Construction, particularly in the urban environment, often exposes nearby structures and facilities to hazards, some of which are difficult to predict precisely and manifest as the work progresses. Impacts of this nature are associated with excavations, tunneling and foundation construction methods, and often require monitoring of potentially impacted structures and facilities. A monitoring program may include a variety of means of observation and measurements, including periodic visual and photographic observations of the work and adjacent properties, survey readings and instrumentation to measure displacement, vibrations, groundwater levels and other phenomena.
Commonly used construction monitoring techniques have been available for over 50 years. Technology has reduced the cost and expanded the options for monitoring programs. In spite of this, the full benefit of construction monitoring is often not realized. Here are a few things that can be wrong with the monitoring program for your project:
It doesn’t exist (but the risk does)
Not everyone in the construction industry understands the potential benefits of construction monitoring. It is well understood in the tunneling and underground construction sectors, but less well understood in the general building and municipal sectors. Many owners fail to understand their exposure to construction risk and how monitoring can help manage that risk. They may not take interest in whether the monitoring programs for their projects are effective. Unsophisticated contractors view monitoring as a nuisance that draws attention to immaterial impacts from their work, causing alarm, delays and added costs. These attitudes can lead to inadequately resourced monitoring or no monitoring being performed at all.
Unless it is required by the owner or an agency having jurisdiction, many contractors will not perform monitoring, even when engaging in high-risk work. Sometimes this is a matter of overconfidence, but often it is a matter of not understanding unfamiliar risks. For example, monitoring might not be required to construct a small detached apartment building. But consider the same project with a lot-line earth-retaining structure to support an adjacent property. In this case, a risk exists for which early warning might be crucial to prevent property damage and injury. The need for monitoring is driven by the hazards and potential consequences of the work, not strictly by the size or cost of the project as a whole.
It measures the wrong things
Suppose you want to monitor an unreinforced masonry building for vibrations during pile driving. The energy used to drive the pile will be transmitted to the ground and cause the ground to move, which will propagate as a wave from its source. The ground motion will displace the structure causing an inertial response that will generate forces in the lateral force resisting system for the building – in this case the masonry walls, which are also likely the most brittle structural components of the building. Therefore, the ideal location for monitoring vibrations might be the walls near the bottom of the building, if not the foundations, where the force effects would be expected to be greatest. The floors will have a different response that might not be representative of the effects on the masonry walls.
When planning a monitoring program, some thought should go into determining which construction impacts are of interest and how best to monitor them. The ideal means or locations for monitoring a particular construction effect, might not always be practical but the trade-offs should always be understood. I have seen instrumentation installers select instruments and monitoring point locations based solely on the convenience of installation. If the installer does not understand or does not care about what needs to be monitored and why, then there is no other basis for this decision. I have also seen monitoring plans developed by geotechnical engineers where various types of instruments are sprinkled over the site almost randomly until the plan ‘looks’ complete – not unlike the way some firms plan subsurface explorations. This approach is sure to collect some data, but extracting insight from the data will be more difficult. If your monitoring program fails to capture the actual phenomena of interest and instead measures something else, it is not going to be cost-effective and may not yield timely and actionable information.
The data is not available
Regardless of the form of the data, monitoring does not do anyone any good if the data is not distributed to the appropriate stakeholders. In the mid-2000s, I was involved with a project where monitoring was performed by an old-school surveying firm that worked entirely by hand. It sometimes took more than a week for measurements to get out of the surveyor’s field book and displacement measurements transmitted. Delays like this could have lead to the contractor unwittingly causing movement of the adjacent historic structure, resulting in damage. If some adverse event occurs during construction, timely data processing and distribution is necessary to implement contingency plans before too much damage is done.
Modern automated data collection tools and the internet have made timely data distribution easier, assuming that it is shared with the right people. One of the functions of a monitoring program is to provide objective evidence that can be used to expeditiously resolve claims. However, for this to work, all relevant stakeholders must have access to the data. For example, some contractors and owners refuse to share preconstruction survey reports with the owners of the subject properties or provide an incomplete version of the report. This may result in the facts of a claim remaining disputed longer than necessary, increasing the costs for all concerned.
The data is not interpreted
Like other data-collection efforts, the objective of construction monitoring is insight, not data. The purpose is to answer relevant questions: Is the ground moving in response to excavation? Has the adjacent structure been subjected to damaging vibrations? But beyond that, it is often necessary to know why some observed condition or measurement has occurred. Interpretation of the data is used to provide these answers.
Interpretation may not be provided due to cost or risk considerations. If the construction contractor is required to provide monitoring, they may not have the incentive to pay for interpretation of the data. In addition, low-cost monitoring firms often do not have the budget or in-house structural and geotechnical engineering support for interpretation to not be a liability. Therefore, not infrequently, monitoring reports are little more than data dumps or provide no more interpretation than comparing observations with baseline values and threshold levels. The problem is that without interpretation no assessment is made of the risk associated with a threshold event, the likely cause of the event and corrective action required.
In addition, if no one is looking at and thinking about the data, errors can be missed. On one of my projects, optical monitoring of a sheet pile cofferdam suggested that it was lifting out of the ground. When I learned that the surveyor had placed control points immediately outside the cofferdam, I immediately understood that the points had settled. After some argument, I was able to convince the surveyor to backsight to other control points for every reading, thus eliminating the erroneous uplift readings. In this case attempting to understand an anomalous set of readings uncovered an error, which could be corrected, instead of reporting a false threshold level reading.
No risk management context
What all of these common shortcomings of project monitoring programs have in common is that they fail to understand monitoring in its proper risk management context. While proper project design and construction planning can eliminate or reduce the likelihood and consequence of hazards, it may not be possible to reduce the likelihood of all hazards to tolerable levels. This leaves residual risks which have to be managed. Protective measures can be developed to reduce the likelihood or consequences of hazards. Risks that cannot be reduced by protective measures require control and monitoring provides the empirical basis for risk control decisions. But risk control cannot be accomplished through monitoring alone. Operating procedures and contingency plans must be in place that provide for corrective measures to be taken should a hazard develop, but before the effects become severe. The role of monitoring is to provide early detection of deviation from expected performance, which is necessary for the timely implementation of contingency measures and other controls.
Construction monitoring should be performed if a substantial retained risk is associated with the work after risk elimination and reduction measures have been taken. It should measure the anticipated effects of the potential hazards in a manner that reflect the risks involved. The cost of monitoring and other trade-offs should be considered relative to these risks. For a small risk, an indirect and infrequent measurement may be acceptable, for a larger risk more rigorous tactics may be warranted. The data should be available, with interpretation, to all relevant stakeholders.
Construction monitoring is fundamental to controlling construction risk and must be planned to reflect the hazards associated with the work to be performed and the expected impacts on nearby structures and facilities. It should be developed in and should serve a broader project risk management context, providing timely, actionable information to stakeholder for making risk control decision.
The information and statements in this document are for information purposes only and do not comprise the professional advice of the author or create a professional relationship between reader and author.