Applying LiDAR Technology to Structural Monitoring

Certainty 3Ds latest development project for Retention Wall Monitoring reinforces the application of LiDAR technology directly in engineering and construction operations. At Certainty 3D, we always consider the associated value proposition when developing application specific tools. This value proposition is significant.

Our client, an internationally known construction company and TopoDOT user, explained the significance of wall movement. The possibility of wall failure aside, they informed us that all retention walls settle over time. Multi-million dollar road construction projects cannot start until this settling has stopped. Thus there are huge returns in a fast field to finish process. Accompanying these returns, however is pressure for well-defined workflows, integration into existing processes, and above all correct results.

Clearly just about any LiDAR scanner can acquire data faster than traditional survey methods. At first glance, a properly registered point cloud seems to solve all the problems. But we found it necessary to look much deeper into the process. We began by understanding the nature of the wall itself. How the wall moves? How is the wall constructed? What wall features represent wall movement? How can those features be accurately identified within the LiDAR Data?

The following image demonstrates the nature of the challenge. TopoDOTs View 1 shows where the image in View 3 is located within the project. In view 3 there two point clouds, taken several weeks apart, red being first and yellow second. A section of these point clouds is mapped to the calibrated image. View 2 shows the cross section of the point cloud data along the wall plane. Closer examination of the data reveals the challenges.

The wall is not flat. The vertical ridge pattern along the wall results in a rather thick point cloud. This precludes any simplistic selection of individual points within the point cloud and calls for more sophisticated processing techniques in order to describe the wall surface.

The horizontal joints are clearly evident within the point cloud data and are the only wall features available to measure vertical motion or settling.

A closer look at this particular section shows motion of the wall moving away from its original position as shown in figure 3, while the joint location indicates vertical movement or settling. These motions can easily be measured manually in TopoDOT using standard MicroStation measurement tools.

These manual measurements are easily done within TopoDOT. However these measurements must be repeated at each rigid panel section along the entire wall. Thus there could be hundreds of such measurements per wall. Repetition of this process for many walls over weeks or months makes such manual measurements impractical. Certainty 3D responded with a high level of automation.

The challenges for the Certainty 3D development team could be summarized as:

1) Extract features representing the wall surface for each point cloud

2) Identify joint locations accurately for each point cloud

3) Communicate those results to the user for easy and accurate interpretation

The TopoDOT wall monitoring tool will automatically apply statistical filtering algorithms to describe each wall surface representing the respective point clouds. Shown in Figure 4, View 2, TopoDOT extracts a single line representing the original red point cloud. The second point cloud is represented as individual point elements. These points are then color coded according to their distance from the first line. View 3 along with the legend communicates clearly the movement of the upper panels away from the original wall scan by up to 4 hundredths of a foot.

TopoDOT tools will at the same time identify the center of each joint and measure the vertical displacement of each joint automatically. Again this is communicated effectively to the user as a displacement number and direction at the joint as shown in View 3.

As a last step, TopoDOT automates the entire process across the wall. TopoDOT accepts the wall design file alignment which includes the placement of each panel column at the stations. In the following view, wall movement up to distances of about one tenth of a foot (30mm) are clearly described by overlaying the colored displacements over the image. TopoDOT provides this information in a clear manner that is easily interpreted with a minimum of extracted data. Note also that TopoDOT also automatically pulls these point values into standard Excel spreadsheets at each panel station for further analysis and documentation. Time required to run this analysis?about 12 seconds.

The real exciting news for the LiDAR community to take away from this development is LiDAR data rapidly becoming an integral part of design and construction processes. The value proposition of LiDAR data within these applications assures that this technology will spread rapidly across all operations.

About the Author

Ted Knaak

Ted Knaak started his professional career as a control systems analyst for General Electric Space Systems Division through the eighties. In 1990, he moved to Florida where he was exposed to lidar technology at Schwartz Electro-Optics serving as marketing/sales and program manager. Mr. Knaak founded Riegl USA in 1993 and has since been actively involved with lidar and laser scanning technology. In 2011 he founded Certainty 3D, a company focused on data processing software and technology solutions for the laser scanning industry. Certainty 3D’s primary product is TopoDOT™ a point cloud/calibrated image processing program for extracting CAD models from LiDAR data. Mr. Knaak holds MSEE and MSME degrees from Brown and Drexel universities.
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