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“Eppur si Muove” And Yet, It Moves

12/06/2022 | Vaughn J. Mantor

No one knows for sure if Galileo Galilei really said “Eppur si Muove” (And Yet, It Moves), so it’s a point of contention among history buffs. You know the story; Galileo was imprisoned and forced to recant the publication of his astronomical research that showed the Earth revolved around the sun, not the other way around, which was the canonical view in the early 17th century. Supposedly, Galileo said or wrote the Italian phrase after his recantation.

What did Galileo have that allowed him to assert something that his more famous contemporary, Tyco Brahe, could not? Technology. Specifically, the telescope. With his telescope, which he made himself, Galileo was the first to see the detail of Jupiter and its moons. (The four moons which Galileo saw are still called The Galilean Moons. A modest, amateur reflector telescope is easily strong enough to let you see just what Galileo discovered more than 400 years ago.)

We humans detect movement with our senses, but our senses can detect only a limited range of speed or acceleration. You cannot see the bullet fired from a rifle nor the blooming of a rose, except with instruments.

New technology, i.e. laser scanning, lets us see and measure things that could not be seen before.

These examples we’ve collected are taken from our experiences in laser scanning in the last twenty years. Measuring movement, and other things that are not visible to the unaided senses, have saved a lot of time and money.

“I’m sorry I’m late. A hill fell on my car.”

For many of those who work in engineering and construction, knowledge of movement, however slight, makes all the difference. For example, retaining walls are subjected to large pressures from the earth and rocks that they hold back. A failure is, at best, a costly inconvenience, at worst, a life-threatening disaster.

Drivers along this stretch of the Hudson River above New York City were oblivious to the dangers right above them.

The retaining wall started to move long before it collapsed. But the movement was so small and diffuse that it was not observable with the naked eye. Our instruments can measure distances down to the millimeter. In such circumstances, laser scanning can detect the movement and the speed and the acceleration. Here is a method we commonly use.

First, we establish a number (depending on the length of the wall) of fixed positions that face the retaining wall, and we put permanent markers in the ground at each position. From each position, we scan the wall. We combine the data from all scanning positions into a coordinate system appropriate for the project.

Next, in consultation with engineers and owners, we return at intervals to rescan the wall from the same positions we used previously. By returning at regular intervals, we can show the speed at which the wall is moving and whether it’s accelerating or slowing, i.e. the first derivative. As we emphasized in our earlier article, “Know What AND Know When,” the earlier such scanning begins, the better.

Laser scanning should be started even after a collapse has occurred to assess how much of the remaining wall is still in its original place.

A less heart-stopping example.

Sometimes, a limited movement is desirable, but not too much. It’s easy to measure such with laser scanning, and the number of measurements is vastly larger than by any other means, and it’s cheaper, too. The picture below shows a laser scan of a bridge truss.

In Arizona, at a major expansion of the Tucson Convention Center, the engineers designed a long series of roof trusses. Each truss was supposed to have a specific camber once installed, and a slightly different camber under load. Two scans, one after truss installation and one under load, revealed the precise camber of every truss all along its length. By over laying the two point clouds, the precise differences in camber can be seen easily and measured accurately on any computer screen. Such information would have been time-consuming, expensive, difficult, and dangerous to accrue by any manual method.

“My office chair rolls away when I stand up.”

A government building for processing paperwork was built on fill. Later, those who worked there noticed cracks opening in the structure of the building, including the floor. Laser scanning at regular intervals delivered measurements to assess:

1. Where the building was sinking.

2 The rate of change.

3. The rate that cracks were opening.

4. The levelness and the flatness of the floors.

5. The twist of walls, windows, and doors.

6. How out of plumb the walls were.

7. The rate of change of all these elements.

The engineers used all this information to design methods to shore up the building.

“This bolt just popped out of that joint!”

The movements of structures are not always linear. Those working in a machine shop on the site of an open pit mine saw and heard some strange things. Bolts popping out of the structural steel, cracks in the concrete floor, machines going out of plumb, and other peculiarities. Laser scanning revealed that the building was sinking, unevenly, and that the ground around the building was also moving laterally and vertically. The building was being pushed by all sorts of pressures from multiple directions. Using the information from laser scanning, the engineers and owners decided on the most cost-effective solution to this difficulty, build a new shop elsewhere.

Straight, square, plumb, flat, truly round?

While the detection of movement requires more than one scan, even a single scan can reveal things that are not evident to the unaided eye. The cabinet maker uses a straightedge, winding sticks, squares, and such to check for straight, square, plumb, and flat because he’s working on material only a few feet long or shorter. But how does one check for straight, square, etc. of a beam fifty feet long and fifty feet above grade? Every laser scan reveals irregularities in the structure and the components.

Recently, Verify 3D scanned a large hotel in Colorado for a hotel developer as preparation for a major renovation. After the scanning was complete and the point cloud and Revit model were delivered to the client, the client asked for a map of the flatness of the parking garage. This map would let the designer place drainage holes at the lowest points in the floor.

In less than one hour, Verify 3D created the map, accurate to less than 1/2 inch over the length of the garage.

In a way, the new map of the flatness of the floor is also measuring movement even though it was only one scan. The original design of the parking garage put drainage grates and pipes at the low points in the floor. The laser measurements offer clear evidence of any movement that would affect runoff.

The claustrophobic may want to skip this final example. Detecting movement, however slight, in an underground mine is a matter of life and death. Obviously. For several reasons, underground mines are laser scanned at intervals. Here is a brief video, made entirely from laser scanning data, of a mine we scanned recently.

Verify 3D has been scanning mines for more than a decade. We maintain MSHA certification.

Whether it moves or is still, laser scanning can tell you things your eyes simply cannot see.

If you’d like a personal explanation or demonstration of the ways laser scanning can help you, or if you’d like to speak with one of our clients, see our contact information below.

Because our experience in this technology dates to 2002, Verify 3D knows the most appropriate equipment to use on each project and how to use it for the best benefit to our clients. Much of the time, we use a Leica P-30 laser scanner, Leica Cyclone software for registration, and Revit for CAD modeling, but not exclusively. We use other makes and models of scanners, other registration software, and other CAD modeling tools, depending on the needs of our clients.

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