The Importance of Vibration Monitoring for Commercial Construction and Demolition
Commercial construction can often present a damage risk to surrounding structures. This is especially true in urban environments where high rises are being erected in close proximity to existing buildings. And, with the unprecedented growth in commercial development, new construction often occurs in increasingly condensed spaces where older structures are scraped to make room for redevelopment.
These tighter tolerances, however, have increased the potential for construction-induced vibration damage and claims, particularly from demolition and specific construction activities. Therefore, it is increasingly important to actively monitor and analyze the impact of construction vibration on adjacent and nearby structures.
What is Vibration Monitoring?
Vibration monitoring uses state-of-the-art seismographic equipment to detect and record ground movements caused by construction or demolition activities. As seismic waves travel out from a vibration source, individual soil or rock particles move back and forth over very small distances. Their velocities can vary from barely measurable to over five inches per second. The maximum velocity at which these particles move is defined as the peak particle velocity (PPV). In addition to the PPV, the duration of vibrations are also measured.
Both ground vibration and air overpressure are measured during blasting vibration monitoring. The peak sound pressure level (PSPL) is recorded in decibels using linear microphones. Air pressure from an implosion has the potential to blow out windows, thus it is important to record the PSPL.
Pie Consulting & Engineering performed vibration and air overpressure monitoring during the implosion collapse of the Old University of Colorado Hospital’s eight story Biomedical Research building in Denver.
Vibration monitoring system equipment typically consists of sensor units and a recording device. Sensor units measure the acceleration and duration of ground vibrations. The recording device contains signal processing and data storage devices. When a sensor detects a ground vibration above a prescribed threshold, a signal is sent to the recording device. Data are processed and then compared with predetermined guidelines. Vibration monitoring is conducted on a continuous basis for ongoing activities such as excavation and foundation work or for a single event such as blasting or demolition.
Although vibration monitoring can be conducted at the source, it is best practice to place sensors on or as close to the actual structures that are a concern. Vibrations are observed in real time and alert levels are set to inform the project team of approaching threshold vibrations. Vibration measurements are continuously documented to demonstrate the contractor’s adherence to pre-specified vibration threshold limits.
Effects of Vibration on Surrounding Structures
Buildings in the vicinity of a construction site respond to vibrations with varying results: no perceptible effects at the lowest levels, felt vibrations at moderate levels, and possible damage at the highest levels. Fortunately, ground vibrations from construction activities rarely reach the levels that can damage structures. Many construction activities create vibrations, which can be felt, but felt vibrations will not always cause damage.
Every structure will behave differently depending on what vibration levels it is subjected to. Commercial buildings typically can withstand 2-to-4 inches per second PPV without any structural effect; however, the maximum recommended PPV exposure for many historical buildings is only .2 inches per second. So, special care must be taken to protect buildings that have low structural vibration tolerance. In addition, vibration monitoring of medical buildings housing extremely vibration-sensitive equipment is also critical.
Although earthmovers, excavators, and drill rigs produce some level of vibration, pile driving is typically the greatest source of vibration associated with equipment used during a construction project. Whereas a large bulldozer or caisson driller can produce a vibration level of .09 PPV at 25 feet from the source, the vibration level of an impact pile driver can reach an upper range of 1.6 PPV depending on soil conditions. These values exceed the typical minimum threshold for damage to historic buildings; however, these values are still below the 2.0 PPV typical threshold for potential building damage to modern commercial structures.
Pre-Construction Assessment and Post-Condition Report
Just as important as vibration monitoring during the construction phase, a pre-construction survey of the existing structures will establish a baseline of damage that exists prior to the start of any construction or demolition. It is often difficult to differentiate between pre-existing damage and damage caused as a result of construction since few structures are completely free of defects or signs of distress. This assessment should include a thorough site walkthrough, complete documentation (notes, photographs, videos) of existing distress, and measurements of pre-existing cracks in foundations. Solid documentation of existing conditions and attention to detail is critical. Even a small crack in a building can lead to alleged vibration damage of the structure.
It is also a good practice to install vibration monitors a month prior to the start of construction activity to establish baseline ground motions caused by buses, cars, trucks, rail, and other non-construction vibration sources near the structures of concern. In a dense urban environment, routine traffic often causes some degree of vibration regardless of what construction is occurring.
Following completion of construction or demolition, a post-condition report documents any changes to the existing condition of monitored structures. This report includes all data collected from the pre-construction assessment, the levels of vibrations maintained during the actual vibration monitoring, and field observations of construction activities. Before and after records are compared to support or disprove a claim.
What Pie Can Do For You
Pie, a leading provider of building and forensic science services, understands the importance of vibration monitoring before and during construction or demolition, in conjunction with condition assessments before and after construction or demolition. Critical data assists the construction teams in limiting the exposure of construction-related vibrations to surrounding facilities.