San Dieguito to Sorrento Valley Double Track-Del Mar Tunnels Alternatives Analysis Report Review Notes

San Dieguito to Sorrento Valley Double Track-Del Mar Tunnels Alternatives Analysis Report

Prepared for San Diego Association of Governmets by HDR August 2023

The Alternatives Document here: https://www.sandag.org/-/media/SANDAG/D ... -09-01.pdf

Abstract

This Alternatives Analysis Report documents the analysis of conceptual alignment alternatives for relocating the existing singletrack alignment of the Los Angeles-San Diego-San Luis Obispo Rail Corridor through the City of Del Mar and City of San Diego. Five alignment alternatives were refined based on a previous conceptual engineering study and evaluated against a set of performance criteria. Two of these alternatives were advanced to 10 percent conceptual engineering and were further analyzed for engineering and environmental considerations. Based on feedback from stakeholders and community groups, four additional potential tunnel portal locations were then also evaluated to further minimize impacts on the community and private properties. Additional conceptual alignments were looked at from a high level to demonstrate potential connection between various portal locations.

Initial Alternative Summary:

• CDM - A Tunnel under Camino Del Mar (a road) between the Del Mar Fairgrounds (Milepost 243) and the Los Penasquitos Lagoon.
• CCHS - A tunnel further west between the Del Mar Fairgrounds, under a good chunk of San Diego, all the way to MP 248.
• A tunnel still further west, with the southern end aligning with I-5 (with various points to place the opening as variants)
• A tunnel that hugs or is under I-5 for most of the way south.

The first two got forwarded to the next round of study.

Summary of the Report

The key points from the San Dieguito to Sorrento Valley Double Track - Del Mar Tunnels Alternatives Analysis Report are:

• SANDAG is studying alternatives for relocating the existing single-track rail alignment of the LOSSAN corridor between MP 242.8 to 247.8 in Del Mar and San Diego from the coastal bluffs to a new double-track tunnel alignment. This will enhance safety, reliability and capacity.
• Five conceptual tunnel alignment alternatives were evaluated 
• Camino Del Mar (CDM), 
• Crest Canyon Higher Speed (CCHS), 
• Crest Canyon Above Carmel Valley Road
• Crest Canyon Below Carmel Valley Road, and 
• Interstate 5 (I-5).
• CDM and CCHS were advanced to 10% conceptual engineering based on an alternatives evaluation process considering factors like travel time, costs, environmental impacts, right-of-way needs, etc.
• The 10% designs were then optimized to minimize impacts by reducing the tunnel diameter and modifying the track elevation in relation to sea level rise. Additional north and south portal locations were also assessed.
• The project will significantly improve rail operations by straightening curves to allow speeds up to 110 mph, eliminating a major single-track bottleneck, and planning for sea level rise impacts. Travel time savings of 1.8-4.5 minutes are estimated.
• The preliminary construction cost estimates are $1.56 billion for CDM and $1.48 billion for CCHS (2022 dollars). Construction is estimated to take up to 7 years.
• Next steps include selecting a preferred alternative, conducting preliminary engineering and environmental reviews, securing full funding, and amending rail operating agreements to allow the reduced tunnel size. Construction could be completed by 2035.  

Selection Criteria and Alternative Scores

The route selection criteria, their weightings, and the scores of the various alternatives are provided in Table 3-14 of the alternatives analysis report. The criteria and weightings were developed through a workshop process with the Project Development Team (PDT), which included representatives from SANDAG, NCTD, MTS, LOSSAN, SCRRA, Caltrans, FRA, and BNSF.

The PDT initially identified a list of potential criteria and then selected 11 criteria to use in evaluating the alternative alignments. Each agency represented on the PDT then independently ranked the criteria in order of importance from 1 to 11. The rankings were averaged to determine the overall weighting of each criterion, as shown in the "Weight" column of the table.

