Seismic design & performance considerations for the Bay Delta Conservation Plan’s twin tunnels
At the June 24th meeting of Metropolitan Water District’s Special Committee on the Bay Delta, committee members heard a presentation on the seismic performance for the proposed twin tunnels, the centerpiece of the Bay Delta Conservation Plan. The presentation included details on the seismic design criteria and the construction of the tunnel and its lining system, as well as the historic seismic performance of tunnels.
(Note: This is part one of two-part coverage. Part 2 will cover the committee’s review of the BDCP’s Implementation Agreement, and will post on Wednesday.)
“One of the strengths of Metropolitan is in its staff, and an example of that is Howard Lum,” said Randall Neudeck. “Howard came to us about 20 years ago; he worked for a premier tunnel design firm at that time. He has done multiple projects throughout the nation, and specifically in LA, he’s worked on the MTA Gold Line, he’s worked on our inland feeder arrowhead tunnels, he’s worked on Lake Mathews outlet facilities. He’s published research on tunnel design extensively, and he’s also a professor at Cal State Los Angeles.”
Howard Lum then took the podium, and began by presenting a slide with the four options for alignment of the tunnel in the Delta. The green line on the map is the pipeline/tunnel option which consists entirely of tunnels and will be focus of today’s presentation, he said. He noted the other options that were considered: the western alignment shown in blue consisting of 36 miles of canal and 17 miles of tunnel; the eastern alignment in yellow, consisting primarily of canal embankments, and a thru-Delta option, shown in green and blue, which essentially separated corridors for fish and water conveyance.
The proposed Delta tunnels would actually be two systems of tunnels: the north tunnels and the main tunnels, he said. “The north tunnels essentially connect the three river intakes into the Intermediate Forebay; they are 20-29 foot diameter tunnels that are roughly about 14 miles long,” said Mr. Lum. “Then from the Intermediate Forebay, the twin tunnels, 30 miles long, 40 feet inside diameter, and roughly about 150 feet underground, that connect from Intermediate Forebay all the way down into Clifton Court Forebay.”
There’s been a lot of tunneling work conducted in the United States in the past century, and tunnel technology has evolved quite a bit during that time, said Mr. Lum. “For the Delta tunnels concept, they’re proposing to stay at the practice as well as state of the art tunneling concept,” he explained. “It essentially consists of a tunnel boring machine, or TBM, that moves ahead of the concrete segmental lining system. The front part of the tunnel boring machine is roughly about 10 to 15 meters long, and following that is the tunnel lining system which is made out of concrete segments. The state of the art portion of the process is the earth pressure balancing tunnel boring machine that’s ahead of the tunnel lining system that allows the tunnel boring machine to stabilize the ground as this advances. The system we have for the Delta program is a one-pass system.”
In the conceptual design phase, a lot of effort was put into the viability and feasibility of the tunnel options. DWR performed preliminary ground exploration, and there were detailed evaluations of seismic parameters and site-specific geologic conditions that characterize what kind of ground motion and what kind of earthquakes we might have to deal with for the Delta tunnel alignment, he said.
Tunnel seismic design criteria
The seismic design of the tunnel is based on three criteria: water system reliability, Bay Area seismicity, and tunnel system performance. Mr. Lum then discussed each of these in detail.
Water system reliability
“Like all water conveyance systems at Metropolitan and across the state, a water conveyance system has a higher standard of seismic performance than any regular structure or buildings under the modern building code,” he said. “After an earthquake event, the water system shall and must remain operational, and that has always been a criterion for Metropolitan in designing any water conveyance and distribution structures.” To ensure such reliability, the Delta tunnel is designed based on the Maximum Considered Earthquake, which is the maximum event that’s likely to occur within the lifetime of the structure, he said.
“The other part is that operational performance is essential and the system has to remain operational right after a major event,” he said. “We cannot have any system downtime. Those are the two primary areas of system reliability that’s been incorporated into the design.”
