By all accounts, Dr. W. David Carrier III is a preeminent geotechnical expert on the science of soil. He's been awarded the Norman Medal, which is a prize from the American Society of Civil Engineers that recognizes authors of papers that make a significant contribution to the profession. He's worked for NASA, and even authored a paper about building conditions on the moon entitled, "Geotechnical Engineering Considerations for Extraterrestial Infrastructure Construction." (In 2002, he penned an article recommending saying farewell to using the older Hazen formula for determining sand permeability and instead using the newer Kozeny-Carman formula that was entitled, simply, "Goodbye, Hazen, Hello Kozeny-Carman.")
But when it comes to what's causing the cracking at Tampa Bay Water's C.W. "Bill" Young Regional Reservoir, the good doctor doesn't have an official diagnosis.
For this case, Carrier's most important role was overseeing the permitting of the Tampa Bay Reservoir for the Florida Dept. of Environmental Protection, the agency responsible for permitting the project. He reviewed design documents and offered feedback about various issues during that period of the project. And after permitting was completed and the reservoir became operational, Carrier continued as an inspector of the facility. In all, he's spent nearly 10 years checking up on the reservoir.
One of Dr. David Carrier's photos of cracking at Tampa Bay Water's reservoir. Photo courtesy Tampa Bay Water.
In fact, Carrier was the first person to investigate the cracking, in January 2007. As he's testified in court, upon an initial examination, Dr. Carrier quickly formulated a diagnosis of excess pore pressure. According to this theory, water was staying trapped in the embankment's soil wedge as the reservoir pool was being drawn down. In short, the water in the pool dropped more quickly than that in the embankment.
The trapped water builds up pressure in the embankment, the theory goes, causing the settlement cracks that are the focus of the ongoing federal trial.
According to HDR's original design documents for the reservoir, this shouldn't be happening. The water in the embankment should lower at the same rate as the water in the pool. In at least one instance, Carrier testified, the difference between the two water levels was as much as 8 ft.
Still, Carrier testified, a couple of issues with the pore pressure theory kept nagging at him. For starters, piezometers installed by HDR in the cracked areas failed to detect any increases in pore pressure upon drawdown. (On redirect examination by Tampa Bay Water attorneys on March 19, Carrier admitted that the pore pressure may have been relieved by the cracks, which were in place during monitoring.)
More problematic for Carrier seemed to be a more fundamental question: If it's excess pore pressure—which would be an issue inherent to design—why isn't it happening throughout the entirety of the reservoir's embankment, instead of just in the two areas equaling roughly 10% of the embankment area.
Despite being a witness called by Tampa Bay Water attorneys, Dr. Carrier testified he no longer supports the utility's theory that excess pore pressure is to blame. And that led to TBW attorney David Forziano asking this last question of Carrier under direct examination:
"Have you formed any opinion with an engineering certainty of what's causing the cracking?"
Editor's Note: Twitter users can access ENR's ongoing coverage of the case of Tampa Bay Water v. HDR Engineering by linking to the #ReservoirTrial hashtag. ALSO - Though ENR has reported on the testimony of the trial's first three witnesses (Gerald Seeber, Amanda Rice and now Dr. David Carrier), our future coverage will not be as frequent. The trial, which began March 12, is expected to last four to six weeks.