Most people who visit this city park 👇 would never know that under the tennis courts and concrete slab is a massive underground water tank in need of a seismic upgrade. But this subterranean municipal reservoir is where many of those visitors actually get their water from.
No one had been inside this water tank for a quarter of a century, but now it needed general maintenance and a seismic upgrade to protect against natural soil degradation and intense ground movement from earthquakes. Enhanced Scanning was called to the job to locate post-tensioned cables and rebar so that the structure could be reinforced for continued use.
Here’s a quick rundown: In lots of earthquake-prone locations, infrastructure has been in place since before updated earthquake regulations (or “seismic provisions”) were introduced. Since then, new engineering techniques and advanced materials are also available. And with the recent development of performance-based earthquake engineering (PBEE), municipalities are gradually retrofitting older structures to better withstand large earthquakes.
PBEE was spurred by the California earthquakes in Loma Prieta in 1989 and Northridge in 1994, which damaged infrastructure in the affected areas so severely that it was no longer functional. Four performance objectives were eventually created to limit loss of life, loss of structure, and loss of functionality:
Enhanced Scanning routinely conducts concrete scanning for seismic upgrades, but this underground water tank was larger than most of our previous retrofit projects. Just days before, the tank had been full of potable drinking water for local residents. But, by the time we arrived on site, it had been drained so that we could climb down the hatch to access the concrete walls and floor.
Because it had only been a few days since the tank held thousands of gallons of water, the humidity level was extremely high, making it even more challenging to scan in the enclosed space. These concrete structures are notorious for containing large amounts of steel, and we needed to find and document its positioning.
One surprising discovery we made was that the tank was built with vertically-oriented post-tensioned cables in the walls. Despite all the seismic retrofitting jobs we’ve scanned, we had never seen this orientation before. Normally, we see the cables in floors or ceilings bearing the weight of the suspended concrete under the inexorable force of gravity. In this scenario, the weight of the water pressing down was much less of a concern than was the tremendous pressure exerted on the sides of the tank by the height of the water column. The vertically-oriented post-tensioned cables made this concrete scanning job much more interesting than most concrete scanning projects.
We arrived on site with our finely-tuned GPR scanning equipment to locate rebar and post-tensioned cable in the storage tank walls and floor. We scanned a section of the floor and wall to determine the positioning and orientation of the rebar, which would inform the engineers’ work during the retrofit. While we were not made aware of the level of PBEE targeted in the seismic upgrade, the information provided from a detailed GPR scan was instrumental in helping make the retrofit possible.
Without a thorough GPR scanning report, the engineers wouldn’t have the necessary information to develop an accurate retrofit plan. Since they can’t see through walls, they needed to hire someone with the equipment that could (technically, the GPR receives the reflected electromagnetic waves that it transmits into a subsurface—but “potato, potahto”).
Enhanced Scanning not only has the high-frequency GPR equipment to “see” through concrete, we also have the skills, patience, and attention to detail required to complete such a huge scanning project with precision. No matter how big or small your project, we’ll approach your scanning job in exactly the same way.
If your concrete structure needs a seismic upgrade, contact us today to get your engineer the information they need to develop a detailed retrofit plan.