other surveying operations included the monitoring of the cranes, some weighing as much as 1,000 tons, that were erected on the site to pull down and remove portions of the remaining debris. As the area where the buildings stood essentially became an open pit, there was concern that the weight of the cranes would exert pressure on the slurry wall that held back the Hudson River. As the removal of material created a deeper and deeper hole, the potential for the cranes to collapse the wall inward increased. Bench marks were established on the cranes and periodic observations were taken to detect sinking or shifting of the cranes. This work was carried out by running level loops to the cranes and back.

A haul road was constructed to allow the trucks to take the debris to a barge located just north of the site. This road was placed over a pedestrian walkway/bikeway along the Hudson River bulkhead. Similar to the concern about the cranes, there was concern that the rows of trailers being off-loaded would exert pressure off of the bulkhead. A base line was established with points set in the pavement to take elevations and horizontal offsets to points on the bulkhead.

Location of slurry wall and basement walls.

Establishing Survey Control

One of the first tasks required of the consultants working on the night shift was to establish survey control around the entire site. There was a fair amount of both horizontal and vertical control near the site. LKB workers had set some new control in the area and had also set some of the original monuments near the area. Vollmer workers also had set a fair amount of control along the highway and had recently completed a survey of Battery Park City, the 95-acre site adjacent to the area. Final field edits of the site had been completed only a week earlier with a submission date scheduled for September 12.

While this seemed to be sufficient control to perform the necessary work, the datums used posed some problems. Some of the projects were prepared in recent datums. The Battery Park City survey was prepared in NAD 83 horizontal and NAVD 88 vertical. The Route 9A project, originally surveyed in the 1970s and subsequently in the ’80s and ’90s, was tied to USGS 1927 horizontal and Manhattan Borough President’s datum vertically. Vertically, the two national datums used were NAVD 88 and NGVD 1929, Sandy Hook. The difference between them was approximately 1.1'.

Most of the work in New York City, however, is tied to a specific borough datum. In Manhattan for example, the Manhattan vertical datum is 2.75' above mean sea level at Sandy Hook. Transit Authority vertical datum is 100.1' above the Manhattan vertical datum. Similarly, the Port Authority vertical datum is 297.347' above NGVD 29. The Transit Authority and Port Authority, with tunnels well below sea level, use these datums so engineers and surveyors can work in positive numbers.

Horizontally, there are unique datums as well. The World Trade Center (WTC) site was constructed in the Port Authority System, which in effect is a small adjustment to the Manhattan Coordinate System (Dyckman System). The Dyckman System is a horizontal coordinate system used primarily in mid- to lower Manhattan. The coordinates are expressed in northings and westings. During the 9A project, a right of way survey of the corridor was performed. In order to tie the systems together, over 25 Manhattan monuments were observed in State Plane Coordinates. From this, a local translation was developed between the two systems. The Port Authority used this system to construct the site.

A Department of Design and Construction surveyor takes monitoring readings on one of the slurry walls while connected with a safety line and harness.

Slurry Wall Exposure

One of the first and most critical tasks at the site was the exposure of the slurry wall, a 3' wide x 70' high “bathtub” that was constructed to hold the Hudson River back during the construction of the World Trade Center. The coordinates of the corner points and angle points of the slurry wall were expressed in northings and westings. There was speculation as to whether the coordinates were in fact from the Manhattan System. The conversion to the WTC slurry wall coordinate was applied and the limits of the wall delineated. As the excavation progressed, the wall was exposed to confirm the translation.

Exposing the slurry wall and the excavation within its limits created new surveying challenges. One was the potential movement of the wall. To monitor the wall, survey points were established at 50 ft intervals along the top. Observations taken with a total station were recorded and the data downloaded into an Excel spreadsheet. Comparisons to the initial and most recent readings were made to detect movement.

A Department of Design and Construction surveyor takes monitoring measurements of cranes.
As the excavation continued, it became increasingly dangerous to set up a prism. Surveyors were required to wear harnesses and be tied off to prevent falling into the pit. Eventually, threaded poles were drilled into the top of the wall and hundreds of prisms were permanently mounted along the entire wall.

When the wall was being constructed, cable tie-backs were installed to prevent back pressure from pushing it in. When the towers were constructed, the tie-backs were no longer necessary and were cut. As the debris was removed and more and more of the wall was exposed, it was necessary to put new tie-backs in. The original plan of the wall was used and the tie-backs were replaced in their original locations. To set them vertically required surveyors to transfer elevations from the top of the wall down along the inside face with a steel tape to establish new temporary bench marks on the face of the wall below.

A Department of Design and Construction surveyor works at night, monitoring buildings adjacent to the site.

Underground Column Locations

An interesting task for the surveyors onsite was to gather data on the damaged Cortland Street subway station beneath the site. The DDC team entered the station through a hole in the roof. Once inside the station, they located the subway station columns and provided their coordinates to the night crew.

The positions of the columns were re-laid on the floor of the site. This was necessary to place a matting on the ground above the station. This matting, comprised of large timber beams, was placed to support a crane. The column locations were necessary so the matting would be supported by the columns and not cave in on the station.

During this same period, day crews were working around the perimeter of the site. The collapse of the Twin Towers and #7 World Trade building caused tremendous damage to the utility infrastructure in the area. The utility companies needed to re-route their lines to provide service to their clients who lost electric and telephone service. The surveying teams began collecting data on utility castings and other topographic information. In some cases, entire streets were opened to expose the lines. In these cases, actual locations of the lines were taken and composite utility plans produced.

Surveyors take readings at crane “matting“ over Cortland Street train station. Building #5 was being demolished in the background.
While most of the surveying tasks described required conventional equipment, the conditions were anything but routine. Everyone was required to wear breathing masks. The noise from the heavy equipment and generators made communication very difficult and surveying at night made sighting difficult. Despite these obstacles, the surveying work was performed continually without interruption, aiding the recovery process and greatly enhancing the safety of all the workers on the site. The surveyor’s role was an important one. The monitoring of the buildings provided a measure of safety for the recovery workers in the area. The monitoring of the cranes and the slurry wall gave the structural engineers a level of confidence that the wall would not collapse while continuing to dig deeper into the site. Amid the extremely challenging recovery aspects that resulted from the World Trade Center attacks, surveyors provided the indispensable skills and data from the moment they were brought in.