The evaluation criteria used in the alternatives analysis were a combination of quantitative and qualitative factors, with scores assigned based on a mix of technical analysis and engineering judgment by the Project Development Team (PDT). The report provides some additional information on how each criterion was defined and evaluated, as summarized below:

1. Travel time (14% weight): Quantitative measure of the estimated reduction in travel times through the project area, based on the proposed alignment geometry, grades, and station locations. Travel time estimates were developed using the rail operations modeling software Viriato.
2. Potential environmental impacts (9% weight): Qualitative assessment of the potential impacts on environmentally sensitive areas, mitigation requirements, permitting schedule, and reduction in greenhouse gas emissions. Scores were based on the PDT's prior experience with similar rail projects in the region.
3. Potential right-of-way (ROW) impacts (6% weight): Quantitative and qualitative assessment of the potential temporary and permanent impacts on public and private properties, acquisition costs, and schedule. Scores were based on a preliminary analysis of the affected properties along each alignment alternative.
4. Constructability and duration (7% weight): Qualitative assessment of the construction complexity, geotechnical risks, tunneling methods, and schedule implications of each alternative. Scores were based on the PDT's prior experience with similar projects and a comparison of the potential construction challenges.
5. Other community impacts (4% weight): Qualitative assessment of the potential impacts on local communities, including noise and vibration, visual impacts, and access to beaches and recreational areas. Scores were based on the PDT's knowledge of community concerns and a comparison of the alternatives.
6. Connectivity and travel demand (13% weight): Qualitative assessment of the potential for each alternative to provide connectivity to future mobility hubs and transit services, as identified in SANDAG's 2021 Regional Plan. Scores were based on a comparison of the alternatives in terms of their integration with the regional transportation network.
7. Safety improvements (15% weight): Qualitative assessment of the potential safety benefits of each alternative, such as the elimination of at-grade crossings and the reduction of flood risks. Scores were based on a comparison of the safety features and risk factors associated with each alternative.
8. Capital costs (8% weight): Quantitative measure of the estimated construction costs of each alternative, based on unit prices from the Federal Transit Administration (FTA) cost database and recent projects in the region. Scores were based on a comparison of the total capital costs, including right-of-way and mitigation.
9. Railroad operations during construction (5% weight): Qualitative assessment of the potential impacts on existing rail services during construction, based on the proposed phasing and track capacity. Scores were based on the PDT's review of conceptual phasing plans and a comparison of the alternatives.
10. Operational complexity post-construction (9% weight): Qualitative assessment of the long-term operational and maintenance requirements of each alternative, including tunnel systems, ventilation, and access. Scores were based on the PDT's prior experience with similar rail tunnels and a comparison of the alternatives.
11. Operations and maintenance costs (10% weight): Qualitative assessment of the potential differences in long-term operating and maintenance costs between the alternatives, considering factors such as tunnel systems, bridges, and drainage. Scores were based on the PDT's prior experience and a comparison of the alternatives.

While some of the criteria, such as travel time and capital costs, were evaluated using quantitative methods and data, others relied more heavily on qualitative assessments and engineering judgment, given the conceptual nature of the alternatives and the limited design information available at this stage of the project.

The scores assigned to each alternative for each criterion were determined through a consensus process by the PDT, drawing on their technical expertise, local knowledge, and prior experience with similar projects in the region. The use of a weighted scoring system allowed the PDT to consider a broad range of factors and to prioritize the criteria that were deemed most important for the project's goals and objectives.

It's important to note that the evaluation process documented in the alternatives analysis report represents an initial screening and comparison of the conceptual alternatives, and that more detailed technical studies and stakeholder input will be required in future project phases to refine the design, assess the impacts, and confirm the preferred alternative. The scoring and weighting of the criteria may also evolve as new information becomes available and as the project priorities are further defined through the environmental review and public involvement process.

The alternative alignments were then scored against each criterion on a scale of 1 to 5, with higher scores indicating better performance. The scores were assigned by the PDT through a consensus process, considering the relative advantages and disadvantages of each alternative.

Based on the weighted criteria and scores, 

• the Crest Canyon Higher Speed alternative received the highest total score of 396, 
• followed by the Crest Canyon Above Carmel Valley Road alternative with a score of 347. 
• The Camino Del Mar (CDM) alternative ranked third with a score of 345.

The report notes that while the Crest Canyon Above Carmel Valley Road alternative scored second highest, the PDT decided to advance the CDM alternative instead, as it represented a distinct alignment option from the Crest Canyon alternatives and had only a slightly lower score.

The I-5 alternative received the lowest total score of 223, primarily due to its lower performance on criteria such as travel time, construction impacts, and capital costs.

