Issue for integrity

Brad Christensen and Joseph Phillips / May 14, 2008

A potential bridge-collapse calamity was averted in Arizona through the use of advanced construction-integrity-testing techniques. Major defects that were invisible to the naked eye were discovered in a bridge under construction over the Salt River bed in Phoenix.

The defects were large subsurface voids that were located in a concrete pier shaft by engineering and project management company AMEC. Their timely discovery enabled the hazardous condition to be corrected well in advance of the 35th Avenue Bridge’s opening to the motoring public.

Void check

The bridge is a nine-span AASHTO I-girder bridge that is 1,080 ft long and 91 ft wide. Two types of nondestructive testing were performed by AMEC on eight abutment shafts and 24 pier shafts that were cast in standing water. The abutment shafts were 137 ft deep, and the pier shafts were 113 ft deep. The testing—cross-hole sonic logging/tomography and gamma-density logging—revealed major density drops, velocity reduction and energy loss approximately 25 ft below ground surface in pier shaft 20, indicating significant anomalies in the concrete.

Exploratory core-hole drilling then confirmed the presence of the voids and unconsolidated concrete zones, some of which were as much as 5 ft high and 5 ft wide. AMEC advised the contractor on repair procedures, including the vibration of concrete as it is placed. AMEC and the design engineer closely monitored the repair procedures. The drilled-shaft foundation was chosen as the only structural system that could penetrate the cobbles and boulders in the riverbed and resist scour depths that can go as deep as 45 ft. The design flow for scour was a 500-year event with a flow of 201,025 cu ft per second.

“The cross-hole sonic and gamma-density testing methods utilize different science to evaluate similar properties of concrete in the shaft,” explained Matthew Manthey, field services manager for AMEC’s Tempe, Ariz., office. “The combined technologies reduce false positives and improve confidence when defects are located.”

Cross-hole sonic logging/tomography uses sound waves to verify the integrity of concrete within the drilled shaft inside of the rebar cage. First, access tubes that typically are PVC plastic pipe are attached to the interior of the rebar cage, ideally at evenly spaced 1-ft intervals. After the access tubes are attached, the cage is lowered into the hole and the concrete is added to the shaft. The tubes are then filled with water. Once the concrete has cured, a sound-source probe and a receiver listening probe are lowered down two of the tubes. As the probes are pulled back to the surface at equal speeds, an ultrasonic pulse is passed through the concrete between the probes and all the data is collected on a PC. This proc­ess is repeated for the other tubes to achieve complete coverage through multiple configurations. Longer sound-wave travel times and slower velocities at specific depths indicate irregularities—either pockets of water or air—in the concrete.

For each pier on the 35th Avenue Bridge, 10 access tubes were used, resulting in readings from 45 configurations. Also, a more advanced analysis known as sonic velocity tomography was used to create a three-dimensional graphical display for better visualization of shape, size and location of defects in the shaft. Cross-hole tomography is an imaging method that has been compared to CAT scanning in the medical industry.

For gamma-density logging, the same PVC tubes are used, but without the water, and only one probe is used. This method uses the concept of gamma-ray backscattering to measure the electron density of the materials in the vicinity of the probe. A radioactive source at the bottom of the probe emits gamma rays into the materials surrounding the access tube.

“The gamma rays collide with electrons in the surrounding materials, lose some of their energy and change direction in a manner called ‘Compton scattering,’” explained AMEC Engineer Michael Rucker.

Some of those particles are scattered into a detector located in the probe. The number of these interactions is directly related to the number of electrons in the surrounding materials. Thus, the electron density of materials in close proximity to the access tube, both inside and outside of the rebar cage, is measured. For the 35th Avenue Bridge there were 10 gamma-density configurations, one for each of the tubes. It was used primarily to evaluate the portion of the shaft outside the rebar cage.

Installing confidence

Accurate interpretation of cross-hole sonic and gamma-density data, using complex mathematical proc­esses, was critical to defining the location and extent of anomalies on pier shaft 20. Interpretation included studying waveforms, velocities and the plots of arrival times obtained from cross-hole sonic logging to understand the vertical extent and approximate lateral location of anomalies. The gamma-density data helped to define the horizontal extent of the anomalous zones in the vicinity of each tube.

It also is important to use both techniques together, because each has inherent limitations, as well as strengths. For example, cross-hole sonic logging can be effectively blinded if the access tube becomes debonded from the concrete, a condition that can be caused by contraction of the tube as the concrete cools. However, gamma-density logging can be performed in tubes that have been debonded. The chief limitation of gamma-density logging is that it can only investigate shaft integrity close to the access tubes.

Innovative analysis tools such as the cross-hole sonic and gamma-density methods allow bridge engineers to have a superior understanding of the integrity of their constructed bridge foundation shafts. This understanding increases the level of confidence that the bridge will age and function as designed.

“These testing methods give the engineer a record that the bridge has been built according to plans,” said the structural engineer for the 35th Avenue Bridge project, Jerry Cannon, P.E., of TranSystems Corp. “There’s no other way to verify that it’s good concrete. After all, it’s hard to tell what’s going on when it’s 100 ft underground.”

Sample was not ample

The 35th Avenue Bridge project stands as testament to the necessity of quality integrity testing of drilled-shaft foundations, especially those constructed beneath the groundwater level. The placement of concrete in a wet hole makes the concrete subject to weakening by accidental increase of the water-to-cement ratio or collapse and intrusion of adjacent soil. Had the 35th Avenue Bridge’s defects not been detected, the bridge foundation would have been rendered structurally inadequate with potential catastrophic consequences from river flows during flooding.

Such consequences could involve substantial cost to the city and ultimately the taxpayers for bridge reconstruction and repair, as well as widespread public and media acrimony over the quality of the original construction and the disruption of a transportation route. At worst, the consequences could include serious injury or loss of life and the attendant liability issues.

However, early detection of the pier shaft 20 voids and effective teamwork involving the city of Phoenix, FNF Contracting and TranSystems Corp. enabled the shaft to be restored to integrity within three weeks and with no effect on the bridge’s overall construction schedule.

One key was that the city of Phoenix agreed to have all piers tested, rather than only a sample. “They understood that it only takes one bad apple,” said Manthey. “And you can’t rely on the luck of the draw to find it.”

Bridge failures in Minneapolis and levee failures in New Orleans are reminders that the public expects excellence and tolerates little in the way of excuses when it comes to their safety. In this regard, the approach to testing and analysis on the 35th Avenue Bridge project can serve as a model for project cooperation and drilled-shaft construction in the future. Good presentation of quality data by AMEC combined with good judgment and decisiveness from the owner and contractor led to solid repair that minimized financial impacts and avoided project delays.

About the Author

Christensen is external communications director for AMEC Earth & Environmental and Phillips is materials testing unit manager for AMEC Earth & Environmental’s Arizona operations. They can be reached at 480/940-2320 or by e-mail at [email protected]

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