Posted by: Deirdre Des Jardins | October 8, 2017

WaterFix tunnel lining could leak in a large earthquake in the Delta

The construction of two forty foot diameter tunnels in soft soils consisting of sedimentary layers of sand and peat is a significant engineering challenge. Given the large diameter of the tunnels, the amount of water they will be carrying, and the sedimentary deposits surrounding the tunnels, significant preliminary engineering is required to document that the proposed conceptual design will have sufficient structural integrity to protect the WaterFix tunnels, the water supply, and structures and people on the surface.

1 Sand and silt deposits go down 1,000s  of feet in the Delta     Source: Unruh 2009

California Water Research did an extensive review of the WaterFix tunnel design with Tom Williams, an expert who has consulted on large tunnel and pipeline projects around the world. An examination of DWR’s preliminary engineering documents showed not only that the preliminary engineering is insufficient, but also that cost-cutting could compromise both the lifetime of the WaterFix tunnels and their ability to withstand a large earthquake in the Delta. The Validation Complaint for the WaterFix also has a clause requiring the State Water Project Contractors to pay the WaterFix revenue bond costs whether or not the WaterFix project is completed or maintained in repair. (DWR 2017, p. 10.) This does not exactly inspire confidence.

Tunnel lining design

The WaterFix tunnel lining is proposed to be built out of large segmented concrete pipes. This segmented design, with no inner liner, was chosen on the basis of cost and time to construct, not on strength. (DWR 2010a, p. 9.)


Tunnel segments Source: DWR 2010

A preliminary engineering analysis by the Department of Water Resources showed that the tunnel lining joints could leak in a maximum earthquake in the Delta. (DWR 2010, p. 4-13. ) DWR’s engineers recommended that the option of a second steel liner be kept until the feasibility of the single-pass design was proven. (DWR 2010, p. 10-1. ) A second steel liner would significantly raise the cost of the tunnels and was not kept as an option. Instead the seismic source assumptions in the published WaterFix Conceptual Engineering Report were weakened to a 5% in 50 year event. (DWR 2015b, p. 31.) Seismic source assumptions for the liquefaction analysis were weakened to a 10% in 50 year event. (DWR 2015b, p. 49.) These weakened assumptions mean that the preliminary engineering does not even consider whether the WaterFix tunnels would be repairable after a maximum earthquake in the Delta.

Settlement could also cause the tunnel lining segments to move relative to one another, opening up gaps at the circumferential joints over time. This has caused a shortened expected lifetime for tunnels in deep sedimentary soils in Shanghai. (Xu et. al. 2011.)  Leaks also progressively increase the forces pulling the tunnel segments apart (Yoo 2016.)  The preliminary WaterFix tunnel design does not provide any criteria for maximum differential settlement of the tunnel segments under long term operations. East Bay MUD commented on the tunnel design in 2015, stating:

Long-term degradation of segmental concrete lining may result in failure of the lining. In the event that the tunnel lining fails and results in a tunnel collapse or blowout, a collapse during operations would result in major ground movement extending to the ground surface and potentially sinkholes or blowout.

This potential leakage is of particular concern where the tunnels pass under important structures, including Delta island levees and channels, the Stockton Deep Water Shipping Channel, the Mokelumne aqueduct, and natural gas and other product and services pipelines. Failure under a levee or channel could result in catastrophic flooding, endangering human life. The Final Draft WaterFix Conceptual Engineering Report does not indicate an inspection, monitoring, and remediation program and does not discuss contingencies, controls, and recovery following indication and evidence of leakage of the tunnel lining.

An honest assessment of the benefits of the WaterFix tunnels would disclose the vulnerability of the proposed tunnel lining design to settlement and seismic forces, and provide a long-term inspection, monitoring, and remediation plan for leakage and movement. An honest assessment would also analyze and disclose the expected time to repair the tunnel lining in a maximum earthquake.

References

Department of Water Resources, 2017, Complaint for Validation. Available at http://cms.capitoltechsolutions.com/ClientData/CaliforniaWaterFix/uploads/CWF_Validation_Complaint_.pdf

Department of Water Resources, 2015a, Final Draft Agreement Regarding Construction of Conveyance Project between the Department Of Water Resources and the Conveyance Project Coordination Agency. Available at http://cms.capitoltechsolutions.com/ClientData/CaliforniaWaterFix/uploads/Draft_Final_DCE_Agreement_Combined.pdf

Department of Water Resources, 2015b, Final Draft Conceptual Engineering Report for the Modified Pipeline/Tunnel Option (MPTO). Available at http://www.waterboards.ca.gov/waterrights/water_issues/programs/bay_delta/california_waterfix/exhibits/docs/petitioners_exhibit/dwr/dwr_212.pdf.

Department of Water Resources, 2010, Draft Report Of the Initial Analysis & Optimization of the Pipeline/Tunnel Option. Available at http://www.waterboards.ca.gov/waterrights/water_issues/programs/bay_delta/california_waterfix/exhibits/docs/dd_jardins/DDJ-141%20Initial.pdf

Unruh, Jeffrey, and Hitchcock, Christopher, 2009, Characterization of Potential Seismic Sources in the Sacramento-San Joaquin Delta, California. Available at
https://www.waterboards.ca.gov/waterrights/water_issues/programs/bay_delta/california_waterfix/exhibits/docs/dd_jardins/DDJ-142_Unruh.pdf

Williams, Clyde Thomas, 2017, Testimony for WaterFix Change Petition Hearing. Available at
https://www.waterboards.ca.gov/waterrights/water_issues/programs/bay_delta/california_waterfix/exhibits/docs/dd_jardins/DDJ-163_tw_testimony.pdf

Yoo, Chungsik, 2016, Effect of water leakage in tunnel lining on structural performance of lining in subsea tunnels, Marine Georesources & Geotechnology Vol. 35 , Iss. 3.   Available at http://www.tandfonline.com/doi/abs/10.1080/1064119X.2016.1162235

Xu, Yeshuang & Ma, L & Shen, Shui-Long, 2011, Influential factors on development of land subsidence with process of urbanization in Shanghai. Yantu Lixue/Rock and Soil Mechanics. 32. 578-582. Available at https://www.researchgate.net/publication/288360364_Influential_factors_on_development_of_land_subsidence_with_process_of_urbanization_in_Shanghai



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