WaterFix: tunneling under levees

Construction of two 40′ diameter tunnels in the deep, soft alluvial soils in the Sacramento San-Joaquin Delta is a significant engineering challenge, particularly since it involves tunnel boring under Delta levees. In 2015, Central Delta Water Agency expressed major concerns that the WaterFix Partially Recirculated EIR/EIS failed to “adequately investigate, discuss or analyze, much less mitigate” the risks to Delta levees from tunnel boring. In 2018, San Joaquin County and the San Joaquin County Flood Control agency put on testimony in the State Water Resources Control Board’s WaterFix Change Petition Hearing that the preliminary geotechnical engineering for the Delta tunnels was inadequate and risks to Delta levees had not been addressed. This blog post examines those concerns.

Chapter 9 of the WaterFix Final EIR/EIS includes a discussion of the risks of settlement from tunnel boring. This is the section:

Impact GEO-3: Loss of Property, Personal Injury, or Death from Ground Settlement during Construction of Water Conveyance Features

Two types of ground settlement could be induced during tunneling operations: large settlement and systematic settlement. Large settlement occurs primarily as a result of over-excavation by the tunneling shield. The over-excavation is caused by failure of the tunnel boring machine to control unexpected or adverse ground conditions (for example, running, raveling, squeezing, and flowing ground) or operator error. Large settlement can lead to the creation of voids and/or sinkholes above the tunnel. In extreme circumstances, this settlement can affect the ground surface, potentially causing loss of property or personal injury above the tunneling operation.

Systematic settlement usually results from ground movements that occur before tunnel supports can exit the shield and the tunnel to make full contact with the ground. Soil with higher silt and clay content tend to experience less settlement than sandy soil. (p. 9-195)

The diagram below illustrates the surface settlement trough from tunnel boring. The WaterFix Final EIR/EIS estimates that the maximum systematic settlement could be up to 2.9 inches, with a settlement trough width of 328 to 525 feet.

The WaterFix Final EIR/EIS does not contain any analysis of the effects of settlement from tunnel boring on the Delta levees, implying that it will be done in the future:

the following federal design manuals and professional society and geotechnical literature would be used to predict the maximum amount of settlement that could occur for site-specific conditions, to identify the maximum allowable settlement for individual critical assets, and to develop recommendations for tunneling to avoid excessive settlement, all to minimize the likelihood of loss of property or personal injury from ground settlement above the tunneling operation during construction. (p. 9-288.)

But the WaterFix Final EIR/EIS does not even designate the Delta levees as “critical infrastructure,” stating only that the critical infrastructure will be determined in the future:

Other facilities that may be determined to be critical infrastructure include natural gas pipelines, the proposed EBMUD tunnel, levees, and local electrical distribution and communication lines. (p. 9-288.)

It is unclear why DWR did not designate the Delta levees as critical infrastructure during the 10 years of preliminary engineering for the WaterFix. The WaterFix Final EIR/EIS does discuss the risks of large ground settlements when tunneling, stating:

Operator errors or highly unfavorable/unexpected ground conditions could result in larger settlement. Large ground settlements caused by tunnel construction are almost always the result of using inappropriate tunneling equipment (incompatible with the ground conditions), improperly operating the machine, or encountering sudden or unexpected changes in ground conditions. (p. 9-288, emphasis added.)

Two civil engineers testified in Part 2 of the Water Board’s WaterFix hearing that highly unfavorable and variable ground conditions can be expected in the Delta, and that there are significant risks from tunnel boring. San Joaquin County and the San Joaquin County Flood Control and Water Conservation District put on testimony by Josef Tootle, the Principal Geotechnical Engineer with ENGEO Incorporated, who has 20 years of experience with geotechnical design for projects in the Delta. Tootle testified that recent geotechnical exploration in the Delta by the University of Texas showed that

… the sediments underlying the Delta were both softer and more variable than the engineering consultant had expected. The sediments were in fact so variable that the University of Texas had difficulty interpreting the results at depths greater than 100 feet. (p. 9.)

