Monitoring

   A monitoring program should be established during the consultation and documentation phases and continued until adjacent work is finished. It is undertaken to detect, gauge, record and interpret structural movement, the effects of vibration and other changes to the historic building that result from neighboring construction or demolition work. Data collected during the monitoring program can serve as a baseline for any subsequent movement or changes to site drainage patterns that arise within the first years after construction is completed. Ultimately, monitoring shows the degree to which steps taken to protect an historic structure from adjacent construction are sufficient and successful.

A seismograph records vibrations transmitted at the ground level of an historic building. The instrument is wired to a light and siren designed to warn the excavation crew that vibration levels are approaching preset limits. Additional sensors are often installed in the basement and on sensitive features such as stained glass windows. Photo: Wilson, Ihrig & Associates, Inc.

   Because of liability concerns, those responsible for a new development will often arrange to monitor an adjacent structure. As with a documentation program, the historic property owner may want to hire an independent engineer to review both the monitoring process and the measurements that result.

   The extent of the monitoring program and the tools used will depend upon the scope of the adjacent activity. A basic plan to address concerns over vibration levels may include a single seismograph placed on the structure's basement floor. More comprehensive measurements can be obtained by locating sensors at several points throughout the structure and the ground immediately adjacent to the historic building foundation.

   Whether acceptable vibration levels are mandated by law or left to the discretion of a project engineer, thresholds should take into account surrounding soils, the makeup and condition of the adjacent foundation and the particular vulnerabilities of the historic resource. Construction projects that involve major excavation work next to historic structures should include a program of test blasting before work begins. Testing various charges, delays and blast design configurations will aid in developing a controlled program that limits blast induced damage to a neighboring property.

   Structural movement as described in the preceding section is detected and recorded using a number of different tools. Electronic monitors that feed precise movement measurements to laptop computers can be placed across existing cracks. When budgets are tight or a large number of cracks are involved, inexpensive telltales made from two sheets of overlaid plastic with a grid can also track changes.

Electronic crack monitor and survey targets are shown installed on an existing wall. The crack monitor feeds movement data to a laptop computer. The targets are aligned and measured with optical survey equipment to determine the degree and direction of movement. Photo: McMullan and Associates, Inc.

   Optical survey instruments provide another means of detecting vertical and lateral movement within a historic building. Control points are established and marked by targets or reflectors on the historic structure facade and interior walls before adjacent construction begins. The location of each of these markers is precisely measured at regular intervals. Engineers can use the resulting information to determine whether the markers have shifted from their original positions and, if so, the rate and direction of movement.

   A program of visual inspections undertaken by a qualified conservator or engineer is an important adjunct to technical monitoring procedures. Inspectors should look for newly opened cracks, other signs of settlement and movement, and evidence of increased dampness or water infiltration. Additionally, visual inspections should ensure that temporary protective coverings are secure, that dust and dirt are not accumulating in the historic building, and that fire and hazardous material protection provisions are being upheld. A checklist can be drawn up during the consulting and documentation phases for use during each visual inspection. Such a systematic written record may also prove useful if disputes arise over the timing of and responsibility for damage.

Conclusion

   Protecting a historic building from adjacent construction or demolition activity requires thoughtful planning and cooperation between the developer and the historic property owner. Thorough pre-construction documentation of the historic structure ensures a common understanding of present conditions and suggests appropriate damage prevention measures that can be taken at both the historic site and the construction site. A routine program of visual inspection and vibration and movement monitoring helps insure early detection in cases where the historic building is experiencing effects of neighboring construction work. Early consideration of these issues, before damage takes place or worsens, can allow for the adoption of safeguards that protect the developer's schedule and budget and the physical integrity of the historic structure.

Checklists