By Thomas A. Winant
This presents a challenge to the owner and the tunneling contractor to minimize the impact to the existing structures during the construction of the tubes. Variations in soil conditions, groundwater and geotechnical anomalies all contribute to the settlement that is bound to occur during the mining of massive amounts of material below the surface. Each building has its own characteristics and peculiarities based on its age and design.
In New York, there can be a five-story, non-reinforced masonry walkup on a block and immediately down the road, a 45-story commercial high rise. Each building will react differently to the energy emitted and tunneling work conducted immediately beneath them from the project. Traditional monitoring, which tracks changes to tilt, vibration intensity and crack movement, can offer some indication that something has changed over time. Although this information is important, it falls well short of understanding if the actual structure has been adversely affected. In the case of more compromised structures, many elements can be hidden from view, making visual assessments an incomplete approach. Likewise, traditional monitoring does not address if the structure is in jeopardy.
On New York City’s Second Avenue Subway and East Side Access projects, a new advanced dynamic monitoring technology has been deployed that provides a much deeper understanding of the actual impact to the structure. A New York-based company called STRAAM (Structural Risk Assessment and Management) offers this unique technology and service which the company likens to an EKG of structures.
STRAAM’s technology incorporates 40 years of research relating to the behavior of large structures. Rooted in Advanced Dynamics, Fracture Mechanics and Systems Identification, STRAAM uses highly precise sensors to capture the unique dynamic signature of a structure. Within this signature is a wealth of information regarding the dynamic characteristics of that structure such as stiffness, damping (damage detection) and frequencies of resonance for many modes of vibration. This information represents the current condition of the structure or ‘baseline.’ STRAAM then monitors the structure dynamically, either continuously or periodically, to capture changes in the structure’s signature, as is done with the heart through the use of an EKG.
Damage to the structure shows up in a variety of ways within the signature and must be analyzed by a trained eye. A similar signature analysis process is currently common in large motor maintenance to identify vibrations that can lead to serious problems. What makes STRAAM unique is that the company gathers the information with no mechanical addition of energy and in ranges previously unattainable for structures. Once the data is processed, the results provide a quantifiable and permanent record of the changes to the structural performance. Real-time monitoring allows for perfect correlation between construction activity and changes to the structure’s performance.
For the tunneling projects in New York City, the STRAAM technology and analysis has been used to assess and track the condition of structures that are at risk for potential damage from tunneling activities. Some buildings showed signs of pre-existing weakness or other structural problems prior to commencing work. The MTA hired STRAAM for condition assessments for these buildings to quantify their current condition as a percentage of the design strength. STRAAM established a baseline, which provided the information needed to compare the existing condition of the buildings to the “as designed” condition to see exactly how compromised the structure had become over time. With this information the MTA could make an informed decision as to what approach to take to reinforce the structure to ensure that the building will not be compromised as a result of the tunneling work or related excavation. Some of the structures were strengthened, and this served to reassure the owner that each structure could withstand the impact of construction.
In another case, a 90-year-old historic building began leaning as a result of the tunneling activity, exceeding the thresholds established under the standard monitoring protocol. STRAAM was able to identify and track specific indicators in the dynamic response that would act as “tell tales” for changes in areas of weakness. With this approach, project officials were able to clearly understand that the structure was not in jeopardy, considering the type of work being conducted, regardless of the thresholds being exceeded. This provided an important decision-making tool for the management team who allowed work to continue without delay or remedial repairs.
Dr. Michael Horodniceanu, head of Capital Construction for the MTA and overseeing all of the New York City tunneling work, said that STRAAM “helps take the guess work out of (the process).” The alternative was to stop the entire project, which would have been extremely expensive and would have delayed production.
This new form of dynamic monitoring for structures is adding value to the construction of New York City’s subway projects and can be a great decision-making tool for future tunneling projects around the globe. STRAAM Corp. has broken new ground in this area to benefit the tunneling industry. On a broader perspective, understanding changes to the performance of a structure over time may have an important role in helping to maintain and understand the risk for our entire aging infrastructure. It offers a smarter way to understand how structures are impacted from any outside force, including time.
Thomas A. Winant, P.E., is Director–Business Development for STRAAM Corp.