How vulnerable the area is to earthquakes
Second element is the how vulnerable is the area as far as earthquakes are concerned, he said, presenting a fault map of the Bay Area and the Delta region. The predominant fault is the San Andreas Fault where two major events have occurred – the 1906 San Francisco Earthquake and the 1989 Loma Prieta earthquake, and the both caused a lot of damage to the Bay Area, he said. The other predominant fault is the Hayward Fault in the East Bay, and further east of the Hayward Fault is the Antioch Midland Area, a region with a lot of minor small subduction zones with potential in earthquake magnitudes, he said.
The Delta tunnel does not cross any known active faults based on current study and geologic exploration at this time, he said. Even though it is quite a distance away from the San Andreas and Hayward faults, we consider the Bay Area seismicity as a whole when we’re looking at the Bay Delta seismic vulnerability, he said.
Considering all the seismic risks of the area, the probability of a magnitude 6.5 earthquake in the next 18 years in the whole Bay Delta region is roughly 66%, or a two out of three chance that an earthquake roughly the magnitude of the Northridge earthquake will occur, he said. The tunnels will be designed to withstand the Maximum Considered Earthquake, which is the largest magnitude event based on current seismic analysis and evaluations of the maximum magnitude capable of being generated by each of these faults, which ranges from 7.8 on the San Andreas Fault to 6.5 in the Antioch Midland Zone, he said. The location and the distance of the fault rupture to the tunnels, as well as the site specific ground conditions are considered, he added.
How tunnels perform in earthquakes
There’s been a lot of research conducted on how tunnels perform in earthquakes. Mr. Lum gave a brief summary of the findings:
- Underground structures suffer less damage than any surface structures.
- Deeper tunnels are less vulnerable and will perform better than shallow pipelines. “Sometimes you see pipelines that were damaged in these major events, but again shallow pipelines are very different than deep tunnels that we’re considering,” he said.
- Concrete segment lined tunnel will perform better than any unlined tunnel or rock tunnel with no lining system.
- Segmental liner has better performance than other system. He noted that this is the system that’s being proposed for the Bay Delta tunnels.
- There are localized fault crossings that are used in other tunnels in the state that are crossing active faults, but the Bay Delta tunnel does not cross any known active faults based on current study and evaluations.
Concrete segmental lining system
For this next portion, Mr. Lum showed a video and explained how the tunnel boring machine and segmental lining system are built. This portion of the meeting is best viewed rather than read:
Mr. Lum said there is a full gasket installed around the segments to ensure the tunnel is sealed both inside so that water doesn’t leak out, and outside so that water doesn’t come in. Once the segments are in place at the proper location, a construction worker put the bolts through the bolt pockets, lines them up, and tightens them, forming a complete ring, he said.
“Segmental lining systems are a proven form of technology used in almost all the subway jobs currently going on at this time,” he said. “Essentially, in seismic design for segmental liner, we have to anticipate and determine the maximum ground movement for the magnitude earthquake we are designing for and make sure the rings themselves can accommodate that movement laterally and horizontally as well as that the bolts and all the connections are capable to resist that load. The whole idea here is because the joints have a certain amount of flexibility, it actually increases or enhances the ability to resist earthquake. The concrete provides the rigidity, and yet the bolt and the joint connection allows that little bit of flexibility and accommodation in deformation during a major seismic event.”
How do tunnels perform in earthquakes?
“We all know earthquakes can be devastating,” he said, showing a slide with pictures from major earthquakes that have happened in the past. He noted that all of the earthquakes pictured are within the same magnitude they are designing the tunnels for – between 6 and 8. So how do tunnels perform under those scenarios?
He next presented a slide with a table on it which detailed statistics for past earthquakes. In addition to using the earthquake magnitude to design for tunnels, they consider ground acceleration, which indicates the forces that can be generated to the structure, he said, noting the correlation between the magnitude and the acceleration. He pointed out that after the Northridge earthquake, there was no damage to the LA Metro tunnel, and in the Kobe earthquake of 6.9, there was minor spalling –meaning a little bit of flicking off and cosmetic damage, but the performance of the tunnel was not impacted at all. In the Athens earthquake of 5.9, they had a concrete build collapse but there was no damage to the subway system, he said. “These are all segmental concrete lined tunnels, the same as we’re proposing for the Bay Delta project so we are comparing apples to apples,” he said. The conclusion of that is that segmental liner will experience no or very little damage based on many studies performed by researchers in American Society of Civil Engineers, Japanese Society of Civil Engineers, and many other academic areas, and essentially the .5 Gs is roughly is what they can easily conclude there will be no earthquake damage, he said.
