As a boring machine cuts through the ground beneath Second Avenue in Manhattan to carve a new line to ease subway traffic in New York, a lot of unforeseen movement can happen.
With increased budget compression, controlling construction costs and maintaining infrastructure being most crucial, regardless of project type, Automated Structural Monitoring (ASM) is key to ensure that unforeseen problems don’t arise. With such a component, particularly considered and applied from the beginning of the project, it could prove a boon for design-build projects moving forward — particularly with an eye toward efficiency, cost savings and reduction of time from planning to completion of a project.
ASM is essentially a combination of sensors that can detect movement on any item to which it’s applied. For an example, as the boring machine cuts through the ground underneath Second Avenue, Leica Geosystems utilizes a robotics total station. This uses basically a highly refined, highly accurate laser, which can very accurately find the center of a prism target; the prism target is being located on the structure in question.
So as the construction work continues below, the equipment compares measurement to measurement and sees if there’s been any change in those measurements on structures above to ensure there is no movement of surrounding structures.
ASM not only senses whether there’s been a change in those measurements, it can determine if they have moved compared to the physical surface of the earth. The laser technology that is imbedded into the robotic total stations, combined with the very precise telescope and optics Leica had built into it, enable it to be extremely accurate in finding the center of those targets consistently.
While all structures move, the automated system establishes a baseline over time and can define normal movement, such as movement from thermal expansion and contraction, traffic vibration and other natural occurrences. With that modeled within the system it allows compensation for that natural movement.
From this established baseline, movement as a result of soil or underlying bedrock subsidence can be precisely measured or detected. That subsidence is what the system really targets. And it doesn’t take much movement to be significant. When considering the bedrock on which a building is constructed, it can be a very small amount — perhaps a half inch or less.
Comprehensive Coverage
The ongoing project in Manhattan is a good example. The project itself is enormous — 16 new stations and nearly nine miles of new subway track. As the tunnel-boring machine makes its way through the rock underneath, there is a lot of existing infrastructure above ground. And these existing buildings occupy some of the most substantial real estate in the world.
The project starts at about Second Avenue and 95th Street and it’s essentially designed to do multiple things. One is it’s going to build this line under Second Avenue. Ultimately, a new line will come in underneath the East River and connect the Long Island Railroad to Grand Central Station. In fact, this will include constructing a completely new station beneath the existing Grand Central Station. Essentially the Long Island Railroad would be able to come in to Manhattan and then give riders a choice as to stopping at Grand Central or Penn Station. Ultimately, it will alleviate a lot of east-west traffic in the city.
With ASM, several things come to the construction table to ensure the structures above are intact throughout such projects.
First, it provides a consistency in that once the system is set up, it eliminates the issues associated with individual survey crews who must constantly set up, tear down and set up again. It removes all that inconsistency and uncertainty from the results.
Second, the automated system provides a continuous look at whatever it is monitoring. Developing that crucial baseline ensures a constant look at that particular structure being monitored over a period of time. This provides an essential basis to formulate an opinion as to whether or not there is some deformation activity ongoing that may not be anticipated or detected by the naked eye.
This allows the construction team to take the necessary steps, anything from altering their construction methods to changing the sequence of the project. In the end, it translates to a very cost-effective solution that constantly monitors any changes accurately over a period of time.
With this around-the-clock monitoring along with knowledge of where the tunnel-boring machine is allows precise coordination among all working parts.
Precision Monitoring
When discussing movement, the equipment is recognizing movement in terms of millimeters. The idea is that the building owners and the property owners concerned about tunneling activity underneath can be sure the system is gauging real-time activity.
On large projects, several ASM instruments are placed and kept in place through the entire project. Once an instrument is mounted, it stays. As the project progresses, more pieces of equipment are brought on line to expand with the scope of the project.
From a risk management standpoint, it’s a huge benefit. It basically enables these projects to move forward with a thinner margin of error because of the equipment’s precision.
It reduces risk for the project, the surrounding infrastructure and as a result reduces construction costs and material costs because ASM can more accurately define how much material is needed in a particular place.
The system also helps determine whether items such as additional retaining walls are necessary and how thick they need to be. Rather than overkill in terms of material and time on those particular projects — the shoring up aspect of it — ASM can do it a lot more accurately and a lot more cost-effectively using this monitored baseline.
BIM Relationship
While not necessarily a building information modeling (BIM) component, ASM works alongside the technology. The challenge today is to get the mindset to the point where this particular type of system is actually thought of as a component of the BIM process as well.
There are areas in which it can be included up front as opposed to a little bit later on in the process. ASM typically makes its appearance when the construction is at a point where the building owner’s permission is being sought and they’re seeking releases and sign-offs and the overall project manager realizes there is some risk needing monitoring. It would be very beneficial to have this type of technology put in or included with the BIM up front.
Leica, for example has been very active in trying to work with design firms up front to expand the awareness and get them to a realization point that this is an effective way to go about these projects.
On the technological side, ASM components have been around for quite some time. Leica has performed such monitoring applications for more than a decade on major infrastructure projects, particularly bridges and dams worldwide.
As a result of its constant monitoring, flexibility and contribution early in a project as well as its obvious effect on cost and time savings, ASM is a system tailor made for design-build as well. It’s an area in which Leica is currently working and can show its worth throughout the life of a project while ensuring that the design-build team has its information early and accurately.
Sean Fitzgerald is key account manager, Engineered Solutions, at Leica Geosystems Inc., www.leica-geosystems.com.