Posted by: Deirdre Des Jardins | July 8, 2019

Assessing the Delta tunnel project as a seismic upgrade

Governor Newsom’s April 29, 2019 Executive Order mandated that the California Natural Resources Agency and the California Environmental Protection Agency assess “current planning to modernize conveyance through the Bay Delta with a new single tunnel project.”

Part of the agencies’ assessment of that planning should include an independent, objective assessment of the Delta tunnel’s performance in a large earthquake. The assessment needs to look into why Metropolitan Water District’s analysis of whether the tunnel lining will survive a large earthquake assumes the tunnel is constructed in very dense soil.  This assumption is questionable.

100917_0503_WaterFixtun2.png

In April of 2018, California Water Research collaborated with the Sierra Club and other Southern California environmental groups to send a letter to the General Managers of Metropolitan Water District, and Kern County Water Agency, and the CEO of Santa Clara Valley Water District, explaining that the Delta tunnels were not being designed to withstand a Maximum Considered Earthquake on nearby faults. The letter concluded,  “[t]he Department of Water Resources and Metropolitan Water District must analyze the performance of the Delta tunnels for all seismic hazards in the Delta, including the.. Maximum Considered Earthquake.”

MWD commissioned an evaluation of the performance of the proposed Delta tunnel lining for a Maximum Considered Earthquake in the Delta, a 2% in 50 year, or 1 in 2,475 year event.  The July 2018 analysis concluded the tunnel lining design would perform adequately and no changes to the design were needed. (Arup 2018.)

But there were major limits to the new analysis by Arup. The analysis for a Maximum Considered Earthquake only considered ground shaking at Clifton Court. The analysis also assumed that the soil column at Clifton Court was very dense. Based on this assumption, the analysis concluded that the ground shaking at the depth of the tunnel was about one-third that of the ground shaking on the surface.

chart seismic

As explained below, the assumption that the soil column at Clifton Court is very dense is inconsistent with geologic maps of the deep sedimentary deposits in the Delta. The conclusion that ground shaking at the depth of the tunnel is one-third that of the ground shaking on the surface is also not consistent with measurements from a down-hole seismic array at another site with deep sedimentary deposits, La Cienega in Southern California (Grazier 2004.) Measurements at La Cienega show that for strong earthquakes, ground shaking at the proposed Delta tunnel depths is about 70% of that on the surface (Hu 2005.)

Seismic hazard maps from the California Geologic Survey show no regions in the Delta near Clifton Court Forebay with very dense soils. The CGS map below shows the soil density for areas in the Delta near Clifton Court. (CGS 2018.)   Interpreting the CGS map requires a little explanation.

Geologists and geotechnical engineers estimate the density or stiffness of soils with shear wave velocities (Vs.)  For earthquake engineering, they consider the average shear wave velocity over the top 30 meters (100 feet) of soil.   Average shear velocities above 750 m/s correspond to rock. Average velocities above 360 m/s correspond to very dense soils. Velocities above 180 correspond to stiff soils. Velocities below 180 correspond to soft soils.

The CGS map shows that, in general, the only very dense soils near Clifton Court are in the Diablo Mountain range (at the bottom left corner.)  The soils at Clifton Court are generally stiff (Vs = 294.)  The soils at Bacon Island are generally soft (Vs = 176.)

Woodward quadrangle

Perhaps the sediments Clifton Court Forebay increase in density with depth?   The graphs below show shear velocity vs depth in two boreholes at Bacon Island and Clifton Court, from Kishida 2009. There start to be layers of very dense soil at Clifton Court at about 25 meters (82 feet depth.)  The graph shows essentially no layers of very dense soil for Bacon Island.

Kishida

It is unclear how MWD could have concluded that soils at Clifton Court are “very dense,” except by cherry picking the properties of deep layers of soil.

For evaluation of the seismic performance of the Delta levees in the Delta Risk Management Strategy, on the other hand, DWR’s contractor cherry picked the lowest shear velocity values from specific boreholes.   This was criticized by the US Geologic Survey as not standard engineering practice.

If the DWR’s contractors analyze the seismic performance of the Delta levees as if they were built entirely on soft soils, and MWD’s contractors analyze the performance of the Delta tunnel lining as if it was built entirely in very dense soils, it will likely result in a large over-estimation of the seismic benefits of the Delta tunnel.

An objective, unbiased analysis of the benefits of the Delta tunnel as a seismic upgrade should use the same geotechnical assumptions for both the “without project” conditions and the “with project” conditions.

References

Arup, Seismic Review of Tunnel Liner Performance, Appendix M, Conceptual Engineering Report, California WaterFix Byron Tract Forebay Option,  2018. Available at https://www.waterboards.ca.gov/waterrights/water_issues/programs/bay_delta/california_waterfix/exhibits/docs/petitioners_exhibit/dwr/part2_rebuttal/dwr_1304.pdf

California Geological Survey, Seismic Hazard Zone Report for the Woodward Island 7.5-Minute Quadrangle, Contra Costa County, California 2018.  Available at http://gmw.conservation.ca.gov/shp/EZRIM/Reports/SHZR/SHZR_121_Woodward_Island.pdf

Graizer, V & Shakal, A. Recent Data from CSMIP Instrumented Downhole Arrays. AGU Fall Meeting Abstracts, 2004.  Available at https://www.researchgate.net/publication/252102057_Recent_Data_from_CSMIP_Instrumented_Downhole_Arrays

Hu Jin-jun & Xie Li-li. “Variation of earthquake ground motion with depth,” Acta Seimol. Sin. (2005) 18: 72. DOI:10.1007/s11589-005-0008-x.   Available at http://link.springer.com/article/10.1007/s11589-005-0008-x

Kishida, Tadahiro & Boulanger, Ross & A Abrahamson, Norman & W Driller, Michael & M Wehling, Timothy. Site Effects for the Sacramento-San Joaquin Delta. Earthquake Spectra. (2009) 25 10.1193/1.3111087. Available at https://www.researchgate.net/publication/256932489.

Schaefer, J.  Seismic Review Comments (and Responses) on Technical Memorandum Delta Risk Management Strategy (DRMS) Phase 1 Draft 3 Topical Area: Levee Vulnerability. US Army Corps of Engineers, April 11, 2008.  Available at https://cah2oresearch.com/wp-content/uploads/2019/07/Seismic-Review-Comments-and-Responses.pdf

 


Responses

  1. Thank you. Do you have any information on where the tunnel spoils would be deposied?


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