“The Delta tunnels design is based on 7.8 to 6.5 San Andreas Fault to the Antioch Midland fault. The accelerations equivalent to those magnitudes based on current studies is .27 to .5G, so it falls definitely within the same table of areas,” he said. “The anticipation of that is the Delta tunnels were perform very well using the current concrete segmental liner technology.”
Open channel versus tunnel
Mr. Lum then compared the seismic risks of an open channel embankment and a tunnel.
“An open channel embankment is an above ground structure, so it’s going to have a higher seismic force because the height of the structure will amplify an earthquake,” he said. “Same as when you’re in a high-rise building and you experience higher acceleration of forces, so same thing with open embankment system. There are potential liquefiable areas, so when you have a structure like an embankment, you definitely have to consider liquefaction. The slope stability issues related to open embankments need to be considered under strong earthquake ground motions, and that can be very high forces and lead to some of the potential failure as in some of the other earthquakes.”
As for the tunnels, the proposed Delta tunnel’s design for the Maximum Considered Earthquake and the ground accelerations are definitely within the tunnel lining structure capacity, he said. “This means that based on historic events and based on analytical models, those .5 G is definitely within the structure capability of the tunnel lining system. And because it’s deeper underground, lower acceleration as you go deeper compared to the surface. We did look into liquefaction, and there is no liquefaction expected at this time for the current alignment because the tunnel is well below any liquefiable soil. As you recall, the tunnel is about 150 feet deep and liquefiable soil and the maximum in the Delta region is about 50 to 60 feet deep, so we’re well below any liquefiable layer.”
“As far as constructability goes, there was a lot of study and detail discussion about how constructable the Delta tunnel system is, and rather than going through all those details, I’ll just show you one graph which captures pretty much everything,” Mr. Lum said, presenting a slide depicting the diameters of tunnel projects constructed worldwide. He noted that the measurements are the tunnel boring machine excavator’s outside diameter. “In relationship to all the tunnels that have been completed or undergoing construction, U.S. and worldwide, the Delta tunnel excavator is 45 feet, because our finished inside diameter is 40 feet, and we have to allow for the thickness of those segments, so that’s why it’s a bit bigger. So you can see that the Delta tunnel is right in the middle of the road as far as large diameter tunnels so – it is not extremely large, it’s not small, but it’s very viable, very doable.”
“In summary, the expectation is the Delta segmental liner will perform well in seismic events because tunnels have continuous underground support and that provides stabilization to the tunnel,” said Mr. Lum. “It is a confined space rather than an elevated structure which is subjected to a lot of amplification and swaying.”
There have been advancements in tunnel technology with tunnel boring machines and the ring-build precision system as well as control of ground settlement, which is why there are no so many urban jobs and subway jobs with no significant concerns, he said.
“The segments have been built in the shop so they are high quality, high strength concrete segments under high quality control, so we essentially put those segments out and put it all together inside the tunnel,” he said. “The gasket performance will definitely provide that water tightness … Gasket technology has been proven so that the water tightness can be maintained. The bolted segmental joints provide strength and more importantly the flexibility and the ductility of the system to accommodate earthquake ground motions, and all of these above conclusions are based on both analytical models as well as post earthquake damage observations.”
“That concludes my presentation … “
For more information
- To view Howard Lum’s power point presentation, click here.
- For even more on the design of the tunnels, read this journal article from the Proceedings of the Rapid Excavation and Tunneling Conference 2013; Howard Lum is a co-author: Update on the Pipeline/Tunnel Option for the Delta Habitat Conservation and Conveyance Program
- To view the meeting video and access all meeting materials, click here.
Coming on Wednesday …
Metropolitan’s committee next hears a presentation on the Bay Delta Conservation Plan’s Implementation Agreement.
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