See this page for conceptual drawings of the project, including the alignment of the new bridges and tunnels, from December 23, 2019.
Key excerpts:
Highlights to text by SMK.
This may be the deepest highway tunnel in the U.S.
Trestle upgrades.
Advantages of the bored tunnel alternative.
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F.2.3 SELECTED BORED TUNNEL ALTERNATIVE
The selected bored tunnel alternative will construct two new tunnels crossing Hampton Roads parallel to the west of the existing HRBT using a Tunnel Boring Machine (TBM). HRCP incorporated a bored tunnel construction method during the initial stages of design. The two new tunnels will have an internal diameter of 41.5 feet and be approximately 7,900 feet in length between the launch and reception shafts located on the North and South Islands. The tunnels will vary in depth from
approximately 40 to 150 feet below the water surface. A geologic stratum with weak geotechnical properties exists along a portion of the tunnel alignment just beneath and to the north of South Island. Jet grouting will be used for ground improvement (GI) to strengthen soils in this area prior to advancing the TBM. The anticipated total project schedule encompasses 62 months, with 36 months for tunnel construction only.
Additional Project components will include
full replacement of the North and South Trestle Bridges, expansion of the existing portal islands, and
widening of the Willoughby Bay Trestle Bridges, Bay Avenue Trestle Bridges, and Oastes Creek Trestle Bridges. Also, upland portions of I-64 will be widened to accommodate the additional lanes, the Mallory Street Bridge will be replaced, and the I-64 overpass bridges will be improved.
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Trestle Bridges
The existing two-lane North and South Trestle Bridges will be demolished and reconstructed. The North Trestle Bridges will be replaced by two new four-lane structures.
The two existing two-lane South Trestle Bridges will also be demolished and replaced by a new single eight-lane structure.
The existing Willoughby Bay Trestle Bridge structure will be modified by widening the two existing structures to the outside in both directions to accommodate new travel lanes, shoulders, and new sound walls. The trestle bridges crossing Bay Avenue and Oastes Creek will be similarly expanded.
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F.3 COMPARISON OF ALTERNATIVES
The selected bored tunnel alternative has many advantages when compared to the ITT [Immersed Tube Tunnel] alternative. A tunnel bored underneath the sediment-water interface will avoid substantial in-water impacts related to dredging and avoid direct navigation impacts to the federally- maintained channel. Less disturbance to the channel and open water reduces concerns to commercial ships and military vessels, which will minimize the impact on the economy, tourism, and national security as the tunnel is being constructed.
The bored tunnel construction also reduces overall costs, shortens schedule, and improves worker safety. The use of a bored tunnel approach would substantially reduce the volume of dredging when compared to the ITT approach minimizing the need for ocean disposal. Construction of the bored tunnel underground results in a reduction of noise, dust, and visual impacts. The tunnel itself will not cause impacts to subaqueous bottom, essential fish habitat, or benthic habitat, nor will additional fill or armor stone be placed on top of the tunnels. Finally, the bored tunnel creates substantially less exposure to weather risks such as wind and wave action during construction as the deeper elevations of the tunnel are constructed under the surface of the James River.
The selected bored tunnel has fewer impacts to WOUS [waters of the US] than the ITT, while still meeting the Project purpose and need.
http://www.hrbtexpansion.org/documents/2019/revised_joint_permit_application_and_drawings-december_23,_2019/008_hrbt_jpa_appf_rev1_alternatives_analysis_20191219_1_of_1.pdf