Before work can begin on the complex tracks, tunnels and other structures that will one day make up the California High-Speed Rail system, careful consideration must be given to the impact that construction will have on the corridor.
But the reverse is also true, as designers must consider risks the environment could pose for the stability and reliability of the high-speed rail tracks and tunnels.
“Planning the alignments for California High-Speed Rail system is an arduous task,” said Doug Anderson, senior engineering manager for WSP USA. “There are many stakeholders, and some unusual potential impacts. This results in a range of alternative alignments being proposed.”
One of those areas of concerns was located near a large wind farm near Mojave, California. While the farm itself poses little problem for train tracks or tunnels, a nearby cement plant and quarry company that owns substantial tracts of land in the same area raised some red flags for the designers.
“Blasting is required for the company to extract dolomitic limestone from the quarry to make cement,” Anderson said. “Blasting naturally creates vibration, and designers of the California High-Speed Rail program expressed concern regarding the potential impact of blast vibration on the stability of a high-speed rail tunnel proposed near the quarry.”
WSP recently led a study on behalf of the California High-Speed Rail Authority (Authority) to evaluate two alternate alignments for the tracks near the quarry, and make a recommendation if one was a better fit for the rail segment, or if they would need to head back to the drawing board.
“In the stretch from Bakersfield to Palmdale, the preferred alignment tunnels through the Tehachapi Mountains west of Mojave, and then continues to tunnel south of the mountains,” Anderson said.
The study hit an early roadblock when the quarry declined to share information about its blasting operations. To further complicate matters, seismograph monitoring data was difficult to find, since the study was to take place in an unpopulated area.
It’s not often that earthquakes are helpful. However, in this instance, Anderson and his team believed that the solution to their problem rested with data collected from earthquake activity.
“California is earthquake country, and there are hundreds of seismographs deployed to monitor earthquake activity,” Anderson said. “Drawing on my previous experience working for the U.S. Geological Survey Earthquake Research Center, I was able to make some good contacts and discuss a way to use the earthquake monitoring data for our purposes.”
The Southern California Earthquake Data Center at the California Institute of Technology (CalTech) in Pasadena has an extensive network of seismographs, and provided the research team access to its data, along with instructions on how to extract relevant events.
“As it turned out, one of the seismographs on a ranch near the quarry was located almost directly within one of the proposed alignments,” Anderson said.
Since quarry blasts are basically “noise” for earthquake studies, blast events recorded on the seismic network are filtered out of the earthquake data. Algorithms are used to determine the depth of an event, and shallow or surface events are usually labeled as quarry blasts and removed from the earthquake studies. Fortunately, quarry blast data is still recorded and kept in separate records.
“Since the ranch is about 16,000 feet from the quarry, the data was adjusted using industry-standard constants to provide an estimate of the vibration level at the proposed alignment,” Anderson said.
Collaborating with Xiaomin You of the St. Louis office, who provided computer modelling of blast effects, Anderson was eventually able to determine the range of vibration levels for about 100 events recorded at the ranch.
“Based on my experience with the explosives industry, I suggested some likely blast practices to Xiaomin as boundary conditions for her calculations,” Anderson added.
The study concluded that vibrations from current blasting at the proposed closest tunnel location would be “unlikely to affect tunnel stability at either alternative alignment.” It further suggested that additional evaluation would be necessary to determine the impact that expansion of quarry operations might have, and recommended further pursuit of blast information from the quarry.
“This level is considered well within safe limits for tunnel integrity and stability,” Anderson said. “Discussions regarding how to address impact of future expansion of the quarry on California High-Speed Rail system will certainly be coming in the future.”
The research provided by WSP provides the Authority with a more reliable understanding of the impact of quarry blasting, which Anderson said “improves the client’s negotiation posture in land transfer dealings.”
Though Anderson has worked on blasting-related issues on numerous projects, he said this was the first time he had an opportunity to use earthquake seismographs as a data source.
“It has been exciting to be a part of this important project, and fun to put together the pieces in this part of the project,” Anderson said.
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