Rejected Alternatives

The report discusses three alternatives that were not advanced for further study at the 10% conceptual engineering level: Crest Canyon Above Carmel Valley Road, Crest Canyon Below Carmel Valley Road, and Interstate 5 (I-5). The key factors that led to these alternatives not being selected are:

• 1. Crest Canyon Above Carmel Valley Road:
• Although it scored second highest in the alternatives evaluation, it was not selected because the Project Development Team decided to advance one Crest Canyon alternative and one non-Crest Canyon alternative. The Crest Canyon Higher Speed alternative scored higher and was selected instead.
• Based on the conceptual engineering and preliminary analysis, this alignment could potentially impact the greatest number of private properties and have the highest right-of-way costs compared to the other alternatives.
• 2. Crest Canyon Below Carmel Valley Road:
• Requires tunneling through a liquefaction zone
• More complex operations and higher maintenance costs due to the need for a sump pump because of the vertical profile
• Track profile at the south portal is below the 500-year flood elevation and 100-year flood elevation plus sea level rise, not meeting design criteria and raising safety concerns
•  Second highest construction costs at approximately $2.62 billion
• 3. Interstate 5:
• Slowest travel times due to the length of the alignment and speeds limited to 80/60 mph
• Would require reconstructing approximately 900 feet of the future San Dieguito double track bridge
• Highest capital costs at approximately $3.07 billion, significantly more than the other alternatives
• Challenging construction phasing due to impacts on the San Dieguito Bridge, increasing risk to maintaining rail operations
• Requires a Caltrans permit to tunnel parallel to and under I-5, with risk of delays

In summary, these alternatives were not advanced primarily due to higher costs, greater property impacts, more challenging construction, and/or reduced operational benefits compared to the CDM and CCHS alternatives.

 Continuing Alternatives

The two alternatives selected for further study at the 10% conceptual engineering level were the Camino Del Mar (CDM) and the Crest Canyon Higher Speed (CCHS) alternatives. Here's a summary of their key advantages and disadvantages:

Camino Del Mar (CDM):

• Advantages:
• Tied for fastest travel time (1.8 to 4.5-minute savings compared to current alignment)
• No impact on the proposed San Dieguito Bridge replacement
• Maximizes use of the existing rail corridor within the lagoon
• Lowest potential right-of-way costs/impacts
• Eliminates the at-grade crossing at Coast Boulevard
• Lowest capital costs at approximately $2.39 billion
• Disadvantages:
• Requires major realignment of Jimmy Durante Boulevard
• Requires Carmel Valley Road to be re-routed or temporarily closed during construction
• Requires bridge and berm construction adjacent to the live railroad through Los Peñasquitos Lagoon
• Has the longest length of the alignments in the lagoon, providing fewer opportunities for lagoon rehabilitation
• Potential visual impacts due to the need to grade separate Jimmy Durante Boulevard
• Higher maintenance costs compared to others due to the longest length of bridge in the saltwater lagoon

Crest Canyon Higher Speed (CCHS):

• Advantages:
• Tied for fastest travel time (1.8 to 4.5-minute savings compared to current alignment)
• No impact on the proposed San Dieguito Bridge replacement
• Eliminates the at-grade crossing at Coast Boulevard
• Second lowest capital costs at approximately $2.52 billion
• Minimizes impacts on the lagoon due to shorter length of alignment within the lagoon
• Disadvantages:
• Requires major realignment of Jimmy Durante Boulevard
• Potential visual impacts from the need to grade separate Jimmy Durante Boulevard
• Property interests outside of the existing railroad right-of-way could be required, particularly at the north and south portal locations. The proposed south portal is located on an undeveloped parcel with an open space easement.

Both alternatives provide significant travel time improvements and eliminate the at-grade crossing at Coast Boulevard. However, they both require realignment of Jimmy Durante Boulevard and have potential visual impacts. The CDM alternative has lower costs but more impacts on the lagoon, while the CCHS alternative has slightly higher costs but fewer lagoon impacts and passes under more private property.

Next Steps

The report outlines several next steps and future considerations for advancing the San Dieguito to Sorrento Valley Double Track project. The key next steps include:

1. Further evaluate the portal locations and refine the assumptions of the conceptual alternatives prior to selecting the alternatives to be carried forward into the preliminary engineering and environmental phase.