Tootles’ testimony was corroborated by Chris Neudeck, the District engineer and Local Agency Representative for 26 of the Reclamation Districts in the Delta. Neudeck’s testimony related experience with two “near-misses” in tunnel boring in the Delta. The first was in boring of a 50 inch diameter sewer line for the City of Stockton Municipal Utilities Department beneath Shima Tract’s levee, and the second in boring a 72 inch diameter interconnect pipeline to Contra Costa Water District’s second Delta intake on Victoria Canal. While the WaterFix tunnels are much deeper, they are also much larger.

Tootle also testified that the preliminary engineering for the WaterFix project is inadequate because the geotechnical investigations to date do not meet accepted standards for preliminary engineering:

…the geotechnical site investigations to date do not meet the accepted standards for a project of any size, let alone a major project in the Delta. A registered civil engineer in the State of California who proceeded to detailed design using only the presently available data would likely be judged to not be acting in accordance with the generally accepted standard of care. And, without more detailed designs, the Petitioners’ assurances on multiple issues appear to be meaningless (p. 8.)

Tootle concluded that there is significant risk in tunneling under Delta levees, stating:

there can be little assurance (and Petitioners have provided none) that the ambitious tunneling activities at critical locations, such as under levees, will not result in serious injury to the integrity of the Delta’s complex levee system and other infrastructure essential to public safety and economic productivity in Delta communities (p. 7.)

Tom Williams, a PhD geologist who has consulted on tunnel and pipeline projects all over the world, raised similar concerns in testimony in Part 1 of the WaterFix hearing.about the inadequacy of DWR’s geotechnical site investigations, and the deferment of mitigations for risk to levees and other critical structures.

DWR’s 2010 internal, unreleased preliminary engineering report shows that independent reviewers of the tunnel design recommended that DWR move the tunnel alignment to the east to provide better ground conditions for constructing the tunnel shafts and boring the tunnels. The document shows that the recommendation was rejected because of cost considerations:

Relocating the tunnel to the eastern side of the Delta would put it outside the Conveyance Planning Area and would in all probability cause a delay in the environmental process. An eastern alignment for the tunnel would also be much longer and would cost between $1 billion and $1.5 billion more than the current alignment.

The document also states:

Not enough is known about the geology in the eastern alignment to say that it much better than the current alignment.

The 2010 recommendations of the independent reviewers to move the tunnel alignment to the east were not disclosed by DWR.

DWR’s construction schedule for the WaterFix, included in DWR’s Joint Exercise of Powers Agreement, may increase risks to the Delta levees from tunnel boring. The construction schedule assumes continuous boring of the Delta tunnels, with no breaks for winter months or for times of high water. The continuous construction schedule saves time and money by avoiding winter work stoppages, but at the cost of greatly increased impacts should the integrity of the Delta levees be compromised.

1 Tyler Island repairs during high water in January 2017

The WaterFix Final Draft Conceptual Engineering Report has a safety plan, but it just requires construction of ring levees around the tunnel shafts to protect the tunnels from flooding. There is no safety plan addressing risks to people on Delta islands in the event of a levee breach during tunnel construction.

The WaterFix Final EIR/EIS CEQA conclusion only addresses hazards to WaterFix construction workers and project structures, stating:

DWR has made conformance to geotechnical design recommendations and monitoring an environmental commitment (see Appendix 3B, Environmental Commitments, AMMs, and CMs). Hazards to workers and project structures would be controlled at safe levels and there would be no increased likelihood of loss of property, personal injury or death due to construction of Alternative 4A. The impact would be less than significant. No mitigation is required (p. 9-289.)

Chris Neudeck testified in the Water Board’s WaterFix hearing that the Department of Water Resources had never contacted the Delta Reclamation Districts he represents about monitoring of settlement of the levees during tunneling boring or mitigation of risks.

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