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T-Rex Pounds the Ground in New Zealand
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Image of a 32 ft long, 8 ft wide, 64,000 lb triaxial vibroseis truck, nicknamed T-Rex
Nicknamed T-Rex, this 32 ft long, 8 ft wide, 64,000 lb triaxial vibroseis truck is being used to pounds the soil all around Christchurch, New Zealand, to help researchers determine how the waves from strong earthquakes that begin deep underground will propagate. Courtesy of Brendon Bradley

The enormous truck seen in Christchurch recently was part of a research project to better quantify the soils beneath the city.

April 23, 2013—Although the gargantuan white and orange truck observed around Christchurch, New Zealand, recently had tires 5 ft tall, was shipped from Texas, and is nicknamed T-Rex, it wasn’t in town to crush cars at a monster truck rally. Rather, the truck is a purpose-built research instrument sent from the Network for Earthquake Engineering Simulation at the University of Texas (UT) for a collaborative research project with the University of Canterbury.

The triaxial vibroseis truck, developed by Industrial Vehicles International, in Tulsa, Oklahoma, is 32 ft long, 8 ft wide, and weighs 64,000 lbs. A large hydraulic unit at the truck’s center pounds the soil, creating vibrations that simulate earthquake movement. The three-axis digital control system enables researchers to select vertical, horizontal, or transverse vibrations.

The project was funded by the National Science Foundation—via the George E. Brown Jr. Network for Earthquake Engineering Simulation (NEES)—and the New Zealand Earthquake Commission (EQC). The NEES, led by Purdue University, has 14 sites across the United States including UT, where it is led by professor Kenneth H. Stokoe II, Ph.D., P.E., M.ASCE.

The research team has two goals for the project. First, to better quantify the soil characteristics beneath Christchurch, which was struck by a devastating earthquake in 2011. Second, they want to compare the active source method of testing provided by the truck with the passive source method of testing, in which background noise below the surface is recorded and analyzed.

“One of the benefits of the truck is not only that we can put out a large force, but we can put out a very well-controlled force,” says Brendon Bradley, Ph.D., a senior lecturer in the Civil and Natural Resources Engineering Department at the University of Canterbury. “We can apply, for example, a sine type wave where the frequency of the vibration is absolutely constant. By having a very simple input vibration, we can [make] our analysis much easier.”

An array of recording instruments—in this case approximately 10 m apart and extending 230 m—records the vibration. The team can then perform a surface wave analysis of these recordings and invert the data to extrapolate the soil properties below the test site.

“Depending on the soil, different waves of different frequency will travel at different speeds,” Bradley explains. “We use our fundamental understanding of how the waves move through the ground to actually invert and work out what the layering of the ground [has] to be to produce the signals that we see.”

Better understanding the soils under Christchurch will enable researchers to understand the reflection and refraction of earthquake waves and begin to understand why the east side of the city was damaged to a much greater extent than the west side during the 2011 earthquake, which killed 181 people and destroyed thousands of buildings.

“We knew qualitatively that the soils were different, but this testing allowed us to quantify what those differences were,” Bradley says.

Although analysis of the data from 22 sites around the city will continue into the summer, Bradley says a first-order analysis reveals complex geotechnical conditions beneath the Christchurch. Varying levels of sand from 10 to 40 m deep at the surface are atop a layer of gravel that extends to approximately 400 m deep. Beneath that are repeating interlayered levels of sand, gravel, and peat until bedrock is reached between 1,000 and 1,200 m deep.

“The reason we are interested in that is because when earthquakes occur deep below the surface, say at 5 or 10 kilometer depth, and the seismic waves travel up from that depth toward the surface, those waves travel between different layers of soil and are reflected and refracted,” Bradley says. “When these waves hit a boundary between different soils, they actually change direction and amplitude. Therefore all of the features of the soil actually have a significant effect on the ground shaking.”

The results will also have implications for other earthquake-prone areas, according to Brady R. Cox, Ph.D., P.E., A.M.ASCE, an assistant professor of civil engineering at UT.

“What we learn in Christchurch can be applied to other cities in seismically active areas that haven’t been hit by large earthquakes recently. As earthquake engineers, we have to take the opportunity to learn hard lessons from the past in order to prevent them from happening again in the future,” Cox said in written comments to Civil Engineering online.

The first-order analysis of this active source method testing—which has been extensively validated previously—reveals that the passive source method testing in Christchurch appears to correspond well at approximately 75 percent of testing sites. Additional analysis will look at the 25 percent of sites at which discrepancies were noted and attempt to find the source of the discrepancies.

“Passive source testing is basically very inexpensive,” Bradley says. “You don’t have to have this really large truck to apply the vibrations. All you need are the instruments to record the background noise. But the problem is because you are recording random and small-amplitude noise, the process of doing the data analysis is much more complicated.”

The team hopes to develop robust algorithms to make passive source testing more closely correspond with the results of active source testing.


 

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