2. Conduct preliminary engineering tasks, such as:
   - Refining the single and twin bore tunnel configurations
   - Refining tunnel portal footprints and identifying opportunities to reduce impacts
   - Evaluating alternative delivery methods and determining the preferred contracting approach
   - Developing draft construction phasing plans and schedules
   - Conducting geotechnical investigations and collecting additional data on subsurface conditions, groundwater, and seismic risks
   - Updating the Basis of Design Report and LOSSAN design criteria
   - Preparing a bridge type selection report

3. Conduct environmental studies and reviews, including:
   - Federalizing the project to begin the NEPA process and coordinating with the federal lead agency
   - Conducting cultural resources, hazardous materials, and biological surveys
   - Preparing habitat assessment maps and mapping the limits of waters of the U.S. and California Coastal Commission wetlands
   - Developing a robust risk analysis

4. Engage stakeholders and the public to gather input and feedback on the project alternatives, potential impacts, and mitigation measures.

5. Secure full funding for final design, right-of-way acquisition, and construction phases of the project.

6. Amend the shared use agreement between BNSF, NCTD, and MTS to allow for the smaller tunnel clearance criteria proposed in the conceptual designs.

7. Complete final design, obtain necessary permits and approvals, and proceed to construction, with the goal of completing the project by 2035 as envisioned in SANDAG's 2021 Regional Plan.

These next steps will involve refining the project design, assessing environmental impacts, complying with regulatory requirements, securing funding, and engaging with stakeholders and the public to move the project forward.

Geology and Seismicity

 The alternatives analysis report used available geotechnical data and studies to assess subsurface conditions, groundwater, and seismic risks for the conceptual tunnel alignments. The report references two geotechnical reports prepared specifically for the project:

1. A geologic desk study and geologic reconnaissance performed by Leighton Associates for the Del Mar alternative tunnel alignments (Appendix E of the report).

2. A limited site investigation consisting of four borings, laboratory tests, in-situ tests, and seismic Primary and Secondary (P and S) wave tests performed by Earth Mechanics Inc.

The geologic reconnaissance report by Leighton Associates identified the anticipated geologic setting, material engineering characteristics (such as erodibility, expansion potential, corrosivity, excavation difficulty, and slope stability), and potential seismic hazards along the alternative alignments. It noted that the alignments are situated within the Peninsular Ranges geomorphic province, characterized by uplifted terraces and coastal sedimentary rock.

Regarding groundwater, the report used data from the limited borings and anticipated that groundwater would be encountered just above the contact with the Delmar Formation, likely closer to sea level with tidal fluctuations. A high groundwater elevation of +10 feet above mean sea level and a low groundwater elevation of 0 feet below mean sea level were recommended for design.

The report also discussed potential ground behavior and risks during tunneling, such as ground settlement, tunnel boring machine (TBM) clogging potential, abrasive ground conditions, and the presence of geologic faults or fractures. It noted that additional geotechnical investigations would be needed to better characterize subsurface conditions and inform tunnel design and construction.

While these initial geotechnical studies provided a basis for assessing the general feasibility of the tunnel alternatives, the report acknowledges that more detailed surveys and analyses will be required in future project phases. The next steps outlined in the report include:

• - Determining the geologic subsurface conditions along the proposed tunnel alignments at depth
• - Collecting additional groundwater data using piezometers along the proposed tunnel alignment
• - Collecting data to document variability within subsurface geologic formations
• - Conducting additional investigations of mapped faults crossing the proposed alignment to determine impacts on tunnel design and construction
• - Performing additional in-situ testing to develop subsurface design parameters, including soil modulus and permeability values
• - Collecting additional shear wave velocity data to characterize the subsurface conditions for design and construction

In summary, while some initial geological and seismic surveys were conducted to assess the general feasibility of the tunnel alternatives, more detailed investigations are planned in future project phases to inform the selection of the preferred alternative and support the tunnel design and construction planning.

