By Cheryl Stockton
Modern construction projects are inherently complex. Whether your team is constructing a new building, embarking on extensive renovations of a historic structure, or undertaking a project that combines both renovation and new construction, there are a myriad of challenges to address. One of the most fundamental concerns is what’s happening underneath a project, notably any movement that impacts the safety of the job site. Attention to movement is particularly acute on projects involving renovation or additions to existing structures. It’s paramount that the design build team track movement in the existing structure to avoid costly damage or worse.
Fortunately, new surveying tools and technology have helped to transform monitoring during construction. These tools can provide a wealth of insight for structural engineers and general contractors, especially when they’re working on historic structures. Those historic buildings are often most at risk for structural concerns and movement during construction.
What are these new tools and how are they best utilized in the field?
An ambitious renovation and expansion on the campus of the University of Virginia (UVA) in Charlottesville, VA offers important lessons for structural engineers, surveyors, and general contractors.
Originally built in 1938, Alderman Library serves as UVA’s main library. Initially designed in a horseshoe shape, a major addition to the library in 1967 filled the horseshoe. As the school has grown to become a research powerhouse, the library’s needs have evolved as well. The university wanted to transform the library to better utilize space, make it more efficient, and add amenities. The 1967 addition was targeted for demolition and replacement with a modern structure that would better serve the university. The original 1938 building would be renovated as well, but it was important to the university to retain the historic character of that building.
Thus, the design build team faced the challenge of removing part of the larger library facility and then expanding the library complex while ensuring that the historic structure retained its integrity. The level of construction activity required for this project would certainly impact the historic library structure.
Given the complexity of this project and the significance of the buildings impacted, the team needed to get creative when crafting a monitoring plan. Simply put, a traditional approach wouldn’t cut it. Fortunately, surveying tools and monitoring software enable teams to develop holistic monitoring plans that provide real-time updates and allow extensive collaboration.
Creating a detailed plan is the first step to ensure any level of monitoring is effective. This plan should clearly outline the goal of monitoring, specifications for which monitors will be used, the placement of all monitors and tools, target sight lines, a delineation of responsibilities, and reporting schedule. A thorough monitoring plan can easily include dozens to hundreds of specifications depending on a project’s size and scope.
When crafting that monitoring plan and throughout the project, consistent collaboration with the general contractor is crucial. For the Alderman Library renovation and expansion, the contractor, Skanska, was involved in monitoring strategy and decisions early on. The Skanska team provided additional assistance, such as building wooden platforms and other instrument mounts as well as installing monitoring sensors throughout the construction site.
Reports should be shared with contractors on a consistent basis, ideally weekly. These reports track movement in real-time and offer important insights on site conditions. Ideally, structural engineers and surveyors work together to develop and analyze monitoring data before sharing with the GC. Surveyors can offer important insights and context, especially if unusual movements are reported. For example, early in the Alderman Library project the continuous monitoring noted a sudden spike in elevation in two parts of the column grid along the building’s first floor. Surveyors and structural engineers investigated immediately and determined that some of the survey control points were obscured briefly. Without all of the survey control to help the automated total stations (ATS) re-section its position, there was inaccuracy in the collected data. The data showed that the control points outside of the construction area moved, although the team was sure they didn’t. While it is possible for a survey control point to get bumped or moved, having two in completely different places move the exact same amount at the same time is highly unlikely. Data showed that the sensors at first level grids F/10 and Q/10 correlated with the temporarily inaccurate survey control checks. This meant that an obstruction was impacting the data. Once the obstruction was removed and the ATS regained line-of-sight to all the sensors, the elevation measurement stabilized.
Being able to quickly diagnose and address inaccurate results was crucial to prevent a work stoppage at the site. Had the structure in fact changed elevation by a half of an inch from the initial observation, the project would have been halted.
The Alderman Library monitoring relies on Trimble S6’s for traditional surveying of periodically monitored sensors and more advanced Trimble S7’s for the ATS’s. Each sensor provides a real-time gauge and can report as frequently as every two minutes. The project includes 130 sensors and two ATS’s. There are currently 41 sensors being read automatically and 89 sensors being read periodically. Placing these tools throughout the construction site ensures broad coverage for any potential movement.
This approach also allows the team to more quickly assess movement. For example, sensors noted minor movement by a key column connected to the original library building. Comparing that data to the lack of movement picked up by sensors at other parts of the site confirmed that the column in question was moving. Contractors developed a plan to stabilize the column, thus preventing significant damage and ensuring the building’s integrity.
Finally, monitoring plans and schedules must remain flexible. As construction progresses, different areas of a site will become more or less sensitive to movement. Adjust sensor locations and sight lines to correlate with the shifting zone of influence. For example, the team monitored the horseshoe area of the Alderman Library for movement for approximately nine months. As the project moves to new phases, ATS units will be placed in and around the building’s courtyard because this area will be more sensitive to movement in this new phase. Having a detailed plan in place will help the team anticipate and manage these changes.
Movement monitoring is all about safety. Key work decisions are connected to potential movement at a site, such as slowing work for shutting down a site. Before making these major calls, contractors need collaborating evidence. Utilizing real-time data from sensors placed strategically around a construction site is crucial. A lack of proper monitoring can risk site safety and the integrity of a building, especially a historic structure. Fortunately, new sensors and software provide the clearest view yet of any movement happening during construction.
Structural engineers and surveyors who work together can leverage these tools to monitor movement closely and ensure issues are analyzed and addressed quickly.
Cheryl Stockton is a Survey Team Leader at Draper Aden Associates, a Mid-Atlantic engineering, surveying, and environmental services firm. Based in Charlottesville, VA, she has over 30 years of experience providing surveying services to a wide array of projects. She can be contacted at firstname.lastname@example.org.