Whether there is one Delta tunnel or two, the construction of a large diameter tunnel in Delta soils consisting of sedimentary layers of peat, sand, silt, and clay is a significant engineering challenge.
The Highway 99 tunnel in Seattle, bored by “Big Bertha,” was constructed in glacial deposits with layers of sand, silt, and clay similar to those in the Sacramento-San Joaquin Delta. On January 12, 2016, a large sinkhole formed near Big Bertha’s cutterhead, creating major worries about tunneling under Seattle’s Alaskan Way Viaduct (a raised freeway.) Washington Governor Jay Inslee halted the tunnel boring on January 14, 2016, citing concerns over public safety.
The large sinkhole created by Big Bertha shows the problems that can be created by “loss of ground” when tunneling in sedimentary deposits. Even without “loss of ground” there can still be significant settlement on the surface. With Big Bertha, there were extensive precautions to protect public safety. The Alaskan Way Viaduct was closed for two weeks while Bertha tunneled under the freeway. The freeway was shielded from impacts of settlement by buried concrete pilings installed by the tunnel contractors, and there was extensive grouting to protect against further sinkholes. There were also a large number of measuring devices installed to detect settlement of the freeway and buildings in Bertha’s path. The tunneling was completed without further problems.
The current alignment of the WaterFix main tunnels passes under Delta island levees, State Route 4, State Route 12, the BNSF railroad tracks used by Amtrak, the Mokelumne aqueduct, and natural gas and other product and services pipelines. These are all critical infrastructure in the Delta. But measures to protect this infrastructure from tunneling impacts were not identified in the WaterFix environmental documents.
Tunneling under Delta levees poses a large economic risk. The horizontal and vertical stresses on the levees from tunnel boring could cause cracks, especially in levee areas that are prone to slope instability. Cracks in a levee could result in seepage and failure if they happened during times of high flows in the Delta, or if they happened during times of low flow and were not identified and repaired. These risks were recently discussed in a news segment on KCRA Channel 3.
WaterFix tunnel alignment was moved east to avoid tunneling under US Army Corps of Engineer’s levees
The US Army Corps of Engineers requires a Section 408 permit for tunneling under state-federal project levees. The state-federal project levees in the Delta are on the Sacramento River and Andrus Island. (See map below.)
Kern County Water Agency documented that the WaterFix tunnel alignment was moved east to avoid crossing project levees whenever feasible:
As originally configured, the project’s main 40-foot diameter tunnels crossed under numerous rivers, sloughs and other waterways. At each of these locations, additional construction activities would have been necessary to protect the levees that line each of the waterways while the TBMs were being operated, potentially leading to unnecessary project risks […] Mitigation measures employed during the planning and conceptual engineering process attempted to minimize as many of these issues as possible. The current alignment (1) reduces tunneling under most sensitive surface features and private property, instead tunneling under publically [sic] held lands and avoiding crossing U.S. Army Corps of Engineers (USACE) levees wherever feasible; (2) minimizes the number of water features crossing with the tunnel alignment; and (3) reduces the number of tunnel contracts to avoid unnecessary surface disruptions.
(Kern County Water Agency, California WaterFix Overview)
But if no US Army Corps of Engineers permit is required for tunneling under non-project levees, what are the protections?
The Waterfix Final EIR/EIS refers to Settlements induced by tunneling in Soft Ground, by the International Tunneling and Underground Space Association. With respect to tunneling contracts, it states:
It is common practice in France to include in contractual documents for urban underground works, clauses that specify the maximum admissible ground movements in the area of influence of the works. This aims at assigning responsibilities in the case damages are experienced. As a result, damages related to ground movements falling with the contractual thresholds will be covered by the owner, with the contractor being responsible for damages experienced when the threshold values are exceeded.
The early specifications for the Delta tunnels stated that maximum allowable settlement thresholds should be determined for all structures that needed to be protected and included in the contract documents:
Category: Protection of Adjacent Structures and Property
Approach: A survey of all structures and property along or adjacent to the alignment will need to be performed and any property that needs to be protected will need to be identified.
A program of geotechnical instrumentation and monitoring will need to be developed and included in the plans and specifications in order to help evaluate the settlements induced by the tunneling activities.
Maximum allowable settlement thresholds should be determined and included in the contract documents.
The recommendation for inclusion of maximum settlement thresholds in the contract was not disclosed or carried forward. The WaterFix Final Environmental Impact Report / Environmental Impact Statement only states that a recommendations will be made for “tunneling techniques to avoid excessive settlement.”
the engineer’s recommendations would be documented in a detailed geotechnical report, which will contain site-specific evaluations of the settlement hazard associated with the site-specific soil conditions overlying the tunnel throughout the alignment. The report will also contain recommendations for the type of tunnel boring machine to be used and the tunneling techniques to be applied to avoid excessive settlement for specific critical assets, such as buildings, major roads, natural gas pipelines, electrical and communication lines, aqueducts, bridges, levees, and sensitive satellite dish facilities. (Final EIR/EIS, Chapter 9, p. 9-287.)
There is no indication that criteria for maximum allowable settlement to protect structures on the surface will be determined or included in the contracts. The Final EIR/EIS does not even indicate that levees will be considered to be critical infrastructure that needs to be protected during tunneling:
Given the likely design depth of the tunnel, the amount of settlement beneath developed areas and critical infrastructure (i.e., the village of Hood, SR 4 and SR 12, the EBMUD aqueduct, and a potentially sensitive satellite dish facility) would be minor… 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. (Chapter 9, p. 9-288.)
As the Newsom administration grapples with the implications of downsizing the twin tunnels project to a still-massive one tunnel project, these critical safety issues must be addressed. Sweeping them under the rug poses unacceptable risks from a variety of perspectives.