The alternatives analysis report acknowledges that the project area is located in a geologically unstable and seismically active region, which could affect the feasibility and design of the proposed tunnel alignments. The report discusses several geological and seismic considerations that were taken into account in the initial assessment of the alternatives:

1. Geologic setting: The proposed tunnel alignments are situated within the Peninsular Ranges geomorphic province, which is characterized by uplifted terraces and coastal sedimentary rock. The tunnels would primarily encounter the Torrey Sandstone and Delmar Formation, which consist of sandstone, siltstone, and claystone. The presence of faults, fractures, and variable rock conditions could impact tunnel construction and stability.
2. Seismic hazards: The project area is subject to potential seismic hazards, including strong ground shaking, fault rupture, liquefaction, settlement, and ground deformation. The report notes that these hazards would need to be mitigated through appropriate seismic design practices and adherence to applicable codes and standards.
3. Ground behavior and risks during tunneling: The report discusses several potential risks and challenges associated with tunneling in the project area, such as ground settlement, tunnel boring machine (TBM) clogging, abrasive ground conditions, and the presence of mixed face conditions or groundwater. These factors could affect the selection of tunneling methods, equipment, and support systems, as well as the need for ground improvement or other mitigation measures.
4. Portal stability: The proposed tunnel portal locations may be subject to slope instability, groundwater seepage, and the presence of weak or expansive soils. The report notes that slope stabilization measures, such as ground anchors or shotcrete, may be required to ensure the long-term stability of the portal areas.

While the initial geotechnical studies and assessments conducted for the alternatives analysis provide a basis for evaluating the general feasibility of the proposed tunnel alignments, the report emphasizes that more detailed investigations and analyses will be needed in future project phases to fully characterize the geological and seismic conditions and inform the selection of the preferred alternative.

Some of the key next steps and future considerations related to geology and seismic risks include:

• Conducting additional subsurface investigations, including borings, geophysical surveys, and laboratory testing, to better define the geologic conditions along the proposed tunnel alignments
• Performing detailed seismic hazard analyses to quantify the potential risks and develop appropriate design criteria and mitigation measures
• Evaluating the feasibility and suitability of different tunneling methods, support systems, and ground improvement techniques based on the anticipated ground conditions and construction risks
• Assessing the long-term performance and maintenance requirements of the tunnels and portal structures, considering factors such as ground deformation, groundwater infiltration, and seismic loading

The specific impacts of the geological and seismic conditions on the feasibility and design of the proposed tunnels will depend on the results of these further studies and analyses. It's possible that some of the alternative alignments may prove to be more challenging or costly to construct than others based on the site-specific geological and seismic constraints. The selection of the preferred alternative will need to balance these technical considerations with other factors such as environmental impacts, right-of-way requirements, and community preferences.

Geological and seismic considerations were among the factors used in the evaluation and selection of the alternative tunnel alignments. However, based on the information provided in the alternatives analysis report, it appears that these considerations played a relatively limited role in the initial screening and ranking of the alternatives, as compared to other criteria such as travel time, environmental impacts, right-of-way requirements, and costs.

The report describes the process used by the Project Development Team (PDT) to develop and weight the evaluation criteria for the alternative alignments. The PDT selected 11 criteria, which were then ranked by each participating agency to determine their relative importance. The top-weighted criteria included:

1. Travel time (14%)
2. Safety improvements (15%)
3. Connectivity and travel demand (13%)
4. Operations and maintenance costs (10%)

Geological and seismic factors were not explicitly listed among the 11 evaluation criteria, although they may have been implicitly considered under broader categories such as "constructability, construction impacts, and duration" (weighted at 7%) or "operational complexity" (weighted at 9%).

The report does mention that the initial geotechnical studies and assessments, including the geologic reconnaissance and limited site investigations, were used to support the conceptual engineering and feasibility analysis of the alternative alignments. However, it's not clear from the information provided how much these geological and seismic considerations influenced the scoring and ranking of the alternatives in the evaluation matrix.

It's possible that the geological and seismic risks were considered to be relatively similar across the alternative alignments at this conceptual level of analysis, based on the available data and the general characteristics of the project area. As noted in the report, more detailed geotechnical investigations and seismic hazard analyses will be needed in future project phases to fully assess the impacts of these factors on the feasibility, design, and construction of the preferred alternative.

In summary, while geological and seismic considerations were taken into account in the initial assessment of the alternative tunnel alignments, they do not appear to have been a primary driver in the selection process at this stage of the project. The evaluation and ranking of the alternatives relied more heavily on other criteria related to transportation benefits, environmental impacts, and costs. As the project moves forward, it will be important to give greater weight to the geological and seismic factors in the refinement and optimization of the preferred alternative, to ensure that the final tunnel design is safe, constructible, and resilient to the long-term geological and seismic risks in the project area.

Train Speed and Vibrations

The alternatives analysis report discusses the anticipated train speeds and potential vibration impacts for the proposed tunnel alignments.

Train Speeds:

The report states that the conceptual tunnel alignments were designed to accommodate maximum speeds of 110 miles per hour for passenger trains and 60 miles per hour for freight trains, where possible, based on the proposed track geometry and tunnel configurations. These higher speeds are intended to support the project's goals of reducing travel times and increasing capacity on the LOSSAN rail corridor.

The operational analysis in the report estimates that the proposed tunnel alignments could result in travel time savings of 1.8 to 4.5 minutes between Solana Beach and Sorrento Valley, compared to the existing alignment. The specific travel time savings would depend on the service scenario (all-stop or limited-stop) and the train technology (diesel-electric locomotive or zero-emission multiple unit).

Ground Vibration:

The report includes a summary of a noise and vibration analysis conducted by Entech Consulting Group (Appendix F). The analysis found that the proposed tunnel alignments could have potential noise and vibration impacts on surrounding land uses at the portal locations, although the extent of these impacts is not quantified in detail.

The report notes that several design features could help mitigate potential noise and vibration impacts:

- The at-grade sections at the tunnel entrances would consist of U-walls and cut-and-cover structures, which could reduce noise and vibration levels.
- The track design would incorporate features such as acoustic absorption under the trainsets, tangent track with high-resilience fasteners, and smooth track surfaces to minimize noise and vibration.
- Tunnel ventilation systems at the portals would require attenuators, enclosures, and other abatement features to meet local noise ordinances.

The analysis also states that potential vibration levels and ground-borne noise are not anticipated to exceed Federal Transit Administration (FTA) and Federal Railroad Administration (FRA) threshold levels for trains operating below the ground surface (i.e., within the tunnels).

However, the report does not provide specific data on the intensity or frequency of ground vibrations that would be generated by train operations in the proposed tunnels. More detailed noise and vibration modeling and analysis would likely be conducted in future project phases to quantify the potential impacts and develop appropriate mitigation measures.

It's worth noting that the depth of the tunnels (ranging from 130 to 290 feet below ground surface for the conceptual alignments) could help attenuate ground-borne vibration from train operations, as vibration levels generally decrease with distance from the source. However, site-specific factors such as soil conditions, tunnel design, and the proximity and sensitivity of receptors would need to be considered in a more detailed assessment of potential vibration impacts.

Tunnels

The alternatives analysis report provides some initial information on the characteristics and dimensions of the proposed tunnels for the CDM and CCHS alternatives, which were advanced to 10% conceptual engineering:

Camino Del Mar (CDM) Alternative:

• - Approximately 1.8 miles (9,950 feet) of twin bored tunnels
• - 33-foot internal diameter for each tunnel bore
• - 70-foot track center spacing between the two bores
• - Depths of up to 130 feet below existing ground to the top of the tunnel
• - Cross passages constructed every 800 feet between the two running tunnels for emergency evacuation

Crest Canyon Higher Speed (CCHS) Alternative:

• - Approximately 2.6 miles (13,625 feet) of twin bored tunnels
• - 33-foot internal diameter for each tunnel bore
• - 70-foot track center spacing between the two bores
• - Depths of up to 290 feet below existing ground to the top of the tunnel
• - Cross passages constructed every 800 feet between the two running tunnels for emergency evacuation

After further design refinements to minimize impacts, the tunnel diameters were reduced to 28 feet, and the track center spacing was reduced to 56 feet for both alternatives.

The report notes that tunnel boring machines (TBMs) are considered the most appropriate excavation method for the majority of the tunnel alignments, given the anticipated ground conditions (primarily sedimentary rock of the Torrey Sandstone and Delmar Formations). The cross passages would be mined using sequential excavation method (SEM) techniques.

Regarding equivalent tunnels constructed in California, the report does not provide specific examples or details on construction time, equipment, and resources. However, there are several notable tunnel projects in California that could serve as reference points:

1. Devil's Slide Tunnels (San Mateo County, CA): This project involved the construction of two 4,200-foot-long, 30-foot-diameter tunnels for Highway 1. The tunnels were excavated through granite and sedimentary rocks using the New Austrian Tunneling Method (NATM). Construction began in 2007 and was completed in 2013.
2. Caldecott Tunnel Fourth Bore (Alameda and Contra Costa Counties, CA): This project added a fourth bore to the existing Caldecott Tunnel on Highway 24. The new bore is 3,389 feet long and 41 feet in diameter. It was excavated using a 52-foot-diameter earth pressure balance TBM through sedimentary and volcanic rocks. Construction began in 2010 and was completed in 2013.
3. Central Subway Tunnels (San Francisco, CA): This ongoing project involves the construction of 1.7 miles of twin-bore tunnels for the San Francisco Municipal Transportation Agency's Central Subway light rail extension. The tunnels are being excavated using two 21.5-foot-diameter earth pressure balance TBMs through a mix of soft ground and rock. Construction began in 2013 and is expected to be completed in 2022.

While these projects provide some context for tunnel construction in California, the specific time, equipment, and resources required for the San Dieguito to Sorrento Valley Double Track tunnels will depend on various factors, such as the final tunnel dimensions, ground conditions, construction methods, and site constraints. More detailed planning and analysis will be needed in future project phases to develop accurate estimates of construction duration and resource requirements.

Impact on Fairgrounds and Jimmy Durante Blvd.

The alternatives analysis report discusses the potential impacts on Jimmy Durante Boulevard and access to the Del Mar Fairgrounds for the Camino Del Mar (CDM) and Crest Canyon Higher Speed (CCHS) tunnel alternatives, which were advanced to 10% conceptual engineering.

For both alternatives, the proposed tunnel alignments would cross beneath Jimmy Durante Boulevard near the northern portal location. To accommodate this crossing and maintain adequate vertical clearance for the tunnels, the conceptual designs include raising the profile of Jimmy Durante Boulevard and constructing a roadway bridge over the tunnel approach.

Camino Del Mar (CDM) Alternative:
- The northern tunnel portal would be located just south of the intersection of CDM and Jimmy Durante Boulevard.
- Jimmy Durante Boulevard would be raised by up to 16 feet to pass over the cut-and-cover section of the tunnel approach.
- Retaining walls would be required along both sides of the raised roadway to minimize impacts on adjacent properties and maintain access to the existing railroad right-of-way during construction.
- The conceptual design maintains the existing width and horizontal alignment of Jimmy Durante Boulevard, although the report notes that further optimization of the roadway configuration and intersection layout may be considered in future project phases.

Crest Canyon Higher Speed (CCHS) Alternative:
- The northern tunnel portal would be located east of the intersection of CDM and Jimmy Durante Boulevard.
- Similar to the CDM alternative, Jimmy Durante Boulevard would be raised to pass over the cut-and-cover section of the tunnel approach.
- Retaining walls would be required along the raised roadway to minimize impacts on adjacent properties and maintain access to the existing railroad right-of-way during construction.
- The conceptual design maintains the existing width and horizontal alignment of Jimmy Durante Boulevard, with the possibility of further optimization in future project phases.

For both alternatives, the report acknowledges that construction of the grade separation and roadway modifications at Jimmy Durante Boulevard would require careful phasing and traffic control to maintain access to the Del Mar Fairgrounds and minimize disruption to local circulation. The report notes that construction activities may be subject to seasonal restrictions to avoid impacts on major events at the fairgrounds and peak periods of coastal access.

The specific impacts on fairgrounds access and the duration of construction-related disruptions would depend on factors such as the final design of the grade separation, the construction methods and phasing, and the coordination with fairgrounds operations and other stakeholders. The report recommends that these issues be further evaluated and addressed in future project phases, including the development of detailed construction staging and traffic management plans.

It's worth noting that the alternatives analysis report also considered several other potential northern portal locations, including options within the fairgrounds property and over Jimmy Durante Boulevard, as part of a subsequent design refinement process. These options were evaluated at a conceptual level but were not advanced to the same level of engineering as the CDM and CCHS alternatives. The report notes that the portal location within the fairgrounds could potentially reduce impacts on Jimmy Durante Boulevard and local access but would require additional coordination with fairgrounds operations and may involve more extensive modifications to the existing rail infrastructure